Phenological records in historical documents have been proven to be of unique value for reconstructing past climate changes. As a literary genre,
poetry reached its peak in the Tang and Song dynasties (618–1279 CE) in China. Sources from this period could provide abundant phenological
records in the absence of phenological observations. However, the reliability of phenological records from poems, as well as their processing methods,
remains to be comprehensively summarized and discussed. In this paper, after introducing the certainties and uncertainties of phenological
information in poems, the key processing steps and methods for deriving phenological records from poems and using them in past climate change
studies are discussed: (1) two principles, namely the principle of conservatism and the principle of personal experience, should be followed to
reduce uncertainties; (2) the phenological records in poems need to be filtered according to the types of poems, background information, rhetorical
devices, spatial representations, and human influence; (3) animals and plants are identified at the species level according to their modern
distributions and the sequences of different phenophases; (4) phenophases in poems are identified on the basis of modern observation criteria;
(5) the dates and sites for the phenophases in poems are confirmed from background information and related studies. As a case study, 86 phenological
records from poems of the Tang Dynasty in the Guanzhong region in China were extracted to reconstruct annual temperature anomalies in specific years
in the period between 600 and 900 CE. Following this, the reconstruction from poems was compared with relevant reconstructions in published studies to
demonstrate the validity and reliability of phenological records from poems in studies of past climate changes. This paper reveals that the
phenological records from poems could be useful evidence of past climate changes after being scientifically processed. This could provide an
important reference for future studies in this domain, in both principle and methodology, pursuant of extracting and applying phenological records
from poems for larger areas and different periods in Chinese history.
Introduction
Phenology is the study of recurring biological life cycle stages and the seasonality of non-biological events triggered by environmental changes
(Schwartz, 2003; Richardson et al., 2013). Phenological data derived from historical documents have been widely used as proxies to reflect past
climatic changes around the world, especially in Europe and Asia. The records of grape harvest dates (Chuine et al., 2004; Meier et al., 2007; Maurer
et al., 2009; Daux et al., 2012; Možný et al., 2016; Labbé et al., 2019), grain harvest dates (Nordli, 2001; Kiss et al., 2011; Wetter and
Pfister, 2011; Možný et al., 2012; Pribyl et al., 2012; Brázdil et al., 2018), and ice break-up dates (Tarand and Nordli, 2001; Nordli
et al., 2007; Etien et al., 2008) have been adopted to reconstruct past climate changes in Europe. In Japan, cherry blossom records have been used to
reconstruct spring temperatures dating back to the medieval period (800–1400 CE) (Aono and Kazui, 2008; Aono and Saito, 2010; Aono, 2015).
In China, occasional phenological observations began around 2000 years ago, and they have been recorded in various documents. These documents can be
further divided into sources produced by institutions and sources generated by individuals. The former includes Chinese classical documents and local
gazettes, as well as the archives of the Qing Dynasty (1644–1911 CE) and the Republic of China (1912–1949 CE) (Ge et al., 2008). Based on the
documents produced by institutions, abundant phenological records have been extracted to reconstruct past climate changes in specific regions and
periods in China (Chu, 1973; Ge et al., 2003; Zheng et al., 2005; Hao et al., 2009; Liu et al., 2016; Fei, 2019). However, the phenophases recorded in these documents are mainly non-organic, such as “ice phenology” (the time of freezing and
opening of water-bodies), “snow phenology” (the dates of first and last snowfall), and “frost phenology” (the dates of first and last frosts). The
limited amount of organic phenophases that do occur in these documents are principally “agricultural phenology” (e.g., the commencement dates of
spring cultivation, winter wheat harvest in summer, and millet harvest in autumn). Therefore, it is difficult to compare the phenological data from
documents produced by institutions with those from modern observations, which largely focus on the seasonal changes of natural plants. In contrast,
the phenological information in personal documents (mostly private diaries) is much more varied and includes records about both non-organic and
organic events, such as flowers blossoming, leaf expansion and discoloration, and fruit ripening (Ge et al., 2008; Liu et al., 2014; Zheng et al.,
2014). Using phenological evidence from diaries, many studies have reconstructed past climate changes in different regions and periods in China (Fang
et al., 2005; Xiao et al., 2008; Ge et al., 2014; Wang et al., 2015; Zheng et al., 2018). However, the diaries were most abundant within the past
800 years, especially in the Ming Dynasty (1368–1644 CE), the Qing Dynasty, and the Republic of China. The earliest diary found in China so far (The
Diary of Genzi-Xinchou by Lv Zuqian) only dates back to 1180 CE (Ge et al., 2018). Thus, there is a lack of phenological records on natural plants
and animals before the 1180s.
China enjoyed unprecedented economic prosperity, political stability, and a relatively open society in the Tang and Song dynasties
(618–1279 CE). The Imperial Examination System, a civil service examination system in imperial China for selecting candidates for the state
bureaucracy, had gradually improved, and poetry was incorporated into the examination subjects during this period (Zhang, 2015). In these contexts, as
a literary genre, poetry reached its peak during the Tang and Song dynasties in ancient China. A very diverse array of people in the Tang and Song dynasties, from emperors to
civilians, preferred to record their thoughts and daily lives in poems. Abundant phenological information that was
provided in poems of the Tang and Song dynasties is a valuable source for phenological records in this period.
Although many studies have indicated that there was a Medieval Warm Period (MWP) in China consistent with many other parts of the world, disputes
still exist regarding the start and end time, regional differences, and the extent of warming in different periods of the MWP in China (Zhang et al.,
2003; Yang et al., 2007; Ge et al., 2013). The period of the Tang and Song dynasties coincided with the MWP in China. More reconstruction studies of
the Tang and Song dynasties based on high-resolution proxies will contribute to a better understanding of these controversial issues. Extracting
phenological records from poems of the Tang and Song dynasties is an effective way to improve the resolution of proxy data in this period. However, it
is an extraordinary challenge to extract phenological records from poems due to the use of rhetorical devices, the limitations of poetic rules and
forms, and the conventions of rhymes and sounds in the poems. In addition, the phenological evidence in poems does not always follow modern criteria,
which could lead to considerable uncertainties if the real phenophases in poems were not properly identified.
Chu (1973) laid the foundation for climate reconstructions based on documents and has been highly praised worldwide for his efforts. Although a few
subsequent studies (Man, 1998; Ge et al., 2010) adopted phenological evidence from poems to reconstruct climate changes, further systematic and
specialized research on deriving phenological records from poems of the Tang and Song dynasties still needs to be carried out.
In this study, we first introduce the characteristics of phenological information in poems, including its accessibility and inherent uncertainties. We
then put forward basic principles and key processing steps for extracting phenological records from poems of the Tang and Song dynasties. We also
compare phenological records from poems with other proxies in the reconstruction of past climate changes in the Guanzhong region of central China as a
case study. Our overall objectives are to demonstrate the validity and reliability of phenological records from poems as a proxy of past climate
changes and to provide a reference, both theoretical and methodological, for the extraction and application of phenological records from poems.
The certainties and uncertainties of phenological information in poems from the Tang and Song dynasties
As a special carrier of historical phenological information, poetry has both certainties and uncertainties vis-à-vis applications to past climate
changes. For example, in the study by Chu (1973), which laid the foundation for climate reconstructions based on documents, 17 pieces of evidence were
from poems and 11 of them were phenological information of the Tang and Song dynasties. Most of the phenological information from poems used by Chu
(1973) was valid and the reconstructed results have been verified by other studies, which demonstrates the certainties of phenological records from
poetry. However, other phenological evidence such as the orange trees in the Guanzhong region used by Chu (1973) may be less certain. For instance, some
studies have pointed out that the orange trees in the Guanzhong region recorded in the poems of the Tang Dynasty (618–907 CE) were transplanted from
other places and were taken care of by specialized personnel in the Imperial Palace (Man, 1990; Mu, 1996). Therefore, the certainties and uncertainties
of phenological information in poems from the Tang and Song dynasties need to be analyzed before being used in studies of past climate changes.
The certainties of phenological information from poems
Poetry is one of the major genres of Chinese literature. It expresses peoples' social lives and spiritual worlds with concise and emotive words
according to the requirements of certain syllables, tones, and rhythms. The poetry of the Tang and Song dynasties represents the highest level of
poetry development and has become a treasure of Chinese traditional literature. People in the Tang and Song dynasties exhibited a preference for
recording and sharing their lives and ideas via poems, which is akin to recording in diaries in the later dynasties. Phenology, which could be used to
indicate seasons and guide agricultural activities, is one of the favorite topics of poets. As most of the poems were improvised, they commonly
reflect the real-time experiences of the poets. In addition, the great mass of the poems passed down to contemporary times were written by
well-educated scholars, who were able to describe the phenological phenomena they saw without misusing or abusing words. Thus, poetry is an excellent
carrier of phenological information.
Different types of phenology in poems from the Tang and Song dynasties.
Types of phenology Example quotes from poemsNon-organicphenology of iceAll the springs are frozen and stagnant (Supplement S1-2).phenology of snowIt snows in the eighth lunar month in frontier regions (Supplement S1-3).phenology of frostFrost falls in the eighth lunar month of every year (Supplement S1-4).Organicphenology of agricultureThe people have just finished planting mulberry trees to raise silkworms, and they are going to transplant rice seedling again (Supplement S1-5).phenology of natural plantsUme blossoms begin to bloom in early winter (Supplement S1-6).phenology of animalsThe river reflects the autumn scenery and the geese begin to fly south (Supplement S1-7).
Regarding the different types of poems of the Tang and Song dynasties, phenological information is most abundant in natural poems and realistic
poems. The natural poems describe the force and beauty of nature, such as mountains, rivers, animals, and plants; they contain almost all kinds of
phenological records, spanning both organic and non-organic sources (Table 1). The realistic poems strive for typicality in images, authenticity in details,
and objectivity in descriptions. For example, there is a line in a poem by Bai Juyi that is as follows: “there is a crescent moon on the third night and the
cicada sings for the first time” (Supplement S1-1), which specifically records the phenology of the first call of cicadas. Generally speaking, the
phenological information from poems, especially natural poems and realistic poems, is objective and authentic and can thus be leveraged as a data
source for reconstructing past climatic changes.
The quantity, spatial distribution, and accessibility of phenological records from the poems
By their very nature, Chinese poems have many distinctions in terms of recording phenological information compared to documents produced by
institutions and personal diaries (Table 2). Poems have evident advantages in the quantity and variety of phenological evidence. According to
Quan-Tang-Shi (the poetry of the Tang Dynasty) (Peng et al., 1986) and Quan-Song-Shi (the poetry of the Song Dynasty) (Center for Ancient Classics
and Archives of Peking University, 1999), nearly 50 000 poems from the Tang Dynasty and more than 270 000 poems from the Song Dynasty are
preserved. Numerous phenological records in the poems include not only non-organic events but also a variety of organic phenomena, most of
which concern the phenology of natural plants and animals.
Comparisons among the phenological evidence from poems, diaries, and documents produced by institutions in China.
PoemsDiariesDocuments produced by institutionsTypes of phenological evidenceorganic (phenology of plants and animals) and non-organic (phenology of ice, snow, and frost)organic (phenology of plants and animals) and non-organic (phenology of ice, snow, and frost)mostly non-organic (phenology of ice, snow, and frost) and a few organic (agricultural phenology)Amount of phenological evidencemoremorelessReasons for phenological record-keepingmemory of daily life/expressing feelingsmemory of daily life/observing phenologyrecording extreme climatic events and agriculture-related activitiesFrequency of phenological record-keepingsporadicsporadic to phenophase- specific recurrentphenophase-specific recurrentContinuity of phenological record-keepingintermittentintermittent/less than the lifetime of the observerup to the occurrence of extreme climatic eventsSpecies clarityambiguous to species-specificambiguous to species- specificmost clearPhenophases clarityambiguous to phenophase-specificambiguous to phenophase-specificmost clearSpatial clarityambiguous to inferableclear/inferablemost clearTemporal clarityambiguous to inferableclear/inferablemost clear
The spatial distributions of the phenological records are highly consistent with the ruling regions of the dynasties and there is a positive
relationship between the quantity of records preserved from particular areas and the level of development of those areas. Take the Song Dynasty
(960–1279 CE) as an example: because northern China was dominated by the Jin Dynasty from 1127 to 1279 CE, the phenological records from Quan-Song-Shi
of this period are mainly located in southern China, especially around the city of Hangzhou (the capital city of the Song Dynasty at that time).
In general, the accessibility of phenological records in poems tends to be lower than that of other documents. Unlike documents produced by
institutions in which phenological evidence was recorded by dedicated individuals, the phenological evidence in poems was recorded more
inadvertently. The information about phenophases in poems may be incomplete or ambiguous. For a specific phenophase, a poet usually only recorded it a
few times in poems during his lifetime. Thus, the frequency and continuity of the phenophase in his poems were relatively low. Take the word
“willow” as an example: it was mentioned in 9041 poems in the Quan-Tang-Shi and the Quan-Song-Shi, but clear species names, phenophases, dates, and
sites can be obtained from only 80 (0.88 %) poems. The accessibility of phenological records in poems may vary depending on particular
characteristics of the poets. For example, Li Bai and Du Fu are the most representative romantic poet and realistic poet in the Tang Dynasty,
respectively. According to Quan-Tang-Shi, there were 896 poems written by Li Bai and 1158 poems written by Du Fu. Among them, 23 (2.56 %) poems by
Li Bai and 76 (6.56 %) poems by Du Fu are related to phenology. Thus, the accessibility of phenological information from poems by Du Fu is more than
twice that of Li Bai. Only by integrating the same phenophase recorded by different poets could improvements be made in terms of frequency and
continuity.
The inherent uncertainties of phenological evidence in poems
In addition to the uncertainties arising from data interpretation, calibration, validation, and verification, the extraction of phenological evidence
from poems could also be associated with inherent uncertainties during the identification of species, the identification of phenophases, and the
ascertainment of dates and sites. Such uncertainties need to be identified as a precursor to using phenological records to reconstruct past climate
changes.
Uncertainties in the identification of species
Because the Chinese language has not changed fundamentally during the long history of the country, the people in present-day China can read ancient
poems without too much difficulty. Nevertheless, some changes in meanings and expressions of particular words and phrases still exist. Particular
words or phrases may have several additional meanings in ancient Chinese in comparison to modern usage. For example, the phrase “jin hua” (mainly
refers to golden flower in modern Chinese) has at least four meanings in the Quan-Tang-Shi, but only one of them is a substantial description of
phenology (Table 3).
Different meanings of the Chinese phrase “jin hua” in poems of the Tang Dynasty.
Pinyin of the verseMeanings of “jin hua” in the poemsfan ci huang jin hua (Supplement S1-8)chrysanthemum (inferred from context)sheng li jin hua qiao nai han (Supplement S1-9)decorations on ladies' headwearxuan miao mei jin hua (Supplement S1-10)an alchemistic term for Taoist priestscui wei jin hua bu ci ru (Supplement S1-11)golden patterns on the tails of peacocks
Comparisons among the ancient, modern, and Latin names of several common species.
SpeciesPinyin of ancient namePinyin of modern namesLatin nameAnimalssi jiu, zi gui, du yusi sheng du juanCuculus micropteruscang geng, shang geng, chu que, huang niaohei zhen huang liOriolus chinensisxuan niao, yi niao, luan niao, tian nv, wu yijia yanHirundo rusticatiao, fu yu, ni, qi nvcao chanMogannia conicaPlantsfu qu, fu rong, han danlianNelumbo nuciferalu, wei, jian jialu weiPhragmites australisshan shi liu, ying shan hong, shan zhi zhudu juanRhododendron simsiimu li, ming zha, man zhamu guaChaenomeles sinensis
The different names of some specific species in ancient China have also been simplified and unified in contemporary language. For example, the si
sheng du juan (Cuculus micropterus) had at least three different names during the Tang and Song dynasties (Table 4). It is also noteworthy
that the names of plants and animals in poems were mostly recorded at the genera level due to the lack of modern taxonomic knowledge. Nevertheless,
different species within the same genus may exhibit divergent responses to climate change according to modern phenological studies (Dai et al.,
2013). Thus, large uncertainties exist during the identification of species in poems.
Uncertainties in the judgment of phenophases
Phenophases in poems are not recorded in strict accordance with modern systematic criteria but are described through multiple rhetorical devices such
as metaphor, personification, hyperbole, quote, pun, and rhyme. As such, it is difficult to extract clear phenophases from poems. For example, there is
a line in a poem by the poet Quan Deyu as follows: “peonies occupy the spring breeze with their fragrance alone” (Supplement S1-12), which describes
the phase of peonies flowering. However, the phenophase in this line is equivocal due to the use of personification. To compare the phenological
records from poems with corresponding modern observational phenophases, the exact phenological stages need to be identified from the first flowering
date, the full-flowering date, and the end of flowering date. Therefore, uncertainties may be produced during the identification of specific
phenophases.
Uncertainties in ascertainment of dates
Exact dates are crucial for quantitatively evaluating phenological and climatic changes from past to present. By converting the Chinese lunar calendar
into the modern Gregorian calendar, the phenophases in the poems can be compared with modern observational phenophases. Some poems may contain precise
temporal information. For example, the poet Bai Juyi recorded the following in his poem: “the azalea is falling and the cuckoo is singing in this
year” (Supplement S1-13). The title of this poem is “Farewell spring (written on the 30th day of the third month of the 11th year of the Yuan
He)” – Yuan He is one of the reign titles of the Tang Dynasty, and the corresponding Gregorian date of this poem is 30 April 816 CE. However, the
time of writing was not explicitly recorded for most other poems. Any lack of information concerning year, month, or day may lead to failures in
phenological and climatic reconstructions. For instance, in another poem by Bai Juyi he states “people are busy in the fifth lunar month because the
wheat is yellow in the field” (Supplement S1-14). Here, only information concerning the month was directly presented in this poem, which could
obviously lead to uncertainties when deducing the year and the day. To make matters worse, some poems were written according to the memories or
imaginations of poets. The information from such poems thus needs to be excluded.
Uncertainties in ascertainment of sites
By matching the ancient name of a site with its modern name, the phenophases in poems can be compared with the corresponding observational phenophases
at the same site. However, similar to dates, the sites of phenophases in poems are sometimes missing. Worse still, some names of sites mentioned in
poems are imagined for the purpose of expressing emotions rather than to record real locations. For example, Lu You wrote a verse in his poem which
reads “there are so many willow branches in Ba Qiao, but who would have thought of sending one to me” (Supplement S1-15). Ba Qiao is a location in
Xi'an (a city in central China), which is more than 700 km away from the place where Lu You wrote this poem (Chengdu, China). By describing
the willow branches in his hometown in this poem, the poet expressed his homesickness. When attempting to ascertain sites, these kinds of
uncertainties should be carefully considered and dealt with appropriately.
The methods of processing phenological records in poems from the Tang and Song dynasties for past climate studies
To minimize the uncertainty during the extraction of clear species, phenophases, dates, and sites from poems and to render them comparable with modern
observations, several fundamental principles and processing steps should be put forward.
The basic principles for data processingThe principle of conservatism
The principle of conservatism refers to deducing ambiguous information conservatively in order to keep the characteristics of phenological information
without causing too much deviation. Take the aforementioned poem of Bai Juyi (Supplement S1-14) as an example: the poem was written in 807 CE in
Xi'an according to background information, while the exact date is not recorded. From the poem, we know that the harvest date of wheat in that year
appeared in the fifth lunar month (from 10 June to 8 July in the Gregorian calendar), and thus 10 June, which is the closest to the modern observations
(from 26 May to 8 June with the average of 2 June), can be determined as the date of wheat harvest in 807 CE in Xi'an. It should be noted that if the
recorded period in the poem overlaps with the time of the modern phenophase, the principle of conservatism is inapplicable, and the record in the poem
is invalid.
The principle of personal experience
The principle of personal experience demands that the phenological information described in the poems was being experienced by the poet, thus
excluding records based on imaginations or memories. For example, Yang Wanli recorded a line in his poem which stated that “begonias in my hometown
are flowering on this date and I see them blooming in my dream” (Supplement S1-16). From this, we know that he was not in his hometown when he wrote
this poem. Thus, the phenophase of Begonias in this poem cannot be used. It is more complex to diagnose the information in some poems. For example, Lu
You wrote a poem in 1208 CE wherein the following is recorded: “the Begonias in Biji Fang (place name) are the best in the world. Each branch looks dyed
with scarlet blood” (Supplement S1-17). By looking into the life experience of Lu You, this poem is found to record his memory of
1172 CE. Therefore, this poem cannot be used as phenological evidence either according to the principle of personal experience.
Processing steps of phenological records in poems for climate reconstructions.
The key steps of data processing
On the basis of the foregoing principles, four steps are required for the processing of phenological records in poems (Fig. 1).
Step 1: filtering the records
Filtering the records according to the features of poets and poems.
Poems commonly reflect the thoughts and daily lives of the poets. Thus, the poems written by people in certain professions that have little contact
with phenological events, such as the alchemists mentioned in Table 3, may contain little phenological information. In this way, the poems written
by alchemists can be excluded to improve the accessibility of phenological evidence from the poems. Furthermore, the records can be filtered
according to the styles of poems and the interests or life experiences of the poets. For example, it is more likely that phenological records can be
extracted from pastoral poems than from history-intoned poems.
Filtering the records according to the background information.
According to the background information of a poem, we can judge whether the phenophases in the poem actually happened, thus ensuring the robustness
of phenological evidence. For example, there is a line in a poem by Su Shi as follows: “a few branches of peach blossom outside the bamboo grove,
and the ducks will notice the warming of the river firstly” (Supplement S1-18). This seems to describe the natural phenophases in spring. However,
by looking into the background information, we know that this poem refers to a painting. Therefore, it describes the scenery within the painting
instead of real nature. The record thus needs to be excluded.
Filtering the records according to the rhetorical devices.
Whether the use of rhetorical devices in poems affects the authenticity of phenophases needs to be distinguished. For instance, despite the
rhetorical device of personification used in the aforementioned poem by Quan Deyu (Supplement S1-12), it does reflect the blossom of peonies. Thus,
this poem can be used in the study of past climate changes. The line of Lu Zhaoling saying that “the water in Laizhou (place name) has become
shallower several times and how ripe is the peach fruit” (Supplement S1-19) seems to enquire about the time of the peach phenophase, but
it is actually referring to the myth that peaches mature once every 3000 years in wonderland. The rhetorical device of quotation in this line has
affected the authenticity of phenophases. Thus, this record should be eliminated.
Filtering the records according to the spatial representations.
For a specific species, phenophases vary with latitude, longitude, and elevation. It is necessary to clarify the spatial representation of
phenological records in poems and to select records that are not affected by the local microclimate. For example, Bai Juyi recorded in his poem that
“all the flowers on the plain have withered in the fourth lunar month, but the peaches in the temple on the mountain just begin to bloom”
(Supplement S1-20). This record cannot be directly compared with modern observational data because the difference in altitude is almost
1000 m between the mountain in the poem and the modern observation site on the plain. Other factors that contribute to spatial differences
such as valleys, depressions, and heat island effects are also used to filter the records.
Filtering the records according to human influence.
Human activities, such as cultivation and transplantation, could also affect the phenophases of plants. To accurately reflect climate changes, it is
necessary to filter the records that were affected by human activities. Take the orange trees in the imperial palace of the Tang Dynasty as an
example. Some researchers pointed out that these oranges were transplanted from southern China and could not normally survive the winter on the
Guanzhong Plain. Thus, they were intensively managed by humans. This kind of phenological information in poems cannot be used as an indicator of
climate changes.
Step 2: identifying animals and plants at the species level
There are two principal ways of identifying the animals and plants in poems from the genera level to the species level. The first way involves
identifying the species according to the modern distribution of different species under the genera. For instance, the poet Liu Xian recorded the
following information in his poem: “the flowers of peaches are going to fall while the branches of willow are stretching” (Supplement S1-21). This
poem was written in Xi'an, which is located in the middle reaches of the Yellow River. Historically, the main peach species were Amygdalus davidiana and Amygdalus persica. According to modern species distributions, the former species can be found along the middle and lower
reaches of the Yellow River, while the latter occurs in the Huai River basin (Gong et al., 1983). Thus, the peach in the poem can be identified as
A. davidiana. The second way is to identify the species according to the sequences and correlations of different phenophases. For example,
Gao Shi wrote a poem in Chengdu wherein it is stated that “the green-up of willow leaves and the ume blossoms can't stop me from being sad”
(Supplement S1-22). The ume plant in ancient Chinese language usually refers to Chimonanthus praecox or Armeniaca mume. From the
content of the text, we can infer that the ume blossoms occurred at a similar time to the leaf expansion of willow. According to modern observation
data in Chengdu, the average full leaf expansion date of willow (Salix babylonica) is 23 February, while the average full-flowering dates of
Chimonanthus praecox and Armeniaca mume are 10 January and 10 February, respectively. The average date of full flowering for
A. mume is closer in time with the average date of full leaf expansion for willow. Thus, the ume blossoms in the poem can be identified as
A. mume.
Step 3: identifying the phenophases according to modern observation criteria
By applying the semantic differential technique, which is commonly used in the studies of past climate changes (Academy of Meteorological Science of
China Central Meteorological Administration, 1981; Wang, 1991; Wei et al., 2015; Yin et al., 2016; Su et al., 2018; Fang et al., 2019), the
descriptions in poems are classified and graded according to the criteria of the phenological observation methods in China (Wan and Liu, 1979; Gong
et al., 1983; Fang et al., 2005). Taking the aforementioned poem of Quan Deyu (Supplement S1-12) as an example, a line describes a scene where many
peonies were blooming and filling the spring breeze with strong perfume. By classifying and grading the key words “occupy” and “fragrance” in this
poem with other common descriptions of flowering phases in poems such as “tender”, “sparse”, “flourish”, “dense”, “wither”, and “fallen”,
the description of peony blooming in this poem was most likely to match with the full-flowering date under the modern criterion “more than half of
the flowers have blossomed in the observed species”. Thus, the phenophase in the poem can be identified as the full-flowering date. The
classification and grading results for some representative examples of phenological descriptions in poems are shown in Table 5.
Classification and grading results for representative examples of phenological descriptions in poems.
PhenophaseTranslation of the original versesDescription in the modern observation criteriaFirst songNew cicada tweeted two or three times (Supplement S1-23)The date of first callFirst appearanceNew swallow came ten days before the festival of She (Supplement S1-24)The date of first appearanceFirst leafWillow leaves are tender just as a beauty frowns slightly (Supplement S1-25)The date when the first one or two leaves are spread outFull leaf expansionThe green lotus leaves stretch to the horizon (Supplement S1-26)The date when the leaflets on half of the branches of the observed tree are completely flatFirst floweringThe hibiscus is at the beginning of the red and it covers the palace (Supplement S1-27)The date when the petals of one or several flowers begin to open fullyFull floweringPeonies occupy the spring breeze with their fragrance alone (Supplement S1-12)The date when more than half of the flowers have blossomed in the observed speciesEnd of floweringThe flowers of peach are going to fall while the branches of willow are stretching (Supplement S1-21)The date when there are very few flowers on the observed treesFruit dropThe willows and poplars in the street are shrouded in smog (Supplement S1-28)The date when Salix spp. and Populus spp. begin to have fluffy catkinsStep 4: ascertaining the dates and locations
This step firstly sought temporal information, including clear year, month, and date of the phenophase, from the titles, prefaces, and lines of the
poems. The missing time information could then be deduced by consulting the background information and related studies or estimated reasonably according
to the principle of conservatism. Finally, the time information in the Chinese lunar calendar had to be converted into the modern Gregorian
calendar. For example, the poet Cui Riyong recorded in his poem that “the ume blossoms in the palace smell fragrant and look delicate with the background
of snow” (Supplement S1-29). The title of this poem indicates that this poem records a banquet in the imperial palace on People's Day (Chinese
traditional festival on the seventh day of the first lunar month). From the poem, we do not know which year it was. However, this banquet was also
recorded by Xin Tang Shu (New Books of Tang, a history book of the Tang Dynasty) in the year 730 CE. Hence, we know that this poem was written in
730 CE.
Similarly, the exact location of the sites could be confirmed. It should be checked whether the place names appearing in the poems are real sites for
phenophases. For example, Ba Qiao is not the site of the phenophase for willow in the aforementioned poem by Lu You (Supplement S1-15). Thus, the
record in this poem cannot be used as phenological evidence for past climate studies.
Validation of the phenological records from poems for reconstructing past climate changes: a case study of temperature reconstruction in the Guanzhong region for specific years during 600–900 CE
To test the reliability of phenological records in poems for past climate change studies and the validity of the processing methods outlined in this
study, we extracted 86 phenological records (Appendix A) from poems of the Tang Dynasty to reconstruct the mean annual temperatures in the Guanzhong
region of China during the period of 600–900 CE.
The location of the Guanzhong region for the climatic reconstructions in this study with the modern names of sites mentioned in the poems. Publisher's note: please note that the above figure contains disputed territories.
Study area
The Guanzhong region (33∘35′–35∘50′ N, 106∘18′–110∘37′ E), located in central China
(Fig. 2), was where the capital city of the Tang dynasty was located. Many poets were active here and left many poems describing phenology during the
Tang dynasty. The study area has a continental monsoon climate with mean annual temperatures ranging from 7.8 to 13.5 ∘C and mean
annual precipitation from 500 mm in the northeast to 700 mm in the southwest (Qian, 1991).
Data and methods
Since the 86 records from poems pertain to diverse phenophases, they indicate temperature changes at different times of the year. To obtain a
relatively uniform and comparable series of reconstructed temperatures, the mean annual temperature anomaly was selected as the reconstruction
index. Transfer functions between annual temperature anomalies and corresponding phenophases were established by using modern observation data. The
transfer functions were then applied to reconstruct the annual temperature anomalies (with the reference period of 1961–1990 CE) in the Guanzhong
region during 600–900 CE. The modern phenological and meteorological data used and the detailed methods of the transfer functions are shown in
Appendix B.
Comparison of reconstructed temperature anomalies for 600–900 CE (with respect to the mean climatology between 1961 and 1990): (a) annual mean temperature anomalies reconstructed by phenological records from poems in this study; (b) annual mean temperature anomalies reconstructed using the phenological records from historical documents by Liu et al. (2016); (c) winter half-year temperature anomalies reconstructed from historical documents for the middle and lower reaches of the Yellow and Yangtze Rivers with a 30-year temporal resolution by Ge et al. (2003); and (d) annual mean temperature reconstructed from tree rings for the whole of Asia by Ahmed et al. (2013). Squares denote temperature anomalies reconstructed from poems, circles denote temperature anomalies reconstructed from documents of institutions, triangles denote temperature anomalies reconstructed from both poems and documents of institutions, green denotes temperature anomalies reconstructed by phenophases of wild plants, yellow denotes temperature anomalies reconstructed by agricultural phenophases, blue denotes temperature anomalies reconstructed by non-organic phenophases, red denotes temperature anomalies reconstructed by at least two types of phenophases, the gray area approximates the 95 % confidence interval according to ordinary least-squares (OLS) regression, and the dotted lines indicate the 2 standard deviation range of 1.72 ∘C of the modern period (1951–2013).
Results and comparisons with other reconstructions
Figure 3a shows the reconstructed annual temperature anomalies using the phenological records from poems. For validation purposes, the results were
compared with relevant studies. The first series used for comparison is attributable to Liu et al. (2016), wherein winter half-year (from October to
next April) temperature anomalies were reconstructed by 87 phenological records from historical documents (mostly produced by institutions) for the
period 600–902 CE in the Guanzhong region. The reconstruction by Liu et al. (2016) is a reliable reference not only because of the study area and
period considered coincide but also because the proxies used by that study and ours are phenological records from independent sources. To avoid the
additional influences of reconstruction indicators and transfer functions, the records from Liu et al. (2016) were reconstructed to annual temperature
anomalies (Fig. 3b).
Comparisons of data sources, types, and numbers of records used in Liu et al. (2016) and in this study.
Liu et al. (2016) This studyDocuments of institutionsPoemsTotalPoemsNon-organic phenophases420421Agricultural phenophases241251Phenophases of natural plants5152083Phenophases of animals0001Total71168786
Table 6 shows the historical data sources, types, and quantity of phenological evidence in Liu et al. (2016) and this study. Except for a single
record in one poem (Appendix A, poem No. 13), there is no duplication in records between the two studies. In general,
the two studies are based on similar quantities of evidence, while the data types used in the two studies are quite different. In terms of Liu
et al. (2016), 71 of 87 (nearly 82 %) pieces of phenological data are from documents produced by institutions. Among the 87 pieces of evidence, 67
(more than 77 %) are non-organic phenophases or agricultural phenophases (Fig. 3b). By contrast, the vast majority (more than 96 %) of evidence
from poems in this study are phenophases of wild plants (Fig. 3a). These
differences suggest that the phenological records in poems are effective supplements to historical phenological evidence for the period of the Tang
Dynasty. It is also worth noting that fewer years are reconstructed in this study (36) compared to in Liu et al. (2016) (76), which further supports
the claim that the frequency and continuity of phenological records preserved in poems is more sporadic than that of documents produced by
institutions (Table 2).
To assess the validity of the temperature reconstruction from poems, two more temperature reconstructions by different proxies were leveraged for
comparison. The first was winter half-year temperature anomalies at a 30-year resolution reconstructed from documentary evidence in the middle and
lower reaches of the Yellow and Yangtze Rivers of China (Ge et al., 2003) (Fig. 3c). The second was annual temperature anomalies reconstructed from
tree rings in Asia (Ahmed et al., 2013) (Fig. 3d). All four reconstructions indicated that there were more relatively cold years in the later periods
after around the 800s. Indeed, the coldest years according to all four reconstructions occurred in this period. Before the 800s, the reconstructions
by Liu et al. (2016), Ge et al. (2003), and our study showed more relatively warm temperatures, with the warmest years occurring around the
660s. Furthermore, the amplitude of reconstructed temperature by Liu et al. (2016) was 3.30 ∘C, which was very similar to the
amplitude of reconstructed temperature (3.28 ∘C) in our study. As a benchmark, according to modern data spanning 1951–2013 the
amplitude was 3.97 ∘C. In sum, the similarities between different reconstructions confirm the effectiveness of phenological records
from poems for gauging past climate changes.
Discussion
There are still controversies about how the climate changed during the Tang and Song dynasties (Chu, 1973; Fei et al., 2001; Yang et al., 2002; Ge
et al., 2003; Tan et al., 2003; Thompson et al., 2006; Zhang and Lu, 2007). One of the reasons lies in the lack of sufficient evidence supporting the
climatic reconstructions. Although some studies have reconstructed the temperatures during this period using natural evidence such as tree rings,
pollen, and sediment (Xu et al., 2004; Zhang et al., 2014; Zhu et al., 2019), their results either do not cover the entire period or they have
relatively low temporal resolution. In addition, these natural proxies are mostly collected from uninhabited areas, and thus they are not particularly
amenable to evaluating the interactions between climate change and human activities. In comparison, documentary evidence, which occurs more frequently
and is closer to human life, has become an important data source for reconstructing climate changes in this period. As one of the most popular
literary forms in the Tang and Song dynasties, poetry has huge potential to provide abundant and diverse phenological information, which will
undoubtedly contribute to the study of historical climate change.
Despite this, very few studies have thus far been reported concerning the use of phenological records from poems to quantitatively reconstruct
historical climate change due to the lack of effective methodologies for data extraction. Unlike climate reconstructions using other proxies that have
standard processing methods and clear reference objects, the processing of phenological records from poems is much more complex. For example, dating
tree-ring samples only requires counting the number of annual rings from the outside to the inside or comparing them with a standard
chronology. However, the temporal information in poems cannot be obtained directly from a reference chronology. As already mentioned, the temporal
information in poems may be hidden in the poet's biography, official history books, or some related studies. It is necessary to search through
these materials one by one and make careful comparisons before ascertaining the exact temporal information, and indeed some information is found to be
unrecorded after searching through large amounts of material. This problem also exists when seeking to extract information concerning species,
phenophases, and sites from poems.
We attempt to introduce a standard procedure for extracting phenological records from poems, which could, on the one hand, minimize the uncertainties
of the records, and on the other hand, efficiently filter irrelevant records. By following the principles and steps herein, researchers can understand
where to find the information needed and how to manage the phenological data from poems. The extracted phenological records are comparable with modern
observation data and can be used as a proxy for quantitatively reconstructing climate changes.
Although the validity of phenological records from poems has only been tested in a single area of China in the Tang Dynasty, the methodologies of
extracting and processing phenological records from poems for climate reconstructions proposed in this study could be applied to wider regions and
longer periods. On the one hand, many studies have demonstrated that climate is the primary driver of phenophases over the whole of China (Piao et al.,
2006; Dai et al., 2014; Ge et al., 2015; Tao et al., 2017), which indicates that the phenological records obtained at any place could be used as
evidence of climate changes. On the other hand, historians agree that the feudal society in Chinese history did not fundamentally change during
different dynasties (Liu, 1981; Tian, 1982; Feng, 1994). Although historical China varied its administrated area coverage from dynasty to dynasty, its
core socioeconomics closely aligned with the major agricultural areas throughout history. This geographic and temporal overlap allows for continuous
comparison across the core areas of China (Fang et al., 2019). Correspondingly, the essence of literature, especially poetry, has not changed, though
different types of poetry varied in their popularity between dynasties, e.g., differences in terms of poetic forms, the number of words, and the needs
of rhymes and sounds. Therefore, the phenological records obtained from poems from different periods in core areas of historical China can also be
extracted and processed for climatic reconstruction according to the method in this study.
We only used 86 phenological records extracted from poems to reconstruct the temperature anomalies for a small area in the Tang Dynasty. Although the
uncertainties from transfer functions are shown in Appendix C, there are other uncertainties that are difficult to quantitatively assess. For example,
differences in cultivated plant types and crop management may have an effect on the temperature reconstruction, though many studies show that
phenological changes in cultivated plants are principally driven by climate changes, especially temperature variations (Estrella et al., 2007; Lobell
et al., 2012; Liu et al., 2018). Overall, the reconstruction in this study testifies to the reliability of phenological records from poems in
indicating past climate changes. Nevertheless, there are still many phenological records that remain to be extracted. By rough estimation, the
temporal resolution of the phenological records from poems of the Tang and Song dynasties can reach 20 years or less. In addition, phenological
records from poems of the Tang and Song dynasties are widely distributed, covering almost all the regions of modern China. Take the Song Dynasty
(960–1279 CE) as an example. Although northern China was dominated by the Jin Dynasty from 1127 to 1279 CE, which means that most poems written by
poets living in northern China are not contained in the Quan-Song-Shi, we can try to search from the Quan-Jin-Shi (the Poetry of the Jin Dynasty) (Xue
and Guo, 1995) to add phenological records in northern China. The rich records around the capitals and developed cities are of great value in terms of
comparisons with modern phenological observations. Future work will be focused on extracting more records from poems and developing integration
methods for different phenophases at different sites to explore the overall phenological changes and climate changes over a larger region.
Conclusions
In this study, we put forward a processing method to extract phenological information from poems of the Tang and Song dynasties, which includes two
principles (the principle of conservatism and the principle of personal experience) and four steps: (1) filtering the records based on the features of
poets and poems, the background information, the rhetorical devices, the spatial representations, and human influence; (2) identifying the animals and
plants to the species level; (3) judging the phenophases according to modern observation criteria; and (4) ascertaining times and sites. We then used this
method to extract 86 phenological records from poems of the Guanzhong region in central China and reconstructed the annual mean temperature anomalies
for specific years during 600–900 CE. The reconstructed temperature anomaly series was comparable with that reconstructed by records from documents
in the same area and period, demonstrating that our method is effective and reliable. This paper therefore provides a reference in both theory and
method for the extraction and application of phenological records from poems in studies of past climate changes.
Phenological records from poems used in the reconstruction of this study.
No.Gregorian dateSitePhenophaseTranslation of the original verses128 Jun 618Xi'anEnd flowering date of Punica granatumIt missed the spring because of late blooming (Supplement S1-30).227 Feb 631Xi'anFull leaf expansion date of Salix babylonicaThe leaves of willow welcome the third lunar month and the ume blossoms take the two years apart (Supplement S1-31).327 Feb 631Xi'anFull-flowering date of Armeniaca mumeThe leaves of willow welcome the third lunar month and the ume blossoms take the two years apart (Supplement S1-31).418 Jan 634Xi'anFull-flowering date of Chimonanthus praecoxThere are no leaves on the willow tree, but flowers on the ume tree (Supplement S1-32).527 Apr 636Xi'anFull-flowering date of Juglans regiaPeach flowers blossom for those who are going away (Supplement S1-33).610 Sep 660Xi'anFull-flowering date of Osmanthus fragransOnly Osmanthus blooms near the south hill (Supplement S1-34).731 Aug 664Xi'anEnd flowering date of Osmanthus fragransOsmanthus is at the end of flowering in the moonlight and the ume tree is at the beginning of flowering under the beam (Supplement S1-35).831 Aug 664Xi'anFirst flowering date of Chimonanthus praecoxOsmanthus is at the end of flowering in the moonlight and the ume tree is at the beginning of flowering under the beam (Supplement S1-35).98 Feb 671Xi'anFirst flowering date of Armeniaca mumeUme blossoms early in the palace and the willow is new near the creek (Supplement S1-36).108 Feb 671Xi'anFirst leaf date of Salix babylonicaUme blossoms early in the palace and the willow is new near the creek (Supplement S1-36).1118 Feb 674Xi'anFull leaf expansion date of Salix babylonicaThe wicker swings to show its beauty (Supplement S1-37).1211 Aug 681Xi'anFruit maturity date of Amygdalus davidianaThe peaches in the palace are very luxuriant (Supplement S1-38).136 Apr 707Xi'anEnd flowering date of Amygdalus davidianaThe flowers of peach are going to fall while the branches of willow are stretching (Supplement S1-21).146 Apr 707Xi'anFull leaf expansion date of Salix babylonicaThe flowers of peaches are going to fall while the branches of willow are stretching (Supplement S1-21).154 Feb 708Xi'anFirst leaf date of Salix babylonicaThe delicate wicker on the embankment has not turned yellow (Supplement S1-39).164 Feb 708Xi'anFirst flowering date of Armeniaca mumeThe fragrance of ume blossoms and the color of willows can withstand praise (Supplement S1-40).174 Feb 708Xi'anFirst leaf date of Salix babylonicaThe fragrance of ume blossoms and the color of willows can withstand praise (Supplement S1-40).184 Feb 708Xi'anFirst flowering date of Armeniaca mumeThe fragrance of ume blossoms seems to be obscured by beautiful singing (Supplement S1-41).194 Feb 708Xi'anFirst flowering date of Armeniaca mumeUme blossoms vie to bloom in the palace (Supplement S1-42).204 Feb 708Xi'anFirst flowering date of Armeniaca mumeThe ume blossoms and willows in the palace can recognize the weather (Supplement S1-43).214 Feb 708Xi'anFirst leaf date of Salix babylonicaThe ume blossoms and willows in the palace can recognize the weather (Supplement S1-43).224 Feb 708Xi'anFirst flowering date of Amygdalus davidianaWhy do peaches and plums compete to bloom (Supplement S1-44).234 Feb 708Xi'anFirst flowering date of Prunus salicinaWhy do peaches and plums compete to bloom (Supplement S1-44).
Continued.
No.Gregorian dateSitePhenophaseTranslation of the original verses244 Feb 708Xi'anFirst flowering date of Armeniaca vulgarisNew apricot blossoms adorn the palace and ume blossoms bloom at the feast (Supplement S1-45).254 Feb 708Xi'anFirst flowering date of Armeniaca mumeNew apricot blossoms adorn the palace and ume blossoms bloom at the feast (Supplement S1-45).2610 Feb 709Xi'anFull-flowering date of Chimonanthus praecoxThe flicking of snow on the branches adds to the beauty of ume blossoms (Supplement S1-46).2721 Feb 709Xi'anFirst flowering date of Armeniaca mumeUme blossoms and willow catkins are new (Supplement S1-47).2815 Mar 709Xi'anFull leaf expansion date of Salix babylonicaThe willows leaves are all open over the city (Supplement S1-48).2915 Mar 709Xi'anFull leaf expansion date of Salix babylonicaWillows secretly urge the late spring (Supplement S1-49).3017 Apr 709Xi'anBeginning date of fruit drop of Salix babylonicaThe willow by the river flicks the emperor's goblet (Supplement S1-50).3116 Oct 709Xi'anEnd flowering date of Osmanthus fragransThe Osmanthus fell into the goblet full of wine (Supplement S1-51).324 Mar 710Xi'anFull-flowering date of Armeniaca mumeThe ume blossoms remain white when the cold is over while the willows have not turned yellow when the wind is late (Supplement S1-52).334 Mar 710Xi'anFull leaf expansion date of Salix babylonicaThe ume blossoms remain white when the cold is over while the willows have not turned yellow when the wind is late (Supplement S1-52).344 Mar 710Xi'anFull leaf expansion date of Salix babylonicaThere are thousands of willows unfolding their leaves (Supplement S1-53).3525 Mar 710GuanzhongFull-flowering date of Amygdalus davidianaThere are red flowers all over the ground and the whole banquet is filled with fragrance (Supplement S1-54).3625 Mar 710GuanzhongFull-flowering date of Amygdalus davidianaThe red calyxes bloom against the dawn in the garden (Supplement S1-55).3725 Mar 710GuanzhongFull-flowering date of Amygdalus davidianaThe peach blossoms are bright and seem to have brilliance (Supplement S1-56).3825 Mar 710GuanzhongFull-flowering date of Amygdalus davidianaCountless flowers bloom among the flowers by the water (Supplement S1-57).3925 Mar 710GuanzhongFull-flowering date of Amygdalus davidianaThe gorgeous flowers in the garden accompany the beauty (Supplement S1-58).403 Apr 710GuanzhongEnd flowering date of Amygdalus davidianaThe peach blossoms by the Wei River fall into the water (Supplement S1-59).414 Apr 710Xi'anFull-flowering date of Amygdalus davidianaWhen the peaches and plums bloom in spring, the scenery of the capital city is good (Supplement S1-60).424 Apr 710Xi'anFull-flowering date of Prunus salicinaWhen the peaches and plums bloom in spring, the scenery of the capital city is good (Supplement S1-60).434 Apr 710Xi'anBeginning date of fruit drop of Salix babylonicaThe red calyx exudes fragrance and the branches of willows are surrounded by green ribbons (Supplement S1-61).444 Apr 710Xi'anFull-flowering date of Amygdalus davidianaThe red calyx exudes fragrance and the branches of willows are surrounded by green ribbons (Supplement S1-61).455 Apr 710Xi'anEnd flowering date of Armeniaca mumeThe ume blossoms in the palace glowed against the snow and the willow trees in the city were full of smog (Supplement S1-62).465 Apr 710Xi'anBeginning date of fruit drop of Salix babylonicaThe ume blossoms in the palace glowed against the snow and the willow trees in the city were full of smog (Supplement S1-62).
Continued.
No.Gregorian dateSitePhenophaseTranslation of the original verses475 Apr 710Xi'anBeginning date of fruit drop of Salix babylonicaThe willows and ume blossoms in the palace are covered with green ribbons (Supplement S1-63).485 Apr 710Xi'anEnd flowering date of Armeniaca mumeThe willows and ume blossoms in the palace are covered with green ribbons (Supplement S1-63).495 Apr 710Xi'anBeginning date of fruit drop of Salix babylonicaThe willows are covered with green smog (Supplement S1-64).506 Apr 710Xi'anBeginning date of fruit drop of Salix babylonicaThe green ribbons from the willows float at the banquet (Supplement S1-65).516 Apr 710Xi'anEnd flowering date of Amygdalus davidianaRed peach blossoms and emerald green willows adorn the fete (Supplement S1-66).526 Apr 710Xi'anBeginning date of fruit drop of Salix babylonicaRed peach blossoms and emerald green willows adorn the fete (Supplement S1-66).539 May 710Xi'anFirst flowering date of Hibiscus syriacusTrees cover the palace and the hibiscuses start to turn red (Supplement S1-67).5424 Mar 711GuanzhongFull-flowering date of Prunus salicinaThe peach and plum blossoms are lost in their own fragrance (Supplement S1-68).5524 Mar 711GuanzhongFull-flowering date of Amygdalus davidianaThe peach and plum blossoms are lost in their own fragrance (Supplement S1-68).5614 Feb 713Xi'anEnd flowering date of Chimonanthus praecoxThe garden is only accompanied by withered ume blossoms in spring (Supplement S1-69).5728 Feb 713Xi'anFirst leaf date of Salix babylonicaThe branches of willows are fresh (Supplement S1-70).587 Apr 715Xi'anEnd flowering date of Amygdalus davidianaThe pool water is covered with peach blossoms (Supplement S1-71).5929 Jan 730Xi'anFull-flowering date of Chimonanthus praecoxThe ume blossoms in the palace smell fragrant and look delicate with the background of snow (Supplement S1-29).603 Apr 740Xi'anBeginning date of fruit drop of Salix babylonicaPeople at the banquet all resent the falling catkins (Supplement S1-72).6110 Apr 753Xi'anBeginning date of fruit drop of Salix babylonicaThe catkins fall like snowflakes (Supplement S1-73).625 Feb 756Xi'anFull-flowering date of Chimonanthus praecoxThe umes bloom towards the sky (Supplement S1-74).6318 Mar 758Xi'anFirst leaf date of Salix babylonicaThere are thousands of tender branches of willows in the palace (Supplement S1-75).6418 Mar 758Xi'anFull-flowering date of Amygdalus davidianaPeach blossoms are as red as drunk (Supplement S1-76).6515 Apr 758Xi'anEnd flowering date of Amygdalus davidianaThe peach blossoms wither after the catkins (Supplement S1-77).6615 Apr 758Xi'anBeginning date of fruit drop of Salix babylonicaThe peach blossoms wither after the catkins (Supplement S1-77).673 Apr 760Xi'anFull-flowering date of Pyrus betulaefoliaPear flowers bloom during the Cold Food Festival (Supplement S1-78).6818 Mar 762Xi'anFull leaf expansion date of Salix babylonicaFlowers and willows in every village bloom of their own accord (Supplement S1-79).693 Apr 782Xi'anBeginning date of fruit drop of Salix babylonicaIn spring the city is full of flying catkins (Supplement S1-80).7025 Feb 784Xi'anFirst leaf date of Salix babylonicaThe flowers and willows in the capital are fresh (Supplement S1-81).7119 Apr 790Xi'anFull-flowering date of Paeonia suffruticosaPeonies occupy the spring breeze with their fragrance alone (Supplement S1-12).724 Apr 800Xi'anBeginning date of fruit drop of Salix babylonicaThe sycamore blooms after the willow catkins (Supplement S1-82).734 Apr 800Xi'anFirst flowering date of Firmiana platanifoliaThe sycamore blooms after the willow catkins (Supplement S1-82).
Continued.
No.Gregorian dateSitePhenophaseTranslation of the original verses744 Apr 800Xi'anFirst flowering date of Amygdalus davidianaPeach and plum flowers are fresh in every courtyard (Supplement S1-83).754 Apr 800Xi'anFirst flowering date of Prunus salicinaPeach and plum flowers are fresh in every courtyard (Supplement S1-83).764 Apr 800Xi'anFirst flowering date of Paulownia fortuneiPaulownia blooms on Qingming Festival (Supplement S1-84).772 May 805Xi'anEnd flowering date of Paulownia fortuneiThe purple paulownia flowers are falling and the birds are singing (Supplement S1-85).787 Aug 805Xi'anFirst sing date of Cryptotympana atrataA new cicada calls two or three times (Supplement S1-86).791 May 807ZhouzhiEnd flowering date of Paeonia suffruticosaWhen I come back, the peony flowers are all over (Supplement S1-87).8010 Jun 807ZhouzhiBeginning date of winter wheat harvestPeople are busy in the fifth lunar month because the wheat is yellow in the field (Supplement S1-14).8122 Oct 808Xi'anFirst date of frostFrost falls in the ninth lunar month and it turns cold early in autumn (Supplement S1-88).8227 Sep 813Xi'anFull-flowering date of Osmanthus fragransOsmanthus by the railing exudes fragrance (Supplement S1-89).8313 May 815Xi'anBeginning date of fruit drop of Salix babylonicaWillow catkins are flying all over the sky just like snowflakes (Supplement S1-90).843 Apr 820Xi'anFull-flowering date of Armeniaca vulgarisAlthough the apricot blossoms here are better than in other places, I still want to see the flowers in my hometown (Supplement S1-91).8524 Sep 831Xi'anFull-flowering date of Osmanthus fragransThe cold dew wet the Osmanthus quietly (Supplement S1-92).864 Apr 865Xi'anEnd flowering date of Armeniaca vulgarisApricot flowers seem to be sad with me together (Supplement S1-93).The modern data sources and reconstructing method in this study
Modern phenological observation data in Xi'an, which located in the center of Guanzhong region, were derived from the China Phenological Observation
Network (CPON). Xi'an has kept observations every year since 1963, except for the period of 1997–2002. The annual mean temperature data of 1951–2013
in Xi'an was obtained from the Chinese Meteorological Administration. Owing to a lack of data, some modern phenophases were defined based on the
meteorological data. For instance, the modern date of spring cultivation was defined as the first day when the daily mean temperature is
consecutively higher than 5 ∘C for 5 d (Ge et al., 2010). The modern date of millet harvest in autumn is defined as the first
day when the daily mean temperature is continuously lower than 10 ∘C for 5 d (Hao et al., 2009).
After changing the time series of temperature and phenophases to anomalies with respect to the reference period (1961–1990 CE), the transfer
functions between the phenological and temperature anomalies were developed by linear regression, which can be expressed as
y=axi+b,
where y is the annual temperature anomalies and xi is the phenological anomalies for phenophase i. The constants a and b are estimated
using the least-squares method and represent the regression slope and intercept, respectively.
Subsequently, the phenophase-specific transfer functions were applied to each historic phenological anomaly to obtain the annual temperature
anomalies. If there was more than one record in a single year, temperature in that year was calculated as the arithmetic mean of all of the
reconstructed temperatures in that year.
Transfer functions for the temperature reconstructions based on phenological records obtained from Liu et al. (2016) and from poems in this study.
PhenophasesTransfer functionsNumber ofobservationsCorrelationcoefficientsStandard error at 95 %confidence level (∘C)First date of frosty=0.033x+0.423530.432b0.742Last date of frosty=-0.033x+0.38653-0.475b0.724First date of snowy=0.010x-0.023260.467a0.321Last date of snowy=-0.006x-0.01926-0.3350.336First sing date of Cryptotympana atratay=0.013x+0.012150.6380.216Beginning date of spring cultivationy=-0.030x+0.23262-0.396b0.792Beginning date of winter wheat harvesty=-0.084x+1.28422-0.570b0.584Beginning date of millet harvesty=0.024x+0.336610.2310.806First flowering date of Amygdalus davidianay=-0.075x+0.36138-0.573b0.667Full-flowering date of Amygdalus davidianay=-0.086x+0.33138-0.634b0.630End flowering date of Amygdalus davidianay=-0.069x+0.44137-0.531b0.691Fruit maturity date of Amygdalus davidianay=0.022x+0.740130.4950.505First flowering date of Armeniaca mumey=-0.044x+0.62614-0.4360.785Full-flowering date of Armeniaca mumey=-0.055x+0.59014-0.5070.752End flowering date of Armeniaca mumey=-0.061x+0.58614-0.617a0.717First flowering date of Armeniaca vulgarisy=-0.029x+0.11924-0.3200.467Full-flowering date of Armeniaca vulgarisy=-0.045x+0.19620-0.517a0.402End flowering date of Armeniaca vulgarisy=-0.028x+0.13524-0.3310.466First flowering date of Chimonanthus praecoxy=-0.007x+0.669260.1960.845Full-flowering date of Chimonanthus praecoxy=-0.011x+0.77025-0.2180.813First flowering date of Firmiana platanifoliay=-0.016x+0.13514-0.2170.486First flowering date of Hibiscus syriacusy=-0.014x+0.06018-0.4570.456Full-flowering date of Juglans regiay=-0.076x+0.44133-0.663a0.612Full-flowering date of Osmanthus fragransy=-0.069x+0.30617-0.611b0.716End flowering date of Osmanthus fragransy=0.044x+0.486220.497a0.728Full-flowering date of Paeonia suffruticosay=-0.088x+0.30738-0.703b0.581End flowering date of Paeonia suffruticosay=-0.065x+0.49336-0.446b0.731First flowering date of Paulownia fortuneiy=-0.062x+0.68822-0.607a0.813End flowering date of Paulownia fortuneiy=-0.055x+1.10318-0.3820.901First flowering date of Prunus salicinay=-0.068x+0.58513-0.740b0.515Full-flowering date of Prunus salicinay=-0.068x+0.59113-0.779b0.480End flowering date of Punica granatumy=0.056x+0.25721-0.4500.825Full-flowering date of Pyrus betulaefoliay=-0.076x+0.44127-0.698b0.608First leaf date of Salix babylonicay=-0.052x+0.74531-0.471b0.711Full leaf expansion date of Salix babylonicay=-0.042x+0.51137-0.384a0.753Beginning date of fruit drop of Salix babylonicay=-0.091x+1.31217-0.707b0.602
aP<0.05, bP<0.01Note that the original verses and sources of the poems in Chinese used in this paper can be found in the Supplement.
Data availability
All the data used to perform the analysis in this study are described and properly referenced in the paper. The phenological records from poems used to reconstruct the annual temperatures are listed in Appendix A, and all of the original sources of the verses used in this paper are listed in the Supplement in Chinese. The modern phenological data are available from the National Earth System Science Data Center (2020, http://www.geodata.cn/data/datadetails.html?dataguid=5881257&docid=19673). The modern meteorological data are available from the China Meteorological Data Service Center (2021, http://data.cma.cn/en/?r=data/detail&dataCode=SURF_CLI_CHN_MUL_DAY_CES_V3.0).
The supplement related to this article is available online at: https://doi.org/10.5194/cp-17-929-2021-supplement.
Author contributions
YL and ZT contributed to the idea and design of the structure of paper. YL collected and analyzed the data. YL, XF, JD, HW, and ZT wrote the paper.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “International methods and comparisons in climate reconstruction and impacts from archives of societies”. It is not associated with a conference.
Acknowledgements
We would like to thank the anonymous reviewers and editors for their valuable comments.
Financial support
This research has been supported by the National Natural Science Foundation of China (grant nos. 41807438, 41771056), the Strategic Project of Science and Technology of the Chinese Academy of Sciences (grant no. XDA19040101), the National Key Research and Development Program of China (grant no. 2018YFA0606102), Natural Science Basic Research Program of Shaanxi (program no. 2021JQ-793), and the Special Scientific Research Program of Education Department of Shaanxi Provincial Government (grant no. 20JK0877).
Review statement
This paper was edited by Qing Pei and reviewed by three anonymous referees.
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