Much of our knowledge about the impacts of volcanic eruptions on climate comes from proxy records. However, little is known about their impact on the low to mid-latitudes of the Southern Hemisphere. Using superposed epoch analysis, we investigated whether volcanic signals could be identified in annual tree-ring series from eight New Zealand dendrochronological species. We found that most species are reliable recorders of volcanic cooling and that the magnitude and persistence of the post-event response can be broadly linked to plant life history traits. Across species, site-based factors, particularly altitude and exposure to prevailing conditions, are more important determinants of the strength of the volcanic response than species. We then investigated whether chronology selection impacts the magnitude of post-volcanic cooling in tree-ring-based temperature reconstructions by developing two new multispecies reconstructions of New Zealand summer (December–February) temperature with one reconstruction from the pool of all available chronologies, and the other from a selected subset shown to be sensitive to volcanic eruptions. Both reconstructions record temperature anomalies that are remarkably consistent with studies based on instrumental temperature and the ensemble mean response of climate models, demonstrating that New Zealand ring widths are reliable indicators of regional volcanic climate response. However, we also found that volcanic response can be complex, with positive, negative, and neutral responses identified – sometimes within the same species group. Species-wide composites thus tend to underestimate the volcanic response. This has important implications for the development of future tree-ring and multiproxy temperature reconstructions from the Southern Hemisphere.
Emissions from large volcanic eruptions are a key source of temperature and hydroclimate variability on interannual to decadal timescales (Robock, 2005; Iles et al., 2013; Sigl et al., 2015). As few large volcanic eruptions have occurred during the instrumental era, much of our knowledge about volcanic impacts on climate, particularly regional and global temperature, comes from proxy records spanning centuries to millennia (Tejedor et al., 2021; Wilson et al., 2016; D'Arrigo et al., 2013). These records are predominantly high-altitude or high-latitude tree-ring proxies from the Northern Hemisphere (e.g. Briffa et al., 1998; D'Arrigo et al., 2009; Pieper et al., 2014). In comparison, there are very few proxy-based characterisations of the temperature response to volcanic events from the Southern Hemisphere (Tejedor et al., 2021; Neukom et al., 2014). The limited number of studies considering Southern Hemisphere tree-ring proxies have generally not found significant impacts following what are considered “large” volcanic eruptions (Krakauer and Randerson, 2003; Palmer and Ogden, 1992; Allen et al., 2018; Cook et al., 1992). Two eruptions (Santa Maria in 1902 CE and Agung in 1963 CE) have been identified in tree-ring sites spanning South America, but the impacts of other eruptions on growth have proved less conclusive (Villalba and Boninsegna, 1992).
Discovering whether the hemispheres have contrasting sensitivities to volcanic eruptions is vital to understanding future climate projections (Neukom et al., 2014). The muted volcanic impact in Southern Hemisphere proxy reconstructions could be due to a maritime dampening effect on post-eruption cooling and/or the distribution of landmasses toward the Equator (Raible et al., 2016; Krakauer and Randerson, 2003; Allen et al., 2018). Such explanations suggest that the magnitude of Southern Hemisphere cooling is too small to be reliably recorded in tree-ring archives. However, climate models show a clear Southern Hemisphere volcanic signal via reduced mean surface air temperatures (Neukom et al., 2014, 2018). There are several potential explanations for the discrepancy between proxy reconstructions and climate models in the Southern Hemisphere. These include the underestimation of the moderating effects of the ocean on post-eruption cooling in climate models, changes to the hydrological cycle in response to volcanic cooling, uncertainties in volcanic forcing data, and/or proxy noise and spatial distribution (Neukom et al., 2018; Zhu et al., 2020).
The question remains as to whether Southern Hemisphere proxies – specifically tree rings – do record volcanic events. To our knowledge, no studies have explored the factors which influence whether (or not) volcanic signals can be identified using tree-ring data from the Southern Hemisphere. Tree growth depends on a range of environmental and biological factors, and thus careful site and tree selection is necessary to ensure that a specific influence of interest can be studied (Norton and Ogden, 1987). Northern Hemisphere tree-ring studies are predominantly from high latitudes or mid-latitude alpine treeline sites where tree growth is temperature limited (Scuderi, 1990). Around 80 % of chronologies from such sites show significant growth reductions following large eruptions due to unusually low growing-season temperatures (Krakauer and Randerson, 2003). For high-latitude sites, decreased light availability after eruptions also contributes to the strong growth reduction (Tingley et al., 2014). Tree-ring studies from Northern Hemisphere mid-latitude lowland sites have shown that volcanic response is less clear, as temperate-zone trees are less temperature-limited and have more complex relationships with multiple climate variables (Pieper et al., 2014; Wilson et al., 2016).
Exploring possible responses to volcanic eruptions, Pieper et al. (2014) proposed three modes for tree growth in temperate regions: (1) growth reduction due to decreased growing-season temperature, resulting in narrow rings, (2) neutral or no response if the climate sensitivity to volcanic eruptions is insufficient to influence tree growth, and (3) enhanced growth due to an increase in the diffuse light fraction and reduced water stress, resulting in wide rings. Temperate-zone trees from the Southern Hemisphere are also likely to display similar mixed volcanic signals, depending on their relative sensitivity mode and the magnitude of the regional cooling effect. Understanding these factors will help elucidate the proxy archive contribution to the Southern Hemisphere model–data discrepancy. This knowledge will benefit future studies of hemispheric temperatures and help identify which species and/or regions should be prioritised for future proxy development.
Our goal in this study is to assess whether Southern Hemisphere tree rings record past volcanic events using a multispecies network of high-quality,
replicated tree-ring chronologies from New Zealand. This country is a long, narrow, archipelagic landscape stretching from 34 to
47
Tree-ring chronologies have been developed from locations widely distributed throughout New Zealand. Since the initial dendrochronological studies
undertaken by LaMarche et al. (1979), records have been generated from nine endemic species, of which seven are conifers and two are
Nothofagaceae (Table 1). Five main species have been used to develop multi-centennial tree-ring chronologies: kauri ( Can we identify volcanic signals in high-quality tree-ring series from the Southern Hemisphere? Are there differences in the expression of volcanic signals amongst the species? Does chronology selection impact the magnitude of post-volcanic cooling in tree-ring-based temperature reconstructions?
The New Zealand tree-ring chronologies analysed in this study were collated to develop the Eastern Australia and New Zealand Drought Atlas (Palmer
et al., 2015; Fig. 1). Palmer et al. (2015) identified chronologies from the International Tree Ring Data Bank and personal collections, screened the
tree-ring measurements for dating problems using the software program COFECHA (Holmes., 1983; Grissino-Mayer, 2001), and developed site “master”
chronologies from the raw ring widths using the “signal-free” method of standardisation (Melvin and Briffa, 2008). The metadata for all New Zealand
chronologies are provided in Table S1 in the Supplement. As only a single chronology has been
developed from mountain toatoa (
Distribution of tree-ring chronologies in New Zealand. Elevation data sourced from the LINZ Data Service and licensed for reuse under CC BY 4.0.
Distribution, reported climate sensitivities, and key references for New Zealand dendrochronological species.
Selection of volcanic events based on thresholds of peak modelled stratospheric atmospheric optical depth (Toohey and Sigl, 2017), averaged over 30–50
Table 1 summarises the distribution, average climate responses, and main wood properties (average annual ring growth and temporal correlation or
persistence) of the species, as described by the studies documenting the development of the chronologies. In addition, the response of each species to
average New Zealand monthly temperatures, calculated for this study, is also summarised. All species show significant (
Event selection is a significant source of uncertainty in tree-ring studies of volcanic cooling. The choice of volcanic events can greatly influence
the magnitude of average regional cooling identified (Esper et al., 2013; Wilson et al., 2016). In addition, for many events that occurred before
instrumental records, the timing, location, and size of eruptions are uncertain (Timmreck et al., 2021; Garrison et al., 2018). For this analysis, we
are interested in those events which would likely have reduced growing-season temperatures over New Zealand and thus be identifiable as ring-width
anomalies. Therefore, we selected events using a regional volcanic dimming threshold rather than an eruption magnitude. Prior to the instrumental era,
we picked events from the Greenland and Antarctic ice core sulfate aerosol analysis of Toohey and Sigl (2017) based on peak stratospheric
atmospheric aerosol depth (SAOD). We averaged SAOD, modelled using the Easy Volcanic Aerosol module (Toohey et al., 2016), over the latitudinal range
of New Zealand (30 to 50
We tested whether a volcanic signal can be identified in New Zealand tree-ring chronologies using superposed epoch analysis (SEA; Haurwitz and Brier, 1981), a statistical technique widely used to determine the impacts of volcanic eruptions on climate (Rao et al., 2019b; Adams et al., 2003; Scuderi, 1990; Salinger, 1998; Tejedor et al., 2021). The composite response of individual chronologies to the 13 largest eruptions and the 21 full eruption list between 1400 and 1990 CE was studied 0–5 years post-event, with anomalies calculated by subtracting the average of the nearest 5 year background period undisturbed by volcanic forcing (Table S2; Büntgen et al., 2020). Species-level responses were then tested using a composite chronology produced by simple averaging of annual values across sites (Cook and Kairiukstis, 1990). Volcanic responses were categorised as positive or negative if the anomalies exceeded the 5th or 95th percentile response of 10 000 random samples of years undisturbed by volcanic forcing or neutral if they fell between these bounds.
To investigate the influence of chronology selection on the identification of volcanic signals in temperature reconstructions, we report two new reconstructions of New Zealand summer temperatures (December–February). We used the New Zealand average “seven-station” monthly instrumental temperature series (Salinger, 1981; Mullan, 2012), obtained from the New Zealand National Institute of Water and Atmospheric Research (NIWA), to examine the temperature response of the chronologies. Correlations were calculated between autoregressively modelled chronologies and monthly climate data, with each month treated as a separate time series. A 20-month window was selected for correlation analysis, extending from October of the previous growing season to May at the end of the current austral growing season. Two growing seasons were included as significant prior season climate sensitivities have been reported for some species. Based on the response analysis, December–February (DJF) was selected as the seasonal target, as this window captures the strongest correlations across all species (Table 1).
To ensure sufficient overlap between the chronologies and the temperature dataset for calibration and verification, only chronologies extending to or
beyond 1990 CE were retained for the reconstructions. As many sites have not been updated since they were originally sampled in the 1970s and 1980s CE, only 58 of
the 96 chronologies were retained. The first reconstruction (NZall) included the full suite of available chronologies extending to 1990 CE, while the
second (NZsel) was limited to those chronologies that showed a significant volcanic signal using SEA. In each case, only those chronologies
significantly (
The volcanic response in tree-ring reconstructions of temperature was also tested using SEA and the two sets of volcanic eruption years. Further,
variation in the temperature response to different volcanic events was estimated by calculating the 90th percentile bootstrap confidence interval from
1000 replicates drawn without replacement from the event list (Rao et al., 2019b). In each iteration, approximately two-thirds (9 of 13 or 15 of 21) of
the volcanic events were selected. The confidence interval provides some indication of how eruptions of different sizes, locations, and seasonality
may impact the SEA results. To further assess how event selection may have affected the SEA results, the analysis was repeated using volcanic events
selected from the ice core analysis of Crowley and Unterman (2013), using a Southern Hemisphere-wide average threshold of SAOD
We compared the volcanic response seen in our multispecies reconstructions to the ensemble mean DJF response of seven climate models from the Coupled Model Intercomparison Project 5 (CMIP5) suite with Last Millennium (past1000, 850–1850 CE) simulations. The CMIP5 models were forced with either the Gao et al. (2008) or Crowley and Unterman (2013) volcanic forcing series (see Table S4). Data from the historical simulations were appended to extend the dataset from 1850 to 2005 CE.
Mean chronology departures 5 years before and 5 years after the 13 largest eruption years (year 0), separated by tree species. The chronologies contributing to the species-wide composite are shown in black, with the number of chronologies indicated in parentheses. The sensitive chronology composite in shown in blue and the number of contributing chronologies is shown in brackets. Significance bands (dotted grey lines) are the 1st, 5th, 95th, and 99th percentile of 10 000 random samples of non-event years from the species-wide composite.
The results of the superposed epoch analysis for the 13 largest volcanic eruptions between 1400 and 1990 CE are shown in Fig. 3. Two composite
responses are shown for each species: the response averaged across all sites (“all chronology composite”) and the response calculated only from the
site chronologies that individually showed a significant (either positive or negative) response to volcanic eruptions (“sensitive chronology
composite”). Analysis was repeated for the full set of 21 eruptions with SAOD
The species-wide response to volcanic events varied widely between New Zealand dendrochronological species. Three out of eight species, i.e. silver pine, mountain beech, and tānekaha, recorded a composite neutral response. Tānekaha is only weakly correlated to New Zealand average temperatures (Fig. S6b), which may explain its subdued response. However, compared to other species, mountain beech and silver pine both show relatively strong temperature sensitivities (Figs. S1a and S4). As many mountain beech chronologies extend only to the mid-1700s, the species composites were tested against a smaller subset of volcanic events, which may contribute to this result.
Of the remaining five species, one recorded a positive response, while four recorded a negative response. Kauri (Fig. 3a) was the only species to show a
composite positive response to volcanic events, maximal in year
Pink pine, cedar, silver beech, and toatoa show lagged negative responses to volcanic events, with peak negative anomalies recorded in years
In contrast, cedar does not show a consistent species-wide response. Both significant negative and positive responses were recorded in 13 of the 26 chronologies, with the rest showing a neutral response. This is despite a largely consistent within-species temperature sensitivity, which is an inverse response to prior season temperatures (Fig. S5).
The overall muted species response of cedar masks very different individual chronology responses. Cedar ring-width series respond differently to
volcanic events depending on their location, with both very negative and very positive responses recorded. The other species do not show similar
variation. The cedar chronologies have the widest geographical distribution of any species, and thus geographical factors may influence the
variability in response. We used
Five chronology groups were identified via clustering (Fig. 4a), broadly corresponding to differences in region and altitude. North Island
chronologies were distributed in two groups. All chronologies are from montane to subalpine areas above 800
The peak summer period was selected as the seasonal reconstruction target, as the largest number of chronologies across species showed significant
correlations with temperatures between December and February (Table 1). Selecting only those chronologies correlated at
New Zealand average DJF temperature reconstructions. Unfiltered (black) and filtered (20-year spline; blue) mean DJF reconstruction with 90 % uncertainty interval (grey) between 1400 and 2018 CE for
Both New Zealand DJF average temperature reconstructions are shown in Fig. 5 alongside their instrumental fit over the 1911–1990 CE calibration
period. There is good agreement between the reconstructions, with a Pearson
Increasing temperatures are observable in both reconstructions from around 1950 CE, matching the trend in instrumental temperatures. Prior to the
instrumental period, temperatures were higher than average for a sustained period during the 16th century and for a shorter period in the early
18th century. Periods of cooler-than-average temperatures have also occurred, starting at
Mean anomalies 5 years before and after 21 eruption years with SAOD
Figure 6 shows the results of the SEA analysis for the two New Zealand temperature reconstructions, for both sets of volcanic events, compared to the
volcanic response of an ensemble of seven CMIP5 model outputs for the New Zealand region. For the 21 events with SAOD
Distribution of tree rings used in
The difference between the NZall and NZsens reconstruction response is minor for both subsets of volcanic events. The anomaly recorded by NZsens is
0.07
Previous studies have not identified significant volcanic responses in Southern Hemisphere tree rings (Krakauer and Randerson, 2003; Palmer and Ogden, 1992) or in the temperature reconstructions based on them (Allen et al., 2018; Cook et al., 1992). In contrast to previous studies, we found that volcanic events can be clearly identified in New Zealand ring widths, although some species are stronger recorders of volcanic signals than others. Unlike Northern Hemisphere high-latitude and tree line sites, which tend to show a consistent reduction in growth due to volcanic cooling and reductions in light availability, no consistent response was identified across New Zealand conifer and Nothofagaceae species. Predominantly negative (pink pine, cedar, toatoa, silver beech), positive (kauri), and neutral (mountain beech, tānekaha, silver pine) responses were recorded. As most New Zealand chronology sites have been sampled from localised areas of residual forest that are restricted compared to their natural distributional range, it is difficult to distinguish between species-related sensitivities to volcanic eruptions and regional climate factors that may control the response. In reality, it is the combination of biological characteristics, including intrinsic species sensitivity, regional climate, and site-specific factors (e.g. soils, exposure to prevailing conditions), which determine the observed volcanic response. While necessarily simplified, here we discuss some possible explanatory factors for the species-wide responses.
The species-level results in Figs. 3 and S7 clearly show two response types following volcanic events: a rapid but short-lived response and a delayed
response that begins in year
In contrast, the fast responders both respond and recover more quickly from a detrimental change in conditions (e.g. mountain beech) or can rapidly capitalise on beneficial conditions (e.g. kauri). These are relatively fast-growing species, indicated by wider average ring widths than the stress tolerators, and they have lower persistence (Table 1). Mountain beech is shade intolerant but has several responses to abnormally cold temperatures, including rapid shoot growth and temporarily halting bud formation, which allows it to rebound quickly after a poor summer (Wardle, 1970). Kauri could be considered a stress tolerator due to its affinity for poor soils, occurrence on ridges and slopes, and drought tolerance (McGlone et al., 2017); however, relative to other New Zealand conifers in this study, it is a fast responder.
In contrast to the subdued, persistent decrease in growth shown by the stress tolerator species, the initial decline in toatoa ring width in year
We compared the response at six sites which each have chronologies from two different species (Fig. S8), providing the unique opportunity to compare species differences in volcanic sensitivity directly whilst controlling for most other factors. The three species that are co-located and thus available for site-based comparison (cedar–pink pine and cedar–silver pine) all showed stress-tolerator responses to volcanic eruptions. Pink pine and cedar often grow together in mixed stands. Both species are sensitive to temperature, although pink pine has a maximum correlation to late summer temperature, whereas cedar responds to conditions in the winter prior to the growing season and in spring (Fenwick, 2003). A significant difference in the response between species was observed only at one of three sites. Comparison over additional sites is therefore required to determine whether the difference in seasonal temperature response may result in a difference in the sensitivity of pink pine and cedar to climate disturbance following eruptions. Differences between the cedar and silver pine responses were observed at two of the three sites, with cedar showing greater sensitivity to volcanic eruptions. Silver pine is primarily found in the moist, temperate, low-elevation forests of the western coast of the South Island. It is a shade-tolerant species that grows in highly competitive closed-canopy forests on infertile, poorly drained or waterlogged soils (Wardle, 1977; Cook et al., 2002). It is an exceptionally slow-growing species and shows little year-to year variability in ring width (Table 1). Thus, it is unsurprising that volcanic effects were more readily identified in cedar at the Ahaura and Flagstaff Creek sites (Fig. S8).
An interesting result of this study is the strong positive species-wide response of North Island kauri to volcanic events (Fig. 3a) despite the weak
correlation of the chronologies to monthly temperatures (Fig. S3). Over 70 % of the kauri chronologies recorded a small but significant increase
in ring width in the year following a large eruption (SAOD
This suggests that kauri may capitalise on a decrease in summer evapotranspiration during both El Niño events and following significant eruptions. Maximum kauri growth occurs during spring, with large declines in growth rate over the peak summer months when evapotranspiration exceeds precipitation in the northern North Island (Fowler et al., 2005). Dendrometer band studies suggest that reduced spring and summer moisture stress may delay the cessation of growth, resulting in wider annual rings (Palmer and Ogden, 1983). No summer cessation of growth was observed by Palmer and Ogden (1983) at the highest-altitude site, Mt Moehau (1MOE, Table S1). This site receives moisture from condensation and fog drip, as well as rainfall, reducing the summer precipitation deficit. Plausibly, the increase in diffuse radiation and resulting enhanced photosynthesis (Gu et al., 2003; Robock, 2005) may also contribute to post-event kauri growth. However, tree growth is generally more constrained by the environment than photosynthesis (Fatichi et al., 2019; Zweifel et al., 2021), and thus increased photosynthesis may not necessarily translate into growth (i.e. a wider ring) in the presence of another limiting factor, such as the summer moisture deficit. Additional research is needed to understand the relative importance of temperature, light availability, humidity, and soil moisture to sub-annual growth in kauri.
Many observational and modelling studies propose a link between large tropical volcanic eruptions and sea surface temperature variability in the tropical Pacific, with El Niño-like conditions more likely in the year following a significant event (Emile-Geay et al., 2008; Adams et al., 2003; Khodri et al., 2017; Christiansen, 2008; Miao et al., 2018; McGregor et al., 2010), although this link is not always identifiable in the paleoclimate data (Dee et al., 2020). The three eruptions included in this analysis since 1900 CE co-occurred with an El Niño event, and the 1982/83 CE El Niño is one of the largest on record (Santoso et al., 2017). While we do not wish to debate the eruption–ENSO response as part of this study, these potential interactions complicate our analysis of the volcanic signal in kauri.
In an attempt to distinguish between the effects of El Niño events and volcanic eruptions on kauri growth, we repeated the SEA analysis, removing
the three volcanic eruptions since 1900 CE. A smaller composite ring-width anomaly was recorded without the three events, but the response remained
significant in year
Differences in volcanic response between sites are observed for all species, largely between sites with significant decreases in growth and sites with neutral responses (Figs. 3 and S7). More temperature-limited sites, such as sites at higher elevation and lower latitude, are expected to be more sensitive to volcanic cooling and thus experience the most reduction in growth. Broadly in line with this expectation, chronologies that are highly correlated to monthly temperatures show greater sensitivity to volcanic eruptions (Fig. S15). However, there are many exceptions, both for temperature-sensitive sites with a neutral volcanic response and sites that are only weakly correlated to temperature but that are markedly affected by the climatic changes following volcanic eruptions. Thus, volcanic response cannot be simply interpreted as a response to cooler-than-average temperatures.
Based on the variability in volcanic response observed in cedar (Fig. S8) it is evident that site-related factors can have a substantial impact on the
volcanic response within a species group. This finding was further explored using
Tree growth of species at different sites is limited by a variety of environmental factors, of which temperature and soil moisture are only two (Fritts, 1976). For many New Zealand species, little is known about what types of sites might accentuate these factors and thereby enhance the climatic sensitivity in the tree-ring series (Dunwiddie, 1979). Although the overall Group 2 cedar response was significant, not all high-altitude sites recorded a volcanic signal. Considering the location, aspect, forest characteristics, and soil type at individual cedar sites, we find that exposure to prevailing conditions is the key explanatory variable for the within-species response for sites near the altitudinal limit. Sites that record a significant growth response have high exposure to prevailing winds and are more sensitive to abnormally low growing-season temperatures. In contrast, chronologies from sites characterised by undulating ridgelines and more continuous forest showed a neutral growth response. Sites experiencing mesic conditions and closed-canopy forests tend to show lower sensitivity to adverse environmental conditions, such as low temperatures (Phipps, 1982). Closed-canopy forests are also more likely to be sensitive to increases in the fraction of diffuse radiation driving photosynthesis (Gu et al., 2003; Tingley et al., 2014), and thus the increase in diffuse radiation fraction may compensate for the decrease in temperature to a greater extent compared to sites with more open canopies.
North Island kauri is another species for which exposure appears to be a determining factor in the chronology response to eruptions. For kauri, sites with a strong positive response to volcanic eruptions are coastal sites exposed to prevailing wind conditions or sites limited by poor underlying sediment substrates (e.g. 1TRO, 1KAW; Table S1). In comparison, sites that showed little volcanic response were those on the leeward side of the coastal range, which are buffered by inland microclimate effects (e.g. 1PBL, 1PKF; Table S1). These sites likely experience less water stress during the summer; therefore, we expect that they receive less benefit from reduced evaporative demand related to volcanic cooling, resulting in a neutral response. The importance of aspect to climate sensitivity – particularly when windward sites are exposed to prevailing winds – has been highlighted in many previous studies (e.g. Dang et al., 2007; Rozas et al., 2013). For New Zealand, a thorough exploration of the importance of site-based parameters other than elevation and latitude (e.g. aspect, exposure, soil type) to volcanic sensitivity is limited because these parameters have not been recorded for many sites.
We expected to find a substantially greater volcanic response in NZsens (i.e. limited to only those chronologies with an individual significant
volcanic response) compared to NZall. However, while NZsens shows a larger post-volcanic temperature response, the difference between the two
reconstructions is not significant (Fig. 6). As shown in Fig. 7, both reconstructions are heavily weighted towards the same subset of
chronologies. Since, sites with higher sensitivity (correlation) to temperature in general show higher volcanic response (Fig. S15), limiting NZsens
to only sensitive chronologies has only a small impact on post-eruption temperatures. Another factor leading to the minimal difference between the
reconstructions is that many volcanically sensitive chronologies, particularly kauri, were cored before 1990 CE and therefore not included in either
temperature reconstruction. These sites should be updated with priority for future studies of volcanic impact in the Southern Hemisphere. In
developing NZsens, we used a “volcanic sensitivity” threshold based on the SEA result significance (
When testing the reconstructions using the event list from Toohey and Sigl (2017) (Fig. 6), we concluded that losing reconstruction strength
outweighs the small increase in volcanic sensitivity in NZsens and that it is not beneficial to restrict the predictor pool. However, when we then
repeated the SEA analysis using the event list derived from Crowley and Unterman (2013), the mean response of NZall to the largest subset of
12 events with SAOD
Figure S12c indicates that the SEA compositing procedure can fail when using a small number of events if the volcanic signal is small compared to other sources of interannual variability, especially when not all events have a climate impact. This is one potential reason that this study has identified significant volcanic signals in Southern Hemisphere tree rings when previous studies did not. Before Gao et al. (2008), no comprehensive reconstruction of global aerosol loading was available. Uncertainty in eruption dates and sizes likely contributed to the lack of volcanic signal identified in studies undertaken prior to the release of the “Gao” dataset (e.g. Cook et al., 2002; Villalba and Boninsegna, 1992). Revisiting the data from other major Southern Hemisphere dendrochronology regions (e.g. Tasmania, South America) is therefore an important aspect for future research.
Previous studies that narrowly focused on the impacts of the 1815 CE eruption of Tambora on New Zealand tree rings (Palmer and Ogden, 1992; Norton, 1992) also presented inconclusive results. The authors of these studies were seeking synchronous growth reductions across species, whereas our analysis, with the benefit of much more data, shows responses vary widely between species. Because of this variation in response, studies that rely on compositing across species and regions (e.g. Krakauer and Randerson, 2003) are also likely to underestimate the true volcanic response in Southern Hemisphere tree rings.
In this study we also compared reconstructed temperature anomalies with anomalies from climate models over the New Zealand region for DJF – peak
growing season in the Southern Hemisphere. We found no difference between the magnitude of the year
Unlike some Northern Hemisphere studies, our ring-width temperature reconstructions show no increased persistence in temperature anomalies following eruptive events compared to the climate model ensemble (Fig. 6). Ring widths from New Zealand conifers therefore appear suitable for volcanic investigations. Northern Hemisphere high-altitude and high-latitude trees predominantly used to determine the temperature impacts of volcanic eruptions contain higher biological persistence than the chronologies we used in our temperature reconstructions, influencing their post-eruption response. For example, the average first-order autocorrelation of our predictor chronologies is 0.53 (range 0.15–0.87; SD 0.15) compared to Arctic sites with an average of 0.62 (range 0.15–0.93; SD 0.13; Cropper and Fritts, 1981). Nevertheless, several New Zealand species do show a lagged volcanic response (Figs. 3 and S7) that is not present in the final temperature reconstructions. Methodological decisions play an important role in the persistence of tree-ring-based temperature reconstructions (Büntgen et al., 2021). In our reconstructions, pre-whitening of both the tree-ring predictors and the temperature data, including significant lagged predictors, and the selection of predictors from multiple species all contribute to the responses we identified.
Very few studies have considered whether volcanic signals are identifiable in tree-ring chronologies from the Southern Hemisphere. We investigated whether volcanic events could be identified in New Zealand tree rings using data from eight dendrochronological species. In doing so, we set out to answer the following three questions. First, can volcanic signals be identified in the Southern Hemisphere? Second, are there species-level differences in volcanic signal strength? Finally, does chronology selection impact the magnitude of post-volcanic cooling in temperature reconstructions from tree rings?
In answering the first two questions, we found that New Zealand dendrochronological species are reliable recorders of volcanic cooling but that response varies across species. The magnitude and persistence of the species-wide volcanic response can be broadly linked to plant life history traits. The larger magnitude and more immediate responses are recorded by the “fast-responder” species, such as mountain beech and kauri, and more delayed but persistent responses are recorded by the “stress-tolerator” species, such as silver pine. In general, volcanic events can be more readily observed in the ring widths of fast-responder species, which should be prioritised for future regional or hemispheric studies. Unfortunately, the paucity of information on the ecology of many New Zealand species limits our understanding of how species allocate resources to processes other than cambial growth in response to short-term changes in climatic conditions.
The volcanic response of New Zealand trees is complex, with positive, negative, and neutral responses identified sometimes within the same species group. For subalpine sites, this finding is not dissimilar to previous studies of temperate-zone Northern Hemisphere species. We found that site-related factors have greater control over displayed volcanic responses than species and presented a suite of plausible, testable hypotheses explaining the results. The altitude of the site with respect to the species altitudinal limit and exposure to prevailing conditions are factors thought to determine whether a tree-ring volcanic response could be identified. In some cases, sites near the lower altitudinal limit of the species were also strong responders, suggesting a reduction in summer moisture stress could also be an important factor in post-volcanic growth. Our results indicate that studies intending to utilise tree rings to investigate regional volcanic cooling should carefully consider the characteristics of the sample site. While valid for all dendrochronological studies, it is particularly important for identifying volcanic signals, as we find that the range of temperature-sensitive sites is greater than the range of volcanically sensitive sites.
In answer to the last question, we developed two new reconstructions of New Zealand summer temperature to investigate whether chronology selection impacted the magnitude of post-volcanic cooling. There was little difference in the post-event anomalies, suggesting that limiting the predictor pool for volcanic sensitivity is unnecessary when targeting average growing-season temperatures in New Zealand. Both reconstructions showed temperature anomalies remarkably consistent with studies based on instrumental temperature and the ensemble mean response of CMIP5 climate models. Based on these results, New Zealand ring widths are reliable indicators of regional volcanic climate response.
More broadly, the findings of this study have important implications for the development of future tree-ring or multiproxy hemispheric temperature reconstructions from the Southern Hemisphere, which often incorporate species-specific “master” chronologies (i.e. composite chronologies developed from across many sites) into their predictor pool. As shown in this study, the compositing process can result in reduced volcanic signals when more than one type of response (i.e. positive, negative, or neutral) is recorded by a single species. However, as most New Zealand species-level composites show significant volcanic responses, temperature reconstructions based on composite chronologies should also show the influence of volcanic eruptions.
All data and software used in this study are publicly available. The New Zealand “seven-station” temperature series was downloaded from NIWA at
The supplement related to this article is available online at:
PAH and JGP conceptualised this study, JGP curated the data, and PAH undertook the analysis. PAH wrote the manuscript with contributions from all authors.
The contact author has declared that neither they nor their co-authors have any competing interests.
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This article is part of the special issue “Interdisciplinary studies of volcanic impacts on climate and society”. It is not associated with a conference.
We acknowledge the World Climate research Programme's Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups (listed in Table S4 of this paper) for producing their model output and making it available. The authors thank Kathy Allen, Michael Sigl, and the two anonymous reviewers for their helpful comments that greatly improved this paper.
Philippa A. Higgins is supported by an Australian Government Research Training Scholarship and the UNSW Scientia PhD Scholarship Scheme. Fiona Johnson is supported by the UNSW Scientia Program. Further support was provided by the ARC Centre of Excellence in Australian Biodiversity and Heritage (grant no. CE170100015).
This paper was edited by Michael Sigl and reviewed by two anonymous referees.