Climate-growth Responses Research Articles

Tree-Ring Chronologies from the Upper Treeline in the Russian Altai Mountains Reveal Strong and Stable Summer Temperature Signals

Radial tree growth at high-elevation and high-latitude sites is predominantly controlled by changes in summer temperature. This relationship is, however, expected to weaken under projected global warming, which questions the reliability of tree-ring chronologies for climate reconstructions. Here, we examined the growth–climate response patterns of five tree-ring width (TRW) and maximum latewood density (MXD) chronologies of larch (Larix sibirica) from upper-treeline ecotones in the Altai Mountains, which is a key region for developing millennial-long dendroclimatic records in inner Eurasia. The TRW and MXD chronologies exhibited significant year-to-year coherency within and between the two parameters (p < 0.001). While TRW is mostly influenced by temperature changes during the first half of the growing season from June to July (r = 0.66), MXD is most strongly correlated with May–August temperatures (r = 0.73). All seasonal temperature signals are statistically significant at the 99% confidence level, temporally stable back to 1940 CE, the period with reliable instrumental measurements, and spatially representative for a vast area of inner Eurasia between northeastern Kazakhstan in the west, northern Mongolia in the east, southern Russia in the north and northwestern China in the south. Our findings demonstrate the paleoclimatic potential of TRW and especially MXD chronologies and reject any sign of the ´divergence problem´ at these high-elevation, mid-latitude larch sites.

Altitudinal Difference of Growth–Climate Response Models in the Coniferous Forests of Southeastern Tibetan Plateau, China

Characterized as a climatologically sensitive region, the southeastern Tibetan Plateau (STP) is an ideal location for dendrochronological research. Here, five tree-ring width (TRW) chronologies were developed: three for Picea likiangensis along altitudinal gradients from 3600 to 4400 m a.s.l. and two for Sabina saltuaria and Abies squamata from 4200 m a.s.l. Significant differences in the growth rates and age composition of Picea likiangensis were observed at various elevation gradients. The chronology statistics (mean sensitivity, etc.) fluctuated with the elevation gradient. Picea likiangensis showed distinct growth patterns in response to climatic variability along the altitude gradient: the minimum temperature influenced tree growth at lower and middle altitudes, while higher altitudes were affected by precipitation. The radial growth of different tree species growing in the same region is controlled by the same climatic factors. Sabina saltuaria and Abies squamata exhibited similar growth responses to Picea likiangensis. Stand conditions (wind speeds, slope, and elevation) and biotic factors (the depth of root, forest type, tree age, and sensitivity) can partially explain why the ring width–climate relationships change with altitude.

Physiological meaning of bimodal tree growth-climate response patterns.

Correlation coefficients are widely used to identify and quantify climate signals in proxy archives. Significant relationships between tree-ring chronologies and meteorological measurements are typically applied by dendroclimatologists to distinguish between more or less relevant climate variation for ring formation. While insignificant growth-climate correlations are usually found with cold season months, we argue that weak relationships with high summer temperatures not necessarily disprove their importance for xylogenesis. Here, we use maximum latewood density records from ten treeline sites between northern Scandinavia and southern Spain to demonstrate how monthly growth-climate correlations change from narrow unimodal to wide bimodal seasons when vegetation periods become longer and warmer. Statistically meaningful relationships occur when minimum temperatures exceed 'biological zero' at around 5° C. We conclude that the absence of evidence for statistical significance between tree growth and the warmest summer temperatures at Mediterranean sites is no evidence of absence for the physiological importance of high summer temperatures for ring formation. Since correlation should never be confused with causation, statistical values require mechanistic understanding, and different interpretations are needed for insignificant correlations within and outside the growing season.

Non-linear response of Norway spruce to climate variation along elevational and age gradients in the Carpathians

Climate change, namely increased warming coupled with a rise in extreme events (e.g., droughts, storms, heatwaves), is negatively affecting forest ecosystems worldwide. In these ecosystems, growth dynamics and biomass accumulation are driven mainly by environmental constraints, inter-tree competition, and disturbance regimes. Usually, climate–growth relationships are assessed by linear correlation due to the simplicity and straightforwardness of modeling. However, applying this method may bias results, since the ecological and physiological responses of trees to environmental factors are non-linear, and usually bell-shaped.In the Eastern Carpathian, Norway spruce is at the southeasternmost edge of its natural occurrence; this region is thus potentially vulnerable to climate change. A non-linear assessment of climate–growth relationships using machine-learning techniques for Norway spruce in this area had not been conducted prior to this study. To address this knowledge gap, we analyzed a large tree-ring network from 158 stands, with over 3000 trees of varying age distributed along an elevational gradient. Our results showed that non-linearity in the growth-climate response of spruce was season-specific: temperatures from the previous autumn and current growing season, along with water availability during winter, induced a bell-shaped response. Moreover, we found that at low elevations, spruce growth was mainly limited by water availability in the growing season, while winter temperatures are likely to have had a slight influence along the entire elevational gradient. Furthermore, at elevations lower than 1400 m, spruce trees were also found to be sensitive to previous autumn water availability. Overall, our results shed new light on the response of Norway spruce to climate in the Carpathians, which may aid in management decisions.

Elevation-dependent growth trends of Picea orientalis (L.) Peterm forests in the Fırtına Creek Basin (Rize/Turkey)

Changes in climatic conditions have ecological and economic consequences or impact for trees and forest stands. In this respect, it is crucial to understand the radial growth trends of trees, their ecological response across elevation gradients, and the possible impacts of climate change on the species. In this study, Picea orientalis (L.) Peterm, located in the Fırtına Creek Basin in the northern part of the Eastern Black Sea Mountains in northeastern Turkey, was investigated along different elevation gradients in the context of climate-growth response. In the study, six site chronologies along an elevation gradient from 900 m to 2050 m were developed. Dendroclimatological methods were applied to assess the elevation-dependent radial growth characteristics of trees. In total, 200 increment cores were collected from 98 living trees. The relationships between climate and tree ring growth along the elevation gradient showed a positive relationship between growth and temperature at upper elevations (1700–1900 m) and a positive relationship between growth and precipitation at middle elevations (1400–1600 m). In particular, January–April mean temperatures and May rainfall were positively correlated with tree-ring growth. At lower elevations (900–1100 m), the correlation coefficients between climate and growth are insignificant because, at these altitudes, the species have optimum ecological conditions. It was also found that mean temperatures between 0 °C and 2 °C in January–April and total precipitation between 60 and 100 mm in January–February positively affect tree growth. However, if these thresholds are exceeded, it is possible that growth may be adversely affected. This research underscores the complex interplay between climate variables and tree growth along elevation gradients, highlighting the need for adaptive management strategies in the face of changing climatic conditions.

Rapid Warming Exacerbates Winter Drought Stress in Trees at High-Altitude Areas in Northeast China

Against the background of global warming, trees in high-latitude and high-altitude areas are more sensitive to rapid warming. Revealing the response patterns of trees at different altitudes to rapid warming in typical alpine mountain environments can help to predict the future distribution of forests in the region and the associated changes in the timberline. This study used tree-ring width data (band sampling) from Dahurian larch (Larix gmelinii Rupr.) along an altitudinal gradient (970–1409 m) on Oakley Mountain to establish 10 chronologies and to analyze the growth–climate response of larch to warming and altitudinal changes along a gradient. The results showed that before rapid warming, larch growth was strongly influenced mainly by precipitation in July–August, while after warming, the growth was controlled mainly by precipitation (snowfall) in winter (October–April) and showed a significant positive correlation with the SPEI in winter and a negative correlation with temperature in February–April (early spring). This indicates that the rapid warming event led to a drastic change in the water heat balance during the pre-growth period of the trees in the study area, which shifted the tree growth from being restricted by drought in the growing season before warming to being restricted by winter drought (i.e., the lagged effect of snowfall in the pre-growing season) after warming and that the trees at low altitudes suffered from more severe winter drought. In the future, as the global climate warms further, the growth of mountain larch at low altitudes (below 1200 m) will continue to decrease, and the coupling of winter snowfall and seasonal (February–April) warming will increase the upper altitude limit at which winter drought occurs for larch in the study area; additionally, larch at higher altitudes will be able to cope with this ecological process better.

A tree ring-based spring temperature reconstruction for the Hindu Kush region in northern Pakistan

Annually resolved and absolutely dated proxy archives are necessary for contextualizing the rate and amplitude of anthropogenic climate change. High-resolution climate reconstructions are particularly important for remote regions where instrumental meteorological observations are limited in space and time. Here, we develop a robust, 260-year-long tree-ring width chronology from 32 Himalayan cedars (Cedrus deodara D. Don), which grew around 3000 m asl in the Kumrat Valley of the Hindu Kush region in northern Pakistan. Growth-climate response analyses reveal a significant negative correlation of -0.631 between our new standard chronology and March-April minimum temperatures (p < 0.001, n = 51). This inverse relationship between radial tree growth and early spring temperatures, together with positive correlations against precipitation totals at the onset of the growing season, suggest that warming-induced drought is the most limiting factor of sub-alpine forest growth in the Kumrat Valley. We then use the inverse growth-temperature association to reconstruct March-April minimum temperatures from 1812 to 2018 CE. The reconstruction explains 40 % of the instrumental record back to 1967, and exhibits the coldest and warmest spring conditions in 1815–1864 (6.67 ± 0.21°C) and 1918–1927 (8.11 ± 0.43°C), respectively. This pioneering study is indicative of the dendroclimatological potential in the Hindu Kush, where samples from living and relict trees of different species and elevational bands should be collected for advanced temperature and hydroclimate reconstructions.

Growth-climate relationships of four tree species in the subtropical evergreen broad-leaved forests in Southwest China

Subtropical forests are an important carbon sink, yet our understanding on tree growth response to inter-annual climate variability remains limited. This study investigated the growth-climate relationships of four evergreen broad-leaved tree species (Stewartia pteropetiolata W. C. Cheng, Schima noronhae Reinw. ex Bl. Bijdr, Machilus gamblei King ex J. D. Hooker, and Machilus yunnanensis Lec) in the subtropical evergreen broad-leaved forests (EBFs) in the Ailao Mountains of Yunnan province, southwest China. We constructed tree ring-width residual chronologies for each species and correlated them with daily and monthly climate data. Ring-width chronologies of S. pteropetiolata and S. noronhae were positively correlated with precipitation, relative humidity, and self-calibrated Palmer drought index (scPDSI) during autumn of the previous year and spring-to-early summer of the current year, indicating that these two Theaceae species were sensitive to moisture availability. Tree radial growth of two Machilus species was primarily limited by excessive precipitation and dense cloud cover during the rainy season in this middle montane cloud forest. Moving correlations between climate variables and radial growth of all species were temporally unstable, with a weakening response to climate variability in recent decades. We highlight the dendroclimatic potential of evergreen broad-leaved tree species in moist subtropical forests. The findings offer a pertinent perspective, emphasizing the necessity of separately considering distinct ecological indicators for trees in order to obtain a comprehensive understanding of climate-growth responses within highly diverse subtropical forests.

Variable climate-growth relationships of quaking aspen (Populus tremuloides) among Sky Island mountain ranges in the Great Basin, Nevada, USA

The Great Basin is an arid province located in the interior western United States. The region encompasses millions of hectares and quaking aspen (Populus tremuloides Michx.) forests comprise a minor portion of the total area. However, montane aspen forests play a disproportionately large role in providing ecosystem services in the region, including water retention, biodiversity, wildlife habitat, livestock forage, and recreational uses. With warming temperatures, increasing evaporative demand, and heightened precipitation variability, the future of aspen has become a critical concern. Using dendroecological approaches, we assessed growth patterns of 20 aspen stands across three geographically isolated “sky island” mountain ranges spanning portions of the north-central Great Basin. We anticipated that the growth of Great Basin aspen would be strongly influenced by regional climatic patterns and largely in synchrony. Results revealed a more complex growth dynamic that varied among mountain ranges and across environmental gradients. In particular, aspen climate-growth relationships in the slightly dryer Ruby Mountains were strongly and positively correlated (r > 0.5) with previous fall to winter moisture availability. The Jarbidge Mountains had a positive but modest relationship with previous fall to winter moisture availability (r > 0.3). Climate-growth response in the Santa Rosa Mountains, the wettest range, showed no significant response to moisture availability during any time period examined but had greater tree-ring growth with warmer May temperatures. Although tree-ring centennial (1910 – 2010) growth trends were positive for all three mountain ranges, only the Santa Rosa Mountains maintained a positive recent growth trend (1970 – 2010). Moreover, distinct temporal shifts in tree growth-climate relationships in each mountain range suggest potentially unique aspen population adaptations to climate variability. For instance, in two of the mountain ranges, there was a shift from positive/neutral to negative growth relationships with temperature starting around the 1963 – 1987 time period, while tree growth also began simultaneously responding more positively to moisture availability. These growth shifts and observed enhanced sensitivities to monthly and seasonal climate variables over time may reflect dynamic tree growth responses caused by ongoing global climate change, but that may be tempered by local or regional factors, such as the relative availability and timing of soil moisture provided by spring snowmelt. A better understanding of biogeographic variation and causality in aspen growth could provide multiple management pathways governed by resilience characteristics in the face of future anthropogenic and climatic threats.

The intra-annual rhythm of Pinus sylvestris growth-climate responses under a warming climate at its southern distribution limits

Warming climate has posed a pressing threat on boreal forests and an improved understanding of the intra-annual pattern of climatic influences on the tree growth can help interpret the response of boreal forest to climate change. Here, we systematically examined the growth-climate relationship, cambium phenology, and xylem cell dynamics of Pinus sylvestris (PS) to disentangle its intra-annual growth rhythm to external drivers and internal physiological process at its southern distribution limits across Eurasia. We showed the intra-annual rhythm of PS to temperature and precipitation is anti-phase and synchronous at its southern limits. Temperature had both promoting and inhibiting effects on PS growth over growing season where the growth-temperature response gradually changed from positive in winter-early spring to negative in late spring-summer, and then returned to positive in autumn. Precipitation enhanced PS growth in late spring-summer. Temperature dominated the intra-annual growth-climate response rhythm, and the response shifted to negative when temperature reached 13.72 °C in spring-summer and shifted to positive when temperature reached 17.41 °C in summer-autumn in Shenyang, respectively. A high water demand for the rapid earlywood cell formation in spring-summer and heat requirement for cambial cell division in summer-autumn caused the shifts in growth-temperature response, respectively. Climatic warming advances and prolongs the time of summer water availability that limits PS growth at its southern distribution limits and thus warming climate may pose a greater threat to the southern population of PS.

Shifting climatic responses of tree rings and NDVI along environmental gradients

Variations in the growth of aboveground biomass compartments such as tree stem and foliage significantly influence the carbon cycle of forest ecosystems. Yet the patterns of climate-driven responses of stem and foliage and their modulating factors remain poorly understood. In this study, we investigate the climatic response of Norway spruce (Picea abies) at 138 sites covering wide spatial and site fertility gradients in temperate forests in Central Europe. To characterize the annual growth rate of stem biomass and seasonal canopy vigor, we used tree-ring chronologies and time-series of NDVI derived from Landsat imagery. We calculated correlations of tree-ring width and NDVI with mean growing season temperature and standardized precipitation evapotranspiration index (SPEI). We evaluated how these climate responses varied with aridity index, soil category, stand age, and topographical factors. The results show that the climate-growth responses of tree rings shift from positive to negative for SPEI and from negative to positive for temperature from dry (warm) to wet (cold) areas. By contrast, NDVI revealed a negative response to temperature across the entire climatic gradient. The negative response of NDVI to temperature likely results from drought effects in warm areas and supporting effects of cloudy conditions on foliage greenness in wet areas. Contrary to NDVI, climate responses of tree rings differed according to stand age and were unaffected by local topographical features and soil conditions. Our findings demonstrate that the decoupling of stem and foliage climatic responses may result from their different climatic limitation along environmental gradients. These results imply that in temperate forest ecosystems, the canopy vigor may show different trends compared to stem growth under ongoing climate change.

Increasing precipitation variability and climate-growth responses of five tree species in North Carolina, USA

We examined the effects of increasing summertime precipitation variability from 1950 to 2022 on the radial-growth responses of five tree species native to central North Carolina, USA. Tree-ring data were collected from chestnut oak, post oak, longleaf pine, shortleaf pine, and Virginia pine and processed following standard dendrochronology procedures. Adjusted latewood chronologies for each species were created and correlated with either monthly or multi-monthly combinations of summertime precipitation for above average (AA, > 1 σ above mean), below average (BA, <−1 σ below mean), and near average (NA, <−1, and 1 > σ) precipitation years. June–September precipitation variability and other summertime monthly combinations significantly increased during the study period, with a 10.2% increase in AA/BA years during the 21st century. Climate-growth correlations ranged from 0.40–0.51 using all years within the study period. However, using AA and BA years exclusively, climate-growth responses ranged from 0.44–0.71, with post oak and longleaf pine experiencing significantly higher correlations. No significant changes in climate-growth responses occurred for chestnut oak, shortleaf pine, and Virginia pine. These findings suggest the effects of increased precipitation variability on climate-growth responses are species-dependent and affected by the precipitation classification (i.e. AA or BA years). These responses help explain temporal variations in the strength of climate-growth responses, particularly for some species, and offer additional considerations for dendroclimatological research.

Application Potential of the Blue Intensity Parameter for Estimation of Tree Radial Growth Climatic Response on the Crimean Peninsula

Application Potential of the Blue Intensity Parameter for Estimation of Tree Radial Growth Climatic Response on the Crimean Peninsula

Linkage between growth phenology and climate-growth responses along landscape gradients in boreal forests

Boreal forests represent an important carbon sink and, therefore, significantly contribute to climate change mitigation. Tree-ring width series of boreal species reflect climate variation at the moment of tree-ring formation but also lagged climatic effects from dormancy preceding tree-ring formation and antecedent growing seasons. However, little is known about how the growth sensitivity to climate in specific intra-annual periods varies across the landscape. Here, we assessed growth responses to climate variation during the 45 months preceding the tree-ring formation for nine boreal stands of Picea glauca and Picea mariana distributed along the gradients of elevation and slope aspect. We combined process-based modeling of wood formation and remote sensing data to determine growth phenology at each site. Next, we classified intra-annual seasons with significant climate-growth correlations based on the timing of dormancy and growth periods. Both the phenology and the climate-growth relationships systematically shifted with elevation and, to a lower extent, also with slope orientation at the treeline. The mean duration of the growing season varied between 100 days at treelines above 900 m and 160 days at lowlands below 500 m. The growth at treelines was stimulated by temperature in the summer of the tree-ring formation year and two years before tree-ring formation. The period of significant climate-growth correlations during the current summer did not exceed three months in agreement with the local duration of the growing season. The growth of trees in lower elevations was instead stimulated by high temperature during the dormancy periods but restricted by high temperature in antecedent summer seasons. In conclusion, our study highlights the linkage between the timing of climate-growth sensitivity and growth phenology, primarily determined by proximity to the treeline. Consequently, accounting for landscape gradients in growth phenology is crucial for upscaling the climatic limits of boreal stands' growth as climate change progresses.

Growth-climate responses of Picea sitchensis [Bong.] Carr. versus Picea abies [L.] Karst. in the British Isles and Central europe

Abstract Introduced tree species have become increasingly important in the context of the ongoing climate change. This paper focuses on the dendrochronology of the most widespread introduced tree species in the British Isles – Sitka spruce (Picea sitchensis [Bong.] Carr.) – in comparable soil conditions in England, Czechia, and Slovakia. The research aims to evaluate the growth dynamics and the influence of climatic factors on this tree species while comparing it with economically main tree species in Europe – Norway spruce (Picea abies [L.] Karst.). Based on the analysis of 150 increment cores, the radial growth of Sitka spruce was on average 24.2% higher than that of Norway spruce. The highest increments in 52 to 62-year-old stands were achieved in England by both Sitka spruce (8.7 mm) and Norway spruce (7.0 mm). In terms of negative pointer years (NPYs), there was no difference in the number of years with a significantly low increment between the two species at any site. The lowest effect of climatic factors on growth was found in Czechia, while the highest was in England. Higher resistance to climate was found for Sitka compared to Norway spruce. In general, the main limiting factor for the growth was the lack of precipitation in the previous year’s vegetation season, or heavy frost in England. In Central Europe, due to low precipitation, Sitka spruce will not be a substantial introduced tree species in the future, but on suitable sites, it can achieve high production potential and play a significant role for increasing stand diversity in the face of climate change.

Dendrochronology in the tropics using tree-rings of Pinus kesiya

The recent decade has witnessed considerable progress in the number of tree-ring studies using the tropical conifer taxa, Pinus kesiya. Several tree-ring networks have been established in less explored regions in Northeast India, Southwest China and Vietnam. The seasonal climate response of P. kesiya tree-rings has been examined and used to reconstruct temperature and soil moisture variability over the past century and augment the short instrumental records in South and Southeast Asia. In addition to standard approaches, the application of stable isotope, wood density, and blue intensity measurements indicates a significant development in P. kesiya studies. This review elaborates the future prospects of using multiple tree-ring parameters to establish discrete proxies besides tree-ring width. We recommend blue intensity as a cost-effective alternative to quantitative wood anatomy in tropical pines, and call for routine assessments of the temporal stability of climate-growth responses to identify and study potentially non-stationary climate signals. Efforts should be made towards developing extensive networks of long P. kesiya tree-ring chronologies to extend regional climate reconstructions.

Dendroclimatic response of Pinus tabuliformis Carr. along an altitudinal gradient in the warm temperate region of China.

Global climate change can affect the sensitivity of tree radial growth to climate factors, but the specific responses of tree radial growth to microclimate along the altitudinal gradient in the long term are still unclear. In this study, the tree-ring width chronologies of Pinus tabuliformis Carr. in Shanxi Province of China were studied at three altitude gradients (1200-1300m (low altitude), 1300-1400m (medium altitude) and 1400-1500m (high altitude)) during 1958-2017. The results showed that (1) the climate background could be divided into two periods based on the Mann-Kendall test analysis: 1958-1996 was a stable period (mean annual temperature (MAT)=10.25°C, mean annual precipitation (MAP)=614.39 mm), and 1997-2017 was a rapid change period (MAT=10.91°C, MAP=564.70 mm), indicating a warming and drying trend in the study region. (2) The radial growth of P. tabuliformis at different altitudes showed inconsistent variation patterns. The tree radial growth at low and medium altitudes (CV=27.01% for low altitude and CV=24.69% for medium altitude) showed larger variation amplitudes during the rapid change period than that in the stable period (CV=12.40% for low altitude and CV=18.42% for medium altitude). In contrast to the increasing trend, the tree radial growth rates at the high altitude showed a decreasing trend across years. (3) In the stable period, the radial growth of P. tabuliformis at the low altitude showed a significantly negative response to temperature and a positive response to precipitation in May and June. The tree radial growth at the medium altitude was positively related to precipitation in June and minimum temperature in February. The tree growth at the high altitude was mainly positively correlated with the temperature in May and August. In the rapid change period, the radial growth of P. tabuliformis at the low altitude was affected by more meteorological factors than that in the stable period. Medium-altitude trees were positively influenced by precipitation in June and minimum temperature in January, whereas high-altitude trees responded positively to wind speed in February. (4) Along altitudinal gradients, tree radial growth was more related to temperature than precipitation in the stable period. The tree radial growth at the high altitude during the rapid change period was only affected by wind speed in February, whereas the tree radial growth at low and medium altitudes was mainly affected by temperature to a similar extent during the two periods. The study indicated that tree growth-climate response models could help deeply understand the impact of climate change on tree growth adaptation and would be beneficial for developing sustainable management policies for forest ecosystems in the transition zone from warm-temperate to subtropical climates.

The increasing drought sensitivity of silver fir (Abies alba Mill.) is evident in the last two decades

Silver fir (Abies alba Mill.) is still counted among drought-tolerant tree species. However, its ability to cope with the recent extremely dry period has not yet been sufficiently studied. The objective of research was to analyse differences in the climate-growth response between silver fir, Norway spruce (Picea abies L. Karst.) and European larch (Larix decidua Mill.) growing in areas with large-scale disintegration of spruce stands. In 2019–2021, the increment cores were sampled at 16 sites along the altitudinal gradient of 340–775 m a.s.l. in different regions of the Czech Republic affected by bark beetle outbreak. The radial growth pattern of fir was compared with that of spruce or larch growing under the same site conditions. In fir, the missing rings were frequently recorded during the period of peak SO2 pollution load in 1966–1985, but they were rarely identified in recent years. In spruce and larch, missing rings were less common and occurred mainly in the recent dry period after 2015. Fir was less sensitive to summer drought compared to larch and especially to spruce, which showed high sensitivity to summer drought regardless of the altitude. The significant positive response of fir to summer precipitation was recorded at sites up to 450 m a.s.l., however, its sensitivity to drought has increased in the last two decades. Hence, when considering the wider use of fir, it is necessary to respect its ecological requirements as much as possible in order to preserve its vitality and production potential in changing climatic conditions.

Variability in Tree-ring Width and NDVI Responses to Climate at a Landscape Level

Inter-annual climatically driven growth variability of above-ground biomass compartments (for example, tree stems and foliage) controls the intensity of carbon sequestration into forest ecosystems. However, understanding the differences between the climatic response of stem and foliage at the landscape level is limited. In this study, we examined the climate-growth response of stem and leaf biomass and their relationship for Pinus sylvestris (PISY) and Picea abies (PCAB) in topographically complex landscapes. We used tree-ring width chronologies and time series of the normalized difference vegetation index (NDVI) derived from high-resolution Landsat scenes as proxies for stem and leaf biomass, respectively. We then compared growth variability and climate-growth relationships of both biomass proxies between topographical categories. Our results show that the responses of tree rings to climate differ significantly from those found in NDVI, with the stronger climatic signal observed in tree rings. Topography had distinct but species-specific effects: At moisture-limited PISY stands, stem biomass (tree rings) was strongly topographically driven, and leaf biomass (NDVI) was relatively insensitive to topographic variability. In landscapes close to the climatic optimum of PCAB, the relationship between stem and leaf biomass was weak, and their correlations with climate were often inverse, with no significant effects of topography. Different climatic signals from NDVI and tree rings suggest that the response of canopy and stem growth to climate change might be decoupled. Furthermore, our results hint toward different prioritizations of biomass allocation in trees under stressful conditions which might change allometric relationships between individual tree compartments in the long term.

Ecological and methodological drivers of non-stationarity in tree growth response to climate.

Radial tree growth is sensitive to environmental conditions, making observed growth increments an important indicator of climate change effects on forest growth. However, unprecedented climate variability could lead to non-stationarity, that is, a decoupling of tree growth responses from climate over time, potentially inducing biases in climate reconstructions and forest growth projections. Little is known about whether and to what extent environmental conditions, species, and model type and resolution affect the occurrence and magnitude of non-stationarity. To systematically assess potential drivers of non-stationarity, we compiled tree-ring width chronologies of two conifer species, Picea abies and Pinus sylvestris, distributed across cold, dry, and mixed climates. We analyzed 147 sites across the Europe including the distribution margins of these species as well as moderate sites. We calibrated four numerical models (linear vs. non-linear, daily vs. monthly resolution) to simulate growth chronologies based on temperature and soil moisture data. Climate-growth models were tested in independent verification periods to quantify their non-stationarity, which was assessed based on bootstrapped transfer function stability tests. The degree of non-stationarity varied between species, site climatic conditions, and models. Chronologies of P. sylvestris showed stronger non-stationarity compared with Picea abies stands with a high degree of stationarity. Sites with mixed climatic signals were most affected by non-stationarity compared with sites sampled at cold and dry species distribution margins. Moreover, linear models with daily resolution exhibited greater non-stationarity compared with monthly-resolved non-linear models. We conclude that non-stationarity in climate-growth responses is a multifactorial phenomenon driven by the interaction of site climatic conditions, tree species, and methodological features of the modeling approach. Given the existence of multiple drivers and the frequent occurrence of non-stationarity, we recommend that temporal non-stationarity rather than stationarity should be considered as the baseline model of climate-growth response for temperate forests.