High-fidelity representation of climate variations by Amburana cearensis tree-ring chronologies across a tropical forest transition in South America

Numerous ring-width chronologies from different species have recently been developed in diverse tropical forests across South America. However, the temporal and spatial climate signals in these tropical chronologies is less well known. In this work, annual growth rings of Amburana cearensis, a widely distributed tropical tree species, were employed to estimate temporal and spatial patterns of climate variability in the transition from the dry Chiquitano (16–17°S) to the humid Guarayos-southern Amazon (14–15°S) forests. Four well-replicated chronologies (16–21 trees, 22–28 radii) of A. cearensis were compared with temperature and precipitation records available in the region. The interannual variations in all four A. cearensis tree-ring chronologies are positively correlated with precipitation and negatively with temperature during the late dry-early wet season, the classic moisture response seen widely in trees from dry tropical and temperate forests worldwide. However, the chronologies from the dry Chiquitano forests of southern Bolivia reflect the regional reduction in precipitation during recent decades, while the chronologies from the tropical lowland moist forests in the north capture the recent increase in precipitation in the southern Amazon basin. These results indicate that A. cearensis tree growth is not only sensitive to the moisture balance of the growing season, it can also record subtle differences in regional precipitation trends across the dry to humid forest transition. Comparisons with previously developed Centrolobium microchaete chronologies in the region reveal a substantial common signal between chronologies in similar environments, suggesting that regional differences in climate are a major drivers of tree growth along the precipitation gradient. The difficulty of finding A. cearensis trees over 150-years old is the main limitation involved in the paleoclimate application of this species. The expansion of monocultures and intensive cattle ranching in the South American tropics are contributing to the loss of these old growth A. cearensis trees and the valuable records of climate variability and climate change that they contain.     

Convergence in growth responses of tropical trees to climate driven by water stress

Widely documented for temperate and cold forests in both hemispheres, variations in tree growth responses to climate along environmental gradients have rarely been investigated in the tropics. Seven tree‐ring chronologies of Centrolobium microchaete (Fabaceae) in the Cerrado tropical forests of Bolivia are used to determine the growth responses to climate along a precipitation gradient. Chronologies are distributed from the humid Guarayos forests (annual precipitation > 1600 mm) in the transition to the Amazonia to the dry‐mesic Chiquitos forests (annual precipitation < 1200 mm) in the proximity to the dry Chaco. On a large spatial scale, radial growth is positively influenced by rainfall and negatively by temperature at the end of the dry season. However, this regional pattern in climate‐tree growth relationship shows differences along the precipitation gradient. Relationships with climate are highly significant and extend over longer periods of the year in sites with low rainfall and extremely severe dry seasons. At wet sites, larger water soil capacity and endogenous forest dynamics partially mask the direct influence of climate on tree growth. Stronger similarities in tree‐growth responses to climate occur between sites in the dry Central Chiquitos and in the transition to the Guarayos forests. In contrast, the relationships show fewer similarities between sites in the humid Guarayos. We conclude that growth responses to climate in the tropics are more similar between sites with limited rainfall and severe and prolonged dry seasons. Our study points to a convergence in the patterns of growth responses of tropical trees to climate, modulated by scarce rainfall and marked seasonality. The negative impact of water deficits on tree physiological processes induces not only the documented reduction in forest species richness, but also a convergence in tree‐growth responses to climate in dry tropical forests.     

Adjusting xylem anatomy and growth to inter-annual climate variability in two Fabaceae species (Centrolobium microchaete,Cenostigma pluviosum) from Bolivian dry tropical forests

Climate-growth relationships are strong in seasonally dry tropical forests (SDTFs). To better understand the ecological processes controlling these relationships we need to assess the long-term responses of wood anatomy and radial growth to year-to-year climate variability. We assessed how wood-anatomical traits (mean vessel area –MVA– and vessel density –VD–, percentage of conductive area –CA–, xylem-specific estimated hydraulic conductivity –Ks–) and growth responded to local climate (mean temperature, total precipitation, estimated moisture) variability and teleconnections (Pacific Decadal Oscillation) between 1970 and 2011 in two Fabaceae tree species (Centrolobium microchaete and Cenostigma pluviosum) coexisting in a Bolivian SDTF. We found that C. microchaete produced wider vessels and was more responsive to both local climate conditions and teleconnections than C. pluviosum. In C. microchaete VD positively responded to average temperature in the late-wet season and in the previous dry season, and CA and Ks were higher in years with warmer wet and previous early-dry seasons, as well as in years with higher PDO values. These responses were independent from ring-width variability only for C. microchaete. For C. pluviosum, vessel chronologies were more responsive to local temperature variability, and only MVA and CA positively and negatively responded to moisture in the early-dry and the previous dry seasons, respectively. Our results show that wood hydraulic structure in SDTFs is responsive to climate fluctuations. The combined study of ring width and wood anatomy allows having a more complete picture of the influence of climate on growth, particularly in species as C. microchaete which show low collinearity of tree-ring width and wood plasticity in response to climate variability.     

Treeline dynamics in relation to climatic variability in the central Tianshan Mountains, northwestern China

Aim Climate variability may be an important mediating agent of ecosystem dynamics in cold, arid regions such as the central Tianshan Mountains, north‐western China. Tree‐ring chronologies and the age structure of a Schrenk spruce (Picea schrenkiana) forest were developed to examine treeline dynamics in recent decades in relation to climatic variability. Of particular interest was whether tree‐ring growth and population recruitment patterns responded similarly to climate warming. Location The study was conducted in eight stands that ranged from 2500 m to 2750 m a.s.l. near the treeline in the Tianchi Nature Reserve (43°45′?43°59′ N, 88°00′?88°20′ E) in the central Xinjiang Uygur Autonomous Region, northwestern China. Methods Tree‐ring cores were collected and used to develop tree‐ring chronologies. The age of sampled trees was determined from basal cores sampled as close as possible to the ground. Population age structure and recruitment information were obtained using an age–d.b.h. (diameter at breast height) regression from the sampled cores and the d.b.h. measured on all trees in the plots. Ring‐width chronologies and tree age structure were both used to investigate the relationship between treeline dynamics and climate change. Results Comparisons with the climatic records showed that both the radial growth of trees and tree recruitment were influenced positively by temperature and precipitation in the cold high treeline zone, but the patterns of their responses differed. The annual variation in tree rings could be explained largely by the average monthly minimum temperatures during February and August of the current year and by the monthly precipitation of the previous August and January, which had a significant and positive effect on tree radial growth. P. schrenkiana recruitment was influenced mainly by consecutive years of high minimum summer temperatures and high precipitation during spring. Over the last several decades, the treeline did not show an obvious upward shift and new recruitment was rare. Some trees had established at the treeline at least 200 years ago. Recruitment increased until the early 20th century (1910s) but then decreased with poor recruitment over the past several decades (1950–2000). Main conclusions There were strong associations between climatic change and ring‐width patterns, and with recruitments in Schrenk spruce. Average minimum temperatures in February and August, and total precipitation in the previous August and January, had a positive effect on tree‐ring width, and several consecutive years of high minimum summer temperature and spring precipitation was a main factor favouring the establishment of P. schrenkiana following germination within the treeline ecotone. Both dendroclimatology and recruitment analysis were useful and compatible to understand and reconstruct treeline dynamics in the central Tianshan Mountains.     

CO2 fertilization and enhanced drought resistance in Greek firs from Cephalonia Island,Greece

Growth–climate relationships were investigated in Greek firs from Ainos Mountain on the island of Cephalonia in western Greece, using dendrochronology. The goal was to test whether tree growth is sensitive to moisture stress, whether such sensitivity has been stable through time, and whether changes in growth–moisture relationships support an influence of atmospheric CO₂ on growth. Regressions of tree‐ring indices (ad 1820–2007) with instrumental temperature, precipitation, and Palmer Drought Severity Index (PDSI) indicate that growth is fundamentally limited by growing‐season moisture in late spring/early summer, most critically during June. However, this simple picture obscures a pattern of sharply evolving growth–climate relationships during the 20th century. Correlations between growth and June temperature, precipitation, and PDSI were significantly greater in the early 20th century but later degraded and disappeared. By the late 20th–early 21st century, there remains no statistically significant relationship between moisture and growth implying markedly enhanced resistance to drought. Moreover, growth experienced a net increase over the last half‐century culminating with a sharp spike in ad 1988–1990. This recent growth acceleration is evident in the raw ring‐width data prior to standardization, ruling out artifacts from statistical detrending. The vanishing relationship with moisture and parallel enhancement of growth are all the more notable because they occurred against a climatic backdrop of increasing aridity. The results are most consistent with a significant CO₂ fertilization effect operating through restricted stomatal conductance and improved water‐use efficiency. If this interpretation is correct, atmospheric CO₂ is now overcompensating for growth declines anticipated from drier climate, suggesting its effect is unusually strong and likely to be detectable in other up‐to‐date tree‐ring chronologies from the Mediterranean.     

Patterns and drivers of Araucaria araucana forest growth along a biophysical gradient in the northern Patagonian Andes: Linking tree rings with satellite observations of soil moisture

Araucaria araucana (Araucaria) is a long‐lived conifer growing along a sharp west–east biophysical gradient in the Patagonian Andes. The patterns and climate drivers of Araucaria growth have typically been documented on the driest part of the gradient relying on correlations with meteorological records, but the lack of in situ soil moisture observations has precluded an assessment of the growth responses to soil moisture variability. Here, we use a network of 21 tree‐ring width chronologies to investigate the spatiotemporal patterns of tree growth through the entire gradient and evaluate their linkages with regional climate and satellite‐observed surface soil moisture variability. We found that temporal variations in tree growth are remarkably similar throughout the gradient and largely driven by soil moisture variability. The regional spatiotemporal pattern of tree growth was positively correlated with precipitation (r = 0.35 for January 1920–1974; P < 0.01) and predominantly negatively correlated with temperature (r = ?0.38 for January–March 1920–1974; P < 0.01) during the previous growing season. These correlations suggest a temporally lagged growth response to summer moisture that could be associated with known physiological carry‐over processes in conifers and to a response to moisture variability at deeper layers of the rooting zone. Notably, satellite observations revealed a previously unobserved response of Araucaria growth to summer surface soil moisture during the current rather than the previous growing season (r = 0.65 for 1979–2000; P < 0.05). This new response has a large spatial footprint across the mid‐latitudes of the South American continent (35°–45°S) and highlights the potential of Araucaria tree rings for palaeoclimatic applications. The strong moisture constraint on tree growth revealed by satellite observations suggests that projected summer drying during the coming decades may result in regional growth declines in Araucaria forests and other water‐limited ecosystems in the Patagonian Andes.     

Climate increases regional tree-growth variability in Iberian pine forests

Tree populations located at the geographical distribution limit of the species may provide valuable information about tree‐growth response to changes on climatic conditions. We established nine Pinus nigra, 12 P. sylvestris and 17 P. uncinata tree‐ring width chronologies along the eastern and northern Iberian Peninsula, where these species are found at the edge of their natural range. Tree‐growth variability was analyzed using principal component analysis (PCA) for the period 1885–1992. Despite the diversity of species, habitats and climatic regimes, a common macroclimatic signal expressed by the first principal component (PC1) was found. Moreover, considering the PC1 scores as a regional chronology, significant relations were established with Spanish meteorological data. The shared variance held by the tree chronologies, the frequency of narrow rings and the interannual growth variability (sensitivity) increased markedly during the studied period. This shows an enhancement of growth synchrony among forests indicating that climate might have become more limiting to growth. Noticeably, an upward abrupt shift in common variability at the end of the first half of the 20th century was detected. On the other hand, moving‐interval response functions showed a change in the growth–climate relationships during the same period. The relationship between growth and late summer/autumn temperatures of the year before growth (August–September, negative correlation, and November, positive correlation) became stronger. Hence, water stress increase during late summer previous to tree growth could be linked to the larger growth synchrony among sites, suggesting that climate was driving the growth pattern changes. This agrees with the upward trend in temperature observed in these months. Moreover, the higher occurrence of extreme years and the sensitivity increase in the second half of the 20th century were in agreement with an increment in precipitation variability during the growing period. Precipitation variability was positively related to tree‐growth variability, but negatively to radial growth. In conclusion, a change in tree‐growth pattern and in the climatic response of the studied forests was detected since the mid‐20th century and linked to an increase in water stress. These temporal trends were in agreement with the observed increase in warmer conditions and in precipitation variability.     

Topographic mediation of growth in high elevation foxtail pine (Pinus balfouriana Grev. et Balf.) forests in the Sierra Nevada, USA

Aim Climate variability is an important mediating agent of ecosystem dynamics in cold, semi‐arid regions such as the mountains of western North America. Climatically sensitive tree‐ring chronologies offer a means of assessing the impact of climate variability on tree growth across temporal scales of years to centuries and spatial scales of metres to subcontinents. Our goal was to bring practices from landscape ecology that highlight the impact of landscape heterogeneity on ecological pattern and processes into a dendroclimatic study that shows that the biophysical setting of target trees affects ring‐width patterns. Location This study was conducted at two sites near alpine treeline in the Sequoia National Park, USA (36°30′ 00′ N, 118°30′ 00′ W). Methods We collected stand information and increment cores from foxtail pines (Pinus balfouriana Grev. et Balf.) for eight tree‐ring chronologies in four extreme biophysical settings at two sites using proxies for soil moisture and radiation derived from a digital elevation model. Results Biophysical setting affected forest age–class structure, with wet and bright plots showing high recruitment after 1900 ad , but had no obvious effect on immature stem density (e.g. seedlings). Biophysical setting strongly affected ring‐width patterns, with wet plots having higher correlation with instrumental temperature records while dry plots correlated better with instrumental precipitation records. Ring‐width chronologies from the wet plots showed strong low‐frequency variability (i.e. hundreds of years) while ring‐width chronologies from the dry plots showed strong variability on multidecadal scales. Main conclusions There was a strong association between biophysical setting and age‐class structure, and with ring‐width patterns in foxtail pine. The mediation of ring widths by biophysical setting has the potential to further the understanding of the expression of synoptic‐scale climate across rugged terrain. When combined with remotely sensed imagery, a priori GIS modelling of tree growth offers a viable means to devise first‐order predictions of climatic impacts in subalpine forest dynamics and to develop flexible and powerful monitoring schemes.     

Climate–tree growth relationships of longleaf pine (Pinus palustris Mill.) in the Southeastern Coastal Plain, USA

Knowledge of tree growth/climate response relationships is important to dendroecological studies and dendroclimatic reconstructions, particularly in the Southeastern Coastal Plain where few such studies have been attempted. To this end, we developed tree-ring chronologies of total ring width, earlywood width, and latewood width from longleaf pine (Pinus palustris Mill.) at three sites in the Southeastern Coastal Plain to examine the climate–growth relationships for this tree species. The length of these chronologies is unprecedented for southern pine chronologies in the Southeast. We compared the tree-ring chronologies to monthly temperature, precipitation, Palmer drought severity index (PDSI), and Palmer hydrological drought index (PHDI) data from the pertinent climate divisions. We found that PDSI and PHDI have the highest correlation with longleaf pine growth, and the strongest relationships between longleaf pine growth and these variables occur between July and November. Precipitation in the spring and summer was also positively related to growth at all sites. The relationship between temperature and growth was the weakest among all climate variables, but warm summer temperatures had a consistent, negative relationship with longleaf pine growth. The climate signal in the latewood was generally more robust than for total ring width and earlywood width.     

Effect of climate on tree growth in the Pampa biome of Southeastern South America: First tree-ring chronologies from Uruguay

Tree-ring research in the highland tropics and subtropics represents a major frontier for understanding climate-growth relationships. Nonetheless, there are many lowland regions – including the South American Pampa biome – with scarce tree ring data. We present the first two tree-ring chronologies for Scutia buxifolia in subtropical Southeastern South America (SESA), using 54 series from 29 trees in two sites in northern and southern Uruguay. We cross-dated annual rings and compared tree growth from 1950 to 2012 with regional climate variability, including rainfall, temperature and the Palmer Drought Severity Index – PDSI, the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Overall, ring width variability was highly responsive to climate signals linked to water availability. For example, tree growth was positively correlated with accumulated rainfall in the summer-fall prior to ring formation for both chronologies. Summer climate conditions were key for tree growth, as shown by a negative effect of hot summer temperatures and a positive correlation with PDSI in late austral summer. The El Niño phase in late spring/early summer favored an increase in rainfall and annual tree growth, while the La Niña phase was associated with less rainfall and reduced tree growth. Extratropical climate factors such as SAM had an equally relevant effect on tree growth, whereby the positive phase of SAM had a negative effect over radial growth. These findings demonstrate the potential for dendroclimatic research and climate reconstruction in a region with scarce tree-ring data. They also improve the understanding of how climate variability may affect woody growth in native forests – an extremely limited ecosystem in the Pampa biome.     

Contrasting tree‐ring growth to climate responses of Abies alba toward the southern limit of its distribution area

Silver fir Abies alba is an indigenous tree species present in many southern European mountain forests. Its distribution area and its adaptive capacity to climate variability, expressed in tree‐ring growth series, make it a very suitable target species for studying responses to climate particularly in a complex area like the Mediterranean basin where significant changes are expected. We used a set of 52 site chronologies (784 trees) in the Italian Alps and Apennines (38.1°– 46.6°N and 6.7°– 16.3°E) and temperature and precipitation monthly data for the period 1900–1995. Principal component analyses of the tree‐ring site network was applied to extract common modes of variability in annual radial growth among the chronologies. Climate/growth relationships and their stationarity and consistency over time were computed by means of correlation and moving correlation functions. Tree‐ring chronologies show a clear distinction between the Alpine and the Mediterranean sites and a further separation of the Alpine region in western and eastern sectors. Accordingly, we found different transient and contrasting regional responses in time with the trends found in the Mediterranean sites marking a relaxation of some of the major climate limiting factors recorded prior to the last decades. Species’ sensitivity to global change may result in distinct spatial responses reflecting the complexity of the Mediterranean climate, with large differences between various areas of the basin. It is still unclear if these contrasting tree‐ring growth to climate responses of Abies alba are due to the corresponding separation between the Alpine and Mediterranean climate modes, the atmospheric CO₂ fertilization effect, the environmentally most fitted genetic pools of the southern fir ecotypes or a combination of all factors. Climate–growth analysis based on a wide site network and on long‐term weather records confirmed to be excellent tools to detect spatial and temporal variability of species’ responses to climate.     

Long-term changes in tree-ring–climate relationships at Mt. Patscherkofel (Tyrol,Austria) since the mid-1980s

Although growth limitation of trees at Alpine and high-latitude timberlines by prevailing summer temperature is well established, the loss of thermal response of radial tree growth during last decades has repeatedly been addressed. We examined long-term variability of climate–growth relationships in ring width chronologies of Stone pine (Pinus cembra L.) by means of moving response functions (MRF). The study area is situated in the timberline ecotone (ca. 2,000–2,200 m a.s.l.) on Mt. Patscherkofel (Tyrol, Austria). Five site chronologies were developed within the ecotone with constant sample depth (≥19 trees) throughout most of the time period analysed. MRF calculated for the period 1866–1999 and 1901–1999 for ca. 200- and ca. 100-year-old stands, respectively, revealed that mean July temperature is the major and long-term stable driving force of Pinus cembra radial growth within the timberline ecotone. However, since the mid-1980s, radial growth in timberline and tree line chronologies strikingly diverges from the July temperature trend. This is probably a result of extreme climate events (e.g. low winter precipitation, late frost) and/or increasing drought stress on cambial activity. The latter assumption is supported by a <10% increase in annual increments of ca. 50-year-old trees at the timberline and at the tree line in 2003 compared with 2002, when extraordinary hot and dry conditions prevailed during summer. Furthermore, especially during the second half of the twentieth century, influence of climate variables on radial growth show abrupt fluctuations, which might also be a consequence of climate warming on tree physiology.     

Elevation-dependent climate sensitivity in Eucalyptus pauciflora Sieb. ex Spreng

We examine climate sensitivity in tree-ring chronologies from Eucalyptus pauciflora Sieb. ex Spreng at three elevations—1,350, 1,475 and 1,600 m above the sea level. Consistent with the principle that the sensitivity of tree-ring chronologies increases with proximity to the limits of tree growth, statistics reflecting chronology reliability increased with elevation. Climatological analyses of the three elevation classes revealed that whilst ring width is significantly and positively correlated with maximum air temperature during spring (September–November) in the chronology from the highest elevation class, significant correlations with maximum temperature are not present at low elevations. Similarly, whilst ring width in the chronology from 1,350 m was significantly and positively correlated with precipitation during late summer and early autumn, no significant correlations are evident at higher elevations. Our results illustrate the importance of careful site selection in dendroclimatological studies of eucalypts and demonstrate the potential of E. pauciflora for climatological studies.     

An assessment of Schinopsis brasiliensis Engler (Anacardiacea) for dendroclimatological applications in the tropical Cerrado and Chaco forests,Bolivia

Given the scarcity of instrumental climatic data in the South American tropics, it is valuable to explore the dendrochronological potential of the numerous tree species growing in the region. In this paper, we assessed for the first time the dendrochronological characteristics of Schinopsis brasiliensis, an arboreal species from the dry-tropical Cerrado and Chaco forests in Bolivia and adjacent countries. Similar to most woody species in the Cerrado and Chaco regions, growth rings of S. brasiliensis are delimited by the presence of thin but continuous lines of marginal parenchyma. Based on 22 samples from 15 trees, we present the first ring-width chronology for this species covering the period 1812–2011 (200 years). Additionally, a 106-year floating chronology from S. brasiliensis was developed using cores from four columns from the church of San Miguel, Santa Cruz, built in the period 1720–1740. Standard dendrochronological statistics indicate an important common signal in the radial growth of S. brasiliensis. The comparison of variations in regional climate and ring widths shows that tree growth is directly related to spring-summer rainfall and inversely related to temperature. Following the winter dry season, rainfall in late spring and early summer increases soil water supply, which activates tree growth. In contrast, above-average temperatures during the same period increase evapotranspiration, intensify the water deficit and reduce radial growth. The dependence of S. brasiliensis growth on water supply is evidence of its dendrochronological potential for reconstructing past precipitation variations in the extensive tropical Cerrado and Chaco forest formations in South America. Using wood from historical buildings opens the possibility of extending the chronologies of S. brasiliensis over the past 400–500 years.     

Growth decline of Pinus Massoniana in response to warming induced drought and increasing intrinsic water use efficiency in humid subtropical China

Pinus Massoniana is the most widely distributed coniferous species in southern China and one of the most distributed species for plantation in China. It is not uncertain about the responses of tree growth to the combined effects of regional drying and the increase in the intrinsic water-use efficiency (iWUE) due to increased atmospheric CO₂. This study addressed this issue by comparing the tree growth patterns as represented by three tree-ring width chronologies with climate variables and three iWUE series derived from three tree-ring stable carbon isotope discrimination chronologies (Δ¹³C) from Pinus Massoniana in Daiyun Mountain, central Fujian province of China. Among these chronologies, we reported the first tree-ring carbon isotope discrimination chronologies (Δ¹³C) from Fuzhou area spanning last 210 years. It was found that tree radial growth is mainly limited by dry condition from May to October. Growth limitation by cold condition was only found in one high altitude site (780m) in early spring and late autumn. The tree-ring carbon discrimination was enhanced under conditions with low relative humidity and sufficient sunshine in late summer and autumn. In general, the iWUE showed a significantly increasing trend since the 1850s for all the sites in response to the increase in atmospheric CO₂. However, the growth promotion of the increased iWUE on tree growth could not compensate the growth limitation caused by drought. Especially since the 1960s, a growth decline was found at two drought stressed sites at low altitudes. On the other hand, the increase in temperature of spring and autumn and iWUE has most likely enhanced tree growth at the high altitude site.     

High Arctic summer warming tracked by increased Cassiope tetragona growth in the world's northernmost polar desert

Rapid climate warming has resulted in shrub expansion, mainly of erect deciduous shrubs in the Low Arctic, but the more extreme, sparsely vegetated, cold and dry High Arctic is generally considered to remain resistant to such shrub expansion in the next decades. Dwarf shrub dendrochronology may reveal climatological causes of past changes in growth, but is hindered at many High Arctic sites by short and fragmented instrumental climate records. Moreover, only few High Arctic shrub chronologies cover the recent decade of substantial warming. This study investigated the climatic causes of growth variability of the evergreen dwarf shrub Cassiope tetragona between 1927 and 2012 in the northernmost polar desert at 83°N in North Greenland. We analysed climate–growth relationships over the period with available instrumental data (1950–2012) between a 102‐year‐long C. tetragona shoot length chronology and instrumental climate records from the three nearest meteorological stations, gridded climate data, and North Atlantic Oscillation (NAO) and Arctic Oscillation (AO) indices. July extreme maximum temperatures (JulT_emx), as measured at Alert, Canada, June NAO, and previous October AO, together explained 41% of the observed variance in annual C. tetragona growth and likely represent in situ summer temperatures. JulT_emx explained 27% and was reconstructed back to 1927. The reconstruction showed relatively high growing season temperatures in the early to mid‐twentieth century, as well as warming in recent decades. The rapid growth increase in C. tetragona shrubs in response to recent High Arctic summer warming shows that recent and future warming might promote an expansion of this evergreen dwarf shrub, mainly through densification of existing shrub patches, at High Arctic sites with sufficient winter snow cover and ample water supply during summer from melting snow and ice as well as thawing permafrost, contrasting earlier notions of limited shrub growth sensitivity to summer warming in the High Arctic.     

Climate related causes of distinct radial growth reductions in Pinus cembra during the last 200 yr

Distinct radial growth reductions in Cembran pine (Pinus cembra L.) were studied at the timberline on Mt. Patscherkofel (2246 m a.s.l., Tyrol, Austria), which is situated in the inner-Alpine dry region of the Central Austrian Alps. Six timberline stands with different aspects were sampled and ring-width chronologies developed based on dendroecological techniques. Growth-climate relationships between residual chronologies and climate variables were explored using Pearson product-moment correlation coefficients. P. cembra growth at the timberline appears to be limited by cool summer (June-August) and previous autumn (September-October) temperatures and low precipitation in late winter (March). Timberline stands show concurrent growth depressions lasting ≥ 5 yr during the periods 1815–1823, 1851–1858 and 1913–1920, indicating growth depressions were steadily decreasing. Although evaluation of climate data revealed that these growth depressions can mainly be explained by occurrence of cold growing seasons and therefore the influence of recent climate warming on tree growth is plausible, an effect of tree ageing on climate sensitivity may also be involved. On the other hand, climate extremes do not inevitably induce growth responses as would be expected from growth-climate relationships. This is explained by the occurrence of synergistic and/or compensating effects of growth limiting climate variables and preconditioning of tree growth in previous years. Comparison of growth reductions with two published P. cembra timberline chronologies from inner-Alpine dry locations in the Eastern Alps revealed that investigated stands show the highest climate sensitivity during the last 200 yr. This difference in growth response to climate variability is most probably related to the special climate situation at Mt. Patscherkofel, which is exceptionally windy throughout the year and frequently exposed to warm dry winds (Föhn).     

Forest tree growth response to hydroclimate variability in the southern Appalachians

Climate change will affect tree species growth and distribution; however, under the same climatic conditions species may differ in their response according to site conditions. We evaluated the climate‐driven patterns of growth for six dominant deciduous tree species in the southern Appalachians. We categorized species into two functional groups based on their stomatal regulation and xylem architecture: isohydric, diffuse porous and anisohydric, ring porous. We hypothesized that within the same climatic regime: (i) species‐specific differences in growth will be conditional on topographically mediated soil moisture availability; (ii) in extreme drought years, functional groups will have markedly different growth responses; and (iii) multiple hydroclimate variables will have direct and indirect effects on growth for each functional group. We used standardized tree‐ring chronologies to examine growth of diffuse‐porous (Acer, Liriodendron, and Betula) and ring‐porous (Quercus) species vs. on‐site climatic data from 1935 to 2003. Quercus species growing on upslope sites had higher basal area increment (BAI) than Quercus species growing on mesic, cove sites; whereas, Acer and Liriodendron had lower BAI on upslope compared to cove sites. Diffuse‐porous species were more sensitive to climate than ring porous, especially during extreme drought years. Across functional groups, radial growth was more sensitive to precipitation distribution, such as small storms and dry spell length (DSL), rather than the total amount of precipitation. Based on structural equation modeling, diffuse‐porous species on upslope sites were the most sensitive to multiple hydroclimate variables (r² = 0.46), while ring‐porous species on upslope sites were the least sensitive (r² = 0.32). Spring precipitation, vapor pressure deficit, and summer storms had direct effects on summer AET/P, and summer AET/P, growing season small storms and DSL partially explained growth. Decreasing numbers of small storms and extending the days between rainfall events will result in significant growth reduction, even in regions with relatively high total annual rainfall.     

Bioclimatology of beech (Fagus sylvatica L.) in the Eastern Alps: spatial and altitudinal climatic signals identified through a tree-ring network

Aim To identify the dominant spatial patterns of Fagus sylvatica radial growth in the Eastern Alps, and to understand their relationships to climate variation and bioclimatic gradients. Location Fourteen beech stands in the Eastern Alps, growing between 200 and 1500 m a.s.l. in Italy, Slovenia and Austria. Methods At each site, trees were sampled using increment borers or by taking discs from felled trees. Cores and discs were processed by measuring and crossdating ring width. Ring width series were standardized, averaged, and prewhitened to obtain site chronologies. Hierarchical Cluster Analysis (HCA) and Principal Components Analysis of prewhitened site chronologies were used to identify spatial and altitudinal growth patterns, related to the bioclimatic position of each stand. Bootstrap correlation and response functions were computed between monthly climatic variables and either principal component scores or composite chronologies from stands associated by HCA. The stability of dendroclimatic signals was analyzed by moving correlation functions (MCF). Correlation analysis (teleconnections) based on a data base of 37 Italian and Slovenian beech tree‐ring chronologies revealed the spatial extent of principal component scores. Results Sampled trees were 200–400 years old, representing the oldest beech trees that have been crossdated for the Alps to date. Maximum age was directly related to altitude and to the presence of historical forms of conservation. Tree‐ring parameters varied according to geographic patterns and the age of sampled trees. Stands were bioclimatically organized according to their location, and with reference to their elevation and distance from the Adriatic Sea. A direct response to winter temperature was found at all elevations. The altitudinal gradient ranged from low‐elevation stands, characterized by a Mediterranean‐type, late spring–summer drought signal, to mountain and high‐elevation stands, characterized by a direct response to growing season temperature plus an inverse response to the previous year’s July temperature. The mountain and high‐elevation signal was evident in Austria, the Central Alps and Slovenia, while the low‐elevation signal was confined to mountains adjacent to the Adriatic Sea. MCF revealed trends in the response to climatic factors affecting tree‐ring formation in mountain and high‐mountain stands linked to climatic warming. Main conclusions Dendroclimatic networks can be used for bioclimatic studies of tree populations. A biogeographical separation emerged between the Alps and the Apennines at the upper elevations, while different degrees of mediterraneity distinguished sites at lower elevations. This information will be useful in assessing any future climate‐related bioclimatic shifts, especially for forests at ecotones and along altitudinal gradients.