Growth models can be used to assess forest vulnerability to climate warming. If global warming amplifies water deficit in drought‐prone areas, tree populations located at the driest and southernmost ...distribution limits (rear‐edges) should be particularly threatened. Here, we address these statements by analyzing and projecting growth responses to climate of three major tree species (silver fir, Abies alba; Scots pine, Pinus sylvestris; and mountain pine, Pinus uncinata) in mountainous areas of NE Spain. This region is subjected to Mediterranean continental conditions, it encompasses wide climatic, topographic and environmental gradients, and, more importantly, it includes rear‐edges of the continuous distributions of these tree species. We used tree‐ring width data from a network of 110 forests in combination with the process‐based Vaganov–Shashkin‐Lite growth model and climate–growth analyses to forecast changes in tree growth during the 21st century. Climatic projections were based on four ensembles CO2 emission scenarios. Warm and dry conditions during the growing season constrain silver fir and Scots pine growth, particularly at the species rear‐edge. By contrast, growth of high‐elevation mountain pine forests is enhanced by climate warming. The emission scenario (RCP 8.5) corresponding to the most pronounced warming (+1.4 to 4.8 °C) forecasted mean growth reductions of −10.7% and −16.4% in silver fir and Scots pine, respectively, after 2050. This indicates that rising temperatures could amplify drought stress and thus constrain the growth of silver fir and Scots pine rear‐edge populations growing at xeric sites. Contrastingly, mountain pine growth is expected to increase by +12.5% due to a longer and warmer growing season. The projections of growth reduction in silver fir and Scots pine portend dieback and a contraction of their species distribution areas through potential local extinctions of the most vulnerable driest rear‐edge stands. Our modeling approach provides accessible tools to evaluate forest vulnerability to warmer conditions.
Aim
Climate change is expected to modify growth trends of forests around the world. However, this modification may vary in strength and intensity across a species' biogeographical range. Here, we ...study European populations of silver fir (Abies alba) across its southern distribution limits in Spain, Italy and Romania. We hypothesized that growth trends of silver fir will differ across its distribution range, with a marked decline in growth in drought‐prone regions near the species' southernmost biogeographical limits.
Location
Europe (Spain, Italy, Romania).
Methods
We collected tree‐ring data from at least 1300 silver fir trees located in 111 sites. The dataset was used to assess and model growth trends, quantified as changes in basal area increment, and to determine how growth responds to climate.
Results
We found contrasting patterns of basal area increments among countries and sites. Populations of silver fir located outside the Mediterranean area (e.g. northern Italy, Romania) have shown a clear increase in growth over the last two decades, whereas most populations in Spain and southern Italy have displayed a marked decline in growth since the 1980s. The growth of silver fir forests at the south‐western distribution limit is severely constrained by low spring–summer water availability, whereas growth of silver fir forests in non‐Mediterranean areas is limited by cold conditions in late winter to early spring.
Main conclusions
Climate warming is distinctly modifying growth patterns and responses to climate in silver fir across most of the species' European distribution area. In south‐western Europe the reduction in growth of many populations is related to an observed increase in aridity, whereas in more temperate areas warming is enhancing growth. Our results confirm a decline in the growth of silver fir at its south‐western distribution limits as a consequence of climate warming.
Historical and future trends in net primary productivity (NPP) and its sensitivity to global change are largely unknown because of the lack of long-term, high-resolution data. Here we test whether ...annually resolved tree-ring stable carbon (δ
C) and oxygen (δ
O) isotopes can be used as proxies for reconstructing past NPP. Stable isotope chronologies from four sites within three distinct hydroclimatic environments in the eastern United States (US) were compared in time and space against satellite-derived NPP products, including the long-term Global Inventory Modeling and Mapping Studies (GIMMS3g) NPP (1982-2011), the newest high-resolution Landsat NPP (1986-2015), and the Moderate Resolution Imaging Spectroradiometer (MODIS, 2001-2015) NPP. We show that tree-ring isotopes, in particular δ
O, correlate strongly with satellite NPP estimates at both local and large geographical scales in the eastern US. These findings represent an important breakthrough for estimating interannual variability and long-term changes in terrestrial productivity at the biome scale.
The increasing carbon dioxide (CO₂) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of ...trees: altering the carbon and water fluxes passing through the stomatal pores. However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree‐ring sites located across Europe are investigated to determine the intrinsic water‐use efficiency (iWUE), the ratio of photosynthesis to stomatal conductance from 1901 to 2000. The results were compared with simulations of a dynamic vegetation model (LPX‐Bern 1.0) that integrates numerous ecosystem and land–atmosphere exchange processes in a theoretical framework. The spatial pattern of tree‐ring derived iWUE of the investigated coniferous and deciduous species and the model results agreed significantly with a clear south‐to‐north gradient, as well as a general increase in iWUE over the 20th century. The magnitude of the iWUE increase was not spatially uniform, with the strongest increase observed and modelled for temperate forests in Central Europe, a region where summer soil‐water availability decreased over the last century. We were able to demonstrate that the combined effects of increasing CO₂ and climate change leading to soil drying have resulted in an accelerated increase in iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation–climate feedbacks are currently still poorly constrained by observational data.
Hydroclimate variability in tropical South America is strongly regulated by the South American Summer Monsoon (SASM). However, past precipitation changes are poorly constrained due to limited ...observations and high‐resolution paleoproxies. We found that summer precipitation and the El Niño‐Southern Oscillation (ENSO) variability are well registered in tree‐ring stable oxygen isotopes (δ18OTR) of Polylepis tarapacana in the Chilean and Bolivian Altiplano in the Central Andes (18–22°S, ∼4,500 m a.s.l.) with the northern forests having the strongest climate signal. More enriched δ18OTR values were found at the southern sites likely due to the increasing aridity toward the southwest of the Altiplano. The climate signal of P. tarapacana δ18OTR is the combined result of moisture transported from the Amazon Basin, modulated by the SASM, ENSO, and local evaporation, and emerges as a novel tree‐ring climate proxy for the southern tropical Andes.
Plain Language Summary
Understanding past climatic changes in the Central Andes in tropical South America is of great importance to contextualize current hydroclimatic conditions. Here, we present the first P. tarapacana tree‐ring stable oxygen isotope (δ18OTR) chronologies and analyze their value as environmental records for this region. Locally known as queñoa, P. tarapacana grows in the South American Altiplano from 16°S to 23°S at very high elevations (up to 5,100 m a.s.l), making it the highest elevation tree species worldwide. We analyze P. tarapacana δ18OTR from 1950 to present and find that it registers precipitation changes in the Altiplano and the El Niño ‐ Southern Oscillation (ENSO). We suggest that δ18OTR is likely affected by soil evaporation and leaf transpiration due to the high solar radiation and aridity in the Altiplano, leading to an enrichment in δ18OTR values with a more pronounced effect at the more arid sites. P. tarapacana δ18OTR reflects the atmospheric processes transporting moisture to the Altiplano and the influence of local evaporation. Our findings are relevant for generating robust hydroclimate reconstructions in the Central Andes to improve circulation models and provide better management of water resources in tropical South America.
Key Points
Tree‐ring stable oxygen isotopes of Polylepis tarapacana record austral summer precipitation variability in the South American Altiplano
El Niño‐Southern Oscillation is imprinted in the tree‐ring oxygen isotopes with a stronger signal toward the north of the studied area
We investigated the tree growth and physiological response of five pine forest stands in relation to changes in atmospheric CO₂ concentration (ca) and climate in the Iberian Peninsula using annually ...resolved width and δ¹³C tree-ring chronologies since ad 1600. ¹³C discrimination (Δ≈ci/ca), leaf intercellular CO₂ concentration (ci) and intrinsic water-use efficiency (iWUE) were inferred from δ¹³C values. The most pronounced changes were observed during the second half of the 20th century, and differed between stands. Three sites kept a constant ci/ca ratio, leading to significant ci and iWUE increases (active response to ca); whereas a significant increase in ci/ca resulted in the lowest iWUE increase of all stands at a relict Pinus uncinata forest site (passive response to ca). A significant decrease in ci/ca led to the greatest iWUE improvement at the northwestern site. We tested the climatic signal strength registered in the δ¹³C series after removing the low-frequency trends due to the physiological responses to increasing ca. We found stronger correlations with temperature during the growing season, demonstrating that the physiological response to ca changes modulated δ¹³C and masked the climate signal. Since 1970 higher δ¹³C values revealed iWUE improvements at all the sites exceeding values expected by an active response to the ca increase alone. These patterns were related to upward trends in temperatures, indicating that other factors are reinforcing stomatal closure in these forests. Narrower rings during the second half of the 20th century than in previous centuries were observed at four sites and after 1970 at all sites, providing no evidence for a possible CO₂‘fertilization' effect on growth. The iWUE improvements found for all the forests, reflecting both a ca rise and warmer conditions, seem to be insufficient to compensate for the negative effects of the increasing water limitation on growth.
Arctic warming can influence tundra ecosystem function with consequences for climate feedbacks, wildlife and human communities. Yet ecological change across the Arctic tundra biome remains poorly ...quantified due to field measurement limitations and reliance on coarse-resolution satellite data. Here, we assess decadal changes in Arctic tundra greenness using time series from the 30 m resolution Landsat satellites. From 1985 to 2016 tundra greenness increased (greening) at ~37.3% of sampling sites and decreased (browning) at ~4.7% of sampling sites. Greening occurred most often at warm sampling sites with increased summer air temperature, soil temperature, and soil moisture, while browning occurred most often at cold sampling sites that cooled and dried. Tundra greenness was positively correlated with graminoid, shrub, and ecosystem productivity measured at field sites. Our results support the hypothesis that summer warming stimulated plant productivity across much, but not all, of the Arctic tundra biome during recent decades.
Stable oxygen isotopes measured in tree rings are useful for reconstructing climate variability and explaining changes in physiological processes occurring in forests, complementing other tree-ring ...parameters such as ring width. Here, we analyzed the relationships between different climate parameters and annually resolved tree-ring δ
18
O records (δ
18
O
TR
) from white spruce (
Picea glauca
MoenchVoss) trees located near Tungsten (Northwest Territories, Canada) and used the NASA GISS ModelE2 isotopically-equipped general circulation model (GCM) to better interpret the observed relationships. We found that the δ
18
O
TR
series were primarily related to temperature variations in spring and summer, likely through temperature effects on the precipitation δ
18
O in spring, and evaporative enrichment at leaf level in summer. The GCM simulations showed significant positive relationships between modelled precipitation δ
18
O over the study region and surface temperature and geopotential height over northwestern North America, but of stronger magnitudes during fall-winter than during spring–summer. The modelled precipitation δ
18
O was only significantly associated with moisture transport during the fall-winter season. The δ
18
O
TR
showed similar correlation patterns to modelled precipitation δ
18
O only during spring–summer when water matters more for trees, with significant positive correlations with surface temperature and geopotential height, but no correlations with moisture transport. Overall, the δ
18
O
TR
records for northwestern Canada reflect the same significant large-scale climate patterns as precipitation δ
18
O for spring–summer, and therefore have potential for reconstructing past atmospheric dynamics in addition to temperature variability in the region.
The 1783–1784 CE Laki eruption in Iceland was one of the largest, in terms of the mass of SO2 emitted, high‐latitude eruptions in the last millennium, but the seasonal and regional climate response ...was heterogeneous in space and time. Although the eruption did not begin until early June, tree‐ring maximum latewood density (MXD) reconstructions from Alaska suggest that the entire 1783 summer was extraordinarily cold. We use high‐resolution quantitative wood anatomy, climate model simulations, and proxy systems modeling to resolve the intra‐annual climate effects of the Laki eruption on temperatures over northwestern North America. We measured wood anatomical characteristics of white spruce (Picea glauca) trees from two northern Alaska sites. Earlywood cell characteristics of the 1783 ring are normal, while latewood cell wall thickness is significantly and anomalously reduced compared to non‐eruption years. Combined with complementary evidence from climate model experiments and proxy systems modeling, these features indicate an abrupt and premature cessation of cell wall thickening due to a rapid temperature decrease toward the end of the growing season. Reconstructions using conventional annual resolution MXD likely over‐estimate total growing season cooling in this year, while ring width fails to capture this abrupt late‐summer volcanic signal. Our study has implications not only for the interpretation of the climatic impacts of the Laki eruption in North America, but more broadly demonstrates the importance of timing and internal variability when comparing proxy temperature reconstructions and climate model simulations. It further demonstrates the value of developing cellular‐scale tree‐ring proxy measurements for paleoclimatology.
Key Points
Quantitative wood anatomy, climate model data, and proxy systems modeling show abrupt late summer cooling in Alaska after the Laki eruption
We show the importance of timing and internal variability when comparing proxy temperature reconstructions and climate model simulations
Quantitative wood anatomy provides a better understanding of how traditionally used tree‐ring metrics are affected by volcanic forcing
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