Drought-induced increase in water-use efficiency reduces secondary tree growth and tracheid wall thickness in a Mediterranean conifer

In order to understand the impact of drought and intrinsic water-use efficiency (iWUE) on tree growth, we evaluated the relative importance of direct and indirect effects of water availability on secondary growth and xylem anatomy of Juniperus thurifera, a Mediterranean anisohydric conifer. Dendrochronological techniques, quantitative xylem anatomy, and ¹³C/¹²C isotopic ratio were combined to develop standardized chronologies for iWUE, BAI (basal area increment), and anatomical variables on a 40-year-long annually resolved series for 20 trees. We tested the relationship between iWUE and secondary growth at short-term (annual) and long-term (decadal) temporal scales to evaluate whether gains in iWUE may lead to increases in secondary growth. We obtained a positive long-term correlation between iWUE and BAI, simultaneously with a negative short-term correlation between them. Furthermore, BAI and iWUE were correlated with anatomical traits related to carbon sink or storage (tracheid wall thickness and ray parenchyma amount), but no significant correlation with conductive traits (tracheid lumen) was found. Water availability during the growing season significantly modulated tree growth at the xylem level, where growth rates and wood anatomical traits were affected by June precipitation. Our results are consistent with a drought-induced limitation of tree growth response to rising CO₂, despite the trend of rising iWUE being maintained. We also remark the usefulness of exploring this relationship at different temporal scales to fully understand the actual links between iWUE and secondary growth dynamics.     

Explaining Global Increases in Water Use Efficiency: Why Have We Overestimated Responses to Rising Atmospheric CO2 in Natural Forest Ecosystems?

Background

The analysis of tree-ring carbon isotope composition (δ¹³C) has been widely used to estimate spatio-temporal variations in intrinsic water use efficiency (iWUE) of tree species. Numerous studies have reported widespread increases in iWUE coinciding with rising atmospheric CO₂ over the past century. While this could represent a coherent global response, the fact that increases of similar magnitude were observed across biomes with no apparent effect on tree growth raises the question of whether iWUE calculations reflect actual physiological responses to elevated CO₂ levels.

Methodology/Results

Here we use Monte Carlo simulations to test if an artifact of calculation could explain observed increases in iWUE. We show that highly significant positive relationships between iWUE and CO₂ occur even when simulated data (randomized δ¹³C values spanning the observed range) are used in place of actual tree-ring δ¹³C measurements. From simulated data sets we calculated non-physiological changes in iWUE from 1900 to present and across a 4000 m altitudinal range. This generated results strikingly similar to those reported in recent studies encompassing 22 species from tropical, subtropical, temperate, boreal and mediterranean ecosystems. Only 6 of 49 surveyed case studies showed increases in iWUE significantly higher than predicted from random values.

Conclusions/Significance

Our results reveal that increases in iWUE estimated from tree-ring δ¹³C occur independently of changes in ¹³C discrimination that characterize physiological responses to elevated CO₂. Due to a correlation with CO₂ concentration, which is used as an independent factor in the iWUE calculation, any tree-ring δ¹³C data set would inevitably generate increasing iWUE over time. Therefore, although consistent, previously reported trends in iWUE do not necessarily reflect a coherent global response to rising atmospheric CO₂. We discuss the significance of these findings and suggest ways to distinguish real from artificial responses in future studies.     

Strong overestimation of water‐use efficiency responses to rising CO2 in tree‐ring studies

The carbon isotope ratio (δ¹³C) in tree rings is commonly used to derive estimates of the assimilation‐to‐stomatal conductance rate of trees, that is, intrinsic water‐use efficiency (iWUE). Recent studies have observed increased iWUE in response to rising atmospheric CO₂ concentrations (C_a), in many different species, genera and biomes. However, increasing rates of iWUE vary widely from one study to another, likely because numerous covarying factors are involved. Here, we quantified changes in iWUE of two widely distributed boreal conifers using tree samples from a forest inventory network that were collected across a wide range of growing conditions (assessed using the site index, SI), developmental stages and stand histories. Using tree‐ring isotopes analysis, we assessed the magnitude of increase in iWUE after accounting for the effects of tree size, stand age, nitrogen deposition, climate and SI. We also estimated how growth conditions have modulated tree physiological responses to rising C_a. We found that increases in tree size and stand age greatly influenced iWUE. The effect of C_a on iWUE was strongly reduced after accounting for these two variables. iWUE increased in response to C_a, mostly in trees growing on fertile stands, whereas iWUE remained almost unchanged on poor sites. Our results suggest that past studies could have overestimated the CO₂ effect on iWUE, potentially leading to biased inferences about the future net carbon balance of the boreal forest. We also observed that this CO₂ effect is weakening, which could affect the future capacity of trees to resist and recover from drought episodes.     

Limited evidence for CO2‐related growth enhancement in northern Rocky Mountain lodgepole pine populations across climate gradients

Forests sequester large amounts of carbon annually and are integral in buffering against effects of global change. Increasing atmospheric CO₂ may enhance photosynthesis and/or decrease stomatal conductance (g_s) thereby enhancing intrinsic water‐use efficiency (iWUE), having potential indirect and direct benefits to tree growth. While increasing iWUE has been observed in most trees globally, enhanced growth is not ubiquitous, possibly due to concurrent climatic constraints on growth. To investigate our incomplete understanding of interactions between climate and CO₂ and their impacts on tree physiology and growth, we used an environmental gradient approach. We combined dendrochronology with carbon isotope analysis (δ¹³C) to assess the covariation of basal area increment (BAI) and iWUE over time in lodgepole pine. Trees were sampled at 18 sites spanning two climatically distinct elevation transects on the lee and windward sides of the Continental Divide, encompassing the majority of lodgepole pine's northern Rocky Mountain elevational range. We analyzed BAI and iWUE from 1950 to 2015, and explored correlations with monthly climate variables. As expected, iWUE increased at all sites. However, concurrent growth trends depended on site climatic water deficit (CWD). Significant growth increases occurred only at the driest sites, where increases in iWUE were strongest, while growth decreases were greatest at sites where CWD has been historically lowest. Late summer drought of the previous year negatively affected growth across sites. These results suggest that increasing iWUE, if strong enough, may indirectly benefit growth at drier sites by effectively extending the growing season via reductions in g_s. Strong growth decreases at high elevation windward sites may reflect increasing water stress as a result of decreasing snowpack, which was not offset by greater iWUE. Our results imply that increasing iWUE driven by decreasing g_s may benefit tree growth in limited scenarios, having implications for future carbon uptake potential of semiarid ecosystems.     

Increased water‐use efficiency during the 20th century did not translate into enhanced tree growth

Aim The goals of this study are: (1) to determine whether increasing atmospheric CO₂ concentrations and changing climate increased intrinsic water use efficiency (iWUE, as detected by changes in Δ¹³C) over the last four decades; and if it did increase iWUE, whether it led to increased tree growth (as measured by tree‐ring growth); (2) to assess whether CO₂ responses are biome dependent due to different environmental conditions, including availability of nutrients and water; and (3) to discuss how the findings of this study can better inform assumptions of CO₂ fertilization and climate change effects in biospheric and climate models. Location A global range of sites covering all major forest biome types. Methods The analysis encompassed 47 study sites including boreal, wet temperate, mediterranean, semi‐arid and tropical biomes for which measurements of tree ring Δ¹³C and growth are available over multiple decades. Results The iWUE inferred from the Δ¹³C analyses of comparable mature trees increased 20.5% over the last 40 years with no significant differences between biomes. This increase in iWUE did not translate into a significant overall increase in tree growth. Half of the sites showed a positive trend in growth while the other half had a negative or no trend. There were no significant trends within biomes or among biomes. Main conclusions These results show that despite an increase in atmospheric CO₂ concentrations of over 50 p.p.m. and a 20.5% increase in iWUE during the last 40 years, tree growth has not increased as expected, suggesting that other factors have overridden the potential growth benefits of a CO₂‐rich world in many sites. Such factors could include climate change (particularly drought), nutrient limitation and/or physiological long‐term acclimation to elevated CO₂. Hence, the rate of biomass carbon sequestration in tropical, arid, mediterranean, wet temperate and boreal ecosystems may not increase with increasing atmospheric CO₂ concentrations as is often implied by biospheric models and short‐term elevated CO₂ experiments.     

Long tree‐ring chronologies reveal 20th century increases in water‐use efficiency but no enhancement of tree growth at five Iberian pine forests

We investigated the tree growth and physiological response of five pine forest stands in relation to changes in atmospheric CO₂ concentration (c_a) and climate in the Iberian Peninsula using annually resolved width and δ¹³C tree‐ring chronologies since ad 1600. ¹³C discrimination (Δ≈c_i/c_a), leaf intercellular CO₂ concentration (c_i) 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 c_i/c_a ratio, leading to significant c_i and iWUE increases (active response to c_a); whereas a significant increase in c_i/c_a resulted in the lowest iWUE increase of all stands at a relict Pinus uncinata forest site (passive response to c_a). A significant decrease in c_i/c_a 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 c_a. We found stronger correlations with temperature during the growing season, demonstrating that the physiological response to c_a 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 c_a 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 c_a rise and warmer conditions, seem to be insufficient to compensate for the negative effects of the increasing water limitation on growth.     

Differences in leaf gas exchange strategies explain Quercus rubra and Liriodendron tulipifera intrinsic water use efficiency responses to air pollution and climate change

Trees continuously regulate leaf physiology to acquire CO₂ while simultaneously avoiding excessive water loss. The balance between these two processes, or water use efficiency (WUE), is fundamentally important to understanding changes in carbon uptake and transpiration from the leaf to the globe under environmental change. While increasing atmospheric CO₂ (iCO₂) is known to increase tree intrinsic water use efficiency (iWUE), less clear are the additional impacts of climate and acidic air pollution and how they vary by tree species. Here, we couple annually resolved long-term records of tree-ring carbon isotope signatures with leaf physiological measurements of Quercus rubra (Quru) and Liriodendron tulipifera (Litu) at four study locations spanning nearly 100 km in the eastern United States to reconstruct historical iWUE, net photosynthesis (A_net), and stomatal conductance to water (g_s) since 1940. We first show 16%–25% increases in tree iWUE since the mid-20th century, primarily driven by iCO₂, but also document the individual and interactive effects of nitrogen (NO_x) and sulfur (SO₂) air pollution overwhelming climate. We find evidence for Quru leaf gas exchange being less tightly regulated than Litu through an analysis of isotope-derived leaf internal CO₂ (C_i), particularly in wetter, recent years. Modeled estimates of seasonally integrated A_net and g_s revealed a 43%–50% stimulation of A_net was responsible for increasing iWUE in both tree species throughout 79%–86% of the chronologies with reductions in g_s attributable to the remaining 14%–21%, building upon a growing body of literature documenting stimulated A_net overwhelming reductions in g_s as a primary mechanism of increasing iWUE of trees. Finally, our results underscore the importance of considering air pollution, which remains a major environmental issue in many areas of the world, alongside climate in the interpretation of leaf physiology derived from tree rings.     

From pattern to process: linking intrinsic water‐use efficiency to drought‐induced forest decline

The rise in atmospheric CO₂ concentrations (Ca) has been related to tree growth enhancement and increasing intrinsic water‐use efficiency (iWUE). However, the extent that rising Ca has led to increased long‐term iWUE and whether climate could explain deviations from expected Ca‐induced growth enhancement are still poorly understood. The aim of this research was to use Ca and local climatic variability to explain changes during the 20th century in growth and tree ring and needle δ¹³C in declining and nondeclining Abies alba stands from the Spanish Pyrenees, near the southern distribution limit of this species. The temporal trends of iWUE were calculated under three theoretical scenarios for the regulation of plant‐gas exchange at increasing Ca. We tested different linear mixed‐effects models by multimodel selection criteria to predict basal area increment (BAI), a proxy of tree radial growth, using these scenarios and local temperature together with precipitation data as predictors. The theoretical scenario assuming the strongest response to Ca explained 66–81% of the iWUE variance and 28–56% of the observed BAI variance, whereas local climatic variables together explained less than 11–21% of the BAI variance. Our results are consistent with a drought‐induced limitation of the tree growth response to rising CO₂ and a decreasing rate of iWUE improvement from the 1980s onward in declining A. alba stands subjected to lower water availability.     

Species-dependent responses of juniper and spruce to increasing CO<Subscript>2</Subscript> concentration and to climate in semi-arid and arid areas of northwestern China

Qilian juniper (Sabina przewalskii Kom.) and Qinghai spruce (Picea crassifolia Kom.) represent different tree functional types, which can be found extensively in northwestern China. The former is drought-tolerant, whereas the latter is hygrophilous and shade-tolerant. We compared their intrinsic water-use efficiency (iWUE, inferred from carbon isotopic discrimination, δ¹³C, in their wood) as a function of atmospheric CO₂ concentration, [CO₂], and climate. δ¹³C of spruce was consistently about higher than that of juniper in semi-arid areas but was lower in arid areas. This difference was stable over time and demonstrated strong cross-correlations between species, although some subtle high-frequency (2 or 3 years) variations existed in both species, suggesting that regional climate may control carbon isotope discrimination. The ratio (the [CO₂] values in leaf intercellular and the atmosphere, respectively) of the juniper increased steadily over time, whereas that of the spruce showed a long-term downward trend. IWUE increased at all sites over the 150-year study period, mainly caused by increasing [CO₂]. The relationship between iWUE and [CO₂] reveals that the spruce was more sensitive than the juniper to increasing [CO₂], suggesting a species-specific adaptation to long-term environmental changes. Correlations between the high-frequency variations in stable carbon discrimination (Δ) and climate indicate similar intra-site responses to climate in both species, but different response strengths. Overall, complex interactions of temperature and moisture on stable carbon discrimination during current growth seasons of both species were environmental-determined. Regulation of gas exchange and reduced transpiration may influence water and energy budgets directly; therefore species-dependent responses of trees to elevated CO₂ should be considered in future research on global plant physiological ecology.     

Long‐term increases in intrinsic water‐use efficiency do not lead to increased stem growth in a tropical monsoon forest in western Thailand

Rising atmospheric carbon dioxide [CO₂] can accelerate tree growth by stimulating photosynthesis and increasing intrinsic water‐use efficiency (iWUE). Little evidence exists, however, for the long‐term growth and gas‐exchange responses of mature trees in tropical forests to the combined effects of rising [CO₂] and other global changes such as warming. Using tree rings and stable isotopes of carbon and oxygen, we investigated long‐term trends in the iWUE and stem growth (basal area increment, BAI) of three canopy tree species in a tropical monsoon forest in western Thailand (Chukrasia tabularis, Melia azedarach, and Toona ciliata). To do this, we modelled the contribution of ontogenetic effects (tree diameter or age) and calendar year to variation in iWUE, oxygen isotopes, and BAI using mixed‐effects models. Although iWUE increased significantly with both tree diameter and calendar year in all species, BAI at a given tree diameter was lower in more recent years. For one species, C. tabularis, differences in crown dominance significantly influence stable isotopes and growth. Tree ring Δ¹⁸O increased with calendar year in all species, suggesting that increasing iWUE may have been driven by relatively greater reductions in stomatal conductance – leading to enrichment in Δ¹⁸O – than increases in photosynthetic capacity. Plausible explanations for the observed declines in growth include water stress resulting from rising temperatures and El Niño events, increased respiration, changes in allocation, or more likely, a combination of these factors.     

Growth and carbon isotopes of Mediterranean trees reveal contrasting responses to increased carbon dioxide and drought

Forest dynamics will depend upon the physiological performance of individual tree species under more stressful conditions caused by climate change. In order to compare the idiosyncratic responses of Mediterranean tree species (Quercus faginea, Pinus nigra, Juniperus thurifera) coexisting in forests of central Spain, we evaluated the temporal changes in secondary growth (basal area increment; BAI) and intrinsic water-use efficiency (iWUE) during the last four decades, determined how coexisting species are responding to increases in atmospheric CO₂ concentrations (C _a) and drought stress, and assessed the relationship among iWUE and growth during climatically contrasting years. All species increased their iWUE (ca. +15 to +21 %) between the 1970s and the 2000s. This increase was positively related to C _a for J. thurifera and to higher C _a and drought for Q. faginea and P. nigra. During climatically favourable years the study species either increased or maintained their growth at rising iWUE, suggesting a higher CO₂ uptake. However, during unfavourable climatic years Q. faginea and especially P. nigra showed sharp declines in growth at enhanced iWUE, likely caused by a reduced stomatal conductance to save water under stressful dry conditions. In contrast, J. thurifera showed enhanced growth also during unfavourable years at increased iWUE, denoting a beneficial effect of C _a even under climatically harsh conditions. Our results reveal significant inter-specific differences in growth driven by alternative physiological responses to increasing drought stress. Thus, forest composition in the Mediterranean region might be altered due to contrasting capacities of coexisting tree species to withstand increasingly stressful conditions.     

Increased water‐use efficiency translates into contrasting growth patterns of Scots pine and sessile oak at their southern distribution limits

In forests, the increase in atmospheric CO₂ concentrations (C_a) has been related to enhanced tree growth and intrinsic water‐use efficiency (iWUE). However, in drought‐prone areas such as the Mediterranean Basin, it is not yet clear to what extent this “fertilizing” effect may compensate for drought‐induced growth reduction. We investigated tree growth and physiological responses at five Scots pine (Pinus sylvestris L.) and five sessile oak (Quercus petraea (Matt.) Liebl.) sites located at their southernmost distribution limits in Europe for the period 1960–2012 using annually resolved tree‐ring width and δ¹³C data to track ecophysiological processes. Results indicated that all 10 natural stands significantly increased their leaf intercellular CO₂ concentration (C_i), and consequently iWUE. Different trends in the theoretical gas‐exchange scenarios as a response to increasing C_a were found: generally, C_i tended to increase proportionally to C_a, except for trees at the driest sites in which C_i remained constant. C_i from the oak sites displaying higher water availability tended to increase at a comparable rate to C_a. Multiple linear models fitted at site level to predict basal area increment (BAI) using iWUE and climatic variables better explained tree growth in pines (31.9%–71.4%) than in oak stands (15.8%–46.8%). iWUE was negatively linked to pine growth, whereas its effect on growth of oak differed across sites. Tree growth in the western and central oak stands was negatively related to iWUE, whereas BAI from the easternmost stand was positively associated with iWUE. Thus, some Q. petraea stands might have partially benefited from the “fertilizing” effect of rising C_a, whereas P. sylvestris stands due to their strict closure of stomata did not profit from increased iWUE and consequently showed in general growth reductions across sites. Additionally, the inter‐annual variability of BAI and iWUE displayed a geographical polarity in the Mediterranean.     

Tree-Ring Stable Isotopes Reveal Twentieth-Century Increases in Water-Use Efficiency of Fagus sylvatica and Nothofagus spp. in Italian and Chilean Mountains

Changes in intrinsic water use efficiency (iWUE) were investigated in Fagus sylvatica and Nothofagus spp. over the last century. We combined dendrochronological methods with dual-isotope analysis to investigate whether atmospheric changes enhanced iWUE of Fagus and Nothofagus and tree growth (basal area increment, BAI) along latitudinal gradients in Italy and Chile. Post-maturation phases of the trees presented different patterns in δ¹³C, Δ¹³C, δ¹⁸O, Ci (internal CO₂ concentration), iWUE, and BAI. A continuous enhancement in isotope-derived iWUE was observed throughout the twentieth century, which was common to all sites and related to changes in Ca (ambient CO₂ concentration) and secondarily to increases in temperature. In contrast to other studies, we observed a general increasing trend of BAI, with the exception of F. sylvatica in Aspromonte. Both iWUE and BAI were uncoupled with the estimated drought index, which is in agreement with the absence of enduring decline in tree growth. In general, δ¹³C and δ¹⁸O showed a weak relationship, suggesting the major influence of photosynthetic rate on Ci and δ¹³C, and the minor contribution of the regulation of stomatal conductance to iWUE. The substantial warming observed during the twentieth century did not result in a clear pattern of increased drought stress along these latitudinal transects, because of the variability in temporal trends of precipitation and in specific responses of populations.     

芦芽山华北落叶松(Larix principis-rupprechtii)树轮宽度年表对气候因子的响应

在芦芽山地区采集3个不同海拔的华北落叶松(Larix principis-rupprechtii),在传统去趋势的基础上,采用"signal-free"方法对拟合曲线进行修正,避免了中等频率的气候信息引起的拟合偏差,最终建立3个不同海拔树轮宽度标准年表(STD)。同时以10a为界对上述年表进行滤波处理,得到3个低频年表。年表特征值表明,随着海拔升高,年轮平均轮宽变窄,敏感性和高频信息增强,低频信息减弱,这可能与逐渐恶劣的生境有关。中、低海拔年表的低频信息更一致,中、高海拔的高频信息更接近,而高、低海拔无论是标准年表还是高频、低频年表相似性均较差。树轮气候响应分析显示,低海拔STD年表与5月最低温负相关最为显著,STD和低频年表均与5、6月份土壤温度显著负相关,说明生境暖干,树木主要受生长季的干旱胁迫;中海拔STD年表与当年5月最高温正相关最为显著,STD和低频年表与土壤温度相关均不显著,说明生境逐渐变得冷湿,生长季的低温成为树木生长的限制因子;高海拔STD年表与气象要素相关不显著,低频年表与当年4月土壤温度正相关,说明高海拔最为冷湿,并有季节性冻土分布,生长季的土壤低温成为树木生长的限制因子。因此,全球变暖的趋势将更有利于高海拔树木的生长,而低海拔树木的干旱胁迫进一步加剧。     

Increasing water‐use efficiency and age‐specific growth responses of old‐growth ponderosa pine trees in the Northern Rockies

We examined radial growth responses of ponderosa pine (Pinus ponderosa var. ponderosa) between 1905–1954 and 1955–2004 to determine if the effects of increased intrinsic water‐use efficiencies (iWUE) caused by elevated atmospheric CO₂ concentrations were age‐specific. We collected 209 cores from five sites in the Northern Rockies and calculated iWUE using carbon isotope data from 1850 to 2004. Standardized radial growth responses were age dependent, with older trees exhibiting significantly higher values than younger trees during the later period at four sites and all sites combined. No significant differences in radial growth existed either for the individual sites or combined site during the earlier period. Increases in iWUE during 1955–2004 were 11% greater than during 1905–1954, and pentadal fluctuations in iWUE were significantly correlated with the radial growth of older trees from 1850 to 2004. Radial growth of younger trees and iWUE were not significantly correlated. Our results suggest that: (1) responses to elevated atmospheric CO₂ in old‐growth ponderosa forests are age‐specific; (2) radial growth increases in older trees coincided with increased iWUE; (3) ponderosa had increased growth rates in their third, fourth, and fifth centuries of life; and (4) age‐specific growth responses during 1955–2004 are unique since at least the mid‐16th century.     

Size-dependent responses to summer drought in Scots pine, Norway spruce and common oak

In this study, we provide a detailed analysis of tree growth and water status in relation to climate of three major species of forest trees in lower regions of Bavaria, Southern Germany: Scots pine (Pinus sylvestris), Norway spruce (Picea abies) and common oak (Quercus robur). Tree-ring chronologies and latewood δ¹³C were used to derive measures for drought reaction across trees of different dimensions: growth reduction associated with drought years, long-term growth/climate relations and stomatal control on photosynthesis. For Scots pine, growth/climate relations indicated a stronger limitation of radial growth by high summer temperatures and low summer precipitation in smaller trees in contrast to larger trees. This is corroborated by a stronger stomatal control on photosynthesis for smaller pine trees under average conditions. In dry years, however, larger pine trees exhibited stronger growth reductions. For Norway spruce, a significantly stronger correlation of tree-ring width with summer temperatures and summer precipitation was found for larger trees. Additionally, for Norway spruce there is evidence for a change in competition mode from size-asymmetric competition under conditions with sufficient soil water supply to a more size-symmetric competition under dry conditions. Smaller oak trees showed a weaker stomatal control on photosynthesis under both dry and average conditions, which is also reflected by a significantly faster recovery of tree-ring growth after extreme drought events in smaller oak trees. The observed patterns are discussed in the context of the limitation-caused matter partitioning hypothesis and possible species-specific ontogenetic modifications.     

Disentangling the effects of acidic air pollution,atmospheric CO2, and climate change on recent growth of red spruce trees in the Central Appalachian Mountains

In the 45 years after legislation of the Clean Air Act, there has been tremendous progress in reducing acidic air pollutants in the eastern United States, yet limited evidence exists that cleaner air has improved forest health. Here, we investigate the influence of recent environmental changes on the growth and physiology of red spruce (Picea rubens Sarg.) trees, a key indicator species of forest health, spanning three locations along a 100 km transect in the Central Appalachian Mountains. We incorporated a multiproxy approach using 75‐year tree ring chronologies of basal tree growth, carbon isotope discrimination (?¹³C, a proxy for leaf gas exchange), and δ¹⁵N (a proxy for ecosystem N status) to examine tree and ecosystem level responses to environmental change. Results reveal the two most important factors driving increased tree growth since ca. 1989 are reductions in acidic sulfur pollution and increases in atmospheric CO₂, while reductions in pollutant emissions of NO_x and warmer springs played smaller, but significant roles. Tree ring ?¹³C signatures increased significantly since 1989, concurrently with significant declines in tree ring δ¹⁵N signatures. These isotope chronologies provide strong evidence that simultaneous changes in C and N cycling, including greater photosynthesis and stomatal conductance of trees and increases in ecosystem N retention, were related to recent increases in red spruce tree growth and are consequential to ecosystem recovery from acidic pollution. Intrinsic water use efficiency (iWUE) of the red spruce trees increased by ~51% across the 75‐year chronology, and was driven by changes in atmospheric CO₂ and acid pollution, but iWUE was not linked to recent increases in tree growth. This study documents the complex environmental interactions that have contributed to the recovery of red spruce forest ecosystems from pervasive acidic air pollution beginning in 1989, about 15 years after acidic pollutants started to decline in the United States.     

Variations in Environmental Signals in Tree-Ring Indices in Trees with Different Growth Potential

We analysed two groups of Quercus robur trees, growing at nearby plots with different micro-location condition (W-wet and D-dry) in the floodplain Krakovo forest, Slovenia. In the study we compared the growth response of two different tree groups to environmental variables, the potential signal stored in earlywood (EW) structure and the potential difference of the information stored in carbon isotope discrimination of EW and latewood (LW). For that purpose EW and LW widths and carbon isotope discrimination for the period 1970–2008 AD were measured. EW and LW widths were measured on stained microscopic slides and chronologies were standardised using the ARSTAN program. α-cellulose was extracted from pooled EW and LW samples and homogenized samples were further analysed using an elemental analyser and IRMS. We discovered that W oaks grew significantly better over the whole analysed period. The difference between D and W oaks was significant in all analysed variables with the exception of stable carbon isotope discrimination in latewood. In W oaks, latewood widths correlated with summer (June to August) climatic variables, while carbon isotope discrimination was more connected to River Krka flow during the summer. EW discrimination correlated with summer and autumn River Krka flow of the previous year, while latewood discrimination correlated with flow during the current year. In the case of D oaks, the environmental signal appears to be vague, probably due to less favourable growth conditions resulting in markedly reduced increments. Our study revealed important differences in responses to environmental factors between the two oak groups of different physiological conditions that are preconditioned by environmental stress. Environmental information stored in tree-ring features may vary, even within the same forest stand, and largely depends on the micro-environment. Our analysis confirmed our assumptions that separate EW and LW analysis of widths and carbon isotope discrimination provides complementary information in Q. robur dendroecology.     

Comparison of whole wood and cellulose carbon and oxygen isotope series from Pinus nigra ssp. laricio (Corsica/France)

Stable isotopes in tree rings have widely been used for palaeoclimate reconstructions since tree rings record climatic information at annual resolution. However, various wood components or different parts of an annual tree-ring may differ in their isotopic compositions. Thus, sample preparation and subsequent laboratory analysis are crucial for the isotopic signal retained in the final tree-ring isotope series used for climate reconstruction and must therefore be considered for the interpretation of isotope–climate relationships. This study focuses on wood of Corsican Pine trees (Pinus nigra ssp. laricio) as this tree species allows to reconstruct the long-term climate evolution in the western Mediterranean. In a pilot study, we concentrated on methodological issues of sample preparation techniques in order to evaluate isotope records measured on pooled whole tree-ring cellulose and whole tree-ring bulk wood samples. We analysed 80-year long carbon and oxygen chronologies of Corsican Pine trees growing near the upper tree line on Corsica. Carbon and oxygen isotope records of whole tree-ring bulk wood and whole tree-ring cellulose from a pooled sample of 5 trees were correlated with the climate parameters monthly precipitation, temperature and the self-calibrating Palmer Drought Severity Index (sc-PDSI). Results show that the offsets in carbon and oxygen isotopes of bulk wood and cellulose are not constant over time. Both isotopes correlate with climate parameters from late winter and summer. The carbon and oxygen isotope ratios of cellulose are more sensitive to climatic variables than those of bulk wood. The results of this study imply that extraction of cellulose is a pre-requisite for the reconstruction of high-resolution climate records from stable isotope series of P. nigra ssp. laricio.     

Do centennial tree-ring and stable isotope trends of Larix gmelinii (Rupr.) Rupr. indicate increasing water shortage in the Siberian north?

Tree-ring width of Larix gmelinii (Rupr.) Rupr., ratios of stable isotopes of C (δ¹³C) and O (δ¹⁸O) of whole wood and cellulose chronologies were obtained for the northern part of central Siberia (Tura, Russia) for the period 1864–2006. A strong decrease in the isotope ratios of O and C (after atmospheric δ¹³C corrections) and tree-ring width was observed for the period 1967–2005, while weather station data show a decrease in July precipitation, along with increasing July air temperature and vapor pressure deficit (VPD). Temperature at the end of May and the whole month of June mainly determines tree radial growth and marks the beginning of the vegetation period in this region. A positive correlation between tree-ring width and July precipitation was found for the calibration period 1929–2005. Positive significant correlations between C isotope chronologies and temperatures of June and July were found for whole wood and cellulose and negative relationships with July precipitation. These relationships are strengthened when the likely physiological response of trees to increased CO₂ is taken into account (by applying a recently developed δ¹³C correction). For the O isotope ratios, positive relationships with annual temperature, VPD of July and a negative correlation with annual precipitation were observed. The δ¹⁸O in tree rings may reflect annual rather than summer temperatures, due to the late melting of the winter snow and its contribution to the tree water supply in summer. We observed a clear change in the isotope and climate trends after the 1960s, resulting in a drastic change in the relationship between C and O isotope ratios from a negative to a positive correlation. According to isotope fractionation models, this indicates reduced stomatal conductance at a relatively constant photosynthetic rate, as a response of trees to water deficit for the last half century in this permafrost region.