SummaryWe compiled hydrogen and oxygen stable isotope compositions (δ2H and δ18O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ2H was more closely correlated with δ2H of xylem water or atmospheric vapour, whereas leaf water δ18O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ2H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ18O. We next expressed leaf water as isotopic enrichment above xylem water (Δ2H and Δ18O) to remove the impact of xylem water isotopic variation. For Δ2H, leaf water still correlated with atmospheric vapour, whereas Δ18O showed no such correlation. This was explained by covariance between air relative humidity and the Δ18O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2H and 18O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.

We compiled hydrogen and oxygen stable isotope compositions (δ2 H and δ18 O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ2 H was more closely correlated with δ2 H of xylem water or atmospheric vapour, whereas leaf water δ18 O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ2 H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ18 O. We next expressed leaf water as isotopic enrichment above xylem water (Δ2 H and Δ18 O) to remove the impact of xylem water isotopic variation. For Δ2 H, leaf water still correlated with atmospheric vapour, whereas Δ18 O showed no such correlation. This was explained by covariance between air relative humidity and the Δ18 O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that 2 H and 18 O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.

AbstractAlthough the fields of dendrochronology and light stable-isotope mass spectrometry emerged at different times in the first half of the 20th Century, their convergence with the earliest measurements of isotope composition of tree rings is nowca. 70 years old. Much of the early stable isotope analysis (including on wood) explored natural variation of isotopes in the environment, but those researchers making the measurements were already contemplating the role of the isotope composition of the source substrates (e.g., water and CO2), biochemical fractionation, and environment as contributors to final tree-ring isotope values. Growing interest in tree-ring isotopes was heavily motivated by paleoclimate or paleoatmosphere reconstruction, but this new field rapidly developed to generate greatly improved mechanistic understanding along with expanded applications to physiology, ecology, pollution, and more. This chapter primarily charts the historical progression in tree-ring C-H-O isotope studies over those seven decades, but it also identifies potential productive emerging and future directions.

AbstractThe environmental and physiological interpretation of stable isotope variation in organic matter is affected by many different and interacting factors. This is especially true when considering isotope variation in tree rings, which are influenced not only by leaf-level photosynthetic gas exchange processes but also by post-photosynthetic fractionation. It has been proposed that measuring multiple isotopes on the same sample may constrain such interpretations if one isotope provides independent information about important fractionation events that cause variation in another isotope. Here we describe one such “dual-isotope approach” where oxygen isotope variation (δ18O) is used to probe the effects of stomatal conductance on carbon isotope (δ13C) variation for the same sample. This chapter describes the development of this conceptual model, constraints on model applicability, particularly with respect to tree rings, and how it has been utilized to explore aspects of tree physiology.

We are editing a new book in the Springer Tree Physiology Series entitled “Stable Isotopes in Tree Rings: Inferring Physiological, Climatic and Environmental Responses” due out in 2020. Because trees produce annual growth increments that can be precisely dated, annual and interannual variations in tree ring width and stable carbon, oxygen and hydrogen isotopes provide detailed records of past physiological responses to biotic and abiotic impacts over many decades and centuries. In contrast to non-living chronologies (ice cores, stalagmites etc.), trees modify base physical inputs in response to local microclimates through their physiological response to light, temperature, humidity, water availability, CO2 and nutrients. Although this can make interpretation of isotopic variation in organic matter more complicated, it also means that these proxies can provide a wealth of additional information. Thus, an understanding of the combined physical and biological drivers of isotope fractionation in tree rings is crucial for paleoclimate interpretation. In addition, tree rings and stable isotopes contained therein integrate dynamic environmental, phenological and developmental variation that can be used to study present organism function and recent anthropogenic influences apart from their use as proxies for conditions in the distant past. The last few decades have seen tremendous progress in understanding the mechanisms by which tree physiology modifies stable isotope fractionation in organic matter. This book will be the first to comprehensively cover the field of tree ring stable isotopes. This volume highlights how tree ring stable isotopes have been used to address a range of environmental issues from paleoclimatology to forest management, and anthropogenic impacts on forest growth. It evaluates strengths and weaknesses of isotope applications in tree rings. This book focuses on physiological mechanisms that influence isotopic signals and reflect environmental impacts. Each of the 25 chapters has been authored by leading experts providing the most recent developments in the area.

Summary The ratio of leaf internal (ci) to ambient (ca) partial pressure of CO2, defined here as χ, is an index of adjustments in both leaf stomatal conductance and photosynthetic rate to environmental conditions. Measurements and proxies of this ratio can be used to constrain vegetation model uncertainties for predicting terrestrial carbon uptake and water use. We test a theory based on the least‐cost optimality hypothesis for modelling historical changes in χ over the 1951–2014 period, across different tree species and environmental conditions, as reconstructed from stable carbon isotopic measurements across a global network of 103 absolutely dated tree‐ring chronologies. The theory predicts optimal χ as a function of air temperature, vapour pressure deficit, ca and atmospheric pressure. The theoretical model predicts 39% of the variance in χ values across sites and years, but underestimates the intersite variability in the reconstructed χ trends, resulting in only 8% of the variance in χ trends across years explained by the model. Overall, our results support theoretical predictions that variations in χ are tightly regulated by the four environmental drivers. They also suggest that explicitly accounting for the effects of plant‐available soil water and other site‐specific characteristics might improve the predictions.

To identify source water for trees growing on permafrost in Siberia, we applied mechanistic models that quantify physical and biochemical fractionation processes, leading to oxygen isotope variation (δ18O) in plant organic matter. These models allowed us to investigate the influence of a variety of climatic factors on tree-ring cellulose from two dominant species: Larix cajanderi Mayr. from northeastern Yakutia (69° 22′ N, 148° 25′ E, ~ 250 m a.s.l.) and Pinus sylvestris L. from Central Yakutia (62°14′ N, 129°37′ E, ~ 220 m a.s.l.). The climate of the region is highly continental with short growing seasons, low amount of precipitation and these forest ecosystems are growing on permafrost, which in turn impact the water cycle and climate variation in the δ18O of source water. We compared outputs of the Land surface Processes and eXchanges (LPX-Bern v. 1.3), and Roden-Lin-Ehleringer (RLE) models for the common period from 1945 to 2004. Based on our findings, trees from northeastern and central Yakutia may have access to additional thawed permafrost water during dry summer periods. Owing to differences in the soil structure, active thaw soil depth and root systems of trees at two Siberian sites, Larix cajanderi Mayr. trees can access water not more than from 50 cm depth, in contrast to Pinus sylvestris L. in Central Yakutia which can acquire water from up to 80 cm soil depth. The results enhance our understanding of the growth and survival of the trees in this extreme environment.

Tree-ring carbon isotope discrimination (Δ13C) and oxygen isotopes (δ18O) collected from white pine (Pinus strobus) trees adjacent to Lake Superior show potential to produce the first winter-specific paleoclimate reconstruction with inter-annual resolution for this region. Isotopic signatures from 1976 to 2015 were strongly linked to antecedent winter minimum temperatures (Tmin), Lake Superior peak ice cover, and regional to continental-scale atmospheric winter pressure variability including the North American Dipole. The immense thermal inertia of Lake Superior underlies the unique connection between winter conditions and tree-ring Δ13C and δ18O signals from the following growing season in trees located near the lake. By combining these signals, we demonstrate feasibility to reconstruct variability in Tmin, ice cover, and continental-scale atmospheric circulation patterns (r ≥ 0.65, P < 0.001).

To understand drivers of hydroclimate variability in north-coastal California, we obtained tree cross-sections from eleven coastal redwoods (mean age of 1232 years old) from the northern half of the species range. Tree rings from eight trees were cross-dated and sampled at sub-annual resolution for carbon isotope discrimination (Δ13C) and oxygen isotope composition (δ18O). Tree-ring Δ13C and δ18O, compared to modern climate data, demonstrate these signals primarily record summertime hydroclimate variability—primarily through variables associated with evaporative conditions and/or precipitation. Our 1100-year stable isotope chronologies showed that north-coastal California did not undergo the megadroughts observed elsewhere in California and the western United States. This result implicates extended periods of low winter precipitation, rather than growing season evaporation, as the primary driver of previous megadroughts across California and neighboring regions. Compared to cool conditions prevailing over the Northern Hemisphere during the Little Ice age (1301–1875 of the common era, CE), the frequency of isotopic events of a certain magnitude was greater during periods with warmer Northern Hemisphere temperatures such as the Medieval Climate Anomaly (900–1300 CE) and the modern period (1876 to present). This association between tree-ring isotopic variability and long-term shifts in temperatures is consistent with the expected patterns in mid-latitude hydroclimate variability expected from arctic amplification (i.e., shifts in equator-to-pole temperature differences that modify jet stream speed and amplitude) or amplified quasi-resonant wave activity (i.e., wave-patterns in high-altitude winds that become “trapped” within a certain pattern, thereby producing a longer-duration periods of drought or wetness) across mid-latitudes during the boreal summer.

Tree-ring width and stable isotopic composition are widely used for the reconstruction of environmental conditions. Eco-physiological models simulating delta C-13 and delta O-18 provide tools to constrain the interpretation of measured tree-ring variations and their relationships to environmental variables. Here, we apply biochemical models of photosynthesis and a model of stomatal conductance to simulate the intra-annual dynamics of delta(13) C values in photo assimilates and tree-rings. We use these models to investigate the physiological responses of larch trees growing on permafrost to variability in precipitation and permafrost depth associated with regional temperature and precipitation changes. Tree-ring width, delta C-13 and delta O-18 in wood and cellulose were measured in larch (Larix cajanderi Mayr.) samples from northeastern Yakutia (69 degrees N, 148 degrees E) for the period from 1945 to 2004 and used for comparisons with modeled delta C-13 and delta O-18 data.Mechanistic models that quantify physical and biochemical fractionation processes leading to oxygen isotope variation in organic matter are used to identify source water for trees growing on permafrost in Siberia. These models allowed us to investigate the influence of a variety of climatic factors on Siberian forest ecosystem water relations that impact isotope fractionation.Based on delta C-13 and delta O-18 in tree wood and cellulose measurements as well as outputs from different eco-physiological models, we assume that larch trees from northeastern Yakutia can have limited access to the additional thawed permafrost water during dry summer periods. (C) 2015 Elsevier GmbH. All rights reserved.

The extent to which both water source and atmospheric humidity affect δ(2)H values of terrestrial plant leaf waxes will affect the interpretations of δ(2)H variation of leaf waxes as a proxy for hydrological conditions. To elucidate the effects of these parameters, we conducted a long-term experiment in which we grew two tree species, Populus fremontii and Betula occidentalis, hydroponically under combinations of six isotopically distinct waters and two different atmospheric humidities. We observed that leaf n-alkane δ(2)H values of both species were linearly related to source water δ(2)H values, but with slope differences associated with differing humidities. When a modified version of the Craig-Gordon model incorporating plant factors was used to predict the δ(2)H values of leaf water, all modelled leaf water values fit the same linear relationship with n-alkane δ(2)H values. These observations suggested a relatively constant biosynthetic fractionation factor between leaf water and n-alkanes. However, our calculations indicated a small difference in the biosynthetic fractionation factor between the two species, consistent with small differences calculated for species in other studies. At present, it remains unclear if these apparent interspecies differences in biosynthetic fractionation reflect species-specific biochemistry or a common biosynthetic fractionation factor with insufficient model parameterization.

Tree-ring records can provide valuable information to advance our understanding of contemporary terrestrial carbon cycling and to reconstruct key metrics in the decades preceding monitoring data. The growing use of tree rings in carbon-cycle research is being facilitated by increasing recognition of reciprocal benefits among research communities. Yet, basic questions persist regarding what tree rings represent at the ecosystem level, how to optimally integrate them with other data streams, and what related challenges need to be overcome. It is also apparent that considerable unexplored potential exists for tree rings to refine assessments of terrestrial carbon cycling across a range of temporal and spatial domains. Here, we summarize recent advances and highlight promising paths of investigation with respect to (1) growth phenology, (2) forest productivity trends and variability, (3) CO 2 fertilization and water-use efficiency, (4) forest disturbances, and (5) comparisons between observational and computational forest productivity estimates. We encourage the integration of tree-ring data: with eddy-covariance measurements to investigate carbon allocation patterns and water-use efficiency; with remotely sensed observations to distinguish the timing of cambial growth and leaf phenology; and with forest inventories to develop continuous, annually-resolved and long-term carbon budgets. In addition, we note the potential of tree-ring records and derivatives thereof to help evaluate the performance of earth system models regarding the simulated magnitude and dynamics of forest carbon uptake, and inform these models about growth responses to (non-)climatic drivers. Such efforts are expected to improve our understanding of forest carbon cycling and place current developments into a long-term perspective.

Cellulose δ 18 O and δD can provide insights on climates and hydrological cycling in the distant past and how these factors differ spatially. However, most studies of plant cellulose have used only one isotope, most commonly δ 18 O, resulting in difficulties partitioning variation in δ 18 O of precipitation vs. evaporative conditions that affect leaf water isotopic enrichment. Moreover, observations of pronounced diurnal differences from conventional steady‐state model predictions of leaf water isotopic fractionation have cast some doubt on single isotope modeling approaches for separating precipitation and evaporation drivers of cellulose δ 18 O or δD. We explore a dual isotope approach akin to the concept of deuterium‐excess ( d ), to establish deuterium deviations from the global meteoric water line in leaf water (Δ d l ) as driven by relative humidity (RH). To demonstrate this concept, we survey studies of leaf water δ 18 O and δD in hardwood vs. conifer trees. We then apply the concept to cellulose δ 18 O and δD using a mechanistic model of cellulose δ 18 O and δD to reconstruct deuterium deviations from the global meteoric water line (Δ d c ) in Quercus macrocarpa , Q. robur , and Pseudotsuga menziesii . For each species, Δ d c showed strong correlations with RH across sites. Δ d c agreed well with steady‐state predictions for Q. macrocarpa , while for Q. robur , the relationship with RH was steeper than expected. The slope of Δ d c vs. RH of P. menziesii was also close to steady‐state predictions, but Δ d c were more enriched than predicted. This is in agreement with our leaf water survey showing conifer Δ d l was more enriched than predicted. Our data reveal that applications of this method should be appropriate for reconstructing RH from cellulose δ 18 O and δD after accounting for differences between hardwoods and conifers. Hence, Δ d c should be useful for understanding variability in RH associated with past climatic cycles, across regional climates, or across complex terrain where climate modeling is challenging. Furthermore, Δ d c and inferred RH values should help in constraining variation in source water δ 18 O.

Interannual variability in the oxygen and carbon isotope composition of tree ring cellulose was investigated in coast redwood (Sequoia sempervirens) from three sites in coastal Northern California. Middle and late wood samples from annual tree rings were compared to regional climate indices and gridded ocean‐atmosphere fields for the years 1952–2003. The strongest climate‐isotope relationship (r = 0.72) was found with summer (June–September) daily maximum temperature and middle wood δ13, which also responds positively to coastal sea surface temperature and negatively to summer low cloud frequency. Late wood δ18O reflects a balance between 18O‐enriched summer fog drip and depleted summer rainwater, while a combined analysis of late wood δ18O and δ13C revealed sensitivity to the sign of summer precipitation anomalies. Empirical orthogonal function analysis of regional summer climate indices and coast redwood stable isotopes identified multivariate isotopic responses to summer fog and drought that correspond to atmospheric circulation anomalies over the NE Pacific and NW U.S. The presence of regional climate signals in coast redwood stable isotope composition, consistent with known mechanistic processes and prior studies, offers the potential for high‐resolution paleoclimate reconstructions of the California current system from this long‐lived tree species.