Seasonal origins of soil water used by trees Allen, Scott T; Kirchner, James W; Braun, Sabine ...
Hydrology and earth system sciences,
03/2019, Volume:
23, Issue:
2
Journal Article
Peer reviewed
Open access
Rain recharges soil water storages and either percolates
downward into aquifers and streams or is returned to the atmosphere through
evapotranspiration. Although it is commonly assumed that summer ...rainfall
recharges plant-available water during the growing season, the seasonal
origins of water used by plants have not been systematically explored. We
characterize the seasonal origins of waters in soils and trees by comparing
their midsummer isotopic signatures (δ2H) to seasonal isotopic
cycles in precipitation, using a new seasonal origin index. Across 182 Swiss
forest sites, xylem water isotopic signatures show that summer rain was not
the predominant water source for midsummer transpiration in any of the three
sampled tree species. Beech and oak mostly used winter precipitation, whereas
spruce used water of more diverse seasonal origins. Even in the same plots,
beech consistently used more winter precipitation than spruce, demonstrating
consistent niche partitioning in the rhizosphere. All three species' xylem
water isotopes indicate that trees used more winter precipitation in drier
regions, potentially mitigating their vulnerability to summer droughts. The
widespread occurrence of winter isotopic signatures in midsummer xylem
implies that growing-season rainfall may have minimally recharged the soil
water storages that supply tree growth, even across diverse humid climates
(690–2068 mm annual precipitation). These results challenge common
assumptions concerning how water flows through soils and is accessed by
trees. Beyond these ecological and hydrological implications, our findings
also imply that stable isotopes of δ18O and δ2H in plant
tissues, which are often used in climate reconstructions, may not reflect
water from growing-season climates.
Recent studies have challenged the interpretation of
plant water isotopes obtained through cryogenic vacuum distillation (CVD)
based on observations of a large 2H fractionation. These studies have ...hypothesized the existence of an H-atom exchange between water and organic tissue during CVD extraction with the magnitude of H exchange related to relative water content of the sample; however, clear evidence is lacking. Here, we systematically tested the uncertainties in the isotopic composition of CVD-extracted water by conducting a series of incubation and rehydration experiments using isotopically depleted water, water at natural isotope abundance, woody materials with exchangeable H, and organic materials without exchangeable H (cellulose triacetate and caffeine). We show that the offsets between hydrogen and oxygen isotope ratios and expected reference values (Δ2H and Δ18O) have inversely proportional relationships with the absolute amount of water being extracted, i.e. the lower the water amount, the higher the Δ2H and Δ18O. However, neither Δ2H nor Δ18O values, were related to sample relative water content. The Δ2H pattern was more pronounced for materials with exchangeable H atoms than with non-exchangeable H atoms. This is caused by the combined effect of H exchange during the incubation of materials in water and isotopic enrichments during evaporation and sublimation that depend on absolute water amount. The H exchange during CVD extraction itself was negligible. Despite these technical issues, we observed that the water amount-dependent patterns were much less pronounced for samples at natural isotope abundance and particularly low when sufficiently high amounts of water were extracted (>600 µL). Our study provides new insights into the mechanisms causing isotope fractionation during CVD extraction of water. The methodological uncertainties can be controlled if large samples of natural isotope abundance are used in ecohydrological studies.
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.
Stable isotope analysis is a powerful tool for assessing plant carbon and water relations and their impact on biogeochemical processes at different scales. Our process-based understanding of stable ...isotope signals, as well as technological developments, has progressed significantly, opening new frontiers in ecological and interdisciplinary research. This has promoted the broad utilisation of carbon, oxygen and hydrogen isotope applications to gain insight into plant carbon and water cycling and their interaction with the atmosphere and pedosphere. Here, we highlight specific areas of recent progress and new research challenges in plant carbon and water relations, using selected examples covering scales from the leaf to the regional scale. Further, we discuss strengths and limitations of recent technological developments and approaches and highlight new opportunities arising from unprecedented temporal and spatial resolution of stable isotope measurements.
A growing number of studies have described the direct absorption of water into leaves, a phenomenon known as foliar water uptake. The resultant increase in the amount of water in the leaf can be ...important for plant function. Exposing leaves to isotopically enriched or depleted water sources has become a common method for establishing whether or not a plant is capable of carrying out foliar water uptake. However, a careful inspection of our understanding of the fluxes of water isotopes between leaves and the atmosphere under high humidity conditions shows that there can clearly be isotopic exchange between the two pools even in the absence of a change in the mass of water in the leaf. We provide experimental evidence that while leaf water isotope ratios may change following exposure to a fog event using water with a depleted oxygen isotope ratio, leaf mass only changes when leaves are experiencing a water deficit that creates a driving gradient for the uptake of water by the leaf. Studies that rely on stable isotopes of water as a means of studying plant water use, particularly with respect to foliar water uptake, must consider the effects of these isotopic exchange processes.
In this commentary, we summarize and build upon discussions that
emerged during the workshop “Isotope-based studies of water partitioning and
plant–soil interactions in forested and agricultural ...environments” held in
San Casciano in Val di Pesa, Italy, in September 2017. Quantifying and
understanding how water cycles through the Earth's critical zone is important
to provide society and policymakers with the scientific background to manage
water resources sustainably, especially considering the ever-increasing
worldwide concern about water scarcity. Stable isotopes of hydrogen and
oxygen in water have proven to be a powerful tool for tracking water fluxes in
the critical zone. However, both mechanistic complexities (e.g. mixing and
fractionation processes, heterogeneity of natural systems) and methodological
issues (e.g. lack of standard protocols to sample specific compartments,
such as soil water and xylem water) limit the application of stable water
isotopes in critical-zone
science. In this commentary, we examine some of the
opportunities and critical challenges of isotope-based ecohydrological
applications and outline new perspectives focused on interdisciplinary
research opportunities for this important tool in water and environmental
science.
Carbon isotope ratios (δ¹³C) of heterotrophic and rhizospheric sources of soil respiration under deciduous trees were evaluated over two growing seasons. Fluxes and δ¹³C of soil respiratory CO₂ on ...trenched and untrenched plots were calculated from closed chambers, profiles of soil CO₂ mole fraction and δ¹³C and continuous open chambers. δ¹³C of respired CO₂ and bulk carbon were measured from excised leaves and roots and sieved soil cores. Large diel variations (>5‰) in δ¹³C of soil respiration were observed when diel flux variability was large relative to average daily fluxes, independent of trenching. Soil gas transport modelling supported the conclusion that diel surface flux δ¹³C variation was driven by non‐steady state gas transport effects. Active roots were associated with high summertime soil respiration rates and around 1‰ enrichment in the daily average δ¹³C of the soil surface CO₂ flux. Seasonal δ¹³C variability of about 4‰ (most enriched in summer) was observed on all plots and attributed to the heterotrophic CO₂ source.
Abstract
Newly developed millennial δ
13
C larch tree-ring chronology from Siberia allows reconstruction of summer (July) vapor pressure deficit (VPD) changes in a temperature-limited environment. ...VPD increased recently, but does not yet exceed the maximum values reconstructed during the Medieval Warm Anomaly. The most humid conditions in the Siberian North were recorded in the Early Medieval Period and during the Little Ice Age. Increasing VPD under elevated air temperature affects the hydrology of these sensitive ecosystems by greater evapotranspiration rates. Further VPD increases will significantly affect Siberian forests most likely leading to drought and forest mortality even under additional access of thawed permafrost water. Adaptation strategies are needed for Siberian forest ecosystems to protect them in a warming world.
It is currently not well known how coherent carbon and oxygen isotope chronologies from different species and sites are under temperate climate conditions. Here we investigated nine chronologies from ...Switzerland covering the last two centuries, including three deciduous species (Fagus sylvatica, Fraxinus excelsior, and Quercus petraea) and three conifer species (Abies alba, Picea abies, and Pinus sylvestris) from sites neither strongly limited by temperature nor precipitation. All of the chronologies except Fraxinus were significantly correlated to at least one other chronology. Correlations between different species of the same site were of similar strength to correlations between the sites. We observed a strong common high‐frequency (interannual) signal for the δ13C chronologies, whereas the low‐frequency (decadal‐scale) signal was more similar among the δ18O chronologies. For both carbon and oxygen isotopes, we found significant positive relationships with annual and growing season temperatures and negative relationships with precipitation, again of similar magnitude for all species except for Fraxinus, which contained only minor climatic information. Averaging of all chronologies resulted in an increase in the climatic signal of the mean chronology. The combined δ18O record reflected decadal‐scale temperature variations remarkably well (r = 0.72). However, the relationship between climate and carbon isotopes declined over the last 3 decades of the 20th century, probably related to the steep increase in atmospheric CO2 concentrations, resulting in strongly diverging δ13C trends of the different chronologies. Our study indicates that combining chronologies from different species enhances the potential of isotope studies for extending climate reconstructions into areas of temperate climate.
Stomata control the gas exchange of terrestrial plant leaves, and are therefore essential to plant growth and survival. We investigated gas exchange responses to vapor pressure deficit (VPD) in two ...gray poplar (
) lines: wild type and abscisic acid-insensitive (
) with functionally impaired stomata. Transpiration rate in
increased linearly with VPD, up to about 2 kPa. Above this, sharply declining transpiration was followed by leaf death. In contrast, wild type showed a steady or slightly declining transpiration rate up to VPD of nearly 7 kPa, and fully recovered photosynthetic function afterward. There were marked differences in discrimination against
CO
(Δ
C) and C
OO (Δ
O) between
and wild-type plants. The Δ
C indicated that intercellular CO
concentrations decreased with VPD in wild-type plants, but not in
plants. The Δ
O reflected progressive stomatal closure in wild type in response to increasing VPD; however, in
, stomata remained open and oxygen atoms of CO
continued to exchange with
O enriched leaf water. Coupled measurements of Δ
O and gas exchange were used to estimate intercellular vapor pressure,
In wild-type leaves, there was no evidence of unsaturation of
, even at VPD above 6 kPa. In
leaves,
approached 0.6 times saturation vapor pressure before the precipitous decline in transpiration rate. For wild type, a sensitive stomatal response to increasing VPD was pivotal in preventing unsaturation of
In
, after taking unsaturation into account, stomatal conductance increased with increasing VPD, consistent with a disabled active response of guard cell osmotic pressure.
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