Climatic Influences on Summer Use of Winter Precipitation by Trees

Trees in seasonal climates may use water originating from both winter and summer precipitation. However, the seasonal origins of water used by trees have not been systematically studied. We used stable isotopes of water to compare the seasonal origins of water found in three common tree species across 24 Swiss forest sites sampled in two different years. Water from winter precipitation was observed in trees at most sites, even at the peak of summer, although the relative representation of seasonal sources differed by species. However, the representation of winter precipitation in trees decreased with site mean annual precipitation in both years; additionally, it was generally lower in the cooler and wetter year. Together, these relationships show that precipitation amount influenced the seasonal origin of water taken up by trees across both time and space. These results suggest higher turnover of the plant-available soil-water pool in wetter sites and wetter years

Seasonal Origins of Soil Water Used by Trees

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\u27 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

Surrounding species diversity improves subtropical seedlings’ carbon dynamics

Increasing biodiversity has been linked to higher primary productivity in terrestrial ecosystems. However, the underlying ecophysiological mechanisms remain poorly understood. We investigated the effects of surrounding species richness (monoculture, two- and four-species mixtures) on the ecophysiology of Lithocarpus glaber seedlings in experimental plots in subtropical China. A natural rain event isotopically labelled both the water uptaken by the L. glaber seedlings and the carbon in new photoassimilates through changes of photosynthetic discrimination. We followed the labelled carbon (C) and oxygen (O) in the plant–soil–atmosphere continuum. We measured gas-exchange variables (C assimilation, transpiration and above- and belowground respiration) and δ13C in leaf biomass, phloem, soil microbial biomass, leaf- and soil-respired CO2 as well as δ18O in leaf and xylem water. The 13C signal in phloem and respired CO2 in L. glaber in monoculture lagged behind those in species mixture, showing a slower transport of new photoassimilates to and through the phloem in monoculture. Furthermore, leaf-water 18O enrichment above the xylem water in L. glaber increased after the rain in lower diversity plots suggesting a lower ability to compensate for increased transpiration. Lithocarpus glaber in monoculture showed higher C assimilation rate and water-use efficiency. However, these increased C resources did not translate in higher growth of L. glaber in monoculture suggesting the existence of larger nongrowth-related C sinks in monoculture. These ecophysiological responses of L. glaber, in agreement with current understanding of phloem transport are consistent with a stronger competition for water resources in monoculture than in species mixtures. Therefore, increasing species diversity in the close vicinity of the studied plants appears to alleviate physiological stress induced by water competition and to counterbalance the negative effects of interspecific competition on assimilation rates for L. glaber by allowing a higher fraction of the C assimilated to be allocated to growth in species mixture than in monoculture

Surrounding species diversity improves subtropical seedlings’ carbon dynamics

Increasing biodiversity has been linked to higher primary productivity in terrestrial ecosystems. However, the underlying ecophysiological mechanisms remain poorly understood. We investigated the effects of surrounding species richness (monoculture, two- and four-species mixtures) on the ecophysiology of Lithocarpus glaber seedlings in experimental plots in subtropical China. A natural rain event isotopically labelled both the water uptaken by the L.glaber seedlings and the carbon in new photoassimilates through changes of photosynthetic discrimination. We followed the labelled carbon (C) and oxygen (O) in the plant-soil-atmosphere continuum. We measured gas-exchange variables (C assimilation, transpiration and above- and belowground respiration) and C-13 in leaf biomass, phloem, soil microbial biomass, leaf- and soil-respired CO2 as well as O-18 in leaf and xylem water. The C-13 signal in phloem and respired CO2 in L.glaber in monoculture lagged behind those in species mixture, showing a slower transport of new photoassimilates to and through the phloem in monoculture. Furthermore, leaf-water O-18 enrichment above the xylem water in L.glaber increased after the rain in lower diversity plots suggesting a lower ability to compensate for increased transpiration. Lithocarpus glaber in monoculture showed higher C assimilation rate and water-use efficiency. However, these increased C resources did not translate in higher growth of L.glaber in monoculture suggesting the existence of larger nongrowth-related C sinks in monoculture. These ecophysiological responses of L.glaber, in agreement with current understanding of phloem transport are consistent with a stronger competition for water resources in monoculture than in species mixtures. Therefore, increasing species diversity in the close vicinity of the studied plants appears to alleviate physiological stress induced by water competition and to counterbalance the negative effects of interspecific competition on assimilation rates for L.glaber by allowing a higher fraction of the C assimilated to be allocated to growth in species mixture than in monoculture.Peer reviewe

Malate as a key carbon source of leaf dark-respired CO2 across different environmental conditions in potato plants

Dissimilation of carbon sources during plant respiration in support of metabolic processes results in the continuous release of CO(2). The carbon isotopic composition of leaf dark-respired CO(2) (i.e. δ (13) C (R)) shows daily enrichments up to 14.8‰ under different environmental conditions. However, the reasons for this (13)C enrichment in leaf dark-respired CO(2) are not fully understood, since daily changes in δ(13)C of putative leaf respiratory carbon sources (δ (13) C (RS)) are not yet clear. Thus, we exposed potato plants (Solanum tuberosum) to different temperature and soil moisture treatments. We determined δ (13) C (R) with an in-tube incubation technique and δ (13) C (RS) with compound-specific isotope analysis during a daily cycle. The highest δ (13) C (RS) values were found in the organic acid malate under different environmental conditions, showing less negative values compared to δ (13) C (R) (up to 5.2‰) and compared to δ (13) C (RS) of soluble carbohydrates, citrate and starch (up to 8.8‰). Moreover, linear relationships between δ (13) C (R) and δ (13) C (RS) among different putative carbon sources were strongest for malate during daytime (r(2)=0.69, P≤0.001) and nighttime (r(2)=0.36, P≤0.001) under all environmental conditions. A multiple linear regression analysis revealed δ (13) C (RS) of malate as the most important carbon source influencing δ (13) C (R). Thus, our results strongly indicate malate as a key carbon source of (13)C enriched dark-respired CO(2) in potato plants, probably driven by an anapleurotic flux replenishing intermediates of the Krebs cycle

Malate as a key carbon source of leaf dark-respired CO2 across different environmental conditions in potato plants

Carbon isotope analyses revealed malate as a key carbon source of leaf dark-respired CO2 in potato plants under different temperature and soil moisture conditions during a daily cycl

Technical note: On uncertainties in plant water isotopic composition following extraction by cryogenic vacuum distillation

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 (&gt;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.</p

Technical Note: On Uncertainties in Plant Water Isotopic Composition Following Extraction by Cryogenic Vacuum Distillation

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 (\u3e600 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

Spatial Variation in Throughfall, Soil, and Plant Water Isotopes in a Temperate Forest

Studies of stable isotopes of water in the environment have been fundamental to advancing our understanding of how water moves through the soil‐plant‐atmosphere continuum; however, much of this research focuses on how water isotopes vary in time, rather than in space. We examined the spatial variation in the δ18O and δ2H of throughfall and bulk soil water, as well as branch xylem and bulk leaf water of Picea abies (Norway Spruce) and Fagus sylvatica (Beech), in a 1 ha forest plot in the northern Alps of Switzerland. Means and ranges of water isotope ratios varied considerably among throughfall, soil, and xylem samples. Soil water isotope ratios were often poorly explained by soil characteristics and often not predictable from proximal samples. Branch xylem water isotope values varied less than either soil water or bulk leaf water. The isotopic range observed within an individual tree crown was often similar to that observed among different crowns. As a result of the heterogeneity in isotope ratios, inferences about the depth of plant root water uptake drawn from a two end‐member mixing model were highly sensitive to the soil sampling location. Our results clearly demonstrate that studies using water isotopes to infer root water uptake must explicitly consider how to characterize soil water, incorporating measures of both vertical and lateral variation. By accounting for this spatial variation and the processes that shape it, we can improve the application of water isotopes to studies of plant ecophysiology, ecohydrology, soil hydrology, and paleoclimatology