Xylem Water δ18O Research Articles

Resin acid δ13C and δ18O as indicators of intra-seasonal physiological and environmental variability.

Understanding the dynamics of δ13C and δ18O in modern resin is crucial for interpreting (sub)fossilized resin records and resin production dynamics. We measured the δ13C and δ18O offsets between resin acids and their precursor molecules in the top-canopy twigs and breast-height stems of mature Pinus sylvestris trees. We also investigated the physiological and environmental signals imprinted in resin δ13C and δ18O at an intra-seasonal scale. Resin δ13C was c. 2‰ lower than sucrose δ13C, in both twigs and stems, likely due to the loss of 13C-enriched C-1 atoms of pyruvate during isoprene formation and kinetic isotope effects during diterpene synthesis. Resin δ18O was c. 20‰ higher than xylem water δ18O and c. 20‰ lower than δ18O of water-soluble carbohydrates, possibly caused by discrimination against 18O during O2-based diterpene oxidation and 35%-50% oxygen atom exchange with water. Resin δ13C and δ18O recorded a strong signal of soil water potential; however, their overall capacity to infer intraseasonal environmental changes was limited by their temporal, within-tree and among-tree variations. Future studies should validate the potential isotope fractionation mechanisms associated with resin synthesis and explore the use of resin δ13C and δ18O as a long-term proxy for physiological and environmental changes.

Identification of suites of traits that explains drought resistance and phenological patterns of plants in a semi-arid grassland community

Grassland ecosystems are comprised of plants that occupy a wide array of phenological niches and vary considerably in their ability to resist the stress of seasonal soil-water deficits. Yet, the link between plant drought resistance and phenology remains unclear in perennial grassland ecosystems. To evaluate the role of soil water availability and plant drought tolerance in driving phenology, we measured leaf hydraulic conductance (Ksat), resistance to hydraulic failure (P50), leaf gas exchange, plant and soil water stable isotope ratios (δ18O), and several phenology metrics on ten perennial herbaceous species in mixed-grass prairie. The interaction between P50 and δ18O of xylem water explained 67% of differences in phenology, with lower P50 values associated with later season activity, but only among shallow-rooted species. In addition, stomatal control and high water-use efficiency also contributed to the late flowering and late senescence strategies of plants that had low P50 values and relied upon shallow soil water. Alternatively, plants with deeper roots did not possess drought-tolerant leaves, but had high hydraulic efficiency, contributing to their ability to efficiently move water longer distances while maintaining leaf water potential at relatively high values. The suites of traits that characterize these contrasting strategies provide a mechanistic link between phenology and plant-water relations; thus, these traits could help predict grassland community responses to changes in water availability, both temporally and vertically within the soil profile.

Functional traits of lianas in an Australian lowland rainforest align with post‐disturbance rather than dry season advantage

AbstractLianas are an important component of tropical forests; they alter tree mortality and recruitment and impact biogeochemical cycling. Recent evidence suggests that the abundance of lianas in tropical forests is increasing. To understand and predict the effect of lianas on ecosystem processes in tropical forests, it is important to understand the mechanisms through which they compete with trees. In this study, we investigated the functional traits of lianas and trees in a lowland tropical forest in northeast Queensland, Australia. The site is located at 16.1° south latitude and experiences significant seasonality in rainfall, with pronounced wet and dry seasons. It is also subject to relatively frequent disturbance by cyclones. We asked the question of whether the canopy liana community at this site would display functional traits consistent with a competitive advantage over trees in response to disturbance, or in response to dry season water stress. We found that traits that we considered indicative of a dry season advantage (xylem water δ18O as an indicator of rooting depth; leaf and stem tissue δ13C and instantaneous gas exchange as measures of water‐use efficiency) did not differ between canopy lianas and canopy trees. On the other hand, lianas differed from trees in traits that should confer an advantage in response to disturbance (low wood density; low leaf dry matter content; high leaf N concentration; high mass‐based photosynthetic rates). We conclude that the liana community at the study site expressed functional traits geared towards rapid resource acquisition and growth in response to disturbance, rather than outcompeting trees during periods of water stress. These results contribute to a body of literature which will be useful for parameterising a liana functional type in ecosystem models.

Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest

Abstract. We investigated plant water sources of an emblematic refugial population of Fagus sylvatica (L.) in the Ciron river gorges in south-western France using stable water isotopes. It is generally assumed that no isotopic fractionation occurs during root water uptake, so that the isotopic composition of xylem water effectively reflects that of source water. However, this assumption has been called into question by recent studies that found that, at least at some dates during the growing season, plant water did not reflect any mixture of the potential water sources. In this context, highly resolved datasets covering a range of environmental conditions could shed light on possible plant–soil fractionation processes responsible for this phenomenon. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured fortnightly over an entire growing season. Using a Bayesian isotope mixing model (MixSIAR), we then quantified the relative contribution of water sources for F. sylvatica and Quercus robur (L.) trees. Based on δ18O data alone, both species used a mix of top and deep soil water over the season, with Q. robur using deeper soil water than F. sylvatica. The contribution of stream water appeared to be marginal despite the proximity of the trees to the stream, as already reported for other riparian forests. Xylem water δ18O could always be interpreted as a mixture of deep and shallow soil waters, but the δ2H of xylem water was often more depleted than the considered water sources. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected relatively depleted δ2H of xylem water, as already observed in halophytic and xerophytic species. By means of a sensitivity analysis, we found that the estimation of plant water sources using mixing models was strongly affected by this δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

Hydrological niche segregation defines forest structure and drought tolerance strategies in a seasonal Amazon forest

AbstractThe relationship between rooting depth and above‐ground hydraulic traits can potentially define drought resistance strategies that are important in determining species distribution and coexistence in seasonal tropical forests, and understanding this is important for predicting the effects of future climate change in these ecosystems.We assessed the rooting depth of 12 dominant tree species (representingc. 42% of the forest basal area) in a seasonal Amazon forest using the stable isotope ratios (δ18O and δ2H) of water collected from tree xylem and soils from a range of depths. We took advantage of a major ENSO‐related drought in 2015/2016 that caused substantial evaporative isotope enrichment in the soil and revealed water use strategies of each species under extreme conditions. We measured the minimum dry season leaf water potential both in a normal year (2014; Ψnon‐ENSO) and in an extreme drought year (2015; ΨENSO). Furthermore, we measured xylem hydraulic traits that indicate water potential thresholds trees tolerate without risking hydraulic failure (P50and P88).We demonstrate that coexisting trees are largely segregated along a single hydrological niche axis defined by root depth differences, access to light and tolerance of low water potential. These differences in rooting depth were strongly related to tree size; diameter at breast height (DBH) explained 72% of the variation in the δ18Oxylem. Additionally, δ18Oxylemexplained 49% of the variation in P50and 70% of P88, with shallow‐rooted species more tolerant of low water potentials, while δ18O of xylem water explained 47% and 77% of the variation of minimum Ψnon‐ENSOand ΨENSO.We propose a new formulation to estimate an effective functional rooting depth, i.e. the likely soil depth from which roots can sustain water uptake for physiological functions, using DBH as predictor of root depth at this site. Based on these estimates, we conclude that rooting depth varies systematically across the most abundant families, genera and species at the Tapajós forest, and that understorey species in particular are limited to shallow rooting depths.Our results support the theory of hydrological niche segregation and its underlying trade‐off related to drought resistance, which also affect the dominance structure of trees in this seasonal eastern Amazon forest.Synthesis. Our results support the theory of hydrological niche segregation and demonstrate its underlying trade‐off related to drought resistance (access to deep water vs. tolerance of very low water potentials). We found that the single hydrological axis defining water use traits was strongly related to tree size, and infer that periodic extreme droughts influence community composition and the dominance structure of trees in this seasonal eastern Amazon forest.

Does belowground interaction with Fagus sylvatica increase drought susceptibility of photosynthesis and stem growth in Picea abies?

Mixed stands of European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) Karst.) frequently over-yield, when compared to respective monospecific stands. Over-yielding is attributed to enhanced resource uptake efficiency through niche complementarity alleviating species competition, for example through enhanced root stratification in mixture. Under severe and frequent summer drought, however, water limitation may become crucial in modifying the prevailing competitive interaction in mixed beech-spruce forests. We hypothesize, therefore, that under drought (H I) inter-specific interaction with beech reduces water accessibility for spruce more than intra-specific conditions, thus (H II) exacerbating drought susceptibility of spruce in terms of reduced photosynthesis and stem growth. Reactions at the organ (leaf, fine root), tree and stand scale were analysed in a mature forest with beech-spruce group mixture. Under inter-specific conditions spruce’s fine-root production and depth of water uptake (assessed via δ18O of xylem water) shifted to shallow, drought-prone soil horizons, in agreement with H I. Overall, lowered fine root production and ramification along with a reduction in long-distance explorative ectomycorrhizal types resulted in decreased soil exploitation in spruce when growing together with beech. Spruce’s drought sensitivity was exemplified by a distinct decrease in stomatal conductance, net CO2 uptake rate and stem growth during periods of water limitation. Notwithstanding, species interaction effects were absent in leaf gas exchange and stem diameter growth, during a six-week summer drought period in 2013 as well as in the extremely dry year of 2003, hence rejecting H II. Based on results from soil moisture measurements and water uptake depth, we interpret the conflicting findings for H I and H II to result from: (i) seasonal shifts between positive (during spring drought) and negative (during summer drought) effects of beech neighbourhood on soil water availability for spruce, possibly overriding each other in their effect on annual stem diameter growth and (ii) the group-wise mixture pattern, where spruce is exposed to competition with beech only along group edges, i.e. laterally only, so that the putatively adverse beech effect on water accessibility stays limited. Our results suggest, compared to single tree mixture, group-wise mixture of beech and spruce to be a favourable silvicultural option in the face of climate change.

The role of soil processes in δ18O terrestrial climate proxies

AbstractA paleoclimate interpretation of a terrestrial hydrologic proxy such as the δ18O of tree cellulose or speleothem calcite may be biased or misinterpreted if the isotopic composition of the soil water from which the proxy originated undergoes isotopic exchange or fractionation. In this study, we use a global isotope‐enabled land surface model to investigate how the δ18O of precipitation may be altered in a soil column due to evaporation and vertical moisture transport. In order to assess how precipitation and evaporation contribute to the soil water isotopic variability, we compare seasonal and interannual changes in simulated xylem water δ18O within a control simulation and in a suite of sensitivity experiments where the effects of precipitation δ18O, water vapor δ18O, and soil water evaporation are independently removed. The simulations, carried out for the period 1979 to 2004, reveal that in semiarid regions, such as the southwest United States, the seasonal cycle in xylem water δ18O is strongly affected by evaporative loss during the dry season and evaporation can also constitute as much as 50% of the interannual δ18O variance. Additional simulations, including soil water tagging experiments, indicate that upward fluxes of soil water occur during drier periods. For soil water δ18O profiles that are isotopically more depleted in 18O at depth, this imparts a low isotopic signature to xylem water δ18O during such dry intervals. Hence, without taking into account vertical moisture transport in the soils, low δ18O years could be misinterpreted as wet conditions (due to decreased evaporative enrichment) when instead drier conditions are equally as likely.

Groundwater use by native plants in response to changes in precipitation in an intermountain basin

Many arid basins in western North America are likely to experience future changes in precipitation timing and amount. Where shallow water tables occur, plant acquisition of groundwater and soil water may be influenced by growing season precipitation. We conducted a rainfall manipulation experiment to investigate responses of four common native plant species to ambient, increased, and decreased summer monsoon rainfall. We measured plant xylem pressure potentials (Ψ) and stable oxygen isotope signatures (δ18O) to assess effects of altered precipitation on plant water relations and water acquisition patterns. Reduced rainfall decreased Ψ more in the grasses Sporobolus airoides and Distichlis spicata than the more deeply rooted shrubs Sarcobatus vermiculatus and Ericameria nauseosa. E. nauseosa had little response to natural or experimental differences in available soil water. Plant xylem water δ18O indicated that S. airoides and D. spicata are almost entirely dependent on rain-recharged soil water, while E. nauseosa is almost entirely groundwater-dependent. Sarcobatus vermiculatus used groundwater during dry periods, but utilized precipitation from soil layers after large rainfall events. Persistent changes in precipitation patterns could cause shifts in plant community composition that may alter basin-scale groundwater consumption by native plants, affecting water availability for human and ecosystem uses.

Water relations of evergreen and drought-deciduous trees along a seasonally dry tropical forest chronosequence

Seasonally dry tropical forests (SDTF) are characterized by pronounced seasonality in rainfall, and as a result trees in these forests must endure seasonal variation in soil water availability. Furthermore, SDTF on the northern Yucatan Peninsula, Mexico, have a legacy of disturbances, thereby creating a patchy mosaic of different seral stages undergoing secondary succession. We examined the water status of six canopy tree species, representing contrasting leaf phenology (evergreen vs. drought-deciduous) at three seral stages along a fire chronosequence in order to better understand strategies that trees use to overcome seasonal water limitations. The early-seral forest was characterized by high soil water evaporation and low soil moisture, and consequently early-seral trees exhibited lower midday bulk leaf water potentials (ΨL) relative to late-seral trees (−1.01 ± 0.14 and −0.54 ± 0.07 MPa, respectively). Although ΨL did not differ between evergreen and drought-deciduous trees, results from stable isotope analyses indicated different strategies to overcome seasonal water limitations. Differences were especially pronounced in the early-seral stage where evergreen trees had significantly lower xylem water δ18O values relative to drought-deciduous trees (−2.6 ± 0.5 and 0.3 ± 0.6‰, respectively), indicating evergreen species used deeper sources of water. In contrast, drought-deciduous trees showed greater enrichment of foliar 18O (∆18Ol) and 13C, suggesting lower stomatal conductance and greater water-use efficiency. Thus, the rapid development of deep roots appears to be an important strategy enabling evergreen species to overcome seasonal water limitation, whereas, in addition to losing a portion of their leaves, drought-deciduous trees minimize water loss from remaining leaves during the dry season.

Environmental and physiological controls over oxygen and carbon isotope composition of Tasmanian blue gum, Eucalyptus globulus

We measured oxygen isotope ratios (delta18O) of xylem sap, phloem sap, leaves, wood and bark of Eucalyptus globulus Labill. growing in southwestern Australia. Carbon isotope ratios (delta13C) were measured in the dry matter of phloem sap, leaves and wood. Results were used to test several aspects of a mechanistic model of 18O enrichment and provided insights into post-photosynthetic variations in dry matter delta13C. Xylem water delta18O varied little within the tree crown, whereas variation at the landscape-level was more pronounced, with plantations near the coast being enriched by up to 3 per thousand compared with plantations less than 100 km inland. Phloem water was significantly enriched in 18O compared with xylem water in two of three sampling campaigns; mean enrichments were 0.5 and 0.8 per thousand. Phloem sap sugars exported from E. globulus leaves closely reflected observed leaf water enrichment when diurnal variation in photosynthesis was taken into account. Photosynthetic rates were higher in the morning than in the afternoon, whereas leaf water 18O enrichment increased to maximum values in the afternoon. A non-steady-state model of leaf water 18O enrichment accurately predicted observed values through a full diel cycle. Mean estimates of the proportion of organic oxygen effectively exchanging with xylem water during cellulose synthesis were close to 0.40 for both leaves and wood. Carbon isotope ratios of nascent xylem tissues did not differ from those of phloem sap sugars collected concurrently, whereas nascent leaf tissues were depleted in 13C by 2 per thousand compared with phloem sap sugars, suggesting that, in E. globulus, 13C enrichment of sink tissues compared with source leaves does not result from an enriching process within the sink tissue.

Oxygen isotope content of CO2 in nocturnal ecosystem respiration: 2. Short‐term dynamics of foliar and soil component fluxes in an old‐growth ponderosa pine forest

The oxygen isotope contents (δ18O) of soil, xylem, and leaf water and ecosystem respiration were studied in a ponderosa pine forest during summer 2001. Our goal was to assess whether δ18O of CO2 could be used to quantify the relative contributions of soil and foliar respiration to total nocturnal ecosystem respiration. The δ18O in leaf and soil water showed enrichment over a 2‐week sampling period as the weather became hot and dry (leaves 0.9 to 15.0‰, and soil −10.4 to −3.1‰), while δ18O of xylem water remained constant (−12.9‰). Water in the soil was enriched in 18O near the soil surface (−6.4‰ at 5 cm depth) relative to greater depths (−11.1‰ at 20 cm). The δ18O of ecosystem respiration became gradually enriched over the 2‐week sampling period (from 24.2 initially to 32.9‰ at the end, VSMOW scale). Soil respiration contributed 80 ± 12 percent to the total respiratory flux, close to estimates from scaled‐up chamber data (77% [Law et al., 2001a]). Quantitative application of the isotopic approach to determine respiratory proportions required direct measurement of δ18O of soil and xylem water, air and soil temperature, and humidity. Better estimates of the isotopic signatures of component fluxes could be achieved with additional measurements and more detailed modeling. Results demonstrate that (1) there is variability in δ18O of precipitation inputs to ecosystems, (2) immediately following a precipitation event, δ18O of ecosystem respiration may reflect δ18O of precipitation, (3) periods of hot dry weather can substantially enrich ecosystem water pools and subsequently alter the isotope content of CO2 in ecosystem respiration, and (4) stable oxygen isotopes in CO2 can be used to quantify the foliar and soil components of ecosystem respiration.