Leaf Carbon Isotope Ratios Research Articles

Linking stomatal size and density to water use efficiency and leaf carbon isotope ratio in juvenile and mature trees.

Water-use efficiency (WUE) is affected by multiple leaf traits, including stomatal morphology. However, the impact of stomatal morphology on WUE across different ontogenetic stages of tree species is not well-documented. Here, we investigated the relationship between stomatal morphology, intrinsic water-use efficiency (iWUE) and leaf carbon isotope ratio (δ13C). We sampled 190 individuals, including juvenile and mature trees belonging to 18 temperate broadleaved tree species and 9 genera. We measured guard cell length (GCL), stomatal density (SD), specific leaf area (SLA), iWUE and bulk leaf δ13C as a proxy for long-term WUE. Leaf δ13C correlated positively with iWUE across species in both juvenile and mature trees, while GCL showed a negative and SD a positive effect on iWUE and leaf δ13C. Within species, however, only GCL was significantly associated with iWUE and leaf δ13C. SLA had a minor negative influence on iWUE and leaf δ13C, but this effect was inconsistent between juvenile and mature trees. We conclude that GCL and SD can be considered functional morphological traits related to the iWUE and leaf δ13C of trees, highlighting their potential for rapid phenotyping approaches in ecological studies.

The Variations of Leaf δ13C and Its Response to Environmental Changes of Arbuscular and Ectomycorrhizal Plants Depend on Life Forms

Arbuscular mycorrhiza (AM) and ectomycorrhiza (ECM) are the two most common mycorrhizal types and are paid the most attention to, playing a vital common but differentiated function in terrestrial ecosystems. The leaf carbon isotope ratio (δ13C) is an important factor in understanding the relationship between plants and the environment. In this study, a new database was established on leaf δ13C between AM and ECM plants based on the published data set of leaf δ13C in China's C3 terrestrial plants, which involved 1163 observations. The results showed that the differences in leaf δ13C between AM and ECM plants related closely to life forms. Leaf δ13C of ECM plants was higher than that of AM plants in trees, which was mainly led by the group of evergreen trees. The responses of leaf δ13C to environmental changes were varied between AM and ECM plants. Among the four life forms, leaf δ13C of ECM plants decreased more rapidly than that of AM plants, with an increase of longitude, except for deciduous trees. In terms of the sensitivity of leaf δ13C to temperature changes, AM plants were higher than ECM plants in the other three life forms, although there was no significant difference in evergreen trees. For the response to water conditions, the leaf δ13C of ECM plants was more sensitive than that of AM plants in all life forms, except evergreen and deciduous trees. This study laid a foundation for further understanding the role of mycorrhiza in the relationship between plants and the environment.

Intraspecific and sex-dependent variation of leaf traits along altitude gradient in the endangered dioecious tree Taxus fuana Nan Li & R.R. Mill.

Plant intraspecific trait variation (ITV) including sex-dependent differences are matters of many ecological consequences, from individual to ecosystem, especially in endangered and rare species. Taxus fuana is an endangered dioecious species with small and isolated populations endemic to the Himalayas region. Little is known about its trait variation between sexes, and among populations. In this study, 18 leaf traits from 179 reproductive trees (males and females) along the altitude (2600-3200m a.s.l.) of the T. fuana populations distributed in Gyirong County, Tibet, China, were measured. ITV and sources of variation in leaf traits were assessed. The relationship between leaf traits of males and females and altitude was analyzed separately. Variations in leaf traits of T. fuana ranged from 3.1% to 24.2%, with the smallest in leaf carbon content and the largest in leaf thickness to area ratio. On average 78.13% of the variation in leaf traits was from within populations and 21.87% among populations. The trends in leaf width, leaf nitrogen to phosphorus ratio, leaf carbon to nitrogen ratio, leaf carbon isotope ratio, and leaf nitrogen isotope ratio in relation to altitude were the same for males and females. Leaf length to width ratio varied significantly with altitude only in males, while leaf phosphorus content, leaf nitrogen content, and leaf carbon to phosphorus ratio varied significantly with altitude only in females. The correlation coefficients of most leaf traits of females with altitude were larger than that of males. In the relationship between leaf traits, there was a high similarity among males and females, but the altitude accounted for more explanation in females than in males. Our results suggested that the variation in leaf traits of T. fuana was small and did not dominate the interspecific competition in the local communities. Adaptation to the altitude gradient of T. fuana might be through altering nutrient storage processes and water use efficiency. Adaptation of male and female T. fuana to environmental changes showed differences, where the males were more tolerant and the females responded greatly to altitude. The differences in adaptation strategies between male and female T. fuana may be detrimental to the maintenance of their populations.

Flexible drought deciduousness in a neotropical understory herb.

PremiseAdaptive divergence across environmental gradients is a key driver of speciation. Precipitation seasonality gradients are common in the tropics, yet drought adaptation is nearly unexplored in neotropical understory herbs. Here, we examined two recently diverged neotropical spiral gingers, one adapted to seasonal drought and one reliant on perennial water, to uncover the basis of drought adaptation.MethodsWe combined ecophysiological trait measurements in the field and greenhouse with experimental and observational assessments of real‐time drought response to determine how Costus villosissimus (Costaceae) differs from C. allenii to achieve drought adaptation.ResultsWe found that drought‐adapted C. villosissimus has several characteristics indicating flexible dehydration avoidance via semi‐drought‐deciduousness and a fast economic strategy. Although the two species do not differ in water‐use efficiency, C. villosissimus has a more rapid growth rate, lower leaf mass per area, lower stem density, higher leaf nitrogen, and a strong trend of greater light‐saturated photosynthetic rates. These fast economic strategy traits align with both field‐based observations and experimental dry‐down results. During drought, C. villosissimus displays facultative drought‐deciduousness, losing lower leaves during the dry season and rapidly growing new leaves in the wet season.ConclusionsWe revealed a drought adaptation strategy that has not, to our knowledge, previously been documented in tropical herbs. This divergent drought adaptation evolved recently and is an important component of reproductive isolation between C. villosissimus and C. allenii, indicating that adaptive shifts to survive seasonal drought may be an underappreciated axis of neotropical understory plant diversification.

Water relations and drought response of Pinus strobiformis

Southwestern white pine (Pinus strobiformis Engelm.) faces dual threats of climate change shifting its environmental niche and mortality due to a nonnative, invasive fungal pathogen. To inform efforts to sustain this species, we established experimental field trials in three common gardens along an elevational gradient with drought treatments to assess trait responses in southwestern white pine. We measured predawn and midday water potential on 44 maternal families from 10 populations at each garden. We used regression between predawn and midday water potentials to estimate hydroscape area, an index of stomatal regulation of transpiration. We measured leaf carbon isotope ratio and estimated carbon isotope discrimination and leaf mass per area to understand the effects of gardens and treatments on stomatal aperture and leaf structure. Water stress caused by experimental drought and temperature decreased leaf carbon isotope discrimination and leaf mass per area, indicating formation of thin leaves with low stomatal conductance in response to heat and drought. The hydroscape area of southwestern white pine suggests tight control of transpiration via stomatal closure, similar to other isohydric pines. Families with greater stomatal closure (inferred from carbon isotope ratio) at the warm, dry garden had higher survival than other families, suggesting an important role of isohydry in acclimation of southwestern white pine to expected habitat drying and warming.

Isotopic and morphologic proxies for reconstructing light environment and leaf function of fossil leaves: a modern calibration in the Daintree Rainforest, Australia.

Within closed-canopy forests, vertical gradients of light and atmospheric CO2 drive variations in leaf carbon isotope ratios, leaf mass per area (LMA), and the micromorphology of leaf epidermal cells. Variations in traits observed in preserved or fossilized leaves could enable inferences of past forest canopy closure and leaf function and thereby habitat of individual taxa. However, as yet no calibration study has examined how isotopic, micro- and macromorphological traits, in combination, reflect position within a modern closed-canopy forest or how these could be applied to the fossil record. Leaves were sampled from throughout the vertical profile of the tropical forest canopy using the 48.5 m crane at the Daintree Rainforest Observatory, Queensland, Australia. Carbon isotope ratios, LMA, petiole metric (i.e., petiole-width2 /leaf area, a proposed proxy for LMA that can be measured from fossil leaves), and leaf micromorphology (i.e., undulation index and cell area) were compared within species across a range of canopy positions, as quantified by leaf area index (LAI). Individually, cell area, δ13 C, and petiole metric all correlated with both LAI and LMA, but the use of a combined model provided significantly greater predictive power. Using the observed relationships with leaf carbon isotope ratio and morphology to estimate the range of LAI in fossil floras can provide a measure of canopy closure in ancient forests. Similarly, estimates of LAI and LMA for individual taxa can provide comparative measures of light environment and growth strategy of fossil taxa from within a flora.

Multidecadal records of intrinsic water-use efficiency in the desert shrub Encelia farinosa reveal strong responses to climate change

While tree rings have enabled interannual examination of the influence of climate on trees, this is not possible for most shrubs. Here, we leverage a multidecadal record of annual foliar carbon isotope ratio collections coupled with 39 y of survey data from two populations of the drought-deciduous desert shrub Encelia farinosa to provide insight into water-use dynamics and climate. This carbon isotope record provides a unique opportunity to examine the response of desert shrubs to increasing temperature and water stress in a region where climate is changing rapidly. Population mean carbon isotope ratios fluctuated predictably in response to interannual variations in temperature, vapor pressure deficit, and precipitation, and responses were similar among individuals. We leveraged the well-established relationships between leaf carbon isotope ratios and the ratio of intracellular to ambient CO2 concentrations to calculate intrinsic water-use efficiency (iWUE) of the plants and to quantify plant responses to long-term environmental change. The population mean iWUE value increased by 53 to 58% over the study period, much more than the 20 to 30% increase that has been measured in forests [J. Peñuelas, J. G. Canadell, R. Ogaya, Glob. Ecol. Biogeogr. 20, 597-608 (2011)]. Changes were associated with both increased CO2 concentration and increased water stress. Individuals whose lifetimes spanned the entire study period exhibited increases in iWUE that were very similar to the population mean, suggesting that there was significant plasticity within individuals rather than selection at the population scale.

Occurrence of crassulacean acid metabolism in Colombian orchids determined by leaf carbon isotope ratios

AbstractMany Orchidaceae, especially those occupying periodically dry, epiphytic microhabitats in the humid tropics, are believed to engage in the water-conserving crassulacean acid metabolism (CAM) photosynthetic pathway. However, the photosynthetic pathway has been studied in only c. 5% of all orchid species. Here we extend the survey to 1079 orchid species, mainly from Colombia, by assessing the presence of CAM based on the carbon isotopic signature (δ 13C values) of herbarium specimens. Ninety-six species, representing 8.9% of those analysed, had δ 13C values less negative than −20‰, indicating CAM. Epiphytism was the predominant life form (75.2% of species sampled), and 9.4% of these epiphytes showed a CAM-type isotopic signature. Isotope values suggested CAM in 19 terrestrial orchid species, 14 species from high elevation (2000–3400 m) and species from six genera that were previously unknown to engage in CAM (Jacquiniella, Meiracyllium, Pabstiella, Psychopsis, Pterostemma and Solenidium). We conclude that CAM is the major pathway of carbon acquisition in a small but broadly distributed fraction of tropical orchids and is more prevalent at lower elevations.

Qualitative identification of hydrologically different water sources used by plants in rock-dominated environments

Sources of water used by plants in rock-dominated environments are far less clear than those in relatively homogeneous soils, largely because plants in rocky environments normally expose to more complex substrate conditions and their potential water sources are hard to locate and sample. In the current study, potential water sources for plants growing on cliff-face (a typical kind of rock-dominated environment) were roughly separated into two water pools based on their hydrological differences. A variable pool that is composed of water derived from recent precipitation that has rapid fluctuations in water content due to recharge by precipitation and depletion by transpiration, and a constant pool that is characterized by slower changes in water status because it can only be recharged by precipitation under certain conditions and depleted by transpiration of deep-rooted species. This study was conducted in a typical karst catchment of Northwest Guangxi, Southwest China. Stem water samples for the most common dominant tree species of cliff-faces and rainwater for each rain event were consecutively collected throughout the growing season. A co-occurring shrub species, which was identified to have a shallow root system, was further selected as a reference species. The reliance on different water pools was qualitatively identified based on the deviation of stem water isotope values between current and last sampling times, as well as its relationship to the isotope values of rainfall between these two sampling times. Our results showed that the shrub species continuously relied on water from the variable water pool, which was consistent with its shallow root system. While the dominant tree species shifted from the constant pool during the early wet season to the variable pool during the mid-wet season, then back to the constant pool during the early dry season. Moreover, the divergent reliance on different water pools by the two species was further supported by the species-specific dynamics in the efficiency in water use by various plants (reflected by leaf carbon isotope ratio). Our results provide new insights into the linkages between plant transpiration and hydrologically different water pools, which would be helpful in understanding the hydrological importance of different plant species.

Specific responses of sap flux and leaf functional traits to simulated canopy and understory nitrogen additions in a deciduous broadleaf forest.

To investigate the effects of atmospheric nitrogen (N) deposition on water use characteristics and leaf traits of trees, we performed canopy (C50) and understory (U50) N additions as NH4NO3 of 50 kg N ha-1 year-1 in a deciduous broadleaf forest of central China. We measured xylem sap flux, crown area:sapwood area ratio (Ca:As), specific leaf area (SLA), mass-based leaf nitrogen content (Nmass) and leaf carbon isotope ratio (δ13C) of Liquidambar formosana Hance, Quercus acutissima Carruth. and Quercus variabilis Blume. Functional traits under different N addition treatments and their responses among tree species were compared and the relationship between xylem sap flux and leaf functional traits under N additions were explored. Results showed that under U50 sap-flux density of xylem significantly decreased for three tree species. But the effect of C50 on sap flux was species-specific. The decrease of sap-flux density with N additions might be caused by the increased Ca/As. δ13C remained constant among different N addition treatments. The responses of SLA and Nmass to N additions were species- and N addition approaches-specific. The correlation of xylem sap flux with leaf traits was not found. Our findings indicate that the effects of canopy N addition on xylem sap flux and leaf functional traits were species-specific and it is necessary to employ canopy N addition for exploring the real responses of forest ecosystems to climate changes in the future researches.

Climate and soils together regulate photosynthetic carbon isotope discrimination within C3 plants worldwide

AbstractAimWithin C3 plants, photosynthesis is a balance between CO2 supply from the atmosphere via stomata and demand by enzymes within chloroplasts. This process is dynamic and a complex but crucial aspect of photosynthesis. We sought to understand the spatial pattern in CO2 supply–demand balance on a global scale, via analysis of stable isotopes of carbon within leaves (Δ13C), which provide an integrative record of CO2 drawdown during photosynthesis.LocationGlobalTime period1951–2011.Major taxa studiedVascular plants.MethodsWe assembled a database of leaf carbon isotope ratios containing 3,979 species–site combinations from across the globe, including 3,645 for C3 species. We examined a wide array of potential climate and soil drivers of variation in Δ13C.ResultsThe strongest drivers of carbon isotope discrimination at the global scale included atmospheric pressure, potential evapotranspiration and soil pH, which explained 44% of the variation in Δ13C. Addition of eight more climate and soil variables (each explaining small but highly significant amounts of variation) increased the explained variation to 60%. On top of this, the largest plant trait effect was leaf nitrogen per area, which explained 11% of Δ13C variation.Main conclusions: By considering variation in Δ13C at a considerably larger scale than previously, we were able to identify and quantify key drivers in CO2 supply–demand balance previously unacknowledged. Of special note is the key role of soil properties, with greater discrimination on low‐pH and high‐silt soils. Unlike other plant traits, which show typically wide variation within sets of coexisting species, the global pattern in carbon stable isotope ratios is much more conservative; there is relatively narrow variation in time‐integrated CO2 concentrations at the site of carboxylation among plants in a given soil and climate.

Fog Water Is Important in Maintaining the Water Budgets of Vascular Epiphytes in an Asian Tropical Karst Forests during the Dry Season

Fog may be an important source of water for forest vascular epiphytes on trees, because they lack direct access to sources of soil water, but little is known about the water use proportions from various sources and potential water uptake pathways in epiphytes. Here, we analyzed leaf carbon isotope ratios as a measure of water use efficiency (WUE), proportions of fog, rain, and soil water use, and foliar water uptake (FWU) in species of epiphyte and their host trees in a tropical karst dwarf forest in China during the dry season. We found that the WUE, as represented by leaf δ13C, was generally enriched in the epiphyte species compared to their host trees. Epiphytes used substantial proportions of fog water, whereas water use in the host trees was dominated by soil water. The leaves of epiphytes and host trees absorbed water following immersion in water for 3 h and FWU possibly related to foliar epicuticular structures, such as fungal endophytes. Our results show a divergence of water use strategies between epiphytes and their hosts and highlight the importance of fog water for epiphytes during the dry season and under a climate change scenario with a reduced occurrence of fog events.

Impact of root growth and hydraulic conductance on canopy carbon-water relations of young walnut trees ( Juglans regia L.) under drought

Abstract In addition to soil characteristics, two plant traits control the supply of water from the soil to the canopy: root growth and plant hydraulic conductance. Here we examine the impact of root growth and hydraulic conductance on water uptake and transpiration of walnut under deficit irrigation. A greenhouse experiment was conducted using nine young walnut trees ( Juglans regia L.) grown for three months in transparent pots, equipped with: (i) rhizotron tubes, which allowed for non-invasive monitoring of root growth; (ii) pressure transducer tensiometers, recording soil water potential at soil-root interfaces; (iii) psychrometers attached to mature leaves, measuring stem water potential; and (iv) weighing scales used to determine total plant transpiration. Irrigation treatments consisted of different replenishment levels (100%, 75%, and 50%) of potential transpiration replicated over time. Walnut trees showed rapid physiological acclimation characterized by a fast decline and subsequent stabilization of transpiration rates soon after the beginning of drought stress treatments. We also observed a significant decrease in plant hydraulic conductance with decreasing soil and stem water potential under drought stress. At the end of the experiment, isotopic measurements revealed the integrated effect of physiological acclimation on canopy carbon-water relations. Leaf carbon isotope ratios showed significant increases in water-use efficiency with deficit irrigation levels. Leaf water hydrogen and oxygen isotope ratios confirmed that changes in water use-efficiency were caused by decreases in transpiration. Conversely, root growth was highest under low stress (T100) and lowest under high stress (T50). These results indicate the existence of a fundamental tradeoff between water-use efficiency and root growth, which will be useful to optimize the application of water and improve the design of irrigation systems in walnut orchards.

Spatial patterns of leaf δ13C and its relationship with plant functional groups and environmental factors in China

The leaf carbon isotope ratio (δ13C) is a useful parameter for predicting a plant's water use efficiency, as an indicator for plant classification, and even in the reconstruction of paleoclimatic environments. In this study, we investigated the spatial pattern of leaf δ13C values and its relationship with plant functional groups and environmental factors throughout China. The high leaf δ13C in the database appeared in central and western China, and the averaged leaf δ13C was -27.15‰, with a range from -21.05‰ to -31.5‰. The order of the averaged δ13C for plant life forms from most positive to most negative was subshrubs > herbs = shrubs > trees > subtrees. Leaf δ13C is also influenced by some environmental factors, such as mean annual precipitation (MAP), relative humidity (RH), mean annual temperature (MAT), solar hours (SH), and altitude, although the overall influences are still relatively weak, in particular the influence of MAT and altitude. And we further found that plant functional types are dominant factors that regulate the magnitude of leaf δ13C for an individual site, whereas environmental conditions are key to understanding spatial patterns of leaf δ13C when we consider China as a whole. Ultimately, we conducted a multiple regression model of leaf δ13C with environmental factors and mapped the spatial distribution of leaf δ13C in China by using this model. However, this PLS model overestimated leaf δ13C for most life forms, especially for deciduous trees, evergreen shrubs and subtrees, and thus need more improvement in the future.

Relationship of leaf oxygen and carbon isotopic composition with transpiration efficiency in the C4 grasses Setaria viridis and Setaria italica.

Leaf carbon and oxygen isotope ratios can potentially provide a time-integrated proxy for stomatal conductance (gs) and transpiration rate (E), and can be used to estimate transpiration efficiency (TE). In this study, we found significant relationships of bulk leaf carbon isotopic signature (δ13CBL) and bulk leaf oxygen enrichment above source water (Δ18OBL) with gas exchange and TE in the model C4 grasses Setaria viridis and S. italica. Leaf δ13C had strong relationships with E, gs, water use, biomass, and TE. Additionally, the consistent difference in δ13CBL between well-watered and water-limited plants suggests that δ13CBL is effective in separating C4 plants with different availability of water. Alternatively, the use of Δ18OBL as a proxy for E and TE in S. viridis and S. italica was problematic. First, the oxygen isotopic composition of source water, used to calculate leaf water enrichment (Δ18OLW), was variable with time and differed across water treatments. Second, water limitations changed leaf size and masked the relationship of Δ18OLW and Δ18OBL with E. Therefore, the data collected here suggest that δ13CBL but not Δ18OBL may be an effective proxy for TE in C4 grasses.

Overland water flow contributes little to survival, growth, reproduction, and ecophysiology ofOlneya tesota(Desert Ironwood) trees

Abstract We studied the responses of Desert Ironwood trees (Olneya tesota), located in desert washes of the Sonoran Desert in California, to a reduction in overland water flow. We hypothesized that trees with access to overland water runoff would have different morphological and ecophysiological responses than trees that did not. We studied responses including branch growth, water potentials, leaf carbon isotope ratios, leaf mass per area, flower production, seed production, and tree population densities. Only branch growth and predawn water potentials were found to be significantly different between treatments. Predawn water potentials indicated seasonally increased water stress in trees without access to overland water runoff. Trees with access to overland water runoff also responded with increased branch growth. The lack of differences between treatments observed for all other morphological and ecophysiological responses may indicate that environmental conditions, especially water availability, were si...

Re‐growing a tropical dry forest: functional plant trait composition and community assembly during succession

SummaryA longstanding goal of ecology and conservation biology is to understand the environmental and biological controls of forest succession. However, the patterns and mechanisms that guide successional trajectories, especially within tropical forests, remain unclear.We collected leaf functional trait and abiotic data across a 110‐year chronosequence within a tropical dry forest in Costa Rica. Focusing on six key leaf functional traits related to resource acquisition and competition, along with measures of forest stand structure, we propose a mechanistic framework to link species composition, community trait distributions and forest structure. We quantified the community‐weighted trait distributions for specific leaf area, leaf dry matter concentration, leaf phosphorus concentration, leaf carbon to nitrogen ratio and leaf stable isotopic carbon and nitrogen. We assessed several prominent hypotheses for how these functional measures shift in response to changing environmental variables (soil water content, bulk density andpH) across the chronosequence.Increasingly, older forests differed significantly from younger forests in species composition, above‐ground biomass and shifted trait distributions. Early stages of succession were uniformly characterized by lower values of community‐weighted mean specific leaf area, leaf stable nitrogen isotope and leaf phosphorus concentration. Leaf dry matter concentration and leaf carbon to nitrogen ratio were lower during earlier stages of succession, and each trait reached an optimum during intermediate stages of succession. The leaf carbon isotope ratio was the only trait to decrease linearly with increasing stand age indicating reduced water use efficiency in older forests. However, in contrast with expectations, community‐weighted trait variances did not generally change through succession, and when compared to null expectations were lower than expected.The observed directional shift in community‐weighted mean trait values is consistent with the ‘productivity filtering’ hypothesis where a directional shift in water and light availability shifts physiological strategies from ‘slow’ to ‘fast’. In contrast with expectations arising from niche based ecology, none of the community trait distributions were over‐dispersed. Instead, patterns of trait dispersion are consistent with the abiotic filtering and/or competitive hierarchy hypotheses.

Nitrogen control of <sup>13</sup>C enrichment in heterotrophic organs relative to leaves in a landscape-building desert plant species

Abstract. A longstanding puzzle in isotope studies of C3 plant species is that heterotrophic plant organs (e.g., stems, roots, seeds, and fruits) tend to be enriched in 13C compared to the autotrophic organ (leaves) that provides them with photosynthate. Our inability to explain this puzzle suggests key deficiencies in understanding post-photosynthetic metabolic processes. It also limits the effectiveness of applications of stable carbon isotope analyses in a variety of scientific disciplines ranging from plant physiology to global carbon cycle studies. To gain insight into this puzzle, we excavated whole plant architectures of Nitraria tangutorum Bobrov, a C3 species that has an exceptional capability of fixing sands and building sand dunes, in two deserts in northwestern China. We systematically and simultaneously measured carbon isotope ratios and nitrogen and phosphorous contents of different parts of the excavated plants. We also determined the seasonal variations in leaf carbon isotope ratios on nearby intact plants of N. tangutorum. We found, for the first time, that higher nitrogen contents in heterotrophic organs were significantly correlated with increased heterotrophic 13C enrichment compared to leaves. However, phosphorous contents had no effect on the enrichment. In addition, new leaves had carbon isotope ratios similar to roots but were progressively depleted in 13C as they matured. We concluded that a nitrogen-mediated process, hypothesized to be the refixation of respiratory CO2 by phosphoenolpyruvate (PEP) carboxylase, was responsible for the differences in 13C enrichment among different heterotrophic organs, while processes such as fractionating foliar metabolism and preferentially loading into phloem of 13C-enriched sugars may contribute to the overall autotrophic–heterotrophic difference in carbon isotope compositions.

Patterns of herbivory-induced mortality of a dominant non-native tree/shrub (Tamarix spp.) in a southwestern US watershed

The capacity for plant species or populations to cope with herbivory depends in large part on the complex interactions between resource availability, life history and adaptive strategies to maximize defense and/or tolerance to herbivory. Given these complex interactions, the impacts of repeated herbivory on plant stress and subsequent mortality is often difficult to predict. To better understand relationships between herbivory and environmental condition, we studied the relationship between the non-native shrub/tree tamarisk (Tamarix spp.) and a specialist herbivore, the northern tamarisk leaf beetle (Diorhabda carinulata) released as a biological control agent of Tamarix in the Virgin River watershed in the southwestern United States. The beetle feeds exclusively on Tamarix foliage resulting in complete stand foliage desiccation (i.e. defoliation) that lasts several weeks. Approximately 900 Tamarix plants were surveyed over three consecutive growing seasons for canopy dieback and mortality across 10 sites varying in the number of defoliation events, tree height, soil salinity, soil texture and bulk leaf carbon isotope ratios (δ13C). Canopy dieback increased from 27 % by volume in the spring of 2012 to 41 % and 54 % in 2013 and 2014, respectively. Tree mortality increased from 0 % in 2012 to 6 % and 10 % in 2013 and 2014, respectively. Surprisingly, percent canopy dieback was not related to the number of defoliation events that ranged from 2 to 7 across the 10 sites prior to the 2013 growing season. On the other hand, canopy dieback increased with soil salinity in both 2013 (R 2 = 0.39, F = 5.07, P = 0.055) and 2014 (R 2 = 0.56, F = 10.26, P = 0.015). Canopy dieback in 2013 increased with bulk leaf δ13C (R 2 = 0.38, F = 4.08, P = 0.078), although δ13C also decreased with the number of defoliation events (R 2 = 0.64, F = 14.17, P = 0.0055), suggesting that photosynthetic rate or drought stress (as indicated by leaf δ13C) may serve as a poor predictor for Tamarix canopy dieback in response to defoliation. Percent canopy dieback was correlated with shifts in NDVI measured from annual MODIS imagery (R 2 = 0.61, F = 12.32, P = 0.008), demonstrating that the tree surveys reflect site-scale changes in canopy cover. Results show that patterns of Tamarix canopy dieback and subsequent mortality following episodic defoliation by D. carinulata are likely to vary across broad gradients in soil salinity and other abiotic and biotic factors. Documented impacts of this biocontrol agent reported here will aid management efforts aimed at preserving riparian habitat in the short-term with conservation efforts targeting the removal and control of Tamarix over the long term.

Leaf carbon isotope ratio and water use efficiency of urban roadside trees in summer in Kyoto city

AbstractThe effect of summer climate on leaf carbon isotope composition (δ13C) of the major roadside tree species Prunus × yedoensis (P. yedoensis) was investigated in Kyoto city, Japan, to explore the implications for alterations in urban environments. Temperature and the vapor pressure deficit were higher at sites of higher traffic volumes, possibly affected by a heat island effect. The leaf δ13C of P. yedoensis trees was affected strongly by leaf carbon isotope discrimination (Δ), with much less effect of δ13C on atmospheric CO2. Leaf Δ values in the summer were smaller at sites of higher traffic volumes with high atmospheric temperatures, suggesting a higher long‐term water use efficiency (WUE) at these sites. Gas exchange measurements of P. yedoensis leaves indeed suggested a higher intrinsic WUE at sites of higher traffic volumes with high atmospheric temperatures. These results suggest that leaf Δ is related to the response of WUE to summer climates, and that leaf δ13C in urban areas is a useful tracer for understanding the influences of urban environments on plant photosynthetic processes.