Foliar δ13C Research Articles

Variation patterns in foliar δ13C and δ15N among plant life types along an altitudinal gradient on the Tibetan Plateau

Understanding how foliar δ13C and δ15N vary with the environment is crucial for elucidating the carbon and nitrogen cycle dynamics within ecosystems. Yet, there is limited knowledge about these variations among different plant life types along altitudinal gradients, particularly in subtropical forest ecosystems found in the southeastern Tibetan Plateau. Therefore, we collected leaves from 67 species, including 20 trees, 24 shrubs, and 23 herbs along an altitudinal gradient of 834 m to 3105 m in the subtropical forest of Medog, and investigated their patterns of variation along the altitudinal gradient. Our findings revealed a positive relationship between foliar δ13C and altitude, as well as a negative relationship between foliar δ13C and soil temperature. There was no significant correlation between foliar δ15N and altitude, but a hump-shaped relationship was observed between foliar δ15N and soil moisture. Among the different plant functional groups, the δ13C of shrubs is more sensitive and representative of environmental changes. In addition, we discovered no difference in the δ13C (δ15N) values of leaves among trees, shrubs, and herbs in this study. Still, there were significant differences when compared to the δ13C (δ15N) values of leaves in other typical climatic zones, indicating that climatic zones influenced the δ13C and δ15N values of leaves more than plant functional groups. In conclusion, our results provide new insights into the climatic drivers of the changes in δ13C and δ15N of the different plant life types along the altitudinal gradient in the southeastern Tibetan Plateau.

Greener leaves from northern trees: Latitudinal compensation in riparian cottonwoods

Narrowleaf cottonwoods (Populus angustifolia) occur in disjunct populations along rivers in a north-south corridor of the Rocky Mountain regions from Alberta, Canada, to Arizona, USA. This distribution should allow genetic divergence to provide localized ecophysiological adaptation. To test this, we compared 167 genotypes in ten source populations from across the full latitudinal range of 16° (1800 km). Replicated clonal saplings were grown for three years in a common garden at the northern range limit, where we found that shoot characteristics were correlated with the latitude of origin. Southern populations displayed increased height and stem diameter (populations: r2 = 0.489 and 0.472) due to extended in-leaf growing seasons. However, this produced a phenological mismatch, with increased fall frost injury and shoot die-back. With increasing latitude of origin there were foliar differences across the populations, with higher chlorophyll content (chl, r2 = 0.836), nitrogen (N, r2 = 0.429), and leaf mass per area (LMA, r2 = 0.474). Similar latitudinal correspondences were observed across the individual genotypes, again with the strongest correlation for chl (r2 = 0.188). Across the genotypes, chl was positively correlated with leaf length, width, area, and mass, and especially with foliar N (r2 = 0.354), which was also correlated with foliar δ13C (r2 = 0.230). The heritable correspondences between source latitude and foliar characteristics could enable vigorous photosynthesis during the shorter, northern growing season, providing latitudinal compensation. Conversely, slower photosynthesis would reduce the foliar aging rate, enabling leaf function through the longer, southern growing season. The alternate foliar strategies could accommodate the limited ‘leafspan’, or leaf lifespan. This adaptive variation in foliar physiology would impact natural or assisted northward migration of cottonwood populations in response to climate change. It could also be useful for poplar breeding, with northern genotypes contributing genes to increase photosynthetic capacity. Finally, groupings of the populations based on the combined ecophysiological traits were very consistent with genetically defined ecotypes, supporting the evolution of subspecies types within this obligate riparian tree.

Isoscapes of remnant and restored Hawaiian montane forests reveal differences in biological nitrogen fixation and carbon inputs.

Deforestation and subsequent land-use conversion has altered ecosystems and led to negative effects on biodiversity. To ameliorate these effects, nitrogen-fixing (N2-fixing) trees are frequently used in the reforestation of degraded landscapes, especially in the tropics; however, their influence on ecosystem properties such as nitrogen (N) availability and carbon (C) stocks are understudied. Here, we use a 30-y old reforestation site of outplanted native N2-fixing trees (Acacia koa) dominated by exotic grass understory, and a neighboring remnant forest dominated by A. koa canopy trees and native understory, to assess whether restoration is leading to similar N and C biogeochemical landscapes and soil and plant properties as a target remnant forest ecosystem. We measured nutrient contents and isotope values (δ15N, δ13C) in soils, A. koa, and non-N2-fixing understory plants (Rubus spp.) and generated δ15N and δ13C isoscapes of the two forests to test for (1) different levels of biological nitrogen fixation (BNF) and its contribution to non-N2-fixing understory plants, and (2) the influence of historic land conversion and more recent afforestation on plant and soil δ13C. In the plantation, A. koa densities were higher and foliar δ15N values for A. koa and Rubus spp. were lower than in the remnant forest. Foliar and soil isoscapes also showed a more homogeneous distribution of low δ15N values in the plantation and greater influence of A. koa on neighboring plants and soil, suggesting greater BNF. Foliar δ13C also indicated higher water use efficiency (WUEi) in the plantation, indicative of differences in plant-water relations or soil water status between the two forest types. Plantation soil δ13C was higher than the remnant forest, consistent with greater contributions of exotic C4-pasture grasses to soil C pools, possibly due to facilitation of non-native grasses by the dense A. koa canopy. These findings are consequential for forest restoration, as they contribute to the mounting evidence that outplanting N2-fixing trees produces different biogeochemical landscapes than those observed in reference ecosystems, thereby influencing plant-soil interactions which can influence restoration outcomes.

'Ecophysiological controls on water use of tropical cloud forest trees in response to experimental drought'.

Tropical montane cloud forests (TMCFs) are expected to experience more frequent and prolonged droughts over the coming century, yet understanding of TCMF tree responses to moisture stress remains weak compared to the lowland tropics. We simulated a severe drought in a throughfall reduction experiment (TFR) for two years in a Peruvian TCMF and evaluated the physiological responses several dominant species (Clusia flaviflora, Weinmannia bangii, Weinmannia crassiflora, Prunus integrifolia). Measurements were taken of: i) sap flow ii) diurnal cycles of stem shrinkage, stem moisture variation, and water use; iii) intrinsic water use efficiency (iWUE) estimated from foliar δ13C. In Weinmannia bangii, we used dendrometers and volumetric water content (VWC) sensors to quantify daily cycles of stem water storage. In two years of sap flow (Js) data, we found a threshold response of water use to VPD>1.07kPa independent of treatment, though control trees used more soil water than the treatment trees. The daily decline in water use in the TFR trees was associated with a strong reduction in both morning and afternoon Js rates at a given VPD. Soil moisture also affected the hysteresis strength between Js and VPD. Reduced hysteresis under moisture stress implies that TMCFs are strongly dependent on shallow soil water. Additionally, we suggest that hysteresis can serve as a sensitive indicator of environmental constraints on plant function. Finally, six months into the experiment, the TFR treatment significantly increased iWUE in all study species. Our results highlight the conservative behavior of TMCF tree water use under severe soil drought and elucidates physiological thresholds related to VPD and its interaction with soil moisture. The observed strongly isohydric response likely incurs a cost to the carbon balance of the tree, and reduces overall ecosystem carbon uptake.

Seasonal variations of plant water use in the karst desertification control

Understanding the water use characteristics of plants is crucial for the sustainability of forest water management and vegetation restoration. The vegetation restoration program in the karst desertification areas of southwest China has been implemented for more than two decades, and remarkable achievements have been made in ecological restoration. However, the water use characteristics of revegetation are still poorly understood. We investigated the water uptake patterns and water use efficiency of four woody plants (Juglans regia, Zanthoxylum bungeanum, Eriobotrya japonica, and Lonicera japonica) using stable isotopes (δ2H, δ18O, and δ13C) in combination with the MixSIAR model. The results showed that plants responded to seasonal changes in soil moisture with flexible water uptake patterns. Differences in water use sources among the four plant species during the growing season indicated the occurrence of hydrological niche separation, which is the key to vegetation symbiosis. Throughout the study period, groundwater made the lowest contribution to plants (9.39 %~16.25 %), and fissure soil water made the highest contribution (39.74 %~64.71 %). Among them, shrubs and vines were more dependent on fissure soil water compared to trees (50.52 %~64.71 %). Furthermore, plant foliar δ13C was higher in the dry season than in the rainy season. Evergreen shrubs (−27.94 ‰) exhibited higher water use efficiency compared to other tree species (−30.48 ‰~−29.04 ‰). The water use efficiency of four plants showed seasonal variation and was influenced by the water availability caused by soil moisture. Our study demonstrates that fissure soil water is an important water source for karst desertification revegetation and that seasonal changes in water use characteristics are influenced by species-level water uptake patterns and water use strategies. This study provides a reference for vegetation restoration and water resource management in karst areas.

Warming reduces both photosynthetic nutrient use efficiency and water use efficiency in Mediterranean shrubsWarming reduces nutrient use efficiency

The ratio between net photosynthetic rates and the foliar contents of essential plant macronutrients (N, P, K) is termed photosynthetic nutrient use efficiency (PNutUE). A universal trade-off exists whereby plants cannot maximize their PNutUE and their intrinsic water use efficiency (WUEi, carbon gain per unit water spent) simultaneously, because any increase in intercellular CO2 concentration (ci) through a greater stomatal opening would increase PNutE but also enhances transpiration and therefore decreases WUEi. Rising temperatures associated with climate change can result in large decreases in WUEi in semiarid shrubs through photosynthetic machinery impairment and enhanced stomatal conductance and transpiration, but we know remarkably little about the influence of warming and drought on PNutUE and its interplay with WUEi in dryland vegetation. Using a 6-year (2011–2017) manipulative field experiment, we examined the effects of warming (2.5ºC, W), rainfall reduction (30%, RR), and their combination (W+RR) on the photosynthetic use efficiency of three essential nutrients (PNUE, PPUE and PKUE) and on WUEi in three shrub species growing at two semiarid shrublands for the years 2015–2017. Across species, warming (W and W+RR) reduced PNUE by 42.9%, PPUE by 43.8% and PKUE by 41.5% on average relative to shrubs growing under ambient temperatures, whereas RR did not significantly affect their PNutUE. These drastic reductions in PNutUE with warming were mainly driven by non-stomatal and largely non-nutritional decreases in net photosynthetic rates, which were almost halved in warmed shrubs. The photosynthetic use efficiencies of N, P and K were inversely related to foliar δ13C, a proxy for time integrated WUEi, in both ambient (control and RR) and warmed (W and W+RR) shrubs, but with significantly smaller slopes and intercepts for warmed shrubs. Thus, plants achieve a smaller gain in PNutUE for any given increase in stomatal conductance (and reduction in WUEi) under warmer climatic conditions. The strong negative impact of warming on PNutE, along with the warming-induced shift in the trade-off between PNutUE and WUEi, could be indicative of an increasing inability of native plants to cope with warmer conditions, with dire implications for dryland vegetation productivity and survival under climate change.

Urban CO2 imprints on carbon isotope and growth of Chinese pine in the Beijing metropolitan region

Rapid urbanization has occurred globally and resulted in increasing CO2 emissions from urban areas. Compared to natural forests, urban forests are subject to higher atmospheric CO2 concentrations in view of strong urban-periurban-rural gradients of CO2 emissions. However, relevant insights in the CO2-associated urban imprints on the physiology and growth of regional forests remain lacking. By sampling foliage and tree rings of Chinese pine (Pinus tabuliformis) in the Beijing metropolitan region, China, we explored whether and how urban CO2 emissions affect stable carbon isotope ratios (δ13C) and tree growth spatially and/or temporally. The results indicate a significant decrease in foliar δ13C values towards the urban center and this pattern was mainly explained by the urban-periurban-rural gradients of CO2 emissions as surrogated by trunk road density. Tree-ring δ13C values showed a significant decrease over last four decades and this trend was mainly explained by rising levels of CO2 and secondarily mediated by the variations of aridity index during growing season. Moreover, annual basal area increment of Chinese pine was significantly accelerated during last two decades, being mainly driven by increasing CO2 emissions and secondarily mediated by climate variations. These findings reveal significant CO2-associated imprints of urbanization on plant growth and provide empirical evidences of significant CO2-induced alteration of carbon cycles in urban forests.

Functional traits of riparian trees in the lower Fitzroy River, Western Australia

AbstractRiparian trees are critically important for maintaining the ecological function of freshwater ecosystems. Globally, anthropogenic changes to water regimes are impacting the health and distribution of riparian trees. Understanding the physiological constraints on the distribution of riparian tree species in relation to the water regime is essential for informed water resource management that seeks to limit impacts on riparian trees. To fill an identified knowledge gap for the Fitzroy River, a significant river in the wet–dry tropics of north‐western Australia, we used a trait‐based approach to characterise nine common riparian tree species in relation to their distribution along a hydrological gradient. We assessed key functional traits related to drought and flood flows. Leaf mass per unit area (LMA), leaf dry matter content (LDMC), foliar carbon content (% C) and the ratio of carbon to nitrogen (C:N) are broadly related to plant productivity and durability and may reflect resistance to fluvial stress. Traits related to water availability were stem specific density (SSD), mean xylem vessel diameter and xylem vessel density, as well as foliar δ13C which is related to water use efficiency, and leaf osmotic potential at full hydration (π100) as a measure of drought tolerance. We found that π100, δ13C and SSD values reflected species hydrological habitat preferences, with higher δ13C values and lower π100 for species distributed in the drier floodplain habitats, compared with species constrained to the riverbank. Low SSD values for species close to the riverbank may be indicative of aerenchyma tissue in response to flooding. Differences in leaf trait values were primarily attributed to differences between evergreen Myrtaceous and deciduous non‐Myrtaceous species, rather than hydrological habitat preferences. LMA was greatest for the Myrtaceous study species, with deciduous non‐Myrtaceous species associated with wetter habitats having the lowest values. The physiology of riparian trees in northern Australia is not well described for the majority of species. Our study provides insight into plant functional strategies in response to both flooding and drought and is a critical step in understanding plant responses to future water‐take scenarios to support evidence‐based decision making.

Biomass, Carbon and Nitrogen Partitioning and Water Use Efficiency Differences of Five Types of Alpine Grasslands in the Northern Tibetan Plateau.

(1) Background: Grassland covers most areas of the northern Tibetan Plateau along with important global terrestrial carbon (C) and nitrogen (N) pools, so there is a need to better understand the different alpine grassland growth associated with ecosystem C, N storage and water use efficiency (WUE). (2) Methods: The plant biomass and C, N concentrations, stocks and vegetation WUE of five kinds of alpine grassland types were investigated in northern Tibetan Plateau. (3) Results: The results showed that there were significant differences among five types of alpine grasslands in aboveground biomass (AGB), belowground biomass (BGB), total biomass (TB) and root:shoot (R/S) ratio while the highest value of different indices was shown in alpine meadow type (AM). The AGB and BGB partitioning results significantly satisfied the allometric biomass partitioning theory. The C, N concentrations and C/N of the vegetation in AGB and BGB showed significant grassland type differences. The highest C, N stocks of BGB were in AM which was almost six or seven times more than the C, N stocks of AGB in alpine desert type (AD). There were significant differences in δ13C and intrinsic water use efficiency (WUEi) under five alpine grassland types while the highest mean values of foliar δ13C and WUEi were in AD. Significant negative correlations were found between WUEi and C, N concentrations, C/N of AGB and soil water content (SWC) while the correlation with BGB C/N was not significant. For AGB, BGB, TB and R/S, there were positive correlations with C, N concentrations of AGB, BGB and SWC while it had significant negative correlations with C/N of BGB. (4) Conclusions: With regard to its types, it is suggested that the AM or AS may be an actively growing grassland type in the northern Tibetan Plateau.

Changes in Forest Conditions in a Siberian Larch Forest Induced by an Extreme Wet Event

The taiga forest, a semi-arid and nitrogen-limited ecosystem on permafrost, has changed under extreme wet events. This study aims to understand the changes that occurred in a larch forest in Eastern Siberia after the wet event of 2006–2007. In the summer of 2018, studies were conducted at the Spasskaya Pad Experimental Forest Station near Yakutsk, Russia, where a transect (60 m × 510 m) with 34 plots (30 m × 30 m) was set. It included intact sites and affected sites with different levels of forest damage, owing to the extreme wet event. We observed spatial variations in the normalized difference vegetation index (NDVI) calculated from Landsat satellite-observed data, and the foliar δ13C, δ15N, and C/N (carbon/nitrogen) ratio obtained from the needle samples of 105 mature larch trees. Our results reveal that the affected plots had a lower NDVI than the intact plots, resulting from a difference in tree stand density. In addition, the stand density is suggested to be a controlling factor in the spatial variations in the foliar C/N and δ13C values based on their significant relationships with the NDVI in June. We concluded that the larch trees from the regenerating forests in the affected areas have a higher nitrogen level and light availability (relatively low C/N and high δ13C) because of the slight competition for resources, owing to a low-stand density. This may lead to further succession of the larch forests after the extreme wet event.

Restoring the iconic Ulmus americana to urban landscapes: Early tree growth responds to aboveground conditions

Native trees provide a range of benefits, from supporting native wildlife to climate regulation, and many urban natural resource managers prioritize native tree planting and restoration. Ulmus americana (American elm) was once widely planted in American cities but has been decimated by Dutch elm disease (DED; Ophiostoma ulmi). Our study evaluated U. americana establishment and growth across urban landscapes. We planted ramets from three DED-tolerant U. americana genotypes (RV16, RV474, and Sunfield) along an urbanization gradient in Newark, DE and Philadelphia, PA, and assessed physiological and morphological responses. We analyzed how U. americana clone growth, chlorophyll fluorescence, and foliar chemistry relate to impervious surface area, ozone (O3) concentrations, and soil characteristics. The one-year post planting mortality rate was low (4%) demonstrating these elms can withstand urban environmental conditions when provided ample water supply and protection from deer. As expected, the elms differed in growth rate, chlorophyll fluorescence, and foliar chemistry between the cities. Elms planted in Philadelphia had greater photosynthetic capacity in July (Fv/Fm = 0.76) and September (Fv/Fm = 0.75), while Newark elms had greater photosynthetic capacity in August (Fv/Fm = 0.78). Depleted foliar δ13C signatures in Philadelphia suggest elms are experiencing greater fossil-fuel derived atmospheric CO2 than in Newark, possibly contributing to the greater growth rates observed in Philadelphia compared to Newark. Enriched foliar δ15N and greater foliar %N in Philadelphia clones suggest they are experiencing greater N deposition from NOx-derived sources compared to Newark clones. Clones growing in Philadelphia had greater foliar nutrient concentrations despite growing in soils with greater heavy metal concentrations. These foliar-soil chemistry patterns suggest that clones growing in Philadelphia respond positively to urban environmental conditions in a large city, whereas clones growing in Newark may be experiencing N limitation in the first year of growth after planting.

Functionally dissimilar neighbours increase tree water use efficiency through enhancement of leaf phosphorus concentration

Abstract Water use efficiency (WUE) is central to the global cycles of water and carbon. However, whether increasing tree diversity in forests can increase WUE remains poorly understood because the variation in WUE may result from multiple mechanisms acting concurrently. Disentangling their relative importance represents a major challenge. Here, we conducted a forest biodiversity experiment in subtropical China to assess the effects of neighbouring tree diversity on the foliar WUE of Cunninghamia lanceolata, a widespread tree plantation species in China. We measured foliar δ13C as a proxy for the change in intrinsic WUE. To disentangle the relative importance of water and nutrient‐mediated mechanisms in altering WUE, we determined foliar nutrient concentrations and foliar δ18O. Foliar δ18O was used as a surrogate for stomatal conductance and is known to be related to plant water use. Foliar WUE of focal trees increased with specific root length and leaf nitrogen content dissimilarities between focal trees and neighbours. The positive neighbourhood dissimilarity effect on foliar WUE was stronger under a more shaded neighbourhood. Foliar P concentration increased with neighbourhood specific root length dissimilarity. However, neither neighbourhood trait dissimilarities nor species richness significantly affect foliar δ18O. Results from structural equation model suggested that neighbourhood dissimilarity in specific root length indirectly enhanced foliar WUE and tree growth of focal trees via increasing foliar P concentration. Synthesis. Our results demonstrate the potential role of neighbourhood trait dissimilarity in modulating tree WUE in species‐rich communities. Our finding implies that the functional diversity of tree species at the local neighbourhood level has a fundamental ability to control water and nutrient cycles in forests.

Variability in soil and foliar stable carbon and nitrogen isotope compositions in the winter rainfall biomes of South Africa

We present the stable carbon and nitrogen isotope compositions of 204 plants and 99 soils from South Africa's Fynbos and Succulent Karoo biomes, from which we assess variability in δ13C and δ15N within and between the biomes. Higher foliar δ13C in the Succulent Karoo (−22.4 ± 4.7‰) primarily reflects a greater abundance of CAM photosynthesis relative to Fynbos (−25.7 ± 3.5‰). For C3 plants within the Fynbos Biome, there is no statistically significant relationship between δ13C and climate, suggesting that within the present sampling framework, local-scale variability masks expected climatic controls on C3 plant δ13C. Soil and foliar δ15N are highly variable, but foliar δ15N is significantly higher in the Succulent Karoo (7.7 ± 3.9‰) compared to the Fynbos (4.4 ± 3.3‰). Soil and foliar δ15N correlate, with soils 3 ± 1‰ more enriched in 15N. Both soil and foliar δ15N show stronger correlations with climate than δ13C, and they accord with the previously observed tendency for higher δ15N with greater aridity. The relationship between δ15N and mean annual rainfall is, however, relatively steep (2.4 ± 0.6‰ per 100 mm rainfall), potentially implying additional controls on δ15N variability. These data provide a benchmarking of contemporary isotopic variability, anticipated to support future archaeological, palaeoenvironmental and C and N cycling research.

Genetic Variation in Drought-Tolerance Traits and Their Relationships to Growth in Pinus radiata D. Don Under Water Stress.

The selection of drought-tolerant genotypes is globally recognized as an effective strategy to maintain the growth and survival of commercial tree species exposed to future drought periods. New genomic selection tools that reduce the time of progeny trials are required to substitute traditional tree breeding programs. We investigated the genetic variation of water stress tolerance in New Zealand-grown Pinus radiata D. Don using 622 commercially-used genotypes from 63 families. We used quantitative pedigree-based (Genomic Best Linear Unbiased Prediction or ABLUP) and genomic-based (Genomic Best Linear Unbiased Prediction or GBLUP) approaches to examine the heritability estimates associated with water stress tolerance in P. radiata. Tree seedling growth traits, foliar carbon isotope composition (δ13C), and dark-adapted chlorophyll fluorescence (Y) were monitored before, during and after 10 months of water stress. Height growth showed a constant and moderate heritability level, while the heritability estimate for diameter growth and δ13C decreased with water stress. In contrast, chlorophyll fluorescence exhibited low heritability after 5 and 10 months of water stress. The GBLUP approach provided less breeding value accuracy than ABLUP, however, the relative selection efficiency of GBLUP was greater compared with ABLUP selection techniques. Although there was no significant relationship directly between δ13C and Y, the genetic correlations were significant and stronger for GBLUP. The positive genetic correlations between δ13C and tree biomass traits under water stress indicated that intraspecific variation in δ13C was likely driven by differences in the genotype’s photosynthetic capacity. The results show that foliar δ13C can predict P. radiata genotype tolerance to water stress using ABLUP and GBLUP approaches and that such approaches can provide a faster screening and selection of drought-tolerant genotypes for forestry breeding programs.

Leaf 13C data constrain the uncertainty of the carbon dynamics of temperate forest ecosystems

AbstractStable carbon isotope discrimination occurred in plant biophysical and biogeochemical processes can help understand plant physiology and soil biogeochemistry with respect to carbon cycling. Here, we incorporated the photosynthetic carbon isotope discrimination into a process‐based land surface model (iTEM) to test if stable carbon isotope composition (δ13C) can impose additional constraint on model parameters. Sequential data assimilation was implemented at six eddy covariance flux tower sites using carbon flux observations including gross primary productivity (GPP) and net ecosystem exchange (NEE) with and without considering foliar δ13C (δ13Cf) measurement constraints, respectively. Our model‐data fusion showed that δ13Cf can provide useful constraint on photosynthetic (Vcmax25, the maximum rate of carboxylation at 25°C) and stomatal (g1, the slope of stomatal function) parameters as well as the posterior carbon fluxes. When including δ13Cf measurement, g1 spatially varies among the six sites and is significantly correlated with annual precipitation. We incorporated the statistical relationship between g1 and annual precipitation into iTEM, which is then used to quantify the regional carbon dynamic in temperate forest ecosystems of the Northern Hemisphere. Compared with the simulation only conditioned on carbon flux observations, regional carbon flux estimations performed slightly better against the FLUXCOM products and the uncertainties of modeled carbon fluxes were reduced by 27%. Our study demonstrated that δ13Cf data constrains carbon flux uncertainties across space.

Lianas and trees exhibit divergent intrinsic water‐use efficiency along elevational gradients in South American and African tropical forests

AbstractAimElevational gradients provide excellent opportunities to explore long‐term morphological and physiological responses of plants to environmental change. We determined the difference in the elevational pattern of foliar carbon isotope composition (δ13C) between lianas and trees, and assessed whether this difference arises from changes in photosynthesis or stomatal conductance. We also explored the pattern of nutrient limitations with the elevation of these two growth forms.LocationThe study was conducted in two mountain forests situated in the Neotropics and Palaeotropics.Time periodAugust–September 2015 and August–October 2016.Major taxa studiedLianas and trees.MethodsWe conducted inventories of lianas and trees using standardized techniques along elevational gradients in Ecuador and Rwanda. We determined the values of several foliar traits including δ13C and chemical traits in dominant liana and tree species. We set up Bayesian linear mixed‐effect models to quantify the effects of elevation and growth form on each of the foliar traits , and the difference of the effect of elevation between the two growth forms (lianas and trees).ResultsWe found consistent growth form specific divergences in foliar δ13C and carbon to nitrogen ratio (C : N) responses to elevation. While we noted a meaningful increase in foliar δ13C and C : N with elevation for trees, lianas did not exhibit such a trend. Foliar δ13C and C : N remained relatively constant for lianas along the transects.Main conclusionsLianas operate at relatively constant intrinsic water‐ and nitrogen‐use efficiencies with elevation compared with trees. Altogether, the study suggests the existence of a functional divergence of water and nutrient use strategies between lianas and trees along tropical elevational transects.

Plant functional types broadly describe water use strategies in the Caatinga, a seasonally dry tropical forest in northeast Brazil.

In seasonally dry tropical forests, plant functional type can be classified as deciduous low wood density, deciduous high wood density, or evergreen high wood density species. While deciduousness is often associated with drought‐avoidance and low wood density is often associated with tissue water storage, the degree to which these functional types may correspond to diverging and unique water use strategies has not been extensively tested.We examined (a) tolerance to water stress, measured by predawn and mid‐day leaf water potential; (b) water use efficiency, measured via foliar δ13C; and (c) access to soil water, measured via stem water δ18O.We found that deciduous low wood density species maintain high leaf water potential and low water use efficiency. Deciduous high wood density species have lower leaf water potential and variable water use efficiency. Both groups rely on shallow soil water. Evergreen high wood density species have low leaf water potential, higher water use efficiency, and access alternative water sources. These findings indicate that deciduous low wood density species are drought avoiders, with a specialized strategy for storing root and stem water. Deciduous high wood density species are moderately drought tolerant, and evergreen high wood density species are the most drought tolerant group.Synthesis. Our results broadly support the plant functional type framework as a way to understand water use strategies, but also highlight species‐level differences.

Unraveling the Influence of Land-Use Change on δ13C, δ15N, and Soil Nutritional Status in Coniferous, Broadleaved, and Mixed Forests in Southern China: A Field Investigation.

Natural isotopic abundance in soil and foliar can provide integrated information related to the long-term alterations of carbon (C) and nitrogen (N) cycles in forest ecosystems. We evaluated total carbon (TC), total nitrogen (TN), and isotopic natural abundance of C (δ13C) and N (δ15N) in soil and foliar of coniferous plantation (CPF), natural broadleaved forest (NBF), and mixed forest stands at three different soil depths (i.e., 0–10, 10–20, and 20–40 cm). This study also explored how soil available nutrients are affected by different forest types. Lutou forest research station, located in Hunan Province, central China, was used as the study area. Results demonstrated that the topsoil layer had higher TC and TN content in the mixed forest stand, resulting in a better quality of organic materials in the topsoil layer in the mixed forest than NBF and CPF. In general, soil TC, TN, and δ15N varied significantly in different soil depths and forest types. However, the forest type did not exhibit any significant effect on δ13C. Overall, soil δ13C was significantly enriched in CPF, and δ15N values were enriched in mixed forest. Foliar C content varied significantly among forest types, whereas foliar N content was not significantly different. No big differences were observed for foliar δ15N and δ13C across forest types. However, foliar δ13C and δ15N were positively related to soil δ13C and δ15N, respectively. Foliar N, soil and foliar C:N ratio, soil moisture content (SMC), and forest type were observed as the major influential factors affecting isotopic natural abundance, whereas soil pH was not significantly correlated. In addition, forest type change and soil depth increment had a significant effect on soil nutrient availability. In general, soil nutrient availability was higher in mixed forest. Our findings implied that forest type and soil depth alter TC, TN, and soil δ15N, whereas δ13C was only driven by soil depth. Moreover, plantations led to a decline in soil available nutrient content compared with NBF and mixed forest stands.

Partitioning of Environmental and Taxonomic Controls on Brazilian Foliar Content of Carbon and Nitrogen and Stable Isotopes

The Neotropics harbor some of the most diversified woody species in the world, and to understand the nutrient dynamics in these ecosystems, it is crucial to understand the role of plant taxonomy. In addition, biological nitrogen (N) fixation (BNF) in the tropics is one of the key processes affecting the global N cycle. Our objective was to (i) investigate the role of taxonomy and sampling site as predictors of foliar carbon (C) and N concentration and its stable isotopes (i.e., δ13C and δ15N); (ii) assess differences in foliar N, C:N ratio, and δ15N among three functional groups: species of N2-fixers and non-fixers of the Fabaceae family, as well as non-Fabaceae species; and (iii) examine the effect of wood density on tree foliar properties. We hypothesized that Fabaceae specimens in symbiosis with N2-fixers would possess a higher foliar N than non-fixing plants, including those of the Fabaceae family, as well as high-density trees would have higher foliar C and C:N ratio relative to low-density trees, where the latter invest in nutrients instead of structural C. We used a data set composed of 3,668 specimens sampled in three main biomes of Brazil: Amazon, Atlantic Forest, and Cerrado. The partitioning of variance had a higher influence of taxonomy on leaf C, N, and C:N ratio. Conversely, foliar δ13C and δ15N were environmentally constrained. While family was the most important taxonomy level for C, N, and C:N ratio, species played a major role for δ13C and δ15N. Foliar N followed the pattern fixers > non-fixers > non-Fabaceae, while C:N ratio had an opposite trend. In addition, foliar C was correlated with wood density, where high-density > medium-density and low-density woods. The large variability of δ15N was observed among Fabaceae species, demonstrates the complexity of using δ15N as an indicator of BNF. The higher foliar N of Fabaceae non-fixers than non-Fabaceae specimens support the hypothesis that an N-demanding lifestyle is an inherent pattern in this family. Lastly, although observed in some studies, the prediction of foliar properties using wood density is challenging, and future research on this topic is needed.

Soil Moisture and Soluble Salt Content Dominate Changes in Foliar δ13C and δ15N of Desert Communities in the Qaidam Basin, Qinghai-Tibetan Plateau

Changing precipitation and temperature are principal drivers for nutrient cycling dynamics in drylands. Foliar isotopic carbon (C) and nitrogen (N) composition (δ13C and δ15N) are often used to describe the plant’s water use efficiency and nitrogen use strategy in plant ecology research. However, the drivers and mechanisms under differential foliar δ13C and δ15N among plant species and communities are largely unknown for arid high-elevation regions. This study collected 462 leaf samples of ten top-dominant plant species (two or three replicates per species) across 16 sites in 2005 and 2010 to measure the community-weighted means (CWMs) of foliar δ13C and δ15N, northeastern Qaidam Basin, Qinghai-Tibetan Plateau. Our results showed that the CWM of foliar δ15N was higher in 2005 than in 2010 and was lower in the warm-dry season (July and August) than the cool-wet one (June and September) in 2010. Similarly, the CWM of foliar δ13C was higher in 2005 than in 2010, but no difference between warm-dry and cool-wet seasons in 2010. C4 plants have higher δ13C and generally grow faster than C3 species under warm-wet weathers. This might be why the CWM of foliar δ13C was high, while the CWM of foliar δ15N was low in the wet sampling year (2010). The general linear mixed models revealed that soil moisture was the most critical driver for the CWM of foliar δ15N, which explained 42.1% of the variance alone. However, the total soluble salt content was the crucial factor for the CWM of foliar δ13C, being responsible for 29.7% of the variance. Growing season temperature (GST) was the second most vital factor and explained 28.0% and 21.9% of the variance in the CWMs of foliar δ15N and δ13C. Meanwhile, remarkable differences in the CWMs of foliar δ15N and δ13C were also found at the species level. Specifically, Kalidium gracile and Salsola abrotanoides have higher foliar δ15N, while Ephedra sinica and Tamarix chinensis have lower foliar δ15N than other species. The foliar δ13C of Calligonum Kozlov and H. ammodendron was the highest among the ten species. Except for the foliar δ13C of E. sinica was higher than Ceratoide latens between the two sampling years or between the cool-wet and warm-dry seasons, no significant difference in foliar δ13C was found for other species. Overall, the CWMs of foliar δ15N and δ13C dynamics were affected by soil properties, wet-dry climate change, and species identity in high-elevation deserts on the Qinghai Tibetan Plateau.