Variations in foliar stable carbon isotopes among functional groups and along environmental gradients in China - a meta-analysis

Effects of Short-Term Nitrogen Additions on Biomass and Soil Phytochemical Cycling in Alpine Grasslands of Tianshan, China

The nitrogen deposition process, as an important phenomenon of global climate change and an important link in the nitrogen cycle, has had serious and far-reaching impacts on grassland ecosystems. This study aimed to investigate the survival adaptation strategies of plants of different functional groups under nitrogen deposition, and the study identified the following outcomes of differences in biomass changes by conducting in situ simulated nitrogen deposition experiments while integrating plant nutrient contents and soil physicochemical properties: (1) nitrogen addition enhanced the aboveground biomass of grassland communities, in which Poaceae were significantly affected by nitrogen addition. Additionally, nitrogen addition significantly influenced plant total nitrogen and total phosphorus; (2) nitrogen addition improved the plant growth environment, alleviated plant nitrogen limitation, and promoted plant phosphorus uptake; and (3) there was variability in the biomass responses of different functional groups to nitrogen addition. The level of nitrogen addition was the primary factor affecting differences in biomass changes, while nitrogen addition frequency was an important factor affecting changes in plant community structure.

Climate gradient and leaf carbon investment influence the effects of climate change on water use efficiency of forests: A meta-analysis

Forest ecosystems cover a large area of the global land surface and are important carbon sinks. The water-carbon cycles of forests are prone to climate change, but uncertainties remain regarding the magnitude of water use efficiency (WUE) response to climate change and the underpinning mechanism driving WUE variation. We conducted a meta-analysis of the effects of elevated CO2 concentration (eCO2 ), drought and elevated temperature (eT) on the leaf- to plant-level WUE, covering 80 field studies and 95 tree species. The results showed that eCO2 increased leaf intrinsic and instantaneous WUE (WUEi, WUEt), whereas drought enhanced both leaf- and plant-level WUEs. eT increased WUEi but decreased carbon isotope-based WUE, possibly due to the influence of mesophyll conductance. Stimulated leaf-level WUE by drought showed a progressing trend with increasing latitude, while eCO2 -induced WUE enhancement showed decreasing trends after >40° N. These latitudinal gradients might influence the spatial pattern of climate and further drove WUE variation. Moreover, high leaf-level WUE under eCO2 and drought was accompanied by low leaf carbon contents. Such a trade-off between growth efficiency and defence suggests a potentially compromised tolerance to diseases and pests. These findings add important ecophysiological parameters into climate models to predict carbon-water cycles of forests.

Leaf functional traits have more contributions than climate to the variations of leaf stable carbon isotope of different plant functional types on the eastern Qinghai-Tibetan Plateau

Elucidating the mechanisms that control the leaf stable carbon isotope values (δ¹³C_leaf) is the prerequisite for the widespread application of δ¹³C_leaf. However, the competing effects of physiological and environmental factors on δ¹³C_leaf variations of the different plant functional types (PFTs) have not been disentangled, and the corresponding mechanisms remain unclear. Based on large-scale δ¹³C_leaf measurements on the eastern Qinghai-Tibetan Plateau, the relative contributions and regulatory pathways of leaf functional traits (LFTs) and climatic factors to δ¹³C_leaf variations of the different PFTs were investigated. We found that δ¹³C_leaf of the different PFTs was correlated with annual mean precipitation negatively, but not a simple linear relationship with annual mean temperature and varied by PFTs. Leaf nitrogen content per unit area and leaf mass per area (correlated with δ¹³C_leaf positively) had more substantial effects on the δ¹³C_leaf variations of the different PFTs than other LFTs. The relative contributions of LFTs to the δ¹³C_leaf variations were greater than that of climatic factors, and the direct and indirect effects of climatic factors on δ¹³C_leaf variations varied by PFTs. Our findings provide new insights into understanding key drivers of δ¹³C_leaf variations at the PFT level on a regional scale.

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.

An insect isoscape of UK and Ireland

RATIONALE

The study of insect migration is problematic due to the small size of insects. Stable isotope analysis can be used to elucidate movement, either by geographic assignment of location of a species, or by simply distinguishing migrant from resident populations. There are few isoscapes of any kind in the UK/Ireland available for interrogation. Thus, I have measured stable isotope ratios (of H, C, N and S) of 299 individuals of the non-migratory Brimstone moth (Opisthograptis luteolata) collected from 93 locations around the UK and Ireland by citizen scientists.

METHODS

After removing lipids, stable isotope ratios were measured by continuous flow isotope ratio mass spectrometry, using either a conventional elemental analyser (C, N and S) or a high-temperature, thermal conversion elemental analyser in reductive mode.

RESULTS

Maps (isoscapes) were constructed that illustrate the stable isotope spatial distribution of this insect. These are the first isoscapes of H, C, N and S of biological samples covering both UK and Ireland.

CONCLUSIONS

The insect isoscape patterns can be explained from what we know of moth diet, climate and geology. Sulfur isotopes may be of particular use for distinguishing individuals from areas of unique geology. Isoscape patterns may (with care) predict isotope compositions of other, herbivorous, non-aquatic, chitinous taxa. Such isoscapes, when extended beyond the UK and Ireland, would provide a useful tool to elucidate insect migration.

Spatial patterns of leaf δ13C and δ15N of aquatic macrophytes in the arid zone of northwestern China

Analysis of stable isotope composition is an important tool in research on plant physiological ecology. However, large-scale patterns of leaf-stable isotopes for aquatic macrophytes have received considerably less attention. In this study, we examined the spatial pattern of stable isotopes of carbon (δ13C) and nitrogen (δ15N) of macrophytes leaves collected across the arid zone of northwestern China (approximately 2.4 × 106 km2) and attempted to illustrate its relationship with environmental factors (i.e., temperature, precipitation, potential evapotranspiration, sediment total carbon and nitrogen). Our results showed that the mean values of the leaf δ13C and δ15N in the macrophytes sampled from the arid zone were -24.49‰ and 6.82‰, respectively, which were far less depleted than those measured of terrestrial plants. The order of averaged leaf δ13C from different life forms was as follows: submerged > floating-leaved > emergent. Additionally, our studies indicated that the values of foliar δ13C values of all the aquatic macrophytes were only negatively associated with precipitation, but the foliar δ15N values were mainly associated with temperature, precipitation, and potential evapotranspiration. Therefore, we speculated that water-relation factors are the leaf δ13C determinant of macrophytes in the arid zone of northwestern China, and the main factors affecting leaf δ15N values are the complex combination of water and energy factors.

Global patterns of intraspecific leaf trait responses to elevation

Elevational gradients are often used to quantify how traits of plant species respond to abiotic and biotic environmental variations. Yet, such analyses are frequently restricted spatially and applied along single slopes or mountain ranges. Since we know little on the response of intraspecific leaf traits to elevation across the globe, we here perform a global meta-analysis of leaf traits in 109 plant species located in 4 continents and reported in 71 studies published between 1983 and 2018. We quantified the intraspecific change in seven morpho-ecophysiological leaf traits along global elevational gradients: specific leaf area (SLA), leaf mass per area (LMA), leaf area (LA), nitrogen concentration per unit of area (Narea), nitrogen concentration per unit mass (Nmass), phosphorous concentration per unit mass (Pmass) and carbon isotope composition (δ¹³ C). We found LMA, Narea, Nmass and δ¹³ C to significantly increase and SLA to decrease with increasing elevation. Conversely, LA and Pmass showed no significant pattern with elevation worldwide. We found significantly larger increase in Narea, Nmass, Pmass and δ¹³ C with elevation in warmer regions. Larger responses to increasing elevation were apparent for SLA of herbaceous compared to woody species, but not for the other traits. Finally, we also detected evidences of covariation across morphological and physiological traits within the same elevational gradient. In sum, we demonstrate that there are common cross-species patterns of intraspecific leaf trait variation across elevational gradients worldwide. Irrespective of whether such variation is genetically determined via local adaptation or attributed to phenotypic plasticity, the leaf trait patterns quantified here suggest that plant species are adapted to live on a range of temperature conditions. Since the distribution of mountain biota is predominantly shifting upslope in response to changes in environmental conditions, our results are important to further our understanding of how plants species of mountain ecosystems adapt to global environmental change.

Bedrock geology affects foliar nutrient status but has minor influence on leaf carbon isotope discrimination across altitudinal gradients

Carbon isotope discrimination (Δ¹³C) in plant leaves generally decreases with increasing altitude in mountains. Lower foliar Δ¹³C at high elevation usually is associated with higher leaf mass per area (LMA) in thicker leaves. However, it is unclear if lower foliar Δ¹³C in high-altitude plants is caused by improved photosynthetic capacity as an effect of higher nutrient, especially nitrogen, content in thicker leaves. We investigated trends of foliar Δ¹³C in four species, each belonging to a different plant functional type (PFT), across two altitudinal gradients, each on a different bedrock type (carbonate and silicate bedrock, respectively) in a region of the southern Alps (Italy) where the foliar Δ¹³C was not affected by water limitation. Our objective was to assess whether the altitudinal patterns of foliar Δ¹³C in relation to leaf morphology and foliar nutrients were conditioned by indirect control of bedrock geology on soil nutrient availability. The foliar Δ¹³C of the four species was mainly affected by LMA and, secondarily, by stomatal density (SD) but the relative importance of these foliar traits varied among species. Area-based nutrient contents had overall minor importance in controlling C discrimination. Relationships among foliar Δ¹³C, foliar nutrient content and leaf growth rate strongly depended on soil nutrient availability varying differently across the two gradients. In the absence of water limitation, the foliar Δ¹³C was primarily controlled by irradiance which can shape anatomical leaf traits, especially LMA and/or SD, whose relative importance in determining C isotope discrimination differed among species and/or PFT. Decreasing foliar Δ¹³C across altitudinal gradients need not be determined by improved photosynthetic capacity deriving from higher nutrient content in thicker leaves.

Understanding the wide geographic range of a clonal perennial grass: plasticity versus local adaptation

Both phenotypic plasticity and local adaptation may allow widely distributed plant species to either acclimate or adapt to environmental heterogeneity. Given the typically low genetic variation of clonal plants across their habitats, phenotypic plasticity may be the primary adaptive strategy allowing them to thrive across a wide range of habitats. In this study, the mechanism supporting the widespread distribution of the clonal plant Leymus chinensis was determined, i.e. phenotypic plasticity or local specialization in water use efficiency (WUE; reflected by foliar δ(13)C). To test whether plasticity is required for the species to thrive in different habitats, samples were collected across its distribution in the Mongolian steppe, and a controlled watering experiment was conducted with two populations at two different sites. Five populations were also transplanted from different sites into a control environment, and the foliar δ(13)C was compared between the control and original habitats, to test for local specialization in WUE. Results demonstrated decreased foliar δ(13)C with increasing precipitation during controlled watering experiments, with divergent responses between the two populations assessed. Change in foliar δ(13)C (-3.69 ‰) due to water addition was comparable to fluctuations of foliar δ(13)C observed in situ (-4.83 ‰). Foliar δ(13)C differed by -0.91 ‰ between two transplanted populations; however, this difference was not apparent between the two populations when growing in their original habitats. Findings provide evidence that local adaptation affects foliar δ(13)C much less than phenotypic plasticity. Thus, plasticity in WUE is more important than local adaptation in allowing the clonal plant L. chinensis to occupy a wide range of habitats in the Mongolian steppe.

Relative Roles of Soil Moisture, Nutrient Supply, Depth, and Mechanical Impedance in Determining Composition and Structure of Wisconsin Prairies

Ecologists have long classified Midwestern prairies based on compositional variation assumed to reflect local gradients in moisture availability. The best known classification is based on Curtis’ continuum index (CI), calculated using the presence of indicator species thought centered on different portions of an underlying moisture gradient. Direct evidence of the extent to which CI reflects differences in moisture availability has been lacking, however. Many factors that increase moisture availability (e.g., soil depth, silt content) also increase nutrient supply and decrease soil mechanical impedance; the ecological effects of the last have rarely been considered in any ecosystem. Decreased soil mechanical impedance should increase the availability of soil moisture and nutrients by reducing the root costs of retrieving both. Here we assess the relative importance of soil moisture, nutrient supply, and mechanical impedance in determining prairie composition and structure. We used leaf δ¹³C of C₃ plants as a measure of growing-season moisture availability, cation exchange capacity (CEC) x soil depth as a measure of mineral nutrient availability, and penetrometer data as a measure of soil mechanical impedance. Community composition and structure were assessed in 17 remnant prairies in Wisconsin which vary little in annual precipitation. Ordination and regression analyses showed that δ¹³C increased with CI toward “drier” sites, and decreased with soil depth and % silt content. Variation in δ¹³C among remnants was 2.0‰, comparable to that along continental gradients from ca. 500–1500 mm annual rainfall. As predicted, LAI and average leaf height increased significantly toward “wetter” sites. CI accounted for 54% of compositional variance but δ¹³C accounted for only 6.2%, despite the strong relationships of δ¹³C to CI and CI to composition. Compositional variation reflects soil fertility and mechanical impedance more than moisture availability. This study is the first to quantify the effects of soil mechanical impedance on community ecology.

Effects of sampling method on foliar δ (13)C of Leymus chinensis at different scales

Stable carbon isotope composition (δ (13)C) usually shows a negative relationship with precipitation at a large scale. We hypothesized that sampling method affects foliar δ (13)C and its response pattern to precipitation. We selected 11 sites along a precipitation gradient in Inner Mongolia and collected leaves of Leymus chinensis with five or six replications repeatedly in each site from 2009 to 2011. Additionally, we collected leaves of L. chinensis separately from two types of grassland (grazed and fenced) in 2011. Foliar δ (13)C values of all samples were measured. We compared the patterns that foliar δ (13)C to precipitation among different years or different sample sizes, the differences of foliar δ (13)C between grazed and fenced grassland. Whether actual annual precipitation (AAP) or mean annual precipitation (MAP), it was strongly correlated with foliar δ (13)C every year. Significant difference was found between the slopes of foliar δ (13)C to AAP and MAP every year, among the slopes of foliar δ (13)C to AAP from 2009 to 2011. The more samples used at each site the lower and convergent P-values of the linear regression test between foliar δ (13)C and precipitation. Furthermore, there was significant lower foliar δ (13)C value in presence of grazed type than fenced type grassland. These findings provide evidence that there is significant effect of sampling method to foliar δ (13)C and its response pattern to precipitation of L. chinensis. Our results have valuable implications in methodology for future field sampling studies.

Habitat-specific differences in plasticity of foliar δ (13)C in temperate steppe grasses

A decrease in foliar δ (13)C with increasing precipitation is a common tendency in steppe plants. However, the rate of decrease has been reported to differ between different species or populations. We here hypothesized that plant populations in the same habitat of temperate steppes may not differ in foliar δ (13)C response patterns to precipitation, but could differ in the levels of plasticity of foliar δ (13)C across different habitats. In order to test this hypothesis, we conducted controlled watering experiments in northeast China at five sites along a west-east transect at latitude 44°N, which show substantial interannual fluctuations and intra-annual changes in precipitation among them. In 2001, watering treatment (six levels, three replicates) was assigned to 18 plots at each site. The responses of foliar δ (13)C to precipitation (i.e., the sum of watering and rainfall) were determined in populations of several grass species that were common across all sites. Although similar linear regression slopes were observed for populations of different species growing at the same site, significantly different slopes were obtained for populations of the same species growing at different sites. Further, the slope of the line progressively decreased from Site I to Site V for all species in this study. These results suggest habitat-specific differences in plasticity of foliar δ (13)C in temperate steppe grasses. This indicates that species' δ (13)C response to precipitation is conservative at the same site due to their long-term acclimation, but the mechanism responsible behind this needs further investigations.