Leaf nitrogen:phosphorus stoichiometry across Chinese grassland biomes

Leaf P increase outpaces leaf N in an Inner Mongolia grassland over 27 years

The dynamics of leaf nitrogen (N) and phosphorus (P) have been intensively explored in short-term experiments, but rarely at longer timescales. Here, we investigated leaf N : P stoichiometry over a 27-year interval in an Inner Mongolia grassland by comparing leaf N : P concentration of 2006 with that of 1979. Across 80 species, both leaf N and P increased, but the increase in leaf N lagged behind that of leaf P, leading to a significant decrease in the N : P ratio. These changes in leaf N : P stoichiometry varied among functional groups. For leaf N, grasses increased, woody species tended to increase, whereas forbs showed no change. Unlike leaf N, leaf P of grasses and forbs increased, whereas woody species showed no change. Such changes may reflect N deposition and P release induced by soil acidification over the past decades. The interannual effect of precipitation may somewhat have reduced the soil available N, leading to the more modest increase of leaf N than of leaf P. Thus, leaf N : P stoichiometry significantly responded to long-term environmental changes in this temperate steppe, but different functional groups responded differently. Our results indicate that conclusions of plant stoichiometry under short-term N fertilization should be treated with caution when extrapolating to longer timescales.

Abiotic, biotic, and evolutionary control of the distribution of C and N isotopes in food webs

Ecosystem functioning depends on nutrient cycles and their responses to abiotic and biotic determinants, with the influence of evolutionary legacies being generally overlooked in ecosystem ecology. Along a broad elevation gradient characterized by shifting climatic and grazing environments, we addressed clines of plant N and C∶N content and of δ(13)C and δ(15)N in producers (herbs) and in primary (grasshoppers) and secondary (birds) consumers, both within and between species in phylogenetically controlled scenarios. We found parallel and significant intra- and interspecific trends of isotopic variation with elevation in the three groups. In primary producers, nutrient and isotope distributions had a detectable phylogenetic signal that constrained their variation along the environmental gradient. The influence of the environment could not be ascribed to any single factor, and both grazing and climate had an effect on leaf stoichiometry and, thus, on the resources available to consumers. Trends in consumers matched those in plants but often became nonsignificant after controlling for isotopic values of their direct resources, revealing direct bottom-up control and little phylogenetic dependence. By integrating ecosystem and mechanistic perspectives, we found that nutrient dynamics in food webs are governed at the base by the complex interaction between local determinants and evolutionary factors.

Leaf nitrogen and phosphorus of temperate desert plants in response to climate and soil nutrient availability

In desert ecosystems, plant growth and nutrient uptake are restricted by availability of soil nitrogen (N) and phosphorus (P). The effects of both climate and soil nutrient conditions on N and P concentrations among desert plant life forms (annual, perennial and shrub) remain unclear. We assessed leaf N and P levels of 54 desert plants and measured the corresponding soil N and P in shallow (0-10 cm), middle (10-40 cm) and deep soil layers (40-100 cm), at 52 sites in a temperate desert of northwest China. Leaf P and N:P ratios varied markedly among life forms. Leaf P was higher in annuals and perennials than in shrubs. Leaf N and P showed a negative relationship with mean annual temperature (MAT) and no relationship with mean annual precipitation (MAP), but a positive relationship with soil P. Leaf P of shrubs was positively related to soil P in the deep soil. Our study indicated that leaf N and P across the three life forms were influenced by soil P. Deep-rooted plants may enhance the availability of P in the surface soil facilitating growth of shallow-rooted life forms in this N and P limited system, but further research is warranted on this aspect.

Stoichiometry of root and leaf nitrogen and phosphorus in a dry alpine steppe on the Northern Tibetan Plateau

Leaf nitrogen (N) and phosphorus (P) have been used widely in the ecological stoichiometry to understand nutrient limitation in plant. However,few studies have focused on the relationship between root nutrients and environmental factors. The main objective of this study was to clarify the pattern of root and leaf N and P concentrations and the relationships between plant nitrogen (N) and phosphorus (P) concentrations with climatic factors under low temperature conditions in the northern Tibetan Plateau of China. We conducted a systematic census of N and P concentrations, and the N∶P ratio in leaf and root for 139 plant samples, from 14 species and 7 families in a dry Stipa purpurea alpine steppe on the northern Tibetan Plateau of China. The results showed that the mean root N and P concentrations and the N∶P ratios across all species were 13.05 mg g-1, 0.60 mg g-1 and 23.40, respectively. The mean leaf N and P concentrations and the N∶P ratio were 23.20 mg g-1, 1.38 mg g-1, and 17.87, respectively. Compared to global plant nutrients concentrations, plants distributing in high altitude area have higher N concentrations and N∶P, but lower P concentrations, which could be used to explain normally-observed low growth rate of plant in the cold region. Plant N concentrations were unrelated to the mean annual temperature (MAT). The root and leaf P concentrations were negatively correlated with the MAT, but the N∶P ratios were positively correlated with the MAT. It is highly possible this region is not N limited, it is P limited, thus the temperature-biogeochemical hypothesis (TBH) can not be used to explain the relationship between plant N concentrations and MAT in alpine steppe. The results were valuable to understand the bio-geographic patterns of root and leaf nutrients traits and modeling ecosystem nutrient cycling in cold and dry environments.

Plasticity in latitudinal patterns of leaf N and P of Oryza rufipogon in China

Characterising the adaptability in nature of plant stoichiometric patterns across geographic or environmental gradients is important in advancing our understanding of the organisation of plant-nutrient relationships. We examined correlations between plant nutrient traits, latitude, longitude, climate and soil variables in 34 populations of Oryza rufipogon across its range. We further compared the responses of population transplants at two experimental gardens: one beyond its northern natural range and another near the southern limit, to assess the nature of geographic variation in plant nutrients. The study showed that leaf P of O. rufipogon in the field was negatively correlated with latitude and largely depended on temperature and soil P availability. Leaf N was not related to latitude but was significantly correlated with precipitation and soil N concentration. Leaf N:P ratio was largely determined by absorption efficiency of P. Transplantation revealed that there were no significant associations of leaf nutrients with geographic, climatic or soil variables of origin in either of the experimental gardens, indicating phenotypic plasticity. However, examination of relationships between response ratios of leaf nutrients and change ratio of climate and soil environments, as well as norms of reaction in the transplantation experiment, revealed more complexity, suggesting both substantial genotypic diversity and the existence of genotype × environment interactions in these populations of O. rufipogon. These data indicate that adaptive plasticity response of plants to temperature and soil P availability significantly explain the observed shifts in leaf N, P and N:P of O. rufipogon along latitudinal gradients.

The altitudinal patterns of leaf C∶N∶P stoichiometry are regulated by plant growth form, climate and soil on Changbai Mountain, China

Understanding the geographic patterns and potential drivers of leaf stoichiometry is critical for modelling the nutrient fluxes of ecosystems and to predict the responses of ecosystems to global changes. This study aimed to explore the altitudinal patterns and potential drivers of leaf C∶N∶P stoichiometry. We measured the concentrations of leaf C, N and P in 175 plant species as well as soil nutrient concentrations along an altitudinal transect (500-2300 m) on the northern slope of Changbai Mountain, China to explore the response of leaf C∶N∶P stoichiometry to plant growth form (PGF), climate and soil. Leaf C, N, P and C∶N∶P ratios showed significant altitudinal trends. In general, leaf C and C∶N∶P ratios increased while leaf N and P decreased with elevation. Woody and herbaceous species showed different responses to altitudinal gradients. Trees had the largest variation in leaf C, C∶N and C∶P ratios, while herbs showed the largest variation in leaf N, P and N∶P ratio. PGF, climate and soil jointly regulated leaf stoichiometry, explaining 17.6% to 52.1% of the variation in the six leaf stoichiometric traits. PGF was more important in explaining leaf stoichiometry variation than soil and climate. Our findings will help to elucidate the altitudinal patterns of leaf stoichiometry and to model ecosystem nutrient cycling.

Alpine climate alters the relationships between leaf and root morphological traits but not chemical traits

Leaves and fine roots are among the most important and dynamic components of terrestrial ecosystems. To what extent plants synchronize their resource capture strategies above- and belowground remains uncertain. Existing results of trait relationships between leaf and root showed great inconsistency, which may be partly due to the differences in abiotic environmental conditions such as climate and soil. Moreover, there is currently little evidence on whether and how the stringent environments of high-altitude alpine ecosystems alter the coordination between above- and belowground. Here we measured six sets of analogous traits for both leaves and fine roots of 139 species collected from Tibetan alpine grassland and Mongolian temperate grassland. N, P and N:P ratio of leaves and fine roots were positively correlated, independent of biogeographic regions, phylogenetic affiliation or climate. In contrast, leaves and fine roots seem to regulate morphological traits more independently. The specific leaf area (SLA)-specific root length (SRL) correlation shifted from negative at sites under low temperature to positive at warmer sites. The cold climate of alpine regions may impose different constraints on shoots and roots, selecting simultaneously for high SLA leaves for rapid C assimilation during the short growing season, but low SRL roots with high physical robustness to withstand soil freezing. In addition, there might be more community heterogeneity in cold soils, resulting in multidirectional strategies of root in resource acquisition. Thus our results demonstrated that alpine climate alters the relationships between leaf and root morphological but not chemical traits.

UV radiation is the primary factor driving the variation in leaf phenolics across Chinese grasslands

Due to the role leaf phenolics in defending against ultraviolet B (UVB) under previously controlled conditions, we hypothesize that ultraviolet radiation (UVR) could be a primary factor driving the variation in leaf phenolics in plants over a large geographic scale. We measured leaf total phenolics, ultraviolet-absorbing compounds (UVAC), and corresponding leaf N, P, and specific leaf area (SLA) in 151 common species. These species were from 84 sites across the Tibetan Plateau and Inner Mongolian grasslands of China with contrasting UVR (354 vs. 161 mW/cm(2) on average). Overall, leaf phenolics and UVAC were all significantly higher on the Tibetan Plateau than in the Inner Mongolian grasslands, independent of phylogenetic relationships between species. Regression analyses showed that the variation in leaf phenolics was strongly affected by climatic factors, particularly UVR, and soil attributes across all sites. Structural equation modeling (SEM) identified the primary role of UVR in determining leaf phenolic concentrations, after accounting for colinearities with altitude, climatic, and edaphic factors. In addition, phenolics correlated positively with UVAC and SLA, and negatively with leaf N and N: P. These relationships were steeper in the lower-elevation Inner Mongolian than on the Tibetan Plateau grasslands. Our data support that the variation in leaf phenolics is controlled mainly by UV radiation, implying high leaf phenolics facilitates the adaptation of plants to strong irradiation via its UV-screening and/or antioxidation functions, particularly on the Tibetan Plateau. Importantly, our results also suggest that leaf phenolics may influence on vegetation attributes and indirectly affect ecosystem processes by covarying with leaf functional traits.

No globally consistent effect of ectomycorrhizal status on foliar traits

The concept that ectomycorrhizal plants have a particular foliar trait suite characterized by low foliar nutrients and high leaf mass per unit area (LMA) is widely accepted, but whether this trait suite can be generalized to all ectomycorrhizal clades is unclear. We identified 19 evolutionary clades of ectomycorrhizal plants and used a global leaf traits dataset comprising 11,466 samples across c. 3000 species to test whether there were consistent shifts in leaf nutrients or LMA with the evolution of ectomycorrhiza. There were no consistent effects of ectomycorrhizal status on foliar nutrients or LMA in the 17 ectomycorrhizal/non-ectomycorrhizal pairs for which we had sufficient data, with some ectomycorrhizal groups having higher and other groups lower nutrient status than non-ectomycorrhizal contrasts. Controlling for the woodiness of host species did not alter the results. Our findings suggest that the concepts of ectomycorrhizal plant trait suites should be re-examined to ensure that they are broadly reflective of mycorrhizal status across all evolutionary clades, rather than reflecting the traits of a few commonly studied groups, such as the Pinaceae and Fagales.

Nitrogen and water availability interact to affect leaf stoichiometry in a semi-arid grassland

The effects of global change factors on the stoichiometric composition of green and senesced plant tissues are critical determinants of ecosystem feedbacks to anthropogenic-driven global change. So far, little is known about species stoichiometric responses to these changes. We conducted a manipulative field experiment with nitrogen (N; 17.5 g m(-2) year(-1)) and water addition (180 mm per growing season) in a temperate steppe of northern China that is potentially highly vulnerable to global change. A unique and important outcome of our study is that water availability modulated plant nutritional and stoichiometric responses to increased N availability. N addition significantly reduced C:N ratios and increased N:P ratios but only under ambient water conditions. Under increased water supply, N addition had no effect on C:N ratios in green and senesced leaves and N:P ratios in senesced leaves, and significantly decreased C:P ratios in both green and senesced leaves and N:P ratios in green leaves. Stoichiometric ratios varied greatly among species. Our results suggest that N and water addition and species identity can affect stoichiometric ratios of both green and senesced tissues through direct and interactive means. Our findings highlight the importance of water availability in modulating stoichiometric responses of plants to potentially increased N availability in semi-arid grasslands.

Drivers of morphological diversity and distribution in the Hawaiian fern flora: trait associations with size, growth form, and environment

PREMISE OF THE STUDY

Hawaii is home to 238 native and 35 alien fern and lycophyte taxa distributed across steep gradients in elevation and resource availability. The fern flora spans a wide range of growth forms, with extraordinary diversity in morphology and plant size. Yet the potential factors underlying this diversity have remained enigmatic.

METHODS

We used a trait database generated from the most recent and comprehensive survey of Hawaiian ferns and lycophytes to test hypotheses of size-scaling and trait associations with environment and growth form as factors underlying this diversity. We also tested relationships among morphology, taxon abundance and distribution and identified key differences between native and alien taxa.

KEY RESULTS

Strong trait-trait relationships included geometric scaling of plant dimensions with a tendency for more divided fronds in larger ferns. Trait-environment relationships independent of size included more divided fronds at higher elevation, longer blades in shaded habitats, and fronds with shorter stipes and fewer pinnae in drier habitats. Growth forms differed in mean size with epiphytic and epipetric taxa smaller than terrestrial ferns. Plant size was independent of taxon abundance and distribution across islands, and native and alien ferns did not differ in mean size. Alien taxa were more abundant, especially at lower elevations, apparently due to human land use.

CONCLUSIONS

These relationships point to linkages of fern form and demography with biogeography and highlight potential flora-scale physiological and morphological adaptations in ferns across contrasting environments.

Taxonomic identity, phylogeny, climate and soil fertility as drivers of leaf traits across Chinese grassland biomes

Although broad-scale inter-specific patterns of leaf traits are influenced by climate, soil, and taxonomic identity, integrated assessments of these drivers remain rare. Here, we quantify these drivers in a field study of 171 plant species in 174 sites across Chinese grasslands, including the Tibetan Plateau, Inner Mongolia, and Xinjiang. General linear models were used to partition leaf trait variation. Of the total variation in leaf traits, on average 27% is due to taxonomic or phylogenetic differences among species within sites (pure species effect), 29% to variation among sites within species (pure site effect), 38% to joint effects of taxonomic and environmental factors (shared effect), and 6.2% to within-site and within-species variation. Examining the pure site effect, climate explained 7.8%, soil explained 7.4%, and climate and soil variables together accounted for 11%, leaving 18% of the inter-site variation due to factors other than climate or soil. The results do not support the hypothesis that soil fertility is the "missing link" to explain leaf trait variation unexplained by climatic factors. Climate- and soil-induced leaf adaptations occur mostly among species, and leaf traits vary little within species in Chinese grassland plants, despite strongly varying climate and soil conditions.