Short-Term Nitrogen Addition Does Not Significantly Alter the Effects of Seasonal Drought on Leaf Functional Traits in Machilus pauhoi Kanehira Seedlings

Short-term nitrogen (N) addition did not significantly alter the effects of seasonal drought on the leaf functional traits in Machilus pauhoi Kanehira seedlings in N-rich subtropical China.  Seasonal drought and N deposition are major drivers of global environmental change that affect plant growth and ecosystem function in subtropical China. However, no consensus has been reached on the interactive effects of these two drivers.  We conducted a full-factorial experiment to analyze the single and combined effects of seasonal drought and short-term N addition on chemical, morphological and physiological traits of M. pauhoi seedlings.  Seasonal drought (40% of soil field capacity) had significant negative effects on the leaf N concentrations (LNC), phosphorus (P) concentrations (LPC), leaf thickness (LT), net photosynthetic rate (A), transpiration rate (E), stomatal conductance (Gs), and predawn leaf water potential (ψPD), and significant positive effects on the carbon:N (C:N) ratio and specific leaf area (SLA). Short-term N addition (50 kg N·hm−2·year−1 and 100 kg N·hm−2·year−1) tended to decrease the C:N ratio and enhance leaf nutrient, growth, and photosynthetic performance because of increased LNC, LPC, LT, leaf area (LA), SLA, A, E, and ψPD; however, it only had significant effects on LT and Gs. No significant interactive effects on leaf traits were detected. Seasonal drought, short-term N addition, and their interactions had significant effects on soil properties. The soil total C (STC), nitrate N (NO3−-N) and soil total N (STN) concentrations were the main factors that affected the leaf traits.  Seasonal drought had a stronger effect on M. pauhoi seedling leaf traits than short-term N deposition, indicating that the interaction between seasonal drought and short-term N deposition may have an additive effecton M. pauhoi seedling growth in N-rich subtropical China.

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title: Decline of Leaf Hydraulic Conductance with Dehydration: Relationship to Leaf Size and Venation Architecture1[W][OA]
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title: Effects of drought and nitrogen addition on photosynthetic characteristics and resource allocation of Abies fabri seedlings in eastern Tibetan Plateau
abstract: Abies fabri (Mast.) Craib is an endemic and dominant species in typical sub-alpine dark coniferous forests distributed in mountainous regions of the eastern Tibetan Plateau, China. We investigated the ecophysiological responses of A. fabri seedlings to short-term artificially-applied drought, nitrogen addition alone, and the combination of these treatments. Drought was created by excluding natural precipitation with an automatically controlled plastic roof that covered the seedlings. Nitrogen fertilization was applied weekly by spraying over seedlings with ammonium nitrate solution. Experiment results showed that drought caused a reduction in photosynthetic nitrogen use efficiency and leaf mass per area. Nitrogen addition enhanced photosynthetic performance by increasing net photosynthetic rate. In the drought plots, nitrogen addition increased net photosynthetic rate and instantaneous water use efficiency. These results showed that applied nitrogen improved plant water use efficiency and N accumulation in plant organs under drought conditions. Especially under drought conditions more N was concentrated into needles by applied nitrogen as compared with other organs. In conclusion, our results indicated that the combination of nitrogen addition and drought may result in positive effects on A. fabri seedlings in the short-term.

title: Nitrogen-addition effects on leaf traits and photosynthetic carbon gain of boreal forest understory shrubs
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title: Effects of precipitation regime and soil nitrogen on leaf traits in seasonally dry tropical forests of the Yucatan Peninsula, Mexico
abstract: Leaf traits are closely associated with nutrient use by plants and can be utilized as a proxy for nutrient cycling processes. However, open questions remain, in particular regarding the variability of leaf traits within and across seasonally dry tropical forests. To address this, we considered six leaf traits (specific area, thickness, dry matter content, N content, P content and natural abundance 15N) of four co-occurring tree species (two that are not associated with N2-fixing bacteria and two that are associated with N2-fixing bacteria) and net N mineralization rates and inorganic N concentrations along a precipitation gradient (537–1036 mm per year) in the Yucatan Peninsula, Mexico. Specifically we sought to test the hypothesis that leaf traits of dominant plant species shift along a precipitation gradient, but are affected by soil N cycling. Although variation among different species within each site explains some leaf trait variation, there is also a high level of variability across sites, suggesting that factors other than precipitation regime more strongly influence leaf traits. Principal component analyses indicated that across sites and tree species, covariation in leaf traits is an indicator of soil N availability. Patterns of natural abundance 15N in foliage and foliage minus soil suggest that variation in precipitation regime drives a shift in plant N acquisition and the openness of the N cycle. Overall, our study shows that both plant species and site are important determinants of leaf traits, and that the leaf trait spectrum is correlated with soil N cycling.

title: Responses of Woody Plant Functional Traits to Nitrogen Addition: A Meta-Analysis of Leaf Economics, Gas Exchange, and Hydraulic Traits
abstract: Atmospheric nitrogen (N) deposition has been found to significantly affect plant growth and physiological performance in terrestrial ecosystems. Many individual studies have investigated how N addition influences plant functional traits, however these investigations have usually been limited to a single species, and thereby do not allow derivation of general patterns or underlying mechanisms. We synthesized data from 56 papers and conducted a meta-analysis to assess the general responses of 15 variables related to leaf economics, gas exchange, and hydraulic traits to N addition among 61 woody plant species, primarily from temperate and subtropical regions. Results showed that under N addition, leaf area index (+10.3%), foliar N content (+7.3%), intrinsic water-use efficiency (+3.1%) and net photosynthetic rate (+16.1%) significantly increased, while specific leaf area, stomatal conductance, and transpiration rate did not change. For plant hydraulics, N addition significantly increased vessel diameter (+7.0%), hydraulic conductance in stems/shoots (+6.7%), and water potential corresponding to 50% loss of hydraulic conductivity (P50, +21.5%; i.e., P50 became less negative), while water potential in leaves (−6.7%) decreased (became more negative). N addition had little effect on vessel density, hydraulic conductance in leaves and roots, or water potential in stems/shoots. N addition had greater effects on gymnosperms than angiosperms and ammonium nitrate fertilization had larger effects than fertilization with urea, and high levels of N addition affected more traits than low levels. Our results demonstrate that N addition has coupled effects on both carbon and water dynamics of woody plants. Increased leaf N, likely fixed in photosynthetic enzymes and pigments leads to higher photosynthesis and water use efficiency, which may increase leaf growth, as reflected in LAI results. These changes appear to have downstream effects on hydraulic function through increases in vessel diameter, which leads to higher hydraulic conductance, but lower water potential and increased vulnerability to embolism. Overall, our results suggest that N addition will shift plant function along a tradeoff between C and hydraulic economies by enhancing C uptake while simultaneously increasing the risk of hydraulic dysfunction.

title: Modulation of leaf economic traits and trait relationships by climate
abstract: Our aim was to quantify climatic influences on key leaf traits and relationships at the global scale. This knowledge provides insight into how plants have adapted to different environmental pressures, and will lead to better calibration of future vegetation‐climate models. Location The data set represents vegetation from 175 sites around the world.  For more than 2500 vascular plant species, we compiled data on leaf mass per area (LMA), leaf life span (LL), nitrogen concentration (N mass ) and photosynthetic capacity (A mass ). Site climate was described with several standard indices. Correlation and regression analyses were used for quantifying relationships between single leaf traits and climate. Standardized major axis (SMA) analyses were used for assessing the effect of climate on bivariate relationships between leaf traits. Principal components analysis (PCA) was used to summarize multidimensional trait variation.  At hotter, drier and higher irradiance sites, (1) mean LMA and leaf N per area were higher; (2) average LL was shorter at a given LMA, or the increase in LL was less for a given increase in LMA (LL‐LMA relationships became less positive); and (3) A mass was lower at a given N mass , or the increase in A mass was less for a given increase in N mass . Considering all traits simultaneously, 18% of variation along the principal multivariate trait axis was explained by climate.

title: Phenotypic plasticity and local adaptation in leaf ecophysiological traits of 13 contrasting cork oak populations under different water availabilities.
abstract: Plants distributed across a wide range of environmental conditions are submitted to differential selective pressures. Long-term selection can lead to the development of adaptations to the local environment, generating ecotypic differentiation. Additionally, plant species can cope with this environmental variability by phenotypic plasticity. In this study, we examine the importance of both processes in coping with environmental heterogeneity in the Mediterranean sclerophyllous cork oak Quercus suber. For this purpose, we measured growth and key functional traits at the leaf level in 9-year-old plants across 2 years of contrasting precipitation (2005 and 2006) in a common garden. Plants were grown from acorns originated from 13 populations spanning a wide range of climates along the distribution range of the species. The traits measured were: leaf size (LS), specific leaf area (SLA), carbon isotope discrimination (Delta(13)C) and leaf nitrogen content per unit mass (N(mass)). Inter-population differences in LS, SLA and Delta(13)C were found. These differences were associated with rainfall and temperature at the sites of origin, suggesting local adaptation in response to diverging climates. Additionally, SLA and LS exhibited positive responses to the increase in annual rainfall. Year effect explained 28% of the total phenotypic variance in LS and 2.7% in SLA. There was a significant genotype x environment interaction for shoot growth and a phenotypic correlation between the difference in shoot growth among years and the annual mean temperature at origin. This suggests that populations originating from warm sites can benefit more from wet conditions than populations from cool sites. Finally, we investigated the relationships between functional traits and aboveground growth by several regression models. Our results showed that plants with lower SLA presented larger aboveground growth in a dry year and plants with larger leaf sizes displayed larger growth rates in both years. Overall, the study supports the adaptive value of SLA and LS for cork oak under a Mediterranean climate and their potentially important role for dealing with varying temperature and rainfall regimes through both local adaptation and phenotypic plasticity.

title: Altered leaf functional traits by nitrogen addition in a nutrient-poor pine plantation: A consequence of decreased phosphorus availability
abstract: This study aimed to determine how specific leaf area (SLA) and leaf dry matter content (LDMC) respond to N addition and understory vegetation removal in a 13-year-old Mongolian pine (Pinus sylvestris var. mongolica) plantation. Traits (SLA, LDMC, individual needle dry weight, N and P concentrations) of different-aged needles and their crown-average values were measured, and their relationships with soil N and P availability were examined. N addition and understory removal reduced soil Olsen-P by 15–91%. At the crown level, N addition significantly reduced foliar P concentration (by 19%) and SLA (by 8%), and elevated N concentration (by 31%), LDMC (by 10%) and individual leaf dry weight (by 14%); understory removal did not have a significant effect on all leaf traits. At the needle age level, traits of the previous year’s needles responded more strongly to N addition and understory removal than the traits of current-year needles, particularly SLA and N concentration. SLA and LDMC correlated more closely with soil Olsen-P than with soil inorganic N, and LDMC correlated more closely with soil Olsen-P than SLA did. These results indicate that aggravated P limitation resulting from N addition and understory removal could constrain Mongolian pine growth through their effects on the leaf traits.

title: Combined effect of elevated CO2 concentration and drought on the photosynthetic apparatus and leaf morphology traits in seedlings of yellow poplar
abstract: To counter the threat of drought, which is expected to increase in the future, we analyzed the combined effects of elevated carbon dioxide (CO2) concentration and drought on the photosynthetic apparatus and morphological traits in seedlings of yellow poplar (Liriodendron tulipifera). The plants were grown for four months in a phytotron under different CO2 concentrations [ambient CO2, 430 ppm, AC) and elevated CO2, 640 ppm, EC] and water treatment [field capacity, FC and 50% of field capacity, FC50]. FC50 was sufficient for inducing a reduction in the photosynthetic rate of L. tulipifera. However, under FC50 combined with EC, we observed an increase in photosynthetic rate with increased photopigment content, photochemistry efficiency, and light harvesting ability. Further, decreased specific leaf area and increased wax coverage in EC suggested that EC contributed to protect chloroplasts and reduce water loss. In conclusion, functional improvements of the photosynthetic apparatus with changes in morphological traits were observed under FC50 combined with EC and EC ameliorated the adverse effect of FC50 on the seedlings of L. tulipifera.

title: Morphological and physiological attributes associated to drought tolerance of Ethiopian durum wheat genotypes under water deficit condition
abstract: The experiment was conducted to assess the differential morpho-physiological response to stimulated water deficit and to determine the relationship between some of these morphological and physiological traits and yield components of eighteen durum wheat genotypes grown in pots under lathhouse condition. Water deficit significantly affected gas exchange and chlorophyll fluorescence parameters. It reduced the net photosynthesis rate (Pn), transpiration rate (E) and stomatal conductance (gs) measured both at anthesis and grain-filling stages. Similarly, the value of initial fluorescence (Fo) was increased while variable fluorescence (Fv), maximum fluorescence (Fm) and optimum quantum yield fluorescence (Fv/Fm) were decreased under water deficit. RWC of the leaves was decreased by 36.7% while SLA increased by 12.6% due to moisture stress relative to the well-watered control. No significant correlations were found between chlorophyll fluorescence parameters and grain yield under water deficit condition. Similarly, no significant correlations were found between leaf gas exchange parameters and grain yield. On the other hand, peduncle length and excursion were positively correlated with grain yield while negatively correlated with drought susceptibility index under water deficit condition. Leaf posture and rolling had also a profound effect on grain yield and other attributes. Erectleaved genotypes had more grain yield, HI, kernel numbers per spikelet and grain-filling rate but had lower kernel weight than droopy leaved. Similarly, genotypes exhibited strong leaf rolling under water deficit condition had more grain yield, kernel numbers per spike and water use efficiency. The genetic variability found for leaf posture, leaf rolling, peduncle length and excursion among the Ethiopian durum wheat genotypes suggests the opportunity for selection superior and adapted genotype in water-limited environments. These can be achieved by integrating these morphological traits as indirect selection in conjunction with other yield components.

title: Impacts of drought and nitrogen addition on Calluna heathlands differ with plant life-history stage
abstract: Climate change and atmospheric deposition of nitrogen (N) affect the biodiversity patterns and functions of ecosystems world-wide. While many single-factor studies have quantified ecosystem responses to single global change drivers, less is known about the interaction effects of these drivers on ecosystem functions. 
Here, we present the results of a three-year field and a two-year glasshouse experiment, in which we assessed responses of Calluna vulgaris heathlands to the single and combined effects of drought events (D) and N fertilization (D: 25% precipitation reduction in the field experiment and 20–50% soil water content reduction in the glasshouse experiment; N fertilization: 35 kg N ha−1 year−1). 
We examined the effects of D and N treatments on growth responses of the dominant dwarf shrub Calluna vulgaris (in terms of biomass production and allocation, tissue δ13C signatures and C:N ratios) in relation to two plant life-history stages and different ‘ecotypes’ (sub-Atlantic vs. subcontinental heathlands). 
Plant responses varied strongly with life-history stage, and the interaction of N and D showed lower effects than would be expected based on additive responses to single factors. While D treatments had no effects on Calluna in the building phase (ca. ten-year-old plants), seedlings (particularly one-year-old plants) were highly susceptible to drought. Differences in response patterns were attributable to the high shoot–root ratios typical of young Calluna plants. These ratios decreased with progressing life history as a result of increasing below-ground biomass investments. Below-ground biomass production and shoot–root ratios differed between plants from the different heathland sites. 
Tissue δ13C signatures decreased and C:N ratios increased with plant age as a result of decreasing evaporative demands (per unit root biomass). N fertilization increases the shoot–root ratios and thereby the drought susceptibility of Calluna plants. 
Synthesis. Our findings suggest that plant responses to global change are difficult to anticipate by means of single-factor studies or by focusing on a single life-history stage. This highlights the need for global change research to include multiple factors and life-history stages when assessing an ecosystem's susceptibility to shifts in environmental conditions.

title: Additive effects of simulated climate changes, elevated CO2, and nitrogen deposition on grassland diversity
abstract: Biodiversity responses to ongoing climate and atmospheric changes will affect both ecosystem processes and the delivery of ecosystem goods and services. Combined effects of co-occurring global changes on diversity, however, are poorly understood. We examined plant diversity responses in a California annual grassland to manipulations of four global environmental changes, singly and in combination: elevated CO2, warming, precipitation, and nitrogen deposition. After 3 years, elevated CO2 and nitrogen deposition each reduced plant diversity, whereas elevated precipitation increased it and warming had no significant effect. Diversity responses to both single and combined global change treatments were driven overwhelmingly by gains and losses of forb species, which make up most of the native plant diversity in California grasslands. Diversity responses across treatments also showed no consistent relationship to net primary production responses, illustrating that the diversity effects of these environmental changes could not be explained simply by changes in productivity. In two- to four-way combinations, simulated global changes did not interact in any of their effects on diversity. Our results show that climate and atmospheric changes can rapidly alter biological diversity, with combined effects that, at least in some settings, are simple, additive combinations of single-factor effects.

title: Nitrogen deposition increases susceptibility to drought - experimental evidence with the perennial grass Molinia caerulea (L.) Moench
abstract: AimsWe investigated the response of the perennial grass Molinia caerulea (L.) Moench to combined effects of fertilization (N, P) and drought events. We hypothesized that N fertilization increases, and drought decreases productivity, but that N addition strengthens negative effects caused by drought.MethodsWithin a full-factorial 2-year greenhouse experiment we measured biomass productivity and allocation, tissue nutrient concentrations and nitrogen allocation patterns using 15N as a tracer. fertilization caused a strong increase in productivity, but effects of drought were almost insignificant. However, we found strongly interrelated, non-additive effects of fertilization and drought, expressed by a strong increase of necrotic tissue. Dead aboveground biomass showed the highest values for N and 15N.ConclusionsAccelerated productivity of aboveground tissue under N fertilization resulted in increased evaporative demands and thus higher drought susceptibility. In addition 15N allocation patterns showed that fertilization-drought treatments disenabled plants’ control of their N allocation. Molinia was unable to withdraw leaf N during the dieback of aboveground tissue. Due to the lack of an adaptive strategy to the combined effects of fertilization and drought, increasing summer drought may weaken the competitive performance of species with traits comparable to those of Molinia in N-fertilized environments.

title: Nitrogen deposition and drought events have non-additive effects on plant growth – Evidence from greenhouse experiments
abstract: Climate change and nitrogen deposition affect biodiversity and ecosystem functioning, but interactive effects of these global change drivers are poorly understood. We analysed single and interactive effects of nitrogen (N) fertilisation and drought on the growth performance of Calluna vulgaris. We measured biomass production and allocation, tissue nutrient (N, phosphorus (P) and carbon (C)) concentrations, N allocation patterns (using 15N tracer) and plant's water status (using δ 13C signatures) as response variables in a 2-year greenhouse experiment. N fertilisation increased biomass production and biomass shoot:root ratios. 15N allocation patterns indicated an increasing aboveground N allocation following N fertilisation. Tissue δ 13C signatures were higher in N-fertilised plants. Plant responses to drought were weak. We found strong antagonistic interaction effects of N fertilisation and drought for biomass production. δ 13C values peaked when N-fertilised plants were subjected to drought, indicating that N fertilisation increased the evaporative demands of Calluna plants, likely due to increased biomass shoot:root ratios, which in turn resulted in higher drought susceptibility. As an important consequence, even slight drought events may weaken the competitiveness of Calluna when interacting with enhanced airborne N loads. Single-factor studies, thus, need to be complemented by multi-factor analyses to assess conceivable impacts of co-occurring global change drivers.

title: Does excess nitrogen supply increase the drought sensitivity of European beech (Fagus sylvatica L.) seedlings?
abstract: Climate change and atmospheric deposition of nitrogen affect biodiversity patterns and functions of forest ecosystems worldwide. Many studies have quantified tree growth responses to single global change drivers, but less is known about the interaction effects of these drivers at the plant and ecosystem level. In the present study, we conducted a full-factorial greenhouse experiment to analyse single and combined effects of nitrogen fertilization (N treatment) and drought (D treatment) on 16 morphological and chemical response variables (including tissue δ13C signatures) of one-year-old Fagus sylvatica seedlings originating from eight different seed families from the Cantabrian Mountains (NW Spain). Drought exerted the strongest effect on response variables, reflected by decreasing biomass production and increasing tissue δ13C signatures. However, D and N treatments interacted for some of the response variables, indicating that N fertilization has the potential to strengthen the negative effects of drought (with both antagonistic and amplifying interactions). For example, combined effects of N and D treatments caused a sevenfold increase of necrotic leaf biomass. We hypothesize that increasing drought sensitivity was mainly attributable to a significant reduction of the root biomass in combined N and D treatments, limiting the plants’ capability to satisfy their water demands. Significant seed family effects and interactions of seed family with N and D treatments across response variables suggest a high within-population genetic variability. In conclusion, our findings indicated a high drought sensitivity of Cantabrian beech populations, but also interaction effects of N and D on growth responses of beech seedlings.

title: Effects of experimentally-enhanced precipitation and nitrogen on resistance, recovery and resilience of a semi-arid grassland after drought
abstract: Resistance, recovery and resilience are three important properties of ecological stability, but they have rarely been studied in semi-arid grasslands under global change. We analyzed data from a field experiment conducted in a native grassland in northern China to explore the effects of experimentally enhanced precipitation and N deposition on both absolute and relative measures of community resistance, recovery and resilience—calculated in terms of community cover—after a natural drought. For both absolute and relative measures, communities with precipitation enhancement showed higher resistance and lower recovery, but no change in resilience compared to communities with ambient precipitation in the semi-arid grassland. The manipulated increase in N deposition had little effect on these community stability metrics except for decreased community resistance. The response patterns of these stability metrics to alterations in precipitation and N are generally consistent at community, functional group and species levels. Contrary to our expectations, structural equation modeling revealed that water-driven community resistance and recovery result mainly from changes in community species asynchrony rather than species diversity in the semi-arid grassland. These findings suggest that changes in precipitation regimes may have significant impacts on the response of water-limited ecosystems to drought stress under global change scenarios.

title: Reduced compensatory effects explain the nitrogen-mediated reduction in stability of an alpine meadow on the Tibetan Plateau.
abstract: Many ecosystems are facing strong perturbations such as nitrogen (N) fertilization, which can greatly alter ecosystem stability via different mechanisms. Understanding such mechanisms is critical for predicting how ecosystems will function in the face of global changes. We examined how 8 yr of N fertilization with different N rates (no N addition or N addition at a low, medium or high rate) and different forms of N (ammonium, nitrate or ammonium nitrate) affected the temporal stability of the aboveground biomass of an alpine meadow on the Tibetan Plateau, and tested four mechanisms (diversity effect, mean-variance scaling, compensatory dynamics and dominance effect) that may alter stability. Compared with the control (no N addition), a high N rate did not affect the diversity effect, the mean-variance scaling or the dominance effect, but significantly decreased compensatory dynamics among species and functional groups, which contributed to the reduction in community stability of the alpine meadow. The form of N did not affect any of the four mechanisms and thus did not affect community stability. A high N rate can change community stability by altering compensatory dynamics, whereas the form of N may not have an effect.

title: Increased nitrogen deposition alleviated the adverse effects of drought stress on Quercus variabilis and Quercus mongolica seedlings
abstract: The plasticity response of Quercus variabilis and Quercus mongolica seedlings to combined nitrogen (N) deposition and drought stress was evaluated, and their performance in natural niche overlaps was predicted. Seedlings in a greenhouse were exposed to four N deposition levels (0, 4, 8, and 20 g N m−2 year−1) and two water levels (80 and 50 % field-water capacity). Plant traits associated with growth, biomass production, leaf physiology, and morphology were determined. Results showed that drought stress inhibited seedling performance, altered leaf morphology, and decreased fluorescence parameters in both species. By contrast increased N supply had beneficial effects on the nutritional status and activity of the PSII complex. The two species showed similar responses to drought stress. Contrary to the effects in Q. mongolica, N deposition promoted leaf N concentration, PSII activity, leaf chlorophyll contents, and final growth of Q. variabilis under well-watered conditions. Thus, Q. variabilis was more sensitive to N deposition than Q. mongolica. However, excessive N supply (20 g N m−2 year−1) did not exert any positive effects on the two species. Among the observed plasticity of the plant traits, plant growth was the most plastic, and leaf morphology was the least plastic. Therefore, drought stress played a primary role at the whole-plant level, but N supply significantly alleviated the adverse effects of drought stress on plant physiology. A critical N deposition load around 20 g N m−2 year−1 may exist for oak seedlings, which may more adversely affect Q. variabilis than Q. mongolica.

title: Interactive effects of altered rainfall and simulated nitrogen deposition on seedling establishment in a global biodiversity hotspot
abstract: Understanding the interactive effects of global change drivers on vegetation is critical for ecosystem management and restoration, particularly in the Mediterranean-climate biodiversity hotspots of the world. Climate change, habitat loss and nitrogen deposition have been identified as the key threats to biodiversity loss in these regions, yet their combined effects are poorly understood. We measured the interactive effects of rainfall manipulation (reduction, control, addition) and nitrogen deposition (N addition, N 1 P addition, and unfertilised) on the establishment of 19 Banksia-woodland species planted at three sites in southwestern Australia. Seedling survival and aboveground biomass was increased with water addition but was not affected by rainfall reduction. N addition alone did not impact seedling survival and growth, but interacted with rainfall manipulation and site in unpredictable ways. Treatment effects were context dependent, which we attributed to historic nutrient enrichment and competitive exotic species that prevented seedling establishment. Plant species (n 5 6) varied greatly in their water-use efficiency and nitrogen-use efficiency responses to the imposed treatments, which underscores the difficulty of generalising results to larger numbers of species. Despite our finding that rainfall manipulation and nutrient addition have complex, and in some cases antagonistic effects on seedling survival and growth in Banksia woodlands, our results suggest that local context (i.e. invasive species, land-use history) will have as much influence on seedling establishment as global changes in climate and nitrogen deposition. We call for more field experiments and predictive models to explore further the importance of ecological context in determining the interactive effects of multiple global change drivers on ecosystems. Finally, to realize effective biodiversity conservation, local management interventions that address invasive species and associated habitat degradation will be as critical in the future as they are now.

title: Climate and soil factors influencing seedling recruitment of plant species used for dryland restoration
abstract: Land degradation affects 10–20 % of drylands globally. Intensive land use and management, large-scale disturbances such as extractive operations, and global climate change, have contributed to degradation of these systems worldwide. Restoring these damaged environments is critical to improving ecosystem services and functions, conserve biodiversity, and contribute to climate resilience, food security, and landscape sustainability. Here, we present a case study on plant species of the mining intensive semi-arid Pilbara region in Western Australia that examines the effects of climate and soil factors on the restoration of drylands. We analysed the effects of a range of rainfall and temperature scenarios and the use of alternative soil materials on seedling recruitment of key native plant species from this area. Experimental studies were conducted in controlled environment facilities where conditions simulated those found in the Pilbara. Soil from topsoil (T) stockpiles and waste materials (W) from an active mine site were mixed at different proportions (100 % T, 100 % W, and two mixes of topsoil and waste at 50 : 50 and 25 : 75 ratios) and used as growth media. Our results showed that seedling recruitment was highly dependent on soil moisture and emergence was generally higher in the topsoil, which had the highest available water content. In general, responses to the climate scenarios differed significantly among the native species which suggest that future climate scenarios of increasing drought might affect not only seedling recruitment but also diversity and structure of native plant communities. The use of waste materials from mining operations as growth media could be an alternative to the limited topsoil. However, in the early stages of plant establishment successful seedling recruitment can be challenging in the absence of water. These limitations could be overcome by using soil amendments but the cost associated to these solutions at large landscape scales needs to be assessed and proven to be economically feasible.

title: Forest biomass carbon sinks in East Asia, with special reference to the relative contributions of forest expansion and forest growth.
abstract: Forests play an important role in regional and global carbon (C) cycles. With extensive afforestation and reforestation efforts over the last several decades, forests in East Asia have largely expanded, but the dynamics of their C stocks have not been fully assessed. We estimated biomass C stocks of the forests in all five East Asian countries (China, Japan, North Korea, South Korea, and Mongolia) between the 1970s and the 2000s, using the biomass expansion factor method and forest inventory data. Forest area and biomass C density in the whole region increased from 179.78 × 10(6) ha and 38.6 Mg C ha(-1) in the 1970s to 196.65 × 10(6) ha and 45.5 Mg C ha(-1) in the 2000s, respectively. The C stock increased from 6.9 Pg C to 8.9 Pg C, with an averaged sequestration rate of 66.9 Tg C yr(-1). Among the five countries, China and Japan were two major contributors to the total region's forest C sink, with respective contributions of 71.1% and 32.9%. In China, the areal expansion of forest land was a larger contributor to C sinks than increased biomass density for all forests (60.0% vs. 40.0%) and for planted forests (58.1% vs. 41.9%), while the latter contributed more than the former for natural forests (87.0% vs. 13.0%). In Japan, increased biomass density dominated the C sink for all (101.5%), planted (91.1%), and natural (123.8%) forests. Forests in South Korea also acted as a C sink, contributing 9.4% of the total region's sink because of increased forest growth (98.6%). Compared to these countries, the reduction in forest land in both North Korea and Mongolia caused a C loss at an average rate of 9.0 Tg C yr(-1), equal to 13.4% of the total region's C sink. Over the last four decades, the biomass C sequestration by East Asia's forests offset 5.8% of its contemporary fossil-fuel CO2 emissions.

title: [Characteristics and adaption of seasonal drought in southern China under the background of global climate change. I. Change characteristics of precipitation resource].
abstract: Based on the 1959-2008 precipitation data from 262 meteorological stations in southern China, this paper analyzed the change characteristics of seasonal precipitation trend coefficient, precipitation variability, and annual and decadal precipitation standardized anomalies in this region. In the study period, there was a great difference among the trend of quarter precipitation. In most parts of the region, the precipitation in spring and autumn presented a decreasing trend but that in summer and winter was in adverse; only in southwest part, a slightly different trend was observed. In the whole region, the probability of spring drought decreased, but that of summer drought, autumn drought, and winter drought increased. Spring drought often occurred in south and southwest parts, summer drought and autumn drought often occurred in south part and the middle, lower reaches of Yangtze River, and winter drought expanded from south part to south part and the middle, lower reaches of Yangtze River. The precipitation in spring and autumn was below the normal level after the 1980s, while that in summer and winter was below the normal level before the 1990s, above the normal level in the 1990s, and below the normal level since the 21st century. The decadal change of the seasonal precipitation standardized anomaly in each part of the region was basically consistent, i. e., decreased in autumn and increased in summer and winter.

title: Phylogeographic pattern suggests a general northeastward dispersal in the distribution of Machilus pauhoi in South China
abstract: Machilus pauhoi Kanehira is an important timber species in China. A provenance trial was recently set up to evaluate the growth performance of trees from different localities, with the aim of designing seed transfer guidelines. Here, we tested twelve nuclear microsatellite markers derived from other species of the Lauraceae family and investigated population genetic structure in M. pauhoi. Both the number of observed alleles per locus (Na) and the polymorphic information content (PIC) significantly decreased against the latitude, but showed an insignificant decrease against the longitude. Heterozygosity (Ho) and gene diversity (h) exhibited a weak correlation with geographic location. Private alleles were present in multiple populations, and a moderate level of population genetic differentiation was detected (Gst = 0.1691). The joint pattern of genetic diversity (Na, PIC, Ho, and h) suggests that general northeastward dispersal led to the current distribution of M. pauhoi. Significant but weak effects of isolation-by-distance (IBD) occurred, implicating the mountain ranges as the major barrier to gene flow. Both STRUCTURE and hierarchical clustering analyses showed three distinct groups of populations related to the physical connectivity among mountain ranges. A priority in designing genetic conservation should be given to the populations at the southwest side of the species’ distribution. This conservation strategy can also be combined with the pattern of adaptive genetic variation from the provenance trial for comprehensive genetic resource management of native M. pauhoi.

title: [Characteristics and adaptation of seasonal drought in southern China under the background of climate change. III. Spatiotemporal characteristics of seasonal drought in southern China based on the percentage of precipitation anomalies].
abstract: To analyze the spatiotemporal characteristics and occurrence regularity of seasonal drought can provide theoretical basis for constituting the countermeasures of drought resistance and drought mitigation under the background of global climate change. Based on the 1959-2008 daily precipitation and atmospheric temperature data collected from the meteorological stations in 15 provinces (municipalities, and autonomous regions) of southern China, and using the percentages of precipitation anomalies (Pa) in the national standard "Meteorological Drought Classification", which were locally modified, the drought indices in southern China in 1959-2008 were calculated, and the spatial distribution characteristics of drought frequency in southern China in each year, each season, and each month, as well as the inter-annual changes of the drought intensity and the proportions of the stations with seasonal drought were analyzed. In the study period, the annual drought risk in southern China was generally low. There existed obvious seasonal differences in the spatial distribution characteristics of the drought. Autumn drought was most frequent and most intensive, mainly occurred in the middle and lower reaches of Yangtze River, South China and in the other major agricultural areas, winter drought was also frequent and intensive, mainly occurred in the west of Southwest China and the South China and other winter crop planting areas, while spring drought and summer drought were relatively less frequent or intensive. Spring drought mostly occurred in the southwest of Southwest China, the south of South China, and Huaibei area etc. , and summer drought mostly occurred in the middle and lower reaches of Yangtze River, southeastern coastal area of Fujian, and northeast of Southwest China. The area with drought frequently occurred showed an obvious monthly fluctuation and space transformation, which was decreased with time from November to next May, increased with time from May to November, in the smallest range from April to June, and in the widest range from November to December. The annual drought area showed a slight decrease while the drought intensity should a slight increase, but the situation differed with season, i. e. , spring drought area slightly decreased and the drought intensity weakened, summer drought area had an obvious decrease and the drought intensity weakened, autumn drought area increased obviously and the drought intensity increased, while winter drought area decreased and the drought intensity weakened.

title: [Short-term effects of nitrogen and water treatments on fine root order morphology of Machilus pauhoi seedlings.]
abstract: A factorial nitrogen and water addition experiment was carried out with one year-old Machilus pauhoi seedlings from Suichuan County, Jiangxi Province, with six treatments being established: two water levels with 80% and 40% of field moisture holding capacity and three nitrogen addition levels of 0, 50, 100 kg N·hm-2. Specific root length, specific root area, average diameter and tissue density of three fine root orders of M. pauhoi were measured to understand the main and interactive effects of short-term nitrogen addition and drought stress on root characters of M. pauhoi seedlings. The results showed that average fine root diameter and specific root length differed significantly among fine root orders. With the increases of root orders, the average root diameter increased, with the maximum being present in the third order (0.97 mm), but specific root length decreased, with the minimum being present in the third order (238.99 cm·g-1). No significant effects of nitrogen addition on specific root surface area, average fine root diameter, specific root length and root tissue density were observed. There were significant effects of water treatments on average fine root diameter, specific root length and root tissue density. Drought stress significantly increased average diameter of the third order fine roots of seedlings and decreased root tissue density of the first and second order fine roots. Specific root length of the third order fine roots in arid environments was significantly lower than that under the normal water supply condition. There was no interactive effects of nitrogen addition and drought stress on fine root morphology of M. pauhoi.

title: Response of Water Use Efficiency to Global Environmental Change Based on Output From Terrestrial Biosphere Models
abstract: Author(s): Zhou, S; Yu, B; Schwalm, CR; Ciais, P; Zhang, Y; Fisher, JB; Michalak, AM; Wang, W; Poulter, B; Huntzinger, DN; Niu, S; Mao, J; Jain, A; Ricciuto, DM; Shi, X; Ito, A; Wei, Y; Huang, Y; Wang, G |  Water use efficiency (WUE), defined as the ratio of gross primary productivity and evapotranspiration at the ecosystem scale, is a critical variable linking the carbon and water cycles. Incorporating a dependency on vapor pressure deficit, apparent underlying WUE (uWUE) provides a better indicator of how terrestrial ecosystems respond to environmental changes than other WUE formulations. Here we used 20th century simulations from four terrestrial biosphere models to develop a novel variance decomposition method. With this method, we attributed variations in apparent uWUE to both the trend and interannual variation of environmental drivers. The secular increase in atmospheric CO2 explained a clear majority of total variation (66 ± 32%: mean ± one standard deviation), followed by positive trends in nitrogen deposition and climate, as well as a negative trend in land use change. In contrast, interannual variation was mostly driven by interannual climate variability. To analyze the mechanism of the CO2 effect, we partitioned the apparent uWUE into the transpiration ratio (transpiration over evapotranspiration) and potential uWUE. The relative increase in potential uWUE parallels that of CO2, but this direct CO2 effect was offset by 20 ± 4% by changes in ecosystem structure, that is, leaf area index for different vegetation types. However, the decrease in transpiration due to stomatal closure with rising CO2 was reduced by 84% by an increase in leaf area index, resulting in small changes in the transpiration ratio. CO2 concentration thus plays a dominant role in driving apparent uWUE variations over time, but its role differs for the two constituent components: potential uWUE and transpiration.

title: New handbook for standardised measurement of plant functional traits worldwide
abstract: Plant functional traits are the features (morphological, physiological, phenological) that represent ecological strategies and determine how plants respond to environmental factors, affect other trophic levels and influence ecosystem properties. Variation in plant functional traits, and trait syndromes, has proven useful for tackling many important ecological questions at a range of scales, giving rise to a demand for standardised ways to measure ecologically meaningful plant traits. This line of research has been among the most fruitful avenues for understanding ecological and evolutionary patterns and processes. It also has the potential both to build a predictive set of local, regional and global relationships between plants and environment and to quantify a wide range of natural and human-driven processes, including changes in biodiversity, the impacts of species invasions, alterations in biogeochemical processes and vegetation–atmosphere interactions. The importance of these topics dictates the urgent need for more and better data, and increases the value of standardised protocols for quantifying trait variation of different species, in particular for traits with power to predict plant- and ecosystem-level processes, and for traits that can be measured relatively easily. Updated and expanded from the widely used previous version, this handbook retains the focus on clearly presented, widely applicable, step-by-step recipes, with a minimum of text on theory, and not only includes updated methods for the traits previously covered, but also introduces many new protocols for further traits. This new handbook has a better balance between whole-plant traits, leaf traits, root and stem traits and regenerative traits, and puts particular emphasis on traits important for predicting species’ effects on key ecosystem properties. We hope this new handbook becomes a standard companion in local and global efforts to learn about the responses and impacts of different plant species with respect to environmental changes in the present, past and future.

title: Effects of Nitrogen Supply on Water Stress and Recovery Mechanisms in Kentucky Bluegrass Plants
abstract: Non-irrigated crops in temperate and irrigated crops in arid regions are exposed to an incessant series of drought stress and re-watering. Hence, quick and efficient recuperation from drought stress may be amongst the key determinants of plant drought adjustment. Efficient nitrogen (N) nutrition has the capability to assuage water stress in crops by sustaining metabolic activities even at reduced tissue water potential. This study was designed to understand the potential of proper nutrition management by studying the morphological and physiological attributes, and assimilation of nitrogen in Kentucky bluegrass under drought stress. In present study, one heterogeneous habitat and four treatments homogenous habitats each with four replications were examined during field trial. Drought stress resulted in a significant reduction in the nitrogen content of both mother and first ramets, maximum radius, above and below ground mass, number of ramets per plot, leaf water contents and water potential and increased the carbon content and the C:N ratio in both homogenous and heterogeneous plots compared to well-watered and nutritional conditions. Observation using electron microscopy showed that drought stress shrunk the vessel diameter, circumference and xylem area, but increased the sieve diameter, and phloem area in the leaf crosscutting structure of Kentucky bluegrass, first, second, and third ramet leaf. Thus, it can be concluded that water stress markedly reduced all the important traits of Kentucky bluegrass, however, proper nutritional management treatment resulted in the best compensatory performance under drought assuaging its adversity up to some extent and may be considered in formulating good feasible and cost-effective practices for the environmental circumstances related to those of this study.

title: Short-term effects of soil warming and nitrogen addition on the N:P stoichiometry of Cunninghamia lanceolata in subtropical regions
abstract: AimsIncreasing temperature and nitrogen (N) deposition are major drivers of global change that will influence plant-soil systems. We aimed to understand how plant stoichiometry and nutrient limiting types could change with continued warming and N inputs in subtropical regions.MethodsIn 2014, the experiments were established in 30 mini-plots (2 × 2 m) with the following treatments: control, high N addition, low N addition, warming, warming + high N addition, and warming + low N addition. We sampled the leaf and root of Cunninghamia lanceolata and soils to assess their elemental and stoichiometric variables and δ15N under all six conditions.ResultsBoth experimental warming and N fertilization consistently induced an increase in fine-root N, P, and N:P. The N:P ratio of the mature green-leaf and soil was 7.24–11.63 and 4.79–6.56, respectively. On average, C. lanceolata showed higher proportional P resorption, but lower N resorption. The δ15N enrichment factor significantly increased in the warming and N addition treatments.ConclusionsN addition decrease leaf N content, and increased the plant growth, which was due to the effect of the N dilution of C. lanceolata. In subtropical regions, N-limitation affects the growth of C. lanceolata, and the concurrent increase in warming and N fertilization should help relieve N-limiting conditions.

title: Size-dependent nutrient limitation of tree growth from subtropical to cold temperate forests
abstract: The traditional paradigm is that plant growth at high latitudes is generally nitrogen (N) limited, whereas phosphorus (P) limitation occurs at low latitudes. However, this latitudinal pattern of nutrient limitation is not empirically tested and the underlying mechanisms are far from clear. Here we performed a coordinated experiment of N and/or P addition at three forest sites in China, a subtropical forest, a warm-temperate forest and a cold-temperate forest. By measuring relative growth rate (RGR) and leaf nutrient traits among different tree size groups, we assessed how they vary with nutrient addition and tree sizes and uncovered the likely mechanisms underlying these observed responses. Our results revealed that P addition enhanced the RGR of small trees (DBH 15cm) was not impacted by any nutrient addition treatment or soil nutrient variations at any site. Leaf P concentration and resorption efficiency in both small and large trees mostly showed a linear response to soil available P at subtropical and warm-temperate sites, while leaf N:P ratio in small trees elevated linearly with soil available N:P ratio at a cold-temperate site. Overall, this study presents robust experimental evidence that growth in small trees, not large trees, is primarily limited by P in subtropical and warm-temperate forests, but is co-limited by N and P in cold-temperate forests. This size-dependent nutrient limitation highlights the importance of considering tree size classes when assessing nutrient limitation in forest. A is available for this article.

title: Patterns in the Distribution of Cladocerans (Crustacea: Branchiopoda) in Lakes Across a North–south Transect in Alaska, USA
abstract: The remains of cladocerans were examined from the surface sediments of 51 freshwater sites along a north–south transect spanning Alaska. We identified 27 cladoceran taxa from the sediments, consisting primarily of littoral chydorid species. Variations in cladoceran assemblages were related to measured physical and chemical variables using multivariate techniques. Redundancy analysis (RDA) indicated that lake depth, total phosphorus (TP), and altitude all had a significant influence in determining the composition of cladoceran assemblages. Cladoceran communities in tundra and forest-tundra lakes, which were relatively shallow and nutrient-poor, had relatively low abundances of pelagic Cladocera, and were primarily composed of several littoral chydorid species. Among pelagic cladoceran species, there was a distinct shift in dominance from the Bosminidae in lakes in the southern boreal forest region to Daphniidae in lakes in the northern boreal forest. Daphnia dominated lakes had significantly higher total phosphorus, specific conductivity, and calcium concentrations than lakes dominated by Eubosmina. Overall, the relative importance of physical and chemical factors in structuring cladocerans is similar to other previously studied regions, and suggests the Cladocera may be useful as ecological and paleoenvironmental indicators in this region.

title: R in Action: Data Analysis and Graphics with R
abstract: R is a powerful language for statistical computing and graphics that can handle virtually any data-crunching task. It runs on all important platforms and provides thousands of useful specialized modules and utilities. This makes R a great way to get meaningful information from mountains of raw data. R in Action, Second Edition is a language tutorial focused on practical problems. Written by a research methodologist, it takes a direct and modular approach to quickly give readers the information they need to produce useful results. Focusing on realistic data analyses and a comprehensive integration of graphics, it follows the steps that real data analysts use to acquire their data, get it into shape, analyze it, and produce meaningful results that they can provide to clients. Purchase of the print book comes with an offer of a free PDF eBook from Manning. Also available is all code from the book.

title: Multi-scale evapotranspiration of summer maize and the controlling meteorological factors in north China
abstract: Evapotranspiration ( ET ) scale effect influences the understanding of water and energy balance processes at different scales. However, there are fewer studies about the relationships between multi-scale ET and the controlling meteorological factors. In this study, the variations in leaf and plant transpiration and farmland ET of summer maize were systematically analyzed using three-successive-year data from an irrigation experiment station in north China, and the meteorological factors affecting ET at different scales were analyzed based on multivariate regression. The main findings include (1) the transpiration rate ( T r ) and stomatal conductance ( g s ) of sun and shade leaves reduced with the reduction of photosynthetically active radiation ( PAR ), and the T r and g s in shade leaves were about half of sun leaves. When the differences in PAR were smaller, leaf T r and g s at lower part of canopy were slightly lower than those at middle and upper parts of canopy. (2) The sap flow rate (plant transpiration) of summer maize was related to the net radiation but lagged 1 h. (3) Diurnal variation in latent heat flux ( λET ) of summer maize was similar to that in net radiation but lagged slightly. λET , sensible heat flux and ground heat flux accounted for 66.9–70.7, 23.4–26.1 and 4.0–9.2% of net radiation over the whole growing period, respectively. The Bowen ratio ranged from 0.33 to 0.37 over the whole growing period, and peaked at around 10:00 a.m. (4) Leaf transpiration and farmland ET can be characterized by net radiation, air temperature, vapor pressure deficit and wind speed, but plant transpiration cannot be well characterized by them, possibly due to the change of stored water in the stalk.

title: Genotypic variation in drought response of silver birch (Betula pendula Roth): leaf and root morphology and carbon partitioning
abstract: This study investigates the drought response of four genotypes of Betula pendula with a focus on leaf and root morphological traits, leaf phenology and carbon partitioning between shoot and root. Potted one-year-old clonal plants of four genotypes from regions with low to high annual rainfall (550–1270 mm year−1) were subjected to drought periods of 12–14 weeks in two subsequent years. Well-watered control plants of the four genotypes differed significantly with respect to total leaf area per plant (LA) and specific leaf area (SLA), whereas differences in total fine root surface area (RA), root specific area (SRA), and the fine root:leaf mass ratio (FR:LM) were not significant. Highest LA and SLA were found in the clone originating from the driest environment. In complementary physiological investigations this clone was found to have the highest water use as well which was interpreted as competitive superiority in terms of water consumption. Drought resulted in an increase in SLA in all genotypes, and a decrease in LA. Leaf area reduction was more pronounced in the genotypes from high than in those from low rainfall origin. The ratio of total root to leaf surfaces remained more or less constant after drought application despite an increase in FR:LM. This is explained by a decrease in SRA resulting from a reduced abundance of very small fine rootlets (diameter <0.2 mm) in the drought-treated plants. The loss in total root surface area due to a reduction in finest root mass was compensated for by a relative increase in total root dry mass per plant. Comparison of results from the first and second drought period indicated a marked influence of timing of drought, root system size, and putative root limitation on plant drought response. We conclude that leaf and root morphology, the total leaf and root surfaces, and the morphological response to drought in birch are to a large extent under genetic control.

title: Effects of drought stress and N supply on the growth, biomass partitioning and water-use efficiency of Sophora davidii seedlings
abstract: A greenhouse experiment was conducted in order to understand the adaptation responses to different water and N conditions, and further explore
if additional N supply could improve the water-use efficiency (WUE) and adaptability of Sophora davidii seedlings under dry conditions. Twomonth-
old seedlings were subjected to a completely random design with three water (80, 40 and 20% water field capacity (FC)) and three N supply
(N0: 0, Nl: 92 and Nh:184 mgNkg−1 soil) regimes. Drought stress dramatically decreased seedlings height, basal diameter, leaf number, leaf area,
root length, and biomass production. An increase in below-ground biomass was observed indicating a higher root/shoot ratio (R/S) under drought
stress conditions, and drought further decreased relative water content (RWC) and WUE. On the other hand, S. davidii seedlings exhibited strong
responses to N supply, but the responses were inconsistent with the various N supply levels. Low N supply (Nl) increased seedlings height, basal
diameter, leaf number, leaf area, and biomass production, but decreased root length. In contrast, high N supply (Nh) decreased or had little effect
on these growth characteristics. N supply increased leaf percentages, but decreased fine root percentages. In addition, Nl rather than the other two
N treatments increased leaf area ratio (LAR), leaf/fine root mass ratio (L/FR), R/S and RWC under severe drought stress (20% FC), even though
these parameters could increase with the Nh treatment under well-watered condition (80% FC). Moreover, Nl also increased WUE under three
water conditions, but Nh had little effect on WUE under drought stress conditions (40% FC and 20% FC). The results suggested that water and N
co-limited the growth of S. davidii seedlings, and the seedlings exhibited great positive responses to Nl in this study. Appropriate or low N supply,
therefore, would be recommended to stimulate growth, enhance WUE, alleviate drought stress, and consequently contribute to S. davidii seedling
establishment under dry condition, but excess N supply should be avoided.

title: Nutritional constraints in terrestrial and freshwater food webs
abstract: Biological and environmental contrasts between aquatic and terrestrial systems have hindered analyses of community and ecosystem structure across Earth's diverse habitats. Ecological stoichiometry provides an integrative approach for such analyses, as all organisms are composed of the same major elements (C, N, P) whose balance affects production, nutrient cycling, and food-web dynamics. Here we show both similarities and differences in the C:N:P ratios of primary producers (autotrophs) and invertebrate primary consumers (herbivores) across habitats. Terrestrial food webs are built on an extremely nutrient-poor autotroph base with C:P and C:N ratios higher than in lake particulate matter, although the N:P ratios are nearly identical. Terrestrial herbivores (insects) and their freshwater counterparts (zooplankton) are nutrient-rich and indistinguishable in C:N:P stoichiometry. In both lakes and terrestrial systems, herbivores should have low growth efficiencies (10–30%) when consuming autotrophs with typical carbon-to-nutrient ratios. These stoichiometric constraints on herbivore growth appear to be qualitatively similar and widespread in both environments.

title: Terrestrial phosphorus limitation: mechanisms, implications, and nitrogen-phosphorus interactions.
abstract: Nutrient limitation to primary productivity and other biological processes is widespread in terrestrial ecosystems, and nitrogen (N) and phosphorus (P) are the most common limiting elements, both individually and in combination. Mechanisms that drive P limitation, and their interactions with the N cycle, have received less attention than mechanisms causing N limitation. We identify and discuss six mechanisms that could drive P limitation in terrestrial ecosystems. The best known of these is depletion-driven limitation, in which accumulated P losses during long-term soil and ecosystem development contribute to what Walker and Syers termed a "terminal steady state" of profound P depletion and limitation. The other mechanisms are soil barriers that prevent access to P; transactional limitation, in which weathering of P-containing minerals does not keep pace with the supply of other resources; low-P parent materials; P sinks; and anthropogenic changes that increase the supply of other resources (often N) relative to P. We distinguish proximate nutrient limitation (which occurs where additions of a nutrient stimulate biological processes, especially productivity) from ultimate nutrient limitation (where additions of a nutrient can transform ecosystems). Of the mechanisms that drive P limitation, we suggest that depletion, soil barriers, and low-P parent material often cause ultimate limitation because they control the ecosystem mass balance of P. Similarly, demand-independent losses and constraints to N fixation can control the ecosystem-level mass balance of N and cause it to be an ultimate limiting nutrient.

title: Tight Coupling Between Shoot Level Foliar N and P, Leaf Area, and Shoot Growth in Arctic Dwarf Shrubs Under Simulated Climate Change
abstract: Nutrient availability limits productivity of arctic ecosystems, and this constraint means that the amount of nitrogen (N) in plant canopies is an exceptionally strong predictor of vegetation productivity. However, climate change is predicted to increase nutrient availability leading to increases in carbon sequestration and shifts in community structure to more productive species. Despite tight coupling of productivity with canopy nutrients at the vegetation scale, it remains unknown how species/shoot level foliar nutrients couple to growth, or how climate change may influence foliar nutrients–productivity relationships to drive changes in ecosystem carbon gain and community structure. We investigated the influence of climate change on arctic plant growth relationships to shoot level foliar N and phosphorus (P) in three dominant subarctic dwarf shrubs using an 18-year warming and nutrient addition experiment. We found a tight coupling between total leaf N and P per shoot, leaf area and shoot extension. Furthermore, a steeper shoot length-leaf N relationship in deciduous species (Vaccinium myrtillus and Vaccinium uliginosum) under warming manipulations suggests a greater capacity for nitrogen to stimulate growth under warmer conditions in these species. This mechanism may help drive the considerable increases in deciduous shrub cover observed already in some arctic regions. Overall, our work provides the first evidence at the shoot level of tight coupling between foliar N and P, leaf area and growth i.e. consistent across species, and provides mechanistic insight into how interspecific differences in alleviation of nutrient limitation will alter community structure and primary productivity in a warmer Arctic.

title: Mesophyll conductance to CO 2 : current knowledge and future prospects
abstract: During photosynthesis, CO2 moves from the atmosphere (Ca) surrounding the leaf to the sub-stomatal internal cavities (Ci) through stomata, and from there to the site of carboxylation inside the chloroplast stroma (Cc) through the leaf mesophyll. The latter CO2 diffusion component is called mesophyll conductance (gm), and can be divided in at least three components, that is, conductance through intercellular air spaces (gias), through cell wall (gw) and through the liquid phase inside cells (gliq). A large body of evidence has accumulated in the past two decades indicating that gm is sufficiently small as to significantly decrease Cc relative to Ci, therefore limiting photosynthesis. Moreover, gm is not constant, and it changes among species and in response to environmental factors. In addition, there is now evidence that gliq and, in some cases, gw, are the main determinants of gm. Mesophyll conductance is very dynamic, changing in response to environmental variables as rapid or even faster than stomatal conductance (i.e. within seconds to minutes). A revision of current knowledge on gm is presented. Firstly, a historical perspective is given, highlighting the founding works and methods, followed by a re-examination of the range of variation of gm among plant species and functional groups, and a revision of the responses of gm to different external (biotic and abiotic) and internal (developmental, structural and metabolic) factors. The possible physiological bases for gm, including aquaporins and carbonic anhydrases, are discussed. Possible ecological implications for variable gm are indicated, and the errors induced by neglecting gm when interpreting photosynthesis and carbon isotope discrimination models are highlighted. Finally, a series of research priorities for the near future are proposed.  AN-Ci curves; aquaporins; carbonic anhydrase; CO2 diffusion conductance; isotope discrimination; leaf internal diffusion conductance; photosynthesis.  THE CONCEPT OF MESOPHYLL CONDUCTANCE TO CO2 AND THE EVOLUTION OF ITS PERCEPTION IN PLANT SCIENCE During photosynthesis, CO2 has to move from the atmosphere surrounding the leaf across a boundary layer in the air above the foliage surface to the sub-stomatal internal cavities through the stomata , and from there to the site of carboxylation inside the stroma through the leaf mesophyll . From Fick’s first law of diffusion, the net photosynthetic flux at steady state (AN) can be expressed as: AN = gs (Ca Ci) = gm (Ci Cc), where gs and gm are the stomatal and mesophyll conductance to CO2 diffusion, and Ca, Ci and Cc are the CO2 concentrations in the atmosphere, in the sub-stomatal internal cavity and in the chloroplast stroma, respectively (Long & Bernacchi 2003). While this two-dimensional view of CO2 diffusion is mostly valid when referring to the stomata, it is rather simplistic when referring to the mesophyll, and it may be inadequate to describe a system where there are CO2 sinks and sources distributed along the pathway (Parkhurst 1994). However, although there have been attempts to develop three-dimensional models for CO2 mesophyll diffusion (Parkhurst 1994; Aalto & Juurola 2002; Juurola et al. 2005), at present, they are far from being of general use. Nevertheless, as pointed out by Parkhurst (1994), ‘representing internal diffusion limitations with single numbers seems preferable to neglecting them completely’. Therefore, the present review intends to update the current knowledge on mesophyll conductance from a two-dimensional perspective, considering only vertical CO2 diffusion through leaf mesophyll. Lateral CO2 diffusion inside the mesophyll can also be important for photosynthesis in some cases (Morison et al. 2005; Pieruschka, Schurr & Jahnke 2005), although this aspect has already been reviewed recently (Morison & Lawson 2007). In Gaastra’s (1959) pioneer work on leaf photosynthesis, mesophyll conductance (and its inverse, mesophyll resistance) was defined essentially as a diffusion component of the photosynthesis pathway, and it was considered to be an important factor in determining leaf photosynthesis (actually more important than stomatal conductance). Later, Correspondence: J. Flexas.  Cell and Environment (2008) 31, 602–621 doi: 10.1111/j.1365-3040.2007.01757.x  602 Troughton & Slatyer (1969) extended the use of the term to a ‘mixed’ diffusion-biochemical component, that is, to refer to the initial slope of AN versus Ci relationship. Following Gaastra’s (1959) assumption that CO2 concentration inside the chloroplast was near zero (i.e. at the CO2 compensation concentration), Jones & Slatyer (1972b) proposed a method to separate the transport and carboxylation components of what continued to be called mesophyll or intracellular conductance. Using this method, earlier conclusions by Gaastra that mesophyll transport resistance was a limiting factor for photosynthesis were confirmed (Jones & Slatyer 1972a,b; Samsuddin & Impens 1979), and it was suggested, for the first time, that the leaf internal resistance to CO2 transfer could be variable and could respond, for instance, to water stress (Jones 1973). The nomenclature was further complicated by Samsuddin & Impens (1979), who termed internal resistance to the ‘mixed’ diffusion-biochemical term, while restoring the use of the term mesophyll resistance to refer to the transfer component alone. Such a semantic confusion continues nowadays, and several more recent papers still call ‘mesophyll conductance’ or ‘internal conductance’ to the initial slope of AN versus Ci relationship (e.g. Moldau & Kull 1993; Laisk & Loreto 1996; Eichelmann et al. 2004a,b; Béjaoui et al. 2006). We propose, as already did Farquhar & Sharkey (1982) and Parkhurst (1994), to restrict the term mesophyll conductance to the diffusion of CO2 through leaf mesophyll, both through intercellular air spaces (gias), cell wall (gw) and the intracellular liquid pathway (gliq). That is, mesophyll conductance should be viewed as synonymous of ‘leaf internal diffusion conductance’, while terms such as ‘apparent carboxylation efficiency’ or ‘apparent carboxylation conductance’ may be more adequate to refer to the initial slope of the AN versus Ci relationship. With the introduction of the most commonly used leaf photosynthesis model by Farquhar, von Caemmerer & Berry (1980), the early assumption that CO2 concentration in the chloroplasts was close to zero or to the compensation point was rejected, as was later being confirmed by direct measurements of Ci (Lauer & Boyer 1992). Thereafter, most gas exchange studies have usually assumed that gm is large and constant, that is, that Ci = Cc. However, different studies in the late 80s and early 90s have already suggested that Cc is significantly less than Ci, although not close to zero. Evans et al. (1986) reached this conclusion by comparing online carbon isotope discrimination of photosynthesizing leaves with the theoretical discrimination that would be expected if Ci = Cc. Evans (1983) and Evans & Terashima (1988) reached the same conclusion by comparing the initial slope of the AN versus Ci curve in wheat and spinach leaves with the activity of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) as determined in vitro. With the advent of pulse-amplitude fluorometers, the comparison of chlorophyll fluorescence with gas exchange measurements also indicated that Cc was lower than Ci, and that their difference increased under conditions of water stress or salinity (Bongi & Loreto 1989; Cornic et al. 1989; Di Marco et al. 1990). The water stress-induced decrease of Cc below Ci was independently demonstrated by measuring leaf O discrimination (Renou et al. 1990), and was lately confirmed several times by both chlorophyll fluorescence and leaf O discrimination (Tourneux & Peltier 1995; Sánchez-Rodríguez, Pérez & Martínez-Carrasco 1999). METHODOLOGIAL ADVANCES IN DETERMINING MESOPHYLL CONDUCTANCE AND ITS COMPONENTS The pioneer studies described in the previous section includes the development of several methods that have become usual to estimate mesophyll conductance to CO2.A detailed description of the principles and procedures of these methods has recently been reviewed by Warren (2006a).Therefore, the present review will only mention the existing methods, providing the original and extended references for readers interested in a more detailed description.

title: Nitrogen Metabolism and Photosynthesis in Leymus chinensis in Response to Long-term Soil Drought
abstract: Key enzyme activities related to nitrogen metabolism, gas-exchange, chlorophyll fluorescence, and lipid peroxidation were determined in Leymus chinensis (Trin.) Tzvel. plants under four soil moisture regimes (control: 75%–80% of field moisture capacity, mild drought: 60%–65%, and moderate drought: 50%–55% as well as severe drought: 35%–40%). Severe drought significantly decreased the key enzyme activities of nitrogen anabolism such as nitrate reductase (NR, EC 1.6.6.1), glutamine synthetase (GS, EC 6.3.1.2), and glutamate dehydrogenase (GDH, EC 1.4.1.2) but increased the key enzyme activities of nitrogen catabolism such as asparaginase (AS, EC 6.3.5.4) and endopeptidase (EP, EC 3.4.24.11), especially after long-term soil drought. Plant biomass, leaf-biomass ratio between the green leaf and total plant biomass, net photosynthetic rate, stomatal conductance, the maximal efficiency of PSII photochemistry, the actual quantum yield, and the photochemical quenching were significantly reduced by severe water stress. Plant malondialdehyde (MDA) concentration increased with the increase in water stress, particularly at the late-growth stage. Our results suggest that the key enzymes of nitrogen metabolism may play an important role in the photosynthetic acclimation of L. chinensis plants to long-term soil drought.

title: Effects of nitrogen addition on growth and photosynthetic characteristics of Acer truncatum seedlings
abstract: Increasing levels of atmospheric nitrogen deposition have greatly affected forest trees. Acer truncatum Bunge has a large distribution in northern China, Korea and Japan and plays an important ecological role in forest ecosystems. We investigated the responses of A. truncatum to a broad range of nitrogen addition regimes with a focus on seedling growth, biomass partitioning, leaf morphology, gas exchange physiology and chlorophyll fluorescence physiology. Moderate nitrogen addition promoted shoot height, stem diameter at ground height, total biomass, size of leaves and chlorophyll fluorescence and gas exchange performance, whereas extreme level of nitrogen addition did not result in such facilitation. Chlorophyll content, pattern of biomass partitioning, ratio of leaf length to width, leaf water content, and specific leaf area did not change among the addition regimes. The critical amount of nitrogen deposition should be defined in the context of a certain time period in a particular region for a certain species at a special developmental stage. The critical amount of N deposition that weakens total biomass facilitation in A. truncatum planted in mixed soil of yellow cinnamon soil and humic soil is approximately 10 g N m−2 y−1 during the first growing season.  Morphology, Carbon assimilation, Chlorophyll fluorescence, Photosynthesis, High

title: Photosynthesis and growth responses of Fraxinus mandshurica Rupr. seedlings to a gradient of simulated nitrogen deposition
abstract: an N-deposition simulation experiment, we showed that low to medium addition of N had beneficial effects on growth and photosynthetic rates ofFraxinus mandshuricaRupr. seedlings, while beyond a threshold of 80 kg N ha−1 year−1, performance plateaued and even declined at higher immissions.ContextTemperate forests are shifting from naturally N-limited toward N-saturated status with increasing N deposition. Yet, our knowledge regarding how seedling growth and physiology respond to excessive N input in temperate tree species remains very limited.AimsThe objective of this study was to examine growth and photosynthetic responses of F. mandshurica seedlings to a gradient of simulated N deposition.MethodsWe conducted a 4-year study to investigate growth and photosynthetic responses of F. mandshurica seedlings to a large gradient of simulated N deposition (0, 20, 40, 60, 80, 100, and 120 kg N ha−1 year−1). Biomass accumulation and allocation, photosynthetic gas exchange, expression, and activities of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) in leaves were determined during the fourth growing season. Soil biochemical properties were measured to link them to the alterations in growth and photosynthetic traits across the N addition gradient.ResultsSeedling growth and photosynthesis were dependent upon the rates of N deposition. The maximum rate of carboxylation (Vc,max) and the net photosynthetic rate under saturating light (Asat) reached a maximum under 60 kg N ha−1 year−1. By contrast, high-level N inputs (100 and 120 kg N ha−1 year−1) resulted in suboptimal values in biomass and photosynthetic activity. Nitrogen deposition also modulated the activity and expression of Rubisco in leaves with a maximum around 80–100 kg N ha−1 year−1. Redundancy analysis (RDA) showed that the changes of seedling growth and photosynthesis along the gradient of N deposition were mostly attributed to the variations of soil pH and total N content.ConclusionOur data suggest that the threshold of N deposition is about 80 kg N ha−1 year−1 for F. mandshurica seedlings in this region. Excessive N input decreased performance on the seedling growth and photosynthesis.

title: Nitrogen Deposition and the Biodiversity of Boreal Forests: Implications for the Nitrogen Critical Load
abstract: The critical load concept is used to establish the deposition levels which ecosystems can tolerate without significant harmful effects. Here we summarize work within the Swedish research program Abatement Strategies for Transboundary Air Pollution (ASTA) assessing the critical load of N for boreal forests. Results from both field experiments in an area with low background N deposition in northern Sweden, and from a large-scale monitoring study, show that important vegetational changes start to take place when adding low N doses and that recovery of the vegetation after ceasing N input is a very slow process. The data presented indicate that changes in key ecosystem components occur even at a lower rate of N input than the present recommended empirical critical load for boreal forest understorey vegetation of 10–15 kg N ha−1 yr−1. Based on the data presented, we suggest that the critical load should be lowered to 6 kg N ha−1 yr−1.

title: Insights into the effects of simulated nitrogen deposition on leaf functional traits of Rhus typhina
abstract: The effects of anthropogenic nitrogen (N) deposition on successful plant invaders, particularly potential effects on their leaf functional traits, have stimulated considerable research interest. This study aims to gain insights into the leaf functional traits of the controversial invader Rhus typhina in the presence of a gradient of simulated N deposition (control, 0 g L–1; low N, 5 g L–1; medium N, 10 g L–1; and high N, 25 g L–1). Soil pH is decreased under the growth of R. typhina. The soil acidifi cation mediated by R. typhina may be due to the positive effects of R. typhina on soil ammonium concentration and negative effects on soil nitrate concentration. Soil pH decreased under N fertilization due to the release of free H+ via the nitrifi cation process. Leaf width, leaf chlorophyll and N concentrations, SLA, and single leaf wet weight of R. typhina increased in the presence of all N fertilizers; medium N and high N fertilization also increased leaf length and leaf thickness of R. typhina due to the fertilizing effects of the addition of exogenous N on R. typhina growth. Thus, R. typhina leaves may possess higher resource capture ability as well as higher relative growth rate by reducing material investment per unit area under exogenous N fertilization. Meanwhile, medium N fertilization exerts stronger fertilizing effects on leaf length, leaf width, leaf chlorophyll and N concentrations, single leaf wet weight, and leaf thickness of R. typhina than those of high N fertilization. This is possibly because excess N fertilization could drive some unexpected reverse phenomena on leaf growth of R. typhina. Thus, leaf growth of R. typhina may be presumably attenuated with increasing amounts of anthropogenic N deposited into ecosystems in the future, and thereby pose pronounced effects on its subsequent further invasion.

title: High N, dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought
abstract: Hotter, longer, and more frequent global change-type drought events may profoundly impact terrestrial ecosystems by triggering widespread vegetation mortality. However, severe drought is only one component of global change, and ecological effects of drought may be compounded by other drivers, such as anthropogenic nitrogen (N) deposition and nonnative plant invasion. Elevated N deposition, for example, may reduce drought tolerance through increased plant productivity, thereby contributing to drought-induced mortality. High N availability also often favors invasive, nonnative plant species, and the loss of woody vegetation due to drought may create a window of opportunity for these invaders. We investigated the effects of multiple levels of simulated N deposition on a Mediterranean-type shrubland plant community in southern California from 2011 to 2016, a period coinciding with an extreme, multiyear drought in the region. We hypothesized that N addition would increase native shrub productivity, but that this would increase susceptibility to drought and result in increased shrub loss over time. We also predicted that N addition would favor nonnatives, especially annual grasses, leading to higher biomass and cover of these species. Consistent with these hypotheses, we found that high N availability increased native shrub canopy loss and mortality, likely due to the higher productivity and leaf area and reduced water-use efficiency we observed in shrubs subject to N addition. As native shrub cover declined, we also observed a concomitant increase in cover and biomass of nonnative annuals, particularly under high levels of experimental N deposition. Together, these results suggest that the impacts of extended drought on shrubland ecosystems may be more severe under elevated N deposition, potentially contributing to the widespread loss of native woody species and vegetation-type conversion.

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

title: Land cover change effects on soil chemical and biological properties after planting Mongolian pine (Pinus sylvestris var. mongolica) in sandy lands in Keerqin, northeastern China
abstract: We compared soil moisture content, pH, total organic carbon (Corg), total nitrogen (TN), total phosphorus (TP) and inorganic N (NH4+–N, NO3−–N) concentrations, soil potential C and N mineralization rates, soil microbial biomass C (Cmic), soil metabolic quotient (qCO2), soil microbial quotient (Cmic/Corg) and soil enzyme (urease and invertase) activities in semiarid sandy soils under three types of land cover: grassland, Mongolian pine (Pinus sylvestris var. mongolica) plantation, and elm (Ulmus punila)–grass savanna in southeastern Keerqin, in northeast China. Soil Corg, TN and TP concentrations (0–10, 10–20, 20–40 and 40–60 cm) were lower while soil C/N and C/P ratios were higher in the plantation than in grassland and savanna. The effects of land cover change on NH4+–N and NO3−–N concentrations, soil potential nitrification and C mineralization rates in the surface soil (0–10 cm) were dependent on sampling season; but soil potential N mineralization rates were not affected by land cover type and sampling season. The effects of land cover change on Cmic and qCO2 of surface soil were not significant; but Cmic/Corg were significantly affected by land cover change and sampling season. We also found that land cover change, sampling season and land cover type × sampling season interaction significantly influenced soil enzyme (urease and invertase) activities. Usually soil enzyme activities were lower in the pine plantations than in grassland and savanna. Our results suggest that land cover change markedly influenced soil chemical and biological properties in sandy soils in the semiarid region, and these effects vary with sampling season.

title: A global study of relationships between leaf traits, climate and soil measures of nutrient fertility
abstract: This first global quantification of the relationship between leaf traits and soil nutrient fertility reflects the trade-off between growth and nutrient conservation. The power of soils versus climate in predicting leaf trait values is assessed in bivariate and multivariate analyses and is compared with the distribution of growth forms (as a discrete classification of vegetation) across gradients of soil fertility and climate. Location All continents except for Antarctica.  Data on specific leaf area (SLA), leaf N concentration (LNC), leaf P concentration (LPC) and leaf N:P were collected for 474 species distributed across 99 sites (809 records), together with abiotic information from each study site. Individual and combined effects of soils and climate on leaf traits were quantified using maximum likelihood methods. Differences in occurrence of growth form across soil fertility and climate were determined by one-way ANOVA.  There was a consistent increase in SLA, LNC and LPC with increasing soil fertility. SLA was related to proxies of N supply, LNC to both soil total N and P and LPC was only related to proxies of P supply. Soil nutrient measures explained more variance in leaf traits among sites than climate in bivariate analysis. Multivariate analysis showed that climate interacted with soil nutrients for SLA and area-based LNC. Mass-based LNC and LPC were determined mostly by soil fertility, but soil P was highly correlated to precipitation. Relationships of leaf traits to soil nutrients were stronger than those of growth form versus soil nutrients. In contrast, climate determined distribution of growth form more strongly than it did leaf traits.  We provide the first global quantification of the trade-off between traits associated with growth and resource conservation ‘strategies’ in relation to soil fertility. Precipitation but not temperature affected this trade-off. Continuous leaf traits might be better predictors of plant responses to nutrient supply than growth form, but growth forms reflect important aspects of plant species distribution with climate.

title: Nutrient resorption helps drive intra-specific coupling of foliar nitrogen and phosphorus under nutrient-enriched conditions
abstract: AimsPlant biomass growth, storage, and decomposition connect nitrogen (N) and phosphorus (P) cycles, yet we know relatively little about the dynamics of such coupling under nutrient enriched conditions, and our understanding of the interactive relationships between plant N and P in drylands remains particularly poor.MethodsIn a semiarid steppe of northern China, we examined the effects of single and combined N and P additions on soil and plant N and P pools for both mature and senesced leaves in two dominant grasses: Leymus chinensis and Stipa grandis.ResultsNitrogen additions increased N concentrations in mature and senesced leaves for each plant species, and decreased N and P resorption during leaf senescence. The effects of N additions on foliar P concentrations were species-specific, while P additions had no effect on any nutrient characteristics examined. Due to treatment effects on N resorption, N and P concentrations were tightly correlated in senesced leaves but not in mature leaves.ConclusionsTaken together, the results suggest plants in this ecosystem are much more responsive to changing N cycles than P cycles and emphasize the significance of nutrient resorption as an important plant control over the stoichiometric coupling of N and P under nutrient enriched conditions.

title: Temporal variability of foliar nutrients: responses to nitrogen deposition and prescribed fire in a temperate steppe
abstract: Plant nutrient concentrations and stoichiometry drive fundamental ecosystem processes, with important implications for primary production, diversity, and ecosystem sustainability. While a range of evidence exists regarding how plant nutrients vary across spatial scales, our understanding of their temporal variation remains less well understood. Nevertheless, we know nutrients regulate plant function across time, and that important temporal controls could strongly interact with environmental change. Here, we report results from a 3-year assessment of inter-annual changes of foliar nitrogen (N) and phosphorus (P) concentrations and stoichiometry in three dominant grasses in response to N deposition and prescribed fire in a temperate steppe of northern China. Foliar N and P concentrations and their ratios varied greatly among years, with this temporal variation strongly related to inter-annual variation in precipitation. Nitrogen deposition significantly increased foliar N concentrations and N:P ratios in all species, while fire significantly altered foliar N and P concentrations but had no significant impacts on N:P ratios. Generally, N addition enhanced the temporal stability of foliar N and decreased that of foliar P and of N:P ratios. Our results indicate that plant nutrient status and response to environmental change are temporally dynamic and that there are differential effects on the interactions between environmental change drivers and timing for different nutrients. These responses have important implications for consideration of global change effects on plant community structure and function, management strategies, and the modeling of biogeochemical cycles under global change scenarios.

title: Effects of elevated carbon dioxide and nitrogen addition on foliar stoichiometry of nitrogen and phosphorus of five tree species in subtropical model forest ecosystems.
abstract: The effects of elevated carbon dioxide (CO2) and nitrogen (N) addition on foliar N and phosphorus (P) stoichiometry were investigated in five native tree species (four non-N2 fixers and one N2 fixer) in open-top chambers in southern China from 2005 to 2009. The high foliar N:P ratios induced by high foliar N and low foliar P indicate that plants may be more limited by P than by N. The changes in foliar N:P ratios were largely determined by P dynamics rather than N under both elevated CO2 and N addition. Foliar N:P ratios in the non-N2 fixers showed some negative responses to elevated CO2, while N addition reduced foliar N:P ratios in the N2 fixer. The results suggest that N addition would facilitate the N2 fixer rather than the non-N2 fixers to regulate the stoichiometric balance under elevated CO2.

title: Responses of terrestrial ecosystem phosphorus cycling to nitrogen addition: A meta-analysis
abstract: Anthropogenic additions of nitrogen (N) are expected to drive terrestrial ecosystems toward greater phosphorus (P) limitation. However, a comprehensive understanding of how an ecosystem's P cycle responds to external N inputs remains elusive, making model predictions of the anthropogenic P limitation and its impacts largely uncertain. 
 
Location 
Global. 
 
Time period 
1986-2015. 
 
Major taxa studied 
Terrestrial ecosystems. 
 
 
We conducted a meta-analysis including 288 independent study sites from 192 articles to evaluate global patterns and controls of 10 variables associated with ecosystem P cycling under N addition. 
 
 
Overall, N addition increased biomass in plants (+34%) and litter (+15%) as well as plant P content (+17%), while decreasing P concentrations in plants and litter (−8% and −11%, respectively). N addition did not change soil labile P or microbial P, but enhanced phosphatase activity (+24%). The effects of N addition on the litter P pool and soil total P remained unclear due to significant publication biases. The response of P cycling to N addition in tropical forests was different from that in other ecosystem types. N addition did not change plant biomass or phosphatase activity in tropical forests but significantly reduced plant P and soil labile P concentrations. The shift in plant P concentration under N addition was negatively correlated with the N application rate or total N load. N-induced change in soil labile P was strongly regulated by soil pH value at the control sites, with a significant decrease of 14% only in acidic soils (pH < 5.5). 
 
 
Our results suggest that as anthropogenic N enhancement continues in the future it could induce P limitation in terrestrial ecosystems while accelerating P cycling, particularly in tropical forests. A quantitative framework generated on the basis of this meta-analysis is useful for our understanding of ecosystem P cycling with N addition, and for incorporating the anthropogenic P limitation into ecosystem models used to analyse effects of future climate change.

title: Nutrient limitation on terrestrial plant growth--modeling the interaction between nitrogen and phosphorus.
abstract: Growth of plants in terrestrial ecosystems is often limited by the availability of nitrogen (N) or phosphorous (P) Liebig's law of the minimum states that the nutrient in least supply relative to the plant's requirement will limit the plant's growth. An alternative to the law of the minimum is the multiple limitation hypothesis (MLH) which states that plants adjust their growth patterns such that they are limited by several resources simultaneously. We use a simple model of plant growth and nutrient uptake to explore the consequences for the plant's relative growth rate of letting plants invest differentially in N and P uptake. We find a smooth transition between limiting elements, in contrast to the strict transition in Liebig's law of the minimum. At N : P supply ratios where the two elements simultaneously limit growth, an increase in either of the nutrients will increase the growth rate because more resources can be allocated towards the limiting element, as suggested by the multiple limitation hypothesis. However, the further the supply ratio deviates from these supply rates, the more the plants will follow the law of the minimum. Liebig's law of the minimum will in many cases be a useful first-order approximation.

title: Increased N affects P uptake of eight grassland species: the role of root surface phosphatase activity
abstract: Increased N deposition may change species composition in grassland communities by shifting them to P limitation. Interspecifi
c diff erences in P uptake traits might be a crucial yet poorly understood factor in determining the N eff ects. To test
the eff ects of increased N supply (relative to P), we conducted two greenhouse fertilization experiments with eight species
from two functional groups (grasses, herbs), including those common in P and N limited grasslands. We investigated plant
growth and P uptake from two P sources, orthophosphate and not-readily available P (bound-P), under diff erent N supply
levels. Furthermore, to test if the N eff ects on P uptake was due to N availability alone or altered N:P ratio, we examined
several uptake traits (root-surface phosphatase activity, specifi c root length (SRL), root mass ratio (RMR)) under varying
N:P supply ratios. Only a few species ( M. caerulea , A. capillaris , S. pratensis ) could take up a similar amount of P from
bound-P to that from orthophosphate. Th ese species had neither higher SRL, RMR, phosphatase activity per unit root
(Pase root ), nor higher total phosphatase activity ( Pase root times root mass), but higher relative phosphatase activity
( Pase tot divided by biomass) than other species. Th e species common from P-limited grasslands had high Pase rel .
P uptake from bound-P was positively correlated with Pase tot for grasses. High N supply stimulated phosphatase activity
but decreased RMR and SRL, resulting in no increase in P uptake from bound-P. Pase root was infl uenced by N:P supply
ratio, rather than by only N or P level, whereas SRL and RMR was not dominantly infl uenced by N:P ratio. We conclude
that increased N stimulates phosphatase activity via N:P stoichiometry eff ects, which potentially increases plant P uptake
in a species-specifi c way. N deposition, therefore, may alter plant community structure not only by enhancing productivity,
but also by favouring species with traits that enable them to persist better under P limited conditions.

title: EFFECTS OF SOIL NUTRIENT AVAILABILITY ON INVESTMENT IN ACQUISITION OF N AND P IN HAWAIIAN RAIN FORESTS
abstract: We determined the influence of nutrient availability on the mechanisms used by plants to acquire nitrogen and phosphorus from the soil. Extracellular acid phosphatase production, mycorrhizal colonization, and N and P uptake capacities were measured in control, N-, and P-fertilized forests in three sites that varied in nutrient status from N limited to relatively fertile to P limited. Nitrogen fertilization increased extracellular phosphatase activity in all sites. Phosphorus additions consistently reduced phosphatase activity, mycorrhizal colonization, and P uptake capacity across sites. Our results indicate that these plants efficiently allocate resources to nutrient acquisition as suggested by an economic model. Investment in acquisition of a nutrient was greatest when that nutrient was limiting to growth, and plants appeared to allocate excess N to construction of extracellular phosphatases to acquire P. This increase in phosphatase production with N fertilization implies that even P-limited systems might respond to N deposition with greater productivity.

title: Nitrogen inputs accelerate phosphorus cycling rates across a wide variety of terrestrial ecosystems.
abstract: Biologically essential elements--especially nitrogen (N) and phosphorus (P)--constrain plant growth and microbial functioning; however, human activities are drastically altering the magnitude and pattern of such nutrient limitations on land. Here we examine interactions between N and P cycles of P mineralizing enzyme activities (phosphatase enzymes) across a wide variety of terrestrial biomes. • We synthesized results from 34 separate studies and used meta-analysis to evaluate phosphatase activity with N, P, or N×P fertilization. • Our results show that N fertilization enhances phosphatase activity, from the tropics to the extra-tropics, both on plant roots and in bulk soils. By contrast, P fertilization strongly suppresses rates of phosphatase activity. • These results imply that phosphatase enzymes are strongly responsive to changes in local nutrient cycle conditions. We also show that plant phosphatases respond more strongly to fertilization than soil phosphatases. The tight coupling between N and P provides a mechanism for recent observations of N and P co-limitation on land. Moreover, our results suggest that terrestrial plants and microbes can allocate excess N to phosphatase enzymes, thus delaying the onset of single P limitation to plant productivity as can occur via human modifications to the global N cycle.

title: Effects of nitrogen and phosphorus supply on growth and physiological traits of two Larix species
abstract: To investigate how nitrogen (N) and phosphorus (P) fertilization affect plant growth, biomass accumulation and allocation, leaf N and P concentration, and also eco-physiological responses of Larix species with contrasting growth characteristics, slow growing (L. olgensis) and fast growing (L. kaempferi) saplings grown in larch plantation soil were exposed to different N and P regimes. The results showed that N and combined N and P (NP) fertilization promoted growth with a higher biomass accumulation, height growth rate (GRH), net photosynthesis rated (Pn), and photosynthetic P-use efficiency (PPUE) in both species, while P fertilization alone had a positive effect on the growth of L. olgensis but a negative effect on that of L. kaempferi. Moreover, N and NP fertilization increased the photosynthetic N-use efficiency (PNUE) of L. olgensis, while all fertilization treatments decreased that of L. kaempferi. Furthermore, P fertilization increased the N concentration of L. olgensis leaves but had no influence on that of L. kaempferi leaves, while it decreased the NSC concentration of L. olgensis leaves but stimulated the accumulation of starch, fructose and NSC in L. kaempferi leaves. In addition, L. kaempferi with higher GRH showed a greater biomass accumulation, growth, Pn, PNUE, and PPUE when compared to L. olgensis, except for biomass and PPUE under P fertilization and PNUE under N fertilization. These results indicated that the N and P balance rather than the absolute amount of them enhances the growth of L. kaempferi effectively, and P fertilization affects the morphological and physiological traits of the two larch species differently. We propose that the distinct effects of P fertilization on the two larch species were related to different regulation effects of P fertilization on the N concentration, and on the accumulation and transportation of photosynthetic products.

title: Divergent patterns of photosynthetic phosphorus-use efficiency versus nitrogen-use efficiency of tree leaves along nutrient-availability gradients.
abstract: Nitrogen (N) and phosphorus (P) are essential nutrients for photosynthetic carbon assimilation and most frequently limit primary productivity in terrestrial ecosystems. Efficient use of those nutrients is important for plants growing in nutrient-poor environments. 
 
 
 
2.  We investigated the pattern of photosynthetic phosphorus-use efficiency (PPUE) in comparison with photosynthetic nitrogen-use efficiency (PNUE) along gradients of P and N availability across biomes with 340 tree and shrub species. We used both total soil N and P concentration and foliar N/P ratios for indicating nutrient-availability gradients. 
 
 
 
3.  Photosynthetic phosphorus-use efficiency increased with greater leaf mass per area (LMA) toward decreasing P availability. By contrast, PNUE decreased with greater LMA towards decreasing N and P availability. 
 
 
 
4.  The increase in PPUE with decreasing P availability was caused by the net effects of a relatively greater reduction in foliar P concentration and a relatively constant photosynthetic carbon assimilation rate. The decrease in PNUE with decreasing N availability was caused by the effects of a reduction in photosynthetic carbon assimilation rate with greater LMA. 
 
 
 
5.Synthesis. Our results suggest that higher PPUE may be an effective leaf-level adaptation to P-poor soils, especially in tropical tree species. Future research should focus on the difference between PPUE and PNUE in relation to leaf economics, physiology and strategy.

title: Effects of simulated N deposition on foliar nutrient status, N metabolism and photosynthetic capacity of three dominant understory plant species in a mature tropical forest.
abstract: Anthropogenic increase of nitrogen (N) deposition has threatened forest ecosystem health at both regional and global scales. In N-limited ecosystems, atmospheric N input is regarded as an important nutrient source for plant growth. However, it remains an open question on how elevated N deposition affects plant growth in N-rich forest ecosystems. To address this question, we used a simulated N deposition experiment in an N-rich mature tropical forest of southern China, with N addition levels as 0kgNha-1yr-1 (Control), 50kgNha-1yr-1 (Low-N), 100kgNha-1yr-1 (Middle-N) and 150kgNha-1yr-1 (High-N), respectively. We measured foliar nutrient element status (e.g., N, P, K, Ca and Mg), N metabolism and photosynthesis capacity of three dominant understory plant species (Cryptocarya concinna and Cryptocarya chinensis as medium-light species; and Randia canthioides as shade tolerant species) in this forest. Results showed that two years of N addition greatly increased foliar N content, but decreased the content of nutrient cations (e.g., K, Ca and Mg). Nitrogen addition also increased N accumulation as organic forms as soluble protein and/or free amino acid (FAA), but not as chlorophyll in all three species. We further found that the photosynthesis capacity (Pmax) of C. concinna and C. chinensis decreased significantly with elevated N addition, with no effects on R. canthioides. However, photosynthetic nitrogen use efficiency (PNUE) significantly declined with N addition for all three species, with significantly negative relationships between PNUE/Pmax and foliar N content. These findings suggest that excess N inputs can accelerate nutrient imbalance, and inhibit photosynthetic capacity of understory plant species, indicating continuous high N deposition can threat understory plant growth in N-rich tropical forests in the future. Meanwhile, PNUE can be used as a sensitive indicator to assess ecosystem N status under chronic N deposition.

title: Interactive effects of nitrogen deposition and drought-stress on plant-soil feedbacks of Artemisia californica seedlings
abstract: AimsNitrogen (N) deposition and drought are major drivers of global change that will influence plant-soil feedbacks. We investigated how N availability, N-impacted soil communities and drought affect feedback in seedlings of a drought-deciduous mycorrhizal shrub, Artemisia californica.MethodsSeedlings were inoculated with soil from either a high or low deposition site or sterilized inoculum and grown with or without supplemental N and under well-watered or drought-stressed conditions.ResultsInoculum, N and water had interactive effects on feedbacks. Seedlings grown in low deposition inoculum exhibited a neutral to positive feedback under drought and had the highest root to shoot ratios and mycorrhizal colonization. Seedlings inoculated with high N-deposition soil experienced a positive feedback when N fertilized and well-watered, but plants allocated large amounts of biomass to shoots and had a negative response to drought.ConclusionsThe soil community mediates plant response to varying belowground resource availability. We found N-impacted communities may reduce mycorrhizal colonization and allocation to roots and provide less protection against drought. Our results highlight the context dependency of plant-soil feedbacks and the potential for climate change and N deposition to have interactive effects on these relationships.

title: Impacts of altered precipitation, nitrogen deposition and plant competition on a Mediterranean seed bank
abstract: Do increased nitrogen (N) fertilization, altered water supply and increased competition by a nitrophilous plant species alter the establishment of the plant community developed from the soil seed bank present in a semi-arid Mediterranean ecosystem under greenhouse conditions? Does the nitrophilous plant respond in the same way to N addition under field conditions? 
 
 
 
Location 
 
The kermes oak/rosemary shrubland in the Nature Reserve El Regajal-Mar de Ontigola, central Spain. 
 
 
 
Methods 
 
In September 2008, native soil seed bank was collected in open areas dominated by rosemary shrubs. Seeds of the nitrophilous Diplotaxis siifolia were collected in early summer 2008. Pots with aliquots of soil were distributed among eight treatments, which resulted from combinations of two levels of N fertilization, water supply and density of D. siifolia in the seed bank. The plant community was left to grow for 3 mo, when chlorophyll fluorescence measurements and harvests were made. In summer 2008, the soil seed bank density of 0 and 50 kg N·ha−1·yr−1 plots was also manipulated in the field by broadcasting seeds to a density of 8000 D. siifolia seeds·m−2. 
 
 
 
Results 
 
Increased N and competition tended to reduce soil water content in ‘high water’ pots. High water and N fertilization increased total above- and below-ground plant biomass production. Nitrogen fertilization reduced plant root:shoot ratio, had a negative effect on plant richness and, coupled with ‘high competition’ and ‘high water’, reduced forb biomass productivity. The competition treatment decreased chlorophyll fluorescence in grasses. Under field conditions, N fertilization increased D. siifolia density and this effect was dependent on soil phosphorus availability. 
 
 
 
Our results show that nitrophilous plant species such as D. siifolia can be a threat to local plant communities in the context of increased N deposition and altered precipitation events. In particular, local plant communities from semi-arid Mediterranean climates, and especially the forb element, may suffer more from competition in highly polluted and invaded sites during unusually wet years. Finally, we predict a future increase in the relative dominance of these species in semi-arid Mediterranean environments where forb species typically adapted to low-nutrient environments will fail to successfully compete with weeds.

title: Stable Nitrogen and Carbon Pools in Grassland Soils of Variable Texture and Carbon Content
abstract: Nitrogen (N) inputs to many terrestrial ecosystems are increasing, and most of these inputs are sequestered in soil organic matter within 1–3 years. Rapid (minutes to days) immobilization focused previous N retention research on actively cycling plant, microbial, and inorganic N pools. However, most ecosystem N resides in soil organic matter that is not rapidly cycled. This large, stable soil N pool may be an important sink for elevated N inputs. In this study, we measured the capacity of grassland soils to retain 15N in a pool that was not mineralized by microorganisms during 1-year laboratory incubations (called “the stable pool”). We added two levels (2.5 and 50 g N m−2) of 15NH4+ tracer to 60 field plots on coarse- and fine-textured soils along a soil carbon (C) gradient from Texas to Montana, USA. We hypothesized that stable tracer 15N retention and stable bulk soil (native + tracer) N pools would be positively correlated with soil clay and C content and stable soil C pools (C not respired during the incubation). Two growing seasons after the 15N addition, soils (0- to 20-cm depth) contained 71% and 26% of the tracer added to low- and high-N treatments, respectively. In both N treatments, 50% of the tracer retained in soil was stable. Total soil C (r2 = 0.72), stable soil C (r2 = 0.68), and soil clay content (r2 = 0.27) were correlated with stable bulk soil N pools, but not with stable 15N retention. We conclude that on annual time scales, substantial quantities of N are incorporated into stable organic pools that are not readily susceptible to microbial remineralization or subsequent plant uptake, leaching losses, or gaseous losses. Stable N formation may be an important pathway by which rapid soil N immobilization translates into long-term N retention.