Aboveground Net Primary Production Research Articles

Phytolith-Mediated Biocarbon Sequestration

The sequestration of biocarbon primarily arises from natural mechanisms and is acknowledged as a critical measure in mitigating climate change. An inactive form of carbon, known as phytolith occluded carbon, is sequestered within plants and persists in soil for thousands of years following the breakdown of vegetation. Formation, distribution and storage of phytolith occluded carbon are the major factors affecting phytolith-mediated biocarbon sequestration. Crop area, crop duration, aboveground net primary productivity, crop, plant species, plant parts, age of plant, age of plant parts, cell components, phytolith morphotypes, and mean annual precipitation are some of the factors affecting the formation of phytolith occluded carbon. The stability and dissolution of phytolith occluded carbon in the soil are determined by carbon dioxide, temperature, climate zone, topography, soil, and phytolith properties characteristics. The phytolith distribution in the soil is affected by soil type and depth, tillage, bioturbulence, percolation, and phytolith morphotypes. By adopting proper soil and crop management practices phytolith mediated biocarbon sequestration can be enhanced.

Net primary productivity of Betula platyphylla woodland and the response of current-year shoots to vegetation degradation in Mongolian forest steppe

At the southern margin of forest steppe vegetation in Mongolia, Betula platyphylla woodlands occur on north-facing slopes and hill tops, where congeneric shrub Betula fusca often coexists. Woodland degradation results in B. fusca scrub with scattered B. platyphylla trees. We studied structure and productivity of a B. platyphylla woodland and the current-year shoot allometry in the Hustai National Park of central Mongolia. For the B. platyphylla woodland examined, stand basal area and estimated aboveground biomass for stems ≥ 3 cm diameter were 3.9 m2 ha–1 and 13.3 Mg dry mass ha–1, respectively. Leaf area index was 0.93 indicating open canopy layer. Aboveground net primary productivity as the sum of coarse wood production and current-year shoot mass amounted to 1.89 Mg dry mass ha–1 year–1, which was high relative to small aboveground biomass. Analysis of current-year shoot allometry demonstrated that B. platyphylla had more xeromorphic shoots in the scrub, with smaller specific leaf area, lower mass ratio of leaf laminae to supporting parts, and smaller lamina mass against sum of petiole basal area, than in the woodland. Our results indicate that current-year shoot allometry characterizes tree response to woodland degradation at the dry margin of forest steppe.

Plasticity response of desert shrubs to intense drought events at different phenophases under the context of climate change

Global climate change has led to the frequent occurrence of intense drought events in mid-latitude desert ecosystems, coupled with uneven rainfall distribution across phenophases within the year. However, the impact of intense drought events at different phenophases on plant growth and drought resistance strategies still lacks clear conclusions. We selected the typical desert semi-shrubs Artemisia ordosica as our research object, and constructed a rain shelter to simulate intense drought events (without rainfall for 30 consecutive days) during the sprouting, vegetative growth, flowering and fruiting stages. Based on this, we analyzed the differential impacts of intense drought events at different phenophases on the phenotypic characteristics (e.g. shrub height, cover, volume, specific leaf area) and functional traits (ANPP accumulation and allocation) of A. ordosica. The experiment employed a randomized complete block design with three replicates for each treatment. The results indicate that: (1) Under intense drought events at different phenophases, all phenotypic characteristic indicators of A. ordosica significantly decreased. (2) Contrary to the significant negative correlation between twig number and twig size in response to rainfall variations, under intense drought conditions, there is a significant positive correlation between the two, indicating a synergistic effect. (3) The impact of intense drought events at different phenophases on the ANPP (aboveground net primary production) accumulation of A. ordosica varied significantly. The degree of impact is as follows: the flowering and fruiting stage > the sprouting stage > the vegetative growth stage. (4) A. ordosica adapted to intense drought by increasing the proportion of reproductive growth and decreasing the proportion of vegetative growth. Our results reveal the phenotypic and functional trait plasticity response mechanisms of A. ordosica to intense droughts at different phenophases, laying a foundation for predicting the impacts of climate change on desert ecosystems.

The stability of aboveground productivity in a semiarid steppe in China is influenced by the plant community structure

The temporal stability of aboveground net primary productivity is generally regulated by plant community structure and species diversity. There are recurring debates on the mechanisms by which environmental changes impact productivity stability in semiarid steppes. Here we revealed that plant community structure, rather than species diversity, regulates productivity stability in a semiarid Steppe. We conducted a decade-long in situ warming and precipitation addition experiment in a semiarid steppe in Inner Mongolia, China, to monitor plant productivity, species asynchrony, species diversity, and stability. We found that Leymus chinensis productivity increased and the Shannon–Wiener decreased under warming conditions, while Stipa krylovii productivity decreased under precipitation addition. Warming reduced the stability of Stipa krylovii and the plant community stability, whereas it did not affect the stability of L. chinensis. These results deepen our understanding of the regulatory role played by community structure and species diversity regarding productivity stability in semiarid steppes.

Decoupled responses of the stability of above‐ and belowground productivity to drought and clipping in an alpine meadow

Abstract Extreme drought and intensification of land use pose substantial threats to ecosystem stability. However, existing studies that assess ecosystem stability focus primarily on the stability of aboveground net primary productivity (ANPP), while the stability of belowground net primary productivity (BNPP) is often not considered. Here, we examined the response patterns and drivers of ANPP stability and BNPP stability in an alpine meadow to drought and clipping (simulating grazing and mowing) by the implementation of five levels of simulated precipitation (1/12, 1/4, 1/2, 3/4 and ambient precipitation) and clipping (unclipped vs. clipped) treatments over 7 years (2016–2022). The results showed that extreme drought decreased ANPP stability, while moderate drought increased BNPP stability and net primary productivity stability. The findings highlight the importance of jointly considering above‐ and belowground processes when evaluating ecosystem stability. Moreover, clipping did not have an impact on ANPP stability but reduced BNPP stability, and clipping differentially affected the stability of ANPP and BNPP in response to extreme drought. Furthermore, ANPP stability was largely determined by species temporal asynchrony rather than richness, whereas dominant species stability strongly contributed to the variation in BNPP stability of this alpine meadow faced with drought and clipping. Synthesis: In general, these results provide experimental evidence for the decoupling of the responses of above‐ and belowground productivity stability to extreme drought and clipping. Our findings indicate that aboveground responses should not be used as the sole indicator of the whole ecosystem consequences of extreme drought and disturbance factors such as grazing or mowing, emphasizing the need to evaluate ecosystem stability from a whole ecosystem perspective.

Plant height bridges hierarchical community responses to nitrogen enrichment

Abstract Nitrogen enrichment impacts ecosystems productivity by altering different hierarchical community processes, ranging from responses of individual species traits to species reordering and ultimately species turnover. Most previous studies have focussed on examining the responses of these processes separately, but little has been done to examine how hierarchical community responses are linked and contribute to the changes in productivity. To fill this knowledge gap, we conducted a 4‐year nitrogen addition field experiment to investigate the hierarchical community responses of above‐ground net primary productivity (ANPP) in an alpine meadow ecosystem. We found that the individual species response contributed more to ANPP changes under nitrogen addition, while species reordering and species turnover contributed little over the 4‐year experiment. In particular, plant height was a key trait to ultimately explain and bridge nitrogen addition effects on ANPP across different community levels. Nitrogen addition promoted plant height and growth of individual species, and reordered functional groups by increasing taller grasses and sedges and decreasing shorter forbs and legumes. Species turnover had a weak effect on ANPP changes due to low biomass of both lost and gained species. Finally, taller persistent plants promoted ANPP more with nitrogen addition. Synthesis. This study provides a new perspective on how ecosystem productivity responds to nitrogen enrichment in a hierarchical manner linked to plant height, which is helpful for elucidating complex community processes and establishing direct links between plant traits and ecosystem function.

Grassland biomass allocation across continents and grazing practices and its response to climate and altitude

Biomass allocation in grasslands is key to understanding plant response to environmental changes and grazing management. Yet, global studies on how this split between above-ground and below-ground biomass varies across continents and grazing practices are notably scarce. We employ a comprehensive field-oriented grassland database to examine differences in total net primary productivity (TNPP), above-ground net primary productivity (ANPP), below-ground net primary productivity (BNPP), and the BNPP to ANPP ratio across continents and between grazed and non-grazed grasslands. Oceania showed the greatest ANPP (916 g·m−2·a−1), while Asia had the lowest (192 g·m−2·a−1). BNPP values were similar among Oceania, Africa, and Asia (∼600 g·m−2·a−1), significantly exceeding those in Europe (289 g·m−2·a−1) and North America (408 g·m−2·a−1). South America, Africa, and Oceania had the highest TNPP (around 1418 g·m−2·a−1 to 1466 g·m−2·a−1), while Europe had the lowest TNPP (344 g·m−2·a−1). The BNPP to ANPP ratio was highest in South America (4.17) and Asia (3.46). Global differences in TNPP and BNPP between grazed and non-grazed grasslands were minimal, but ANPP (169 g·m−2·a−1 and 198 g·m−2·a−1, respectively) and the BNPP to ANPP ratio (4.87 and 2.40, respectively) differed significantly. Across continents and grazing practices, climate had a greater role in regulating grassland biomass allocation than altitude, and warming may have a more detrimental impact on grazed grasslands than non-grazed grasslands. Distinct biomass allocation trends under various continental and grazing conditions exert effects on the economic and ecological functions of grasslands. Our study underscores the need for balanced grassland utilization strategies at a global scale. This involves optimizing grazing in high ANPP grasslands and protecting those with significant BNPP, thereby contributing to a sustainable and ecologically sound use of grasslands in the future.

Net primary productivity and litter decomposition rates in two distinct Amazonian peatlands.

Measurements of net primary productivity (NPP) and litter decomposition from tropical peatlands are severely lacking, limiting our ability to parameterise and validate models of tropical peatland development and thereby make robust predictions of how these systems will respond to future environmental and climatic change. Here, we present total NPP (i.e., above- and below-ground) and decomposition data from two floristically and structurally distinct forested peatland sites within the Pastaza Marañón Foreland Basin, northern Peru, the largest tropical peatland area in Amazonia: (1) a palm (largely Mauritia flexuosa) dominated swamp forest and (2) a hardwood dominated swamp forest (known as 'pole forest', due to the abundance of thin-stemmed trees). Total NPP in the palm forest and hardwood-dominated forest (9.83 ± 1.43 and 7.34 ± 0.84 Mg C ha-1 year-1, respectively) was low compared with values reported for terra firme forest in the region (14.21-15.01 Mg C ha-1 year-1) and for tropical peatlands elsewhere (11.06 and 13.20 Mg C ha-1 year-1). Despite the similar total NPP of the two forest types, there were considerable differences in the distribution of NPP. Fine root NPP was seven times higher in the palm forest (4.56 ± 1.05 Mg C ha-1 year-1) than in the hardwood forest (0.61 ± 0.22 Mg C ha-1 year-1). Above-ground palm NPP, a frequently overlooked component, made large contributions to total NPP in the palm-dominated forest, accounting for 41% (14% in the hardwood-dominated forest). Conversely, Mauritia flexuosa litter decomposition rates were the same in both plots: highest for leaf material, followed by root and then stem material (21%, 77% and 86% of mass remaining after 1 year respectively for both plots). Our results suggest potential differences in these two peatland types' responses to climate and other environmental changes and will assist in future modelling studies of these systems.

Can we identify tipping points of resilience loss in Mediterranean rangelands under increased summer drought?

Mediterranean ecosystems are predicted to undergo longer and more intense summer droughts. The mechanisms underlying the response of herbaceous communities to such drier environments should be investigated to identify the resilience thresholds of Mediterranean rangelands. A 5-year experiment was conducted in deep and shallow soil rangelands of southern France. A rainout shelter for 75 days in summer imposed drier and warmer conditions. Total soil water content was measured monthly to model available daily soil water. Aboveground net primary production (ANPP), forage quality, and the proportion of graminoids in ANPP were measured in spring and autumn. Plant senescence and plant cover were assessed in summer and spring, respectively. The experimental years were among the driest ever recorded at the site. Therefore, manipulated summer droughts were drier than long-term ambient conditions. Interactions between treatment, community type, and experimental year were found for most variables. In shallow soil communities, spring plant cover decreased markedly with time. This legacy effect, driven by summer plant mortality and the loss of perennial graminoids, led to an abrupt loss of resilience when the extreme water stress index exceeded 37 mm 10 day-1, characterized by a reduction of spring plant cover below 50% and a decreased ANPP in rainy years. Conversely, the ANPP of deep soil communities remained unaffected by increased summer drought, although the presence of graminoids increased and forage nutritive value decreased. This study highlights the role of the soil water reserve of Mediterranean plant communities in modulating ecosystem responses to chronically intensified summer drought. Communities on deep soils were resilient, but communities on shallow soils showed a progressive, rapid, and intense degradation associated with a loss of resilience capacity. Notably, indexes of extreme stress were a better indicator of tipping points than indexes of integrated annual stress. Considering the role of soil water availability in other herbaceous ecosystems should improve the ability to predict the resilience of plant communities under climate change.

Heterogeneity promotes resilience in restored prairie: Implications for the environmental heterogeneity hypothesis.

Enhancing resilience in formerly degraded ecosystems is an important goal of restoration ecology. However, evidence for the recovery of resilience and its underlying mechanisms require long-term experiments and comparison with reference ecosystems. We used data from an experimental prairie restoration that featured long-term soil heterogeneity manipulations and data from two long-term experiments located in a comparable remnant (reference) prairie to (1) quantify the recovery of ecosystem functioning (i.e., productivity) relative to remnant prairie, (2) compare the resilience of restored and remnant prairies to a natural drought, and (3) test whether soil heterogeneity enhances resilience of restored prairie. We compared sensitivity and legacy effects between prairie types (remnant and restored) and among four prairie sites that included two remnant prairie sites and prairie restored under homogeneous and heterogeneous soil conditions. We measured sensitivity and resilience as the proportional change in aboveground net primary productivity (ANPP) during and following drought (sensitivity and legacy effects, respectively) relative to average ANPP based on 4 pre-drought years (2014-2017). In nondrought years, total ANPP was similar between remnant and restored prairie, but remnant prairie had higher grass productivity and lower forb productivity compared with restored prairie. These ANPP patterns generally persisted during drought. The sensitivity of total ANPP to drought was similar between restored and remnant prairie, but grasses in the restored prairie were more sensitive to drought. Post-drought legacy effects were more positive in the restored prairie, and we attributed this to the more positive and less variable legacy response of forb ANPP in the restored prairie, especially in the heterogeneous soil treatment. Our results suggest that productivity recovers in restored prairie and exhibits similar sensitivity to drought as in remnant prairie. Furthermore, creating heterogeneity promotes forb productivity and enhances restored prairie resilience to drought.

Thirty years of forest productivity in a mountainous landscape: The Yin and Yang of topography

AbstractWe measured light‐related patterns of primary productivity within a topographically complex Oregon watershed over a 30‐year period. Second‐growth conifer densities were experimentally altered in 1981. Plots receiving at least 3434 MJ m−2 over a 6‐month growing season averaged 40% greater aboveground net primary productivity (ANPP) than those receiving less light (p = 0.000). Unthinned stands potentially built enough LAI to compensate for low light, but risked mortality that exceeded resilience. The two light levels acted as basins of attraction for other physiological and ecological processes, including size–density relationships and limiting foliar nutrients. Initial (1981) LAI and the irradiation step (above or below 3434 MJ m−2) explained 60% of variation in a 30‐year ANPP. Irradiation within each light group did not affect ANPP. At high irradiation, foliar N/Ca and slope steepness (both negative) explained 58% of the variation in residuals from the initial models, while at low irradiation on north, east, and west aspects, 83% of residual variation was explained by foliar Mg (+), understory cover (+), and 30‐year mortality (−). Light use efficiency (LUE) of fully stocked stands correlated with LAI and foliar N/K. Results suggest that understory influence on tree foliar N (+ or −) enhances ANPP by regulating critical nutrient ratios. Mortality reduced or eliminated differences among thinning levels, which did not vary at low light and only between unthinned and heavily thinned at high light. Values associated with relatively open forests (biodiversity, resilience) may be attained without large sacrifice of long‐term carbon sinks. In our study, light interacts with topography to produce nonlinear dynamics in which small changes in irradiation can have large consequences. Reduced sunlight has been suggested as a geoengineering option to combat global warming. Ecological changes out of proportion to lowered irradiation are a distinct possibility, including sharp reductions in terrestrial carbon sinks.

Changes in productivity partitioning induced by precipitation extremes increase inaccuracy of grassland carbon estimation.

The fraction of net primary productivity (NPP) allocated to belowground organs (fBNPP) in grasslands is a critical parameter in global carbon cycle models; moreover, understanding the effect of precipitation changes on this parameter is vital to accurately estimating carbon sequestration in grassland ecosystems. However, how fBNPP responds to temporal precipitation changes along a gradient from extreme drought to extreme wetness, remains unclear, mainly due to the lack of long-term data of belowground net primary productivity (BNPP) and the fact that most precipitation experiments did not have a gradient from extreme drought to extreme wetness. Here, by conducting both a precipitation gradient experiment (100-500 mm) and a long-term observational study (34 years) in the Inner Mongolia grassland, we showed that fBNPP decreased linearly along the precipitation gradient from extreme drought to extreme wetness due to stronger responses in aboveground NPP to drought and wet conditions than those of BNPP. Our further meta-analysis in grasslands worldwide also indicated that fBNPP increased when precipitation decreased, and the vice versa. Such a consistent pattern of fBNPP response suggests that plants increase the belowground allocation with decreasing precipitation, while increase the aboveground allocation with increasing precipitation. Thus, the linearly decreasing response pattern in fBNPP should be incorporated into models that forecast carbon sequestration in grassland ecosystems; failure to do so will lead to underestimation of the carbon stock in drought years and overestimation of the carbon stock in wet years in grasslands.

Responses of Intraspecific and Interspecific Trait Variations to Nitrogen Addition in a Tibetan Alpine Meadow.

A community functional structure may respond to environmental changes such as nitrogen (N) enrichment by altering intraspecific and interspecific trait variations. However, the relative contributions of both components in determining the community response to N enrichment are unclear. In this study, we measured the plant height (H), leaf area (LA), leaf dry matter content (LDMC), and specific leaf area (SLA) based on a nine-year N addition gradient experiment in an alpine meadow on the Tibetan Plateau. We examined the intraspecific and interspecific variations within and among the communities, the responses of traits in terms of community weighted mean (CWM) and non-weighted mean (CM) to N addition, and the effects of these trait variations on aboveground net primary productivity (ANPP). Our results show that N addition increased the interspecific variation in H while decreasing that of LA within the community, whereas it had no significant effects on the intraspecific variations in the four traits within the community. In contrast, N addition significantly increased the intraspecific variation in H and decreased that of LA among the communities. Moreover, the contribution of intraspecific variation was greater than that of the interspecific variation in terms of CWM for all traits, while the opposite contribution was observed in terms of CM, suggesting that the dominant species would have greater resilience while subdominant species would become less resistant to N addition. Further, intraspecific variations of LA and LDMC within the community played an important role in explaining community productivity. Our results highlight the importance of both intraspecific and interspecific variations in mediating functional trait responses to N enrichment, and intraspecific variation within the communities has important implications for community functioning that should be considered to better understand and predict the responses of the alpine grasslands to N enrichment.

Mowing increased community stability in semiarid grasslands more than either fencing or grazing.

A substantial body of empirical evidence suggests that anthropogenic disturbance can affect the structure and function of grassland ecosystems. Despite this, few studies have elucidated the mechanisms through which grazing and mowing, the two most widespread land management practices, affect the stability of natural grassland communities. In this study, we draw upon 9 years of field data from natural grasslands in northern China to investigate the effects of gazing and mowing on community stability, specifically focusing on community aboveground net primary productivity (ANPP) and dominance, which are two major biodiversity mechanisms known to characterize community fluctuations. We found that both grazing and mowing reduced ANPP in comparison to areas enclosed by fencing. Grazing reduced community stability by increasing the likelihood of single-species dominance and decreasing the relative proportion of nondominant species. In contrast, mowing reduced the productivity of the dominant species but increased the productivity of nondominant species. As a consequence, mowing improved the overall community stability by increasing the stability of nondominant species. Our study provides novel insight into understanding of the relationship between community species fluctuation-stability, with implications for ecological research and ecosystem management in natural grasslands.

Secondary production of the central rangeland region of the United States.

Rangelands are the dominant land use across a broad swath of central North America where they span a wide gradient, from <350 to >900 mm, in mean annual precipitation. Substantial efforts have examined temporal and spatial variation in aboveground net primary production (ANPP) to precipitation (PPT) across this gradient. In contrast, net secondary productivity (NSP, e.g., primary consumer production) has not been evaluated analogously. However, livestock production, which is a form of NSP or primary consumer production supported by primary production, is the dominant non-cultivated land use and an integral economic driver in these regions. Here, we used long-term (mean length = 19 years) ANPP and NSP data from six research sites across the Central Great Plains with a history of a conservative stocking to determine resource (i.e., PPT)-productivity relationships, NSP sensitivities to dry-year precipitation, and regional trophic efficiencies (e.g., NSP:ANPP ratio). PPT-ANPP relationships were linear for both temporal (site-based) and spatial (among site) gradients. The spatial PPT-NSP model revealed that PPT mediated a saturating relationship for NSP as sites became more mesic, a finding that contrasts with many plant-based PPT-ANPP relationships. A saturating response to high growing-season precipitation suggests biogeochemical rather than vegetation growth constraints may govern NSP (i.e., large herbivore production). Differential sensitivity in NSP to dry years demonstrated that the primary consumer production response heightened as sites became more xeric. Although sensitivity generally decreased with increasing precipitation as predicted from known PPT-ANPP relationships, evidence suggests that the dominant species' identity and traits influenced secondary production efficiency. Non-native northern mixed-grass prairie was outperformed by native Central Great Plains rangeland in sensitivity to dry years and efficiency in converting ANPP to NSP. A more comprehensive understanding of the mechanisms leading to differences in producer and consumer responses will require multisite experiments to assess biotic and abiotic determinants of multi-trophic level efficiency and sensitivity.

Understanding how forest ecosystem services are affected by conservation practices and differences in elevation: A study in the Arasbaran biosphere reserve, Iran

Studies addressing how different forest management methods affect the ecosystem services provided by forests in different geographical areas are essential. The objective of this study was to investigate how regulating and supporting services are affected by conservation management and differences in elevation. This was achieved by comparing several ecosystem indicators between the protected and unprotected areas.The forests under study were divided into two elevation ranges (1000–1500 and 1500–2000 m above sea level), and 66 sample plots (area 300 m2) were established in each area. Quantitative and qualitative characteristics of the trees were recorded, and soil and litter samples were obtained in all sample plots. Increment cores were also obtained from the main tree species in the area. In addition, regulating services (regulation of CO2, water and soil) and supporting services (nutrient cycling in soil, and above-ground net primary production) were also evaluated. All of the indicators studied in the regulating services section, except annual carbon sequestration by the trees and the soil loss, in the protected were more than in the unprotected area. No significant differences between the two regions were observed regarding annual carbon sequestration and soil regulation indicators. Regarding the supporting services indicators, total nitrogen, assimilable potassium and average trunk volume of the trees were higher in the protected than in the unprotected area; the opposite was true for the total phosphorus index. The results suggest positive effects of conservation management on regulating and supporting services in the protected area of the Arasbaran forests.The findings of the study indicate that conservation management practices have a positive impact on both regulating and supporting services within the protected area of the Arasbaran forests. This suggests that the implementation of conservation measures in this region has resulted in beneficial outcomes for the ecosystem services provided by the forests. The results highlight the importance of protecting and managing forest areas to enhance their ability to regulate CO2, water, and soil, as well as support nutrient cycling and above-ground net primary production.These outcomes highlight the crucial role of conservation efforts in maintaining and enhancing the overall health and functioning of the Arasbaran forests. It indicates that protective management is essential for promoting key ecosystem services. The study also underscores the importance of considering factors such as altitude and habitat conditions when formulating forest management strategies or plans.

Nitrogen addition alleviates the adverse effects of drought on plant productivity in a temperate steppe.

Drought and nitrogen enrichment could profoundly affect the productivity of semiarid ecosystems. However, how ecosystem productivity will respond to different drought scenarios, especially with a concurrent increase in nitrogen availability, is still poorly understood. Using data from a 4-year field experiment conducted in a semiarid temperate steppe, we explored the responses of aboveground net primary productivity (ANPP) to different drought scenarios and nitrogen addition, and the underlying mechanisms linking soil properties, plant species richness, functional diversity (community-weighted means of plant traits, functional dispersion) and phylogenetic diversity (net relatedness index) to ANPP. Our results showed that completely excluding precipitation in June (1-month intense drought) and reducing half the precipitation amount from June to August (season-long chronic drought) both significantly reduced ANPP, with the latter having a more negative impact on ANPP. However, reducing half of the precipitation frequency from June to August (precipitation redistribution) had no significant effect on ANPP. Nitrogen addition increased ANPP irrespective of drought scenarios. ANPP was primarily determined by soil moisture and nitrogen availability by regulating the community-weighted means of plant height, rather than other aspects of plant diversity. Our findings suggest that precipitation amount is more important than precipitation redistribution in influencing the productivity of temperate steppe, and nitrogen supply could alleviate the adverse impacts of drought on grassland productivity. Our study advances the mechanistic understanding of how the temperate grassland responds to drought stress, and implies that management strategies to protect tall species in the community would be beneficial for maintaining the productivity and carbon sequestration of grassland ecosystems under climate drought.

Testing prescribed burning to shift an agronomic grass community to a diverse native plant community

Prescribed burning can be an effective land management tool. Here, we study changes in plant diversity and composition following experimental fire disturbance in microcosm units extracted from a twenty-five-year-old historically reclaimed grassland located at Highland Valley Copper mine in British Columbia (B.C.), Canada. Experimental microcosm units were dominated by agronomic grass species Elymus lanceolatus, Thinopyrum intermedium and Bromus inermis. The disturbance treatment was fire intensity, represented by three levels (light, moderate, and heavy), replicated six times per treatment. Fire intensity was controlled by modifying the weight of dried litter applied to each microcosm unit (50 g,150 g, 200g), along with the time each grass turf was burned (10 s, 15 s, 20 s). One day after the fire treatment was applied, microcosm units were seeded with a native species mix consisting of six grassland species common to southern B.C. to examine effectiveness of plant establishment postburn. Disturbance treatments resulted in higher overall alpha diversity, richness, evenness, and beta diversity. Plant community changes included colonization of seeded native forbs, grasses, and legumes in response to disturbance. Aboveground net primary productivity (ANPP) was net neutral within the light and moderate burning disturbance treatments but resulted in increased ANPP with heavy disturbance. Litter mass reduced plant diversity and ANPP, indicating that litter was a major factor in plant community dynamics. These results suggest disturbance by burning leads to short term positive plant community response towards increasing diversity of semi-arid grasslands, and aids in shifting plant communities to higher diversity composed of an increase in native plant species. Our results also suggest that without active management the gains observed in native species establishment might quickly be out shadowed and restricted by the previously dominant agronomic plant community.

Later-melting rather than thickening of snowpack enhance the productivity and alter the community composition of temperate grassland

Snowpack is closely related to vegetation green-up in water-limited ecosystems, and has effects on growing-season ecosystem processes. However, we know little about how changes in snowpack depth and melting timing affect primary productivity and plant community structure during the growing season. Here, we conducted a four-year snow manipulation experiment exploring how snow addition, snowmelt delay and their combination affect aboveground net primary productivity (ANPP), species diversity, community composition and plant reproductive phenology in seasonally snow-covered temperate grassland in northern China. Snow addition alone increased soil moisture and nutrient availability during early spring, while did not change plant community structure and ANPP. Instead, snowmelt delay alone postponed plant reproductive phenology, and increased ANPP, decreased species diversity and altered species composition. Grasses are more sensitive to changes in snowmelt timing than forbs, and early-flowering forbs showed a higher sensitivity compared to late-flowering forbs. The effect of snowmelt delay on ANPP and species diversity was offset by snow addition, probably because the added snow unnecessarily lengthens the snow-covering duration. The disparate effects of changes in snowpack depth and snowmelt timing necessitate their discrimination for more mechanistic understanding on the effects of snowpack changes on ecosystems. Our study suggests that it is essential to incorporate non-growing-season climate change events (in particular, snowfall and snowpack changes) to comprehensively disclose the effects of climate change on community structure and ecosystem functions.

Grassland degradation amplifies the negative effect of nitrogen enrichment on soil microbial community stability.

Although nitrogen (N) enrichment is known to threaten the temporal stability of aboveground net primary productivity, it remains unclear how it alters that of belowground microbial abundance and whether its impact can be regulated by grassland degradation. Using data from N enrichment experiments at temperate grasslands with no, moderate, severe, and extreme degradation degrees, we quantified the temporal stability of soil microbial abundance (hereafter 'microbial community stability') using the ratio of the mean quantitative PCR to its standard deviation over 4 years. Both bacterial and fungal community stability sharply decreased when N input exceeded 30 g N m-2 year-1 in non-degraded grasslands, whereas a reduction in this threshold occurred in degraded grasslands. Microbial species diversity, species asynchrony, and species associations jointly altered microbial community stability. Interestingly, the linkages between plant and microbial community stability were strengthened in degraded grasslands, suggesting that plants and soil microbes might depend on each other to keep stable communities in harsh environments. Our findings highlighted the importance of grassland degradation in regulating the responses of microbial community stability to N enrichment and provided experimental evidence for understanding the relationships between plant and microbial community stability.