Increase In Aboveground Net Primary Productivity Research Articles

Moderate grazing weakens legacy effects of grazing history on primary productivity and biodiversity in a meadow grassland

Herbivore grazing has long-lasting effects on the structure and functions of grassland ecosystems, even after the grazing pressure has ceased. These lasting effects are known as grazing legacy effects and significantly affect plant productivity and biodiversity, with the magnitude increased with grazing intensity and further led to grassland degradation. Fencing is a common approach used for restoring degraded grasslands by excluding grazing. Moderate grazing representing a controlled level of grazing intensity is believed to be a useful way to restore ecological functions and thus mitigate grazing legacies. However, how fencing and ongoing moderate grazing affect grazing legacy effects remain unclear and rarely tested with field experiments, particularly with different grazing histories. Here, we investigated aboveground net primary productivity (ANPP), species richness, litter, soil organic carbon (SOC), soil dissolved nitrogen (DON) and hyphal length density (HLD) in 2015 (fencing for 1 year) and 2018 (fencing vs moderate grazing for the subsequent 3 years) after 0, 2, 5, 7 and 12 years of free grazing in a meadow steppe to explore how moderate grazing and fencing affect grazing legacy effects with different histories. We observed a decline in ANPP and species richness with increasing grazing histories, and the magnitude of grazing legacy effects increased accordingly. In most grazing histories, moderate grazing had a greater impact than fencing in reducing the grazing legacy effects on ANPP and species richness. However, with a grazing history of 12 years, moderate grazing led to a smaller increase in ANPP compared to fencing. Additionally, the grazing legacy effects on ANPP and species richness weakened with longer fencing duration for excluding grazing. Litter, SOC, DON and HLD exhibited consistent patterns with the dynamics of ANPP and species richness. Our results indicate that grazing legacy effects on ANPP and species richness intensify with prolonged grazing histories, suggesting that grazing history have stronger impacts on grassland ecosystem functions and structure. The study implies that moderate grazing is an effective strategy for restoring degraded grasslands, particularly for species richness and with short-term grazing histories, instead of relying solely on fencing. The changes of soil variables underlie the grazing legacy effects and the interactions with fencing and moderate grazing.

Recovery through proper grazing exclusion promotes the carbon cycle and increases carbon sequestration in semiarid steppe

Grazing exclusion changes soil physical-chemical characteristics, rapidly affects microbial community composition and function, and alters biogeochemical processes, e.g., carbon (C) cycle, over time. However, the temporal dynamics of CO2 emission and CH4 uptake during grassland restoration chronosequences remain poorly understood. We investigated soil CO2 emission and CH4 uptake, the genes related to CO2 and CH4 production and reduction (cbbL, cbbM, chiA, and pmoA), and associated microbial communities under different durations of grazing exclusion (0, 7, 16, 25, and 38 years) to reveal the mechanisms and potential of soil CO2 emission and CH4 uptake in a semi-arid steppe. The results showed that a proper exclusion period could significantly improve soil physical-chemical conditions, vegetation community, and soil C-cycling. The abundance of C-cycling functional genes (cbbL, cbbM, chiA and pmoA), CH4 uptake and CO2 emission rates showed a single-peak pattern with increasing duration of grazing exclusion, peaking at 16 years and then decreasing in the period between 25 and 38 years, indicating that the effect of exclusion weakened when the exclusion period was too long. The changes in C-cycling functional genes and microbial communities are primarily influenced by aboveground net primary productivity (ANPP), and are associated with CO2, CH4, soil water content (SWC), and soil organic carbon (SOC). Structural equation modeling showed that increases in SOC content and pmoA abundance caused by an increase in ANPP accelerated CO2 emission and CH4 uptake rates, respectively. Our results provide valuable insights into the critical role of grazing exclusion in promoting grassland restoration and carbon sequestration, and have potential implications for sustainable land management practices.

Effect of Nitrogen Application on the Sensitivity of Desert Shrub Community Productivity to Precipitation in Central Asia.

Precipitation variability and nitrogen (N) deposition caused by anthropogenic activities could profoundly impact ecosystem productivity and carbon cycling. In desert ecosystems, vegetation is sensitive to changes in precipitation and N deposition. However, the impacts of large changes in precipitation, especially with a concurrent increase in N content, on plant community remain unclear. In this study, we carried out experiments to monitor the impacts of five precipitation levels and two N levels on the plant community function and composition from the Junggar desert in Central Asia during the period 2018–2019. Our results showed that: (1) Aboveground net primary production (ANPP) significantly increased with increasing precipitation, it followed a positive linear model under normal precipitation range, and nonlinear mode under extreme precipitation events; (2) N application led to an increase in ANPP, but did not significantly improve the sensitivity of ANPP to precipitation change; (3) Changes in N content and precipitation, and their impacts on ANPP were mainly driven by plant density. These results provide a theoretical basis for predict the future dynamics of terrestrial vegetation more accurately under climate change and increasing nitrogen deposition.

Tree species identity drives nutrient use efficiency in young mixed‐species plantations, at both high and low water availability

Abstract Previous studies have demonstrated that tree species diversity increases productivity and may enhance nutrient cycling in forests. The effect of mixing tree species on stand‐level nutrient use efficiency (NutUE) has seldom been studied, and even less so in the context of climate change. Here we present the first study examining how diversity effects on NutUE may be modified by growing season water availability (low vs. high), and importantly, during periodic drought or on water‐limited sites. We tested the interaction of water availability and tree species diversity (i.e. species richness and species identity) on NutUE in two young, experimental plantations located in south‐western France (ORPHEE), and northern Ontario, Canada (IDENT‐SSM). We calculated stand‐level NutUE as above‐ground net primary productivity (ANPP) divided by the product of litterfall mass and macronutrient concentrations, of monocultures and mixed tree communities composed of several temperate tree species, with a focus on birch and pine at both sites. We found significant species richness and water availability effects on NutUE, but they were weakly and inconsistently expressed, detected only for specific nutrients, and differed between the two sites. Species identity had much stronger effects on NutUE when examined using the birch–pine plots at both sites. At ORPHEE, nitrogen use efficiency (NUE) and phosphorus use efficiency (PUE) were significantly greater in the birch–pine mixture than in each monoculture. This was due to an increase in ANPP coupled with a decrease in litter nutrient concentrations in the two‐species plot that was driven by the dominance of pine in the mixture. In the comparatively younger, denser plots at IDENT‐SSM, birch was the dominant species that resulted in positive mixing effects on ANPP and litter nutrient concentrations and a neutral effect of mixing on NUE and PUE in the birch–pine mixture. Overall, the effects of mixing did not differ with water availability treatments, suggesting that species composition of mixtures is more important than water availability for stand‐level NutUE in these young forest communities. Read the free Plain Language Summary for this article on the Journal blog.

Summer grazing by three livestock species at moderate intensity enhances primary productivity and drives community divergence in a semi‐arid steppe

AbstractAimsThe general effects of grazing by large herbivores on plant species composition and productivity are well known. Meanwhile, how different livestock species affect community structure is not fully understood. In northern China, livestock species composition is changing rapidly and hence, it is imperative to understand the resulting effects on grassland productivity and composition.LocationXilingol, Inner Mongolia, China.MethodsWe studied how grazing by three major livestock species affects typical steppe dominated by Leymus chinensis, Stipa grandis, Cleistogenes squarrosa and Carex korshinskyi, in a 4‐year field experiment using a random block‐design with four treatments (sheep, goat, cattle, no grazing), under summer grazing at moderate intensity. We calculated above‐ground net primary productivity (ANPP) and a selectivity index (SI) based on biomass to quantify livestock forage preferences.ResultWe found that (i) sheep preferred Leymus chinensis and mixed subordinate species, cattle preferred Stipa grandis, and goat preferred Carex korshinskyi and Cleistogenes squarrosa. (ii) Consistent with these preferences, sheep increased the relative biomass of Stipa grandis, but reduced that of Carex korshinskyi; cattle reduced the relative biomass of Stipa grandis and Carex korshinskyi but increased that of mixed subordinate species; and goats reduced the relative biomass of Cleistogenes squarrosa and Carex korshinskyi but increased that of mixed subordinate species. (iii) Overall, grazing significantly increased ANPP, mainly due to an increase in ANPP of Leymus chinensis and subordinate species, at the cost of Stipa grandis.ConclusionsSummer grazing by different livestock species at moderate intensity can increase ANPP, while livestock forage preferences can drive divergent changes in species composition. Our results also imply that livestock composition can be used as a mean to manage vegetation dynamics.

Plant quality and primary productivity modulate plant biomass responses to the joint effects of grazing and fertilization in a mesic grassland

AbstractQuestions: Human activities are increasing the density of domestic grazers and global nutrient loads, modifying the main determinants of vegetation community dynamics. Grazing (top‐down control) and nutrient availability (bottom‐up control) may interactively modify plant biomass, which is particularly important in grasslands devoted to livestock production. Here, we aim to understand the interactive effects of grazing and fertilization on grassland plant biomass. We hypothesized that the joint effects of nutrient addition and domestic grazing on above‐ground plant biomass are not additive, but they modify each other through changes in ground‐level light, leaf nutritional quality, above‐ground net primary productivity (ANPP), and below‐ground plant allocation.Location: Flooding Pampa (Buenos Aires, Argentina).Methods: We carried out a factorial experiment of grazing exclusion and fertilization with nitrogen, phosphorus, and potassium + micronutrients during ​several years in a mesic grassland devoted to livestock production.Results: After four years, grazing reduced live above‐ground plant biomass by 52%, and when combined with fertilization this reduction was 70%. Nutrient addition in the grazed grassland increased ANPP and leaf nutrient concentration. These changes in turn intensified grazing pressure and cattle’s plant consumption. By contrast, fertilization did not produce any significant effect on plant biomass or ANPP inside the exclosures, where ground‐level light was low. A structural equation model revealed that the increase in ANPP fostered above‐ground and reduced below‐ground plant biomass.Conclusions: This is the first study conducted in the Pampas grasslands that evaluated the effect of cattle grazing and fertilization on plant communities under field conditions over several years. Grazing and nutrient addition synergistically controlled grassland plant biomass, as the reduction in above‐ground biomass by cattle consumption was greater in fertilized plots. Our results provided empirical evidence that leaf nitrogen and ANPP modulated plant biomass dynamics in grasslands devoted to livestock production in the context of increased nutrient loads in terrestrial ecosystems.

Vascular plant species response to warming and elevated carbon dioxide in a boreal peatland

Peatlands store a significant amount of terrestrial organic carbon in plant biomass and soils. The Spruce and Peatland Responses Under Changing Environments (SPRUCE) project is a warming and elevated carbon dioxide (eCO2) experiment designed to test how the carbon sequestration and storage capacity of peatland ecosystems will respond to climate change. Here, we report changes in the vascular plant community that have occurred during the first five years of SPRUCE. We tracked species composition, diversity, and aboveground net primary production (ANPP) in chambers warmed at a wide range of temperatures (+0, +2.25, +4.5, +6.75, +9 °C), and two CO2 levels (~400 [ambient] and 900 parts per million). We observed an increase in aboveground vascular plant biomass accumulation, due primarily to an increase in shrub abundance. Overall species diversity decreased substantially, likely due in part to shading by increases in shrub density. The main driver of change in the vascular plant community was temperature, with minimal effects of CO2 evident. These results indicate an overall increase in ANPP with warming, but highlight the importance of interactions between direct (warming) and indirect (competition) effects in determining how boreal peatlands will respond to climate change.

Plant growth and aboveground production respond differently to late-season deluges in a semi-arid grassland.

Semi-arid ecosystems are strongly water-limited and typically quite responsive to changes in precipitation amount and event size. In the C4-dominated shortgrass steppe of the Central US, previous experiments suggest that large rain events more effectively stimulate plant growth and aboveground net primary production (ANPP) than an equal amount of precipitation from smaller events. Responses to naturally occurring large events have generally been consistent with experimental results, with the exception of large events occurring later in the growing season (e.g., August). These have been reported as less effective at increasing net C uptake, despite temperatures optimal for C4 plant growth. Since atmospheric warming is increasing the frequency of statistically extreme rain events (deluges) throughout the growing season, how late-season deluges affect semi-arid ecosystems remains to be resolved. We applied deluges in August of three sizes (1.0-2.5 times average August precipitation) to assess the potential for late-season deluges to stimulate plant growth and ANPP. These late-season deluges led to significant "green-up" of this grassland, with new leaf production, and an increase in flowering of the dominant grass species. Further, these responses increased as deluge size increased, suggesting that larger or multiple deluges may lead to even greater growth responses. However, despite strong plant-level responses, no increase in ANPP was measured. Our results confirm that aboveground plant growth in the C4-dominated shortgrass steppe does respond to late-season deluges; however, if there is an increase in plant biomass, net accumulation aboveground is minimal at this time of year.

Experimental warming amplified opposite impacts of drought vs. wet extremes on ecosystem carbon cycle in a tallgrass prairie

Climate warming is leading to greater precipitation variability, resulting in increased frequency and intensity of both drought and wet extremes. However, how these extreme events interact with climate warming and hay-harvest in grasslands to impact ecosystem functions has not yet been well explored. In this study, we took advantage of a long-term experiment to examine how climate warming and clipping (i.e., mimicking hay harvest) regulated impacts of naturally occurring drought and wet extremes on ecosystem CO2 fluxes of a tallgrass prairie in the Great Plains, USA. Warming resulted in net ecosystem carbon release (i.e., positive net ecosystem CO2 exchange, NEE) in the extreme drought year of 2011, but significantly enhanced net carbon uptake in the extremely wet year of 2015 in comparison with NEE in normal years. Warming-induced carbon release in the drought year was due to significantly enhanced ecosystem respiration (ER) from mid-summer to early-autumn, whereas warming-enhanced NEE in the wet year was due to an increase in aboveground net primary production (ANPP) compared to those in normal years. Drought diminished warming-induced increases in ANPP to about one sixth of that in the wet year in the unclipped plots. Interestingly, clipping offset the drought-mediated ecosystem carbon loss by increasing GPP and weakened the wet-enhanced ANPP. Overall, our results suggest that a future, warmer climate may exacerbate carbon losses in terrestrial ecosystems during drought extremes but stimulate the ecosystem carbon sink under wet extremes.

Repackaging precipitation into fewer, larger storms reduces ecosystem exchanges of CO2 and H2O in a semiarid steppe

General circulation models predict that precipitation will become more extreme, i.e. rainfall events of larger size but reduced frequency. Studies in North American grasslands have shown that such repackaging of precipitation into fewer, larger events enhanced above ground net primary productivity (ANPP), likely due to deeper soil moisture infiltration favoring plant water use over evaporation. However, ANPP responses in other regions remain poorly understood, and responses of carbon and water exchanges with the atmosphere remain unknown. Here we manipulated rainfall in a steppe ecosystem of northern China over 4 years to investigate how temporal packaging of precipitation impacts ANPP, evapotranspiration (ET), net ecosystem CO2 exchange (NEE) and the component fluxes gross primary productivity (GPP) and ecosystem respiration (RE). Experimental plots received precipitation equivalent to the 60-year growing-season average of 240 mm, variously packaged into 6, 10, 16, or 24 events representing extreme (P6) to historical average (P24) rainfall frequency. Extraordinarily extreme frequency (6 large events) reduced NEE, GPP, RE, ET and water use efficiency (WUE = |NEE|/ET). The average NEE, GPP and RE declined 35%, 45% and 48% respectively in the P6 treatment as compared to P16, which showed maximum ET and CO2 exchange. After peaking in the 16-event treatment, GPP and WUE in P24 were not distinguishable from P6. These peaks suggest that P16 was optimal for photosynthesis, with sufficiently frequent rain to maintain unregulated plants and adequately deep soil moisture infiltration to favour transpiration, with associated carbon uptake, over evaporation. Path analysis indicated the lower CO2 fluxes were influenced by reduced soil water content and leaf area index and higher soil temperature, with ET regulating the effects of these microclimatic drivers. ANPP showed a monotonic but non-significant decline with decreasing precipitation frequency, consistent with reduced CO2 fluxes. We found an increase in ANPP of xerophyte plants partially compensated for the ANPP decline in the dominant eurytopic xerophyte plants. Our results suggest that extreme temporal repackaging of precipitation into few events with correspondingly long dry intervals may reduce the capacity of steppe ecosystems to assimilate atmospheric CO2, although community diversity may moderate impacts.

Temperature and rainfall interact to control carbon cycling in tropical forests.

Tropical forests dominate global terrestrial carbon (C) exchange, and recent droughts in the Amazon Basin have contributed to short-term declines in terrestrial carbon dioxide uptake and storage. However, the effects of longer-term climate variability on tropical forest carbon dynamics are still not well understood. We synthesised field data from more than 150 tropical forest sites to explore how climate regulates tropical forest aboveground net primary productivity (ANPP) and organic matter decomposition, and combined those data with two existing databases to explore climate - C relationships globally. While previous analyses have focused on the effects of either temperature or rainfall on ANPP, our results highlight the importance of interactions between temperature and rainfall on the C cycle. In cool forests (<20°C), high rainfall slowed rates of C cycling, but in warm tropical forests (>20°C) it consistently enhanced both ANPP and decomposition. At the global scale, our analysis showed an increase in ANPP with rainfall in relatively warm sites, inconsistent with declines in ANPP with rainfall reported previously. Overall, our results alter our understanding of climate - C cycle relationships, with high precipitation accelerating rates of C exchange with the atmosphere in the most productive biome on earth.

Impacts of climate change and human activities on the aboveground production in alpine grasslands: a case study of the source region of the Yellow River, China

Changes in aboveground net primary productivity (ANPP) in alpine grasslands are the consequence of climate change and human activities, but it is difficult to disentangle their relative contributions. Based on monthly remote-sensed vegetation index and meteorological data during the period 1982–2010, we analysed the long-term variation of annual ANPP in the source region of the Yellow River and quantified the effects of climate and human activities including grazing on ANPP variability, using the Carnegie–Ames–Stanford Approach (CASA) model and the ANPP-based residual trend analysis method. Our results suggested that ANPP increased in 80% of alpine grasslands. Areas with negative changes in ANPP were found mainly in the eastern portion of the region, accounting for 0.3% of alpine grasslands. ANPP was positively correlated with the monthly mean temperature from June to September and the sunshine duration in September at a regional scale. Moreover, ANPP was negatively correlated with the total livestock numbers. Using the residual trend analysis method, we demonstrated that climate and human activities accounted for 76.6 and 23.4%, respectively, of the variability in ANPP for the entire study region in 1982–2010. We concluded that climate change alleviated climatic constraints, in particular temperature limitations and sunshine duration, resulting in a significant increase in ANPP. Overgrazing was supposed to be the primary driver for grassland degradation in the eastern region. Our study has implications for grassland management and its sustainability to minimize the risk of grassland degradation and desertification processes in geo-ecologically and socially important regions such as the study region in China.

The effect of nitrogen addition on community structure and productivity in grasslands: A meta-analysis

The ongoing increase in anthropogenic nitrogen (N) addition may have a substantial impact on grassland ecosystems. Understanding the dynamics of the productivity and community structure of grassland ecosystems following N addition is important. We conducted a meta-analysis of the data from 2516 quadrats described in 56 published N-addition studies and aimed to address these questions and further describe the productivity and community structure of grassland ecosystems in response to N addition in China. Our results showed the critical threshold for N-induced productivity loss to grassland was variable. In meadow steppe, when the range of N addition occurred at rates between 7.5 and 20gNm−2yr−1, the aboveground net primary production (ANPP) increased, and below this range, the biomass of the functional groups (forb, grass, and sedge) decreased. Furthermore, above the range, both productivity and structure were negatively related to the rate of N addition. The critical threshold for the N-induced changes in the productivity of the desert steppe was 10.5gNm−2yr−1, at which production increases with no reduction in diversity. Additionally, the increase in ANPP with N addition stabilizes above approximately 12gNm−2yr−1 in desert steppe, and the decrease in species richness with N addition stabilizes above approximately 11gNm−2yr−1. Thus, at approximately 11gNm-2yr−1, further N addition causes no increase in production and a clear reduction in diversity. Our results contribute to a better understanding of the consequences of future climate change in grassland ecosystems.

The response of sub‐Mediterranean grasslands to rainfall variation is influenced by early season precipitation

AbstractQuestionClimate change will likely modify patterns of precipitation, with an expected increase in intra‐annual variability and increased frequency and magnitude of extreme events. The Mediterranean area is expected to be very sensitive to such events because water availability is already limited. However, the effect of precipitation variability on ecosystem services, such as plant productivity, is little known. What is the short‐term effect of an experimental precipitation gradient on above‐ground net primary productivity (ANPP) of two contrasting sub‐Mediterranean grassland ecosystems? How does early season precipitation, i.e. dry or wet spring, influence ANPP? Do the functional groups of grasses and forbs differ in their response?LocationTorricchio Nature Reserve, Central Apennines, Italy.MethodsWe selected two grasslands characterized by contrasting geophysical and soil chemical parameters (north‐ and south‐facing slopes). In both sites, during two climatically different years, mid‐season (summer) precipitation was manipulated in order to obtain a gradient of rainfall availability, comprising additional rainfall, ambient rainfall conditions and rainfall reduction. The above‐ground biomass, subdivided according to the functional groups of forbs and grasses, was collected at the end of each treatment period.ResultsA significant increase in ANPP due to experimental increase in summer rainfall appeared in the year with the wet spring, but only in the mesic north‐facing slope. This response was driven by the increased productivity of perennial forbs, while grasses showed stable above‐ground production. In contrast, in the year with the dry spring, ANPP did not respond to the experimental rainfall gradient, meaning that a dry spring leads to lower ANPP even under increased summer rainfall. The variability of ANPP increased significantly in the xeric south‐facing slope in the year with the wet spring, most likely reflecting indirect effects of small‐scale heterogeneity such as variations in soil depth.ConclusionsIntra‐annual precipitation variation can have noticeable implications for sub‐Mediterranean montane grassland agriculture: to avoid degradation due to overgrazing, livestock pressure should be limited in years with a spring drought, regardless of summer precipitation, especially in mesic grasslands.

Species richness loss after nutrient addition as affected by N:C ratios and phytohormone GA3contents in an alpine meadow community

Aims From the light-competition hypothesis, competition for light is asymmetric and the observed increases in plant-size variability with increasingly denser canopies are primarily due to competition for light. Greater plant height provides pre-emptive access to light and produces increased height differences among species. The question is what produces these differences in plant height or height growth response among species in response to fertilization. Methods In 2009, a field experiment of N, P and N + P enrichments at three levels each was initiated in an alpine meadow on the northeast Qinghai- Tibet Plateau. Effects of fertilization on species richness, aboveground net primary production (ANPP), relative light intensity and plant height of different plant functional groups were determined. Festuca ovina (grass), Kobresia humilis (sedge), Oxytropis ochrocephala (legume), Taraxacum lugubre (rosette forb) and Geranium pylzowianum (upright forb) were selected as exemplars of each of the indicated functional groups. The N: C ratios in aboveground biomass, gibberellic acid (GA3) concentrations in leaves, plant heights and height relative growth rate (RGR) of these exemplar species were analyzed in detail. Important Findings Species richness of grasses significantly increased with increasing N + P levels. Species richness of legumes and upright forbs decreased after N and N + P additions. P addition had no significant effect on species richness. The effects of N + P addition on species richness and ANPP were consistently stronger than those of the single N or P fertilization. Reductions in species richness caused by nutrient addition paralleled the increases in ANPP and decreases in light intensity under the canopies, indicating indirect effect of nutrient addition on species richness via ANPP-induced light competition. The exemplar species that responded most positively to fertilization in height and RGR also displayed stronger increases in their GA3 content and N:C ratios. GA3 concentrations and N: C ratios were positively correlated with height RGR when the data were pooled for all species. The tallest and the fastest-growing grass, F. ovina, had the largest increase in N: C ratios and the highest leaf GA3 concentrations after nutrient addition. These results indicated that differential responses of GA3 concentrations and N: C ratios to fertilization were related to the inequality in plant heights among species.

Different responses of plant species to deferment of sheep grazing in a desert steppe of Inner Mongolia, China

This paper reports the effects of different deferments of sheep grazing on plant variables of desert steppe vegetation in Inner Mongolia during 2010–2012. The study was initiated in May 2010 and comprised five treatments: no grazing (UG), grazing throughout the growing season (G) and grazing deferment for 40, 50 and 60 days (40UG, 50UG and 60UG) from the start of the growing season in a completely randomised block design replicated three times. The plant species were classified into two functional groups (C3 and C4 species) and the relationships among species composition, biomass and annual precipitation were examined in different deferments of sheep grazing. Treatment G significantly decreased the biomass of several species compared with some of the deferred treatments and treatment UG, because of differences in grazing preferences, and, consequently, decreased the aboveground net primary productivity (ANPP). Grazing deferment for 40, 50 and 60 days significantly increased C3-species richness and biomass compared with treatment UG, whereas grazing sheep’s preferences for C4 plants at some growth stages compared with C3 plants, resulted in lower species richness and biomass of C4 plants in treatments G, 40UG and 50UG in a dry year. Similarly, different species responses to treatment 60UG resulted in an increase in ANPP in a year with more precipitation during the growing season. The annual precipitation patterns strongly affected the temporal changes in biomass as well as the responses to grazing, indicating that the plant responses in the desert steppe were co-limited by grazing and precipitation. These findings provide important insights into the management and conservation of desert steppe vegetation in Inner Mongolia.

Mechanisms of grass response in grasslands and shrublands during dry or wet periods

Multi-year climatic periods are expected to increase with global change, yet long-term data are often insufficient to document factors leading to ecological responses. We used a suite of long-term datasets (1993-2010) to examine the processes underlying different relationships between aboveground net primary production (ANPP) and precipitation in wet and dry rainfall periods in shrublands and grasslands in the Chihuahuan Desert. We hypothesized that trends in ANPP can be explained by different processes associated with their dominant grasses [Bouteloua eriopoda (grasslands); Sporobolus flexuosus (shrublands)] and with ecosystem properties that influence soil water dynamics with feedbacks to ANPP. We compared datasets on recruitment and growth for 7 years with no trend in precipitation followed by a 4-year drought and 5 consecutive wet years. We integrated these data in a simulation model to examine the importance of positive feedbacks. In grasslands, ANPP was linearly related to precipitation regardless of rainfall period, primarily as a result of stolon recruitment by B. eriopoda. A lag in responses suggests the importance of legacies associated with stolon density. In shrublands, ANPP was only related to rainfall in the wet period when it increased nonlinearly as the number of wet years increased. Seed availability increased in the first wet year, and seedling establishment occurred 2-4 years later. Increases in biomass, litter and simulated transpiration beginning in the third year corresponded with increases in ANPP. Understanding the processes underlying ecosystem dynamics in multi-year dry or wet periods is expected to improve predictions under directional increases or decreases in rainfall.

Global patterns of nitrogen limitation: confronting two global biogeochemical models with observations

Projections of future changes in land carbon (C) storage using biogeochemical models depend on accurately modeling the interactions between the C and nitrogen (N) cycles. Here, we present a framework for analyzing N limitation in global biogeochemical models to explore how C-N interactions of current models compare to field observations, identify the processes causing model divergence, and identify future observation and experiment needs. We used a set of N-fertilization simulations from two global biogeochemical models (CLM-CN and O-CN) that use different approaches to modeling C-N interactions. On the global scale, net primary productivity (NPP) in the CLM-CN model was substantially more responsive to N fertilization than in the O-CN model. The most striking difference between the two models occurred for humid tropical forests, where the CLM-CN simulated a 62% increase in NPP at high N addition levels (30 g N m(-2) yr(-1)), while the O-CN predicted a 2% decrease in NPP due to N fertilization increasing plant respiration more than photosynthesis. Across 35 temperate and boreal forest sites with field N-fertilization experiments, we show that the CLM-CN simulated a 46% increase in aboveground NPP in response to N, which exceeded the observed increase of 25%. In contrast, the O-CN only simulated a 6% increase in aboveground NPP at the N-fertilization sites. Despite the small response of NPP to N fertilization, the O-CN model accurately simulated ecosystem retention of N and the fate of added N to vegetation when compared to empirical (15) N tracer application studies. In contrast, the CLM-CN predicted lower total ecosystem N retention and partitioned more losses to volatilization than estimated from observed N budgets of small catchments. These results point to the need for model improvements in both models in order to enhance the accuracy with which global C-N cycle feedbacks are simulated.

Feedback from plant species change amplifies CO2 enhancement of grassland productivity

Dynamic global vegetation models simulate feedbacks of vegetation change on ecosystem processes, but direct, experimental evidence for feedbacks that result from atmospheric CO2 enrichment is rare. We hypothesized that feedbacks from species change would amplify the initial CO2 stimulation of aboveground net primary productivity (ANPP) of tallgrass prairie communities. Communities of perennial forb and C4 grass species were grown for 5years along a field CO2 gradient (250-500μLL(-1) ) in central Texas USA on each of three soil types, including upland and lowland clay soils and a sandy soil. CO2 enrichment increased community ANPP by 0-117% among years and soils and increased the contribution of the tallgrass species Sorghastrum nutans (Indian grass) to community ANPP on each of the three soil types. CO2 -induced changes in ANPP and Sorghastrum abundance were linked. The slope of ANPP-CO2 regressions increased between initial and final years on the two clay soils because of a positive feedback from the increase in Sorghastrum fraction. This feedback accounted for 30-60% of the CO2 -mediated increase in ANPP on the upland and lowland clay soils during the final 3years and 1year of the experiment, respectively. By contrast, species change had little influence on the ANPP-CO2 response on the sandy soil, possibly because Sorghastrum increased largely at the expense of a functionally similar C4 grass species. By favoring a mesic C4 tall grass, CO2 enrichment approximately doubled the initial enhancement of community ANPP on two clay soils. The CO2 -stimulation of grassland productivity may be significantly underestimated if feedbacks from plant community change are not considered.

More stable productivity of semi natural grasslands than sown pastures in a seasonally dry climate.

In the Neotropics the predominant pathway to intensify productivity is generally thought to be to convert grasslands to sown pastures, mostly in monoculture. This article examines how above-ground net primary productivity (ANPP) in semi-natural grasslands and sown pastures in Central America respond to rainfall by: (i) assessing the relationships between ANPP and accumulated rainfall and indices of rainfall distribution, (ii) evaluating the variability of ANPP between and within seasons, and (iii) estimating the temporal stability of ANPP. We conducted sequential biomass harvests during 12 periods of 22 days and related those to rainfall. There were significant relationships between ANPP and cumulative rainfall in 22-day periods for both vegetation types and a model including a linear and quadratic term explained 74% of the variation in the data. There was also a significant correlation between ANPP and the number of rainfall events for both vegetation types. Sown pastures had higher ANPP increments per unit rainfall and higher ANPP at the peak of the rainy season than semi-natural grasslands. In contrast, semi-natural grasslands showed higher ANPP early in the dry season. The temporal stability of ANPP was higher in semi-natural grasslands than in the sown pastures in the dry season and over a whole annual cycle. Our results reveal that, contrary to conventional thinking amongst pasture scientists, there appears to be no increase in ANPP arising from replacing semi-natural grasslands with sown pastures under prevailing pasture management practices in seasonally dry climates, while the temporal distribution of ANPP is more even in semi-natural grasslands. Neither sown pastures nor semi-natural grasslands are productive towards the end of the dry season, indicating the potential importance of the widespread practice of retaining tree cover in pastures.