Effects of Extreme Weather Events on Plant Productivity and Tissue Die-Back are Modified by Community Composition

Extreme weather events are expected to increase in frequency and magnitude due to climate change. Their effects on vegetation are widely unknown. Here, experimental grassland and heath communities in Central Europe were exposed either to a simulated single drought or to a prolonged heavy rainfall event. The magnitude of manipulations imitated the local 100-year weather extreme according to extreme value statistics. Overall productivity of both plant communities remained stable in the face of drought and heavy rainfall, despite significant effects on tissue die-back. Grassland communities were more resistant against the extreme weather events than heath communities. Furthermore, effects of extreme weather events on community tissue die-back were modified by functional diversity, even though conclusiveness in this part is limited by the fact that only one species composition was available per diversity level within this case study. More diverse grassland communities exhibited less tissue die-back than less complex grassland communities. On the other side, more diverse heath communities were more vulnerable to extreme weather events compared to less complex heath communities. Furthermore, legumes did not effectively contribute to the buffering against extreme weather events in both vegetation types. Tissue die-back proved a strong stress response in plant communities exposed to 100-year extreme weather events, even though one important ecosystem function, namely productivity, remained surprisingly stable in this experiment. Theories and concepts on biodiversity and ecosystem functioning (insurance hypothesis, redundancy hypothesis) may have to be revisited when extreme weather conditions are considered.     

A field study of the effects of elevated CO2 and plant species diversity on ecosystem-level gas exchange in a planted calcareous grassland

The relationship between plant species diversity and ecosystem CO₂ and water vapour fluxes was investigated for planted calcareous grassland communities composed of 5, 12, or 32 species assembled from the native plant species pool. These diversity manipulations were done in factorial combination with a CO₂ enrichment experiment in order to investigate the degree to which ecosystem responses to elevated CO₂ are altered by a loss of plant diversity. Ecosystem CO₂ and H₂O fluxes were measured over several 24-h periods during the 1994 and 1995 growing seasons. Ecosystem CO₂ assimilation on a ground area basis decreased with decreasing plant diversity in the first year and this was related to a decline in above-ground plant biomass. In the second year, however, CO₂ assimilation was not affected by diversity, and this corresponded to the disappearance of a diversity effect on above-ground biomass. Irrespective of diversity treatment, CO₂ assimilation on a ground area basis was linearly related to peak above-ground biomass in both years. Elevated CO₂ significantly increased ecosystem CO₂ assimilation in both years with no interaction between diversity and CO₂ treatment, and no corresponding increase in above-ground biomass. There were no significant effects of diversity on water vapour flux, which was measured only in the second year. There were indications of a small CO₂ effect on water vapour flux (3–9% lower at elevated CO₂ depending on the light level). Our findings suggest that decreasing plant species diversity may substantially decrease ecosystem CO₂ assimilation during the establishment of such planted calcareous grassland communities, but also suggest that this effect may not persist. In addition, we find no evidence that plant species diversity alters the response of ecosystem CO₂ assimilation to elevated CO₂.     

西藏高原高寒草原生态系统植被C／N值的分布特征及其影响因素

以西藏高原高寒草原生态系统的4个自然地带（高山草原、高山灌丛草甸、山地半荒漠与荒漠以及山地灌丛草原）的19个草地型植被为研究对象,采用野外调查与室内分析相结合的方法,对高寒草原生态系统植被C／N值的分布特征及其影响因素进行了研究。结果表明：西藏高原高寒草原植被C／N值总体上呈现出东西部低而中间高的态势以及斑块状交错分布的格局。不同自然地带间和不同草地型间植被地上部分和根系的C／N值有明显差异,且地上部分的C／N值均大于根系。19个草地型植被地上部分的平均C／N值为34．17,变异系数为35．87％;根系的平均C／N值为29．58,变异系数为40．02％。4个自然地带植被地上部分的平均C／N值为31．98,变异系数为13．82％;根系的平均C／N值为31．86,变异系数为16．92％。回归分析结果显示：植被地上部分C／N值与地上部生物量以及土壤全N和全K含量呈显著正相关、与植被高度呈显著负相关;根系C／N值与海拔和20～30em土壤容重呈显著正相关、与年均降水量和年均蒸发量呈显著负相关,这些因子均为影响西藏高原高寒草原植被C／N值的关键环境因子。总体上看,地理因子、气候因子和土壤物理因子对西藏高原高寒草原生态系统植被C／N值的影响不显著,而植被因子和土壤化学因子则对其有显著影响。     

Disentangling species and functional group richness effects on soil N cycling in a grassland ecosystem

Species richness (SR) and functional group richness (FGR) are often confounded in both observational and experimental field studies of biodiversity and ecosystem function. This precludes discernment of their separate influences on ecosystem processes, including nitrogen (N) cycling, and how those influences might be moderated by global change factors. In a 17‐year field study of grassland species, we used two full factorial experiments to independently vary SR (one or four species, with FGR = 1) and FGR (1–4 groups, with SR = 4) to assess SR and FGR effects on ecosystem N cycling and its response to elevated carbon dioxide (CO₂) and N addition. We hypothesized that increased plant diversity (either SR or FGR) and elevated CO₂ would enhance plant N pools because of greater plant N uptake, but decrease soil N cycling rates because of greater soil carbon inputs and microbial N immobilization. In partial support of these hypotheses, increasing SR or FGR (holding the other constant) enhanced total plant N pools and decreased soil nitrate pools, largely through higher root biomass, and increasing FGR strongly reduced mineralization rates, because of lower root N concentrations. In contrast, increasing SR (holding FGR constant and despite increasing total plant C and N pools) did not alter root N concentrations or net N mineralization rates. Elevated CO₂ had minimal effects on plant and soil N metrics and their responses to plant diversity, whereas enriched N increased plant and soil N pools, but not soil N fluxes. These results show that functional diversity had additional effects on both plant N pools and rates of soil N cycling that were independent of those of species richness.     

Soil biotic processes remain remarkably stable after 100-year extreme weather events in experimental grassland and heath

Climate change will increase the recurrence of extreme weather events such as drought and heavy rainfall. Evidence suggests that extreme weather events pose threats to ecosystem functioning, particularly to nutrient cycling and biomass production. These ecosystem functions depend strongly on below-ground biotic processes, including the activity and interactions among plants, soil fauna, and micro-organisms. Here, experimental grassland and heath communities of three phytodiversity levels were exposed either to a simulated single drought or to a heavy rainfall event. Both weather manipulations were repeated for two consecutive years. The magnitude of manipulations imitated the local 100-year extreme weather event. Heavy rainfall events increased below-ground plant biomass and stimulated soil enzyme activities as well as decomposition rates for both plant communities. In contrast, extreme drought did not reduce below-ground plant biomass and root length, soil enzyme activities, and cellulose decomposition rate. The low responsiveness of the measured ecosystem properties in face of the applied weather manipulations rendered the detection of significant interactions between weather events and phytodiversity impossible. Our data indicate on the one hand the close interaction between below ground plant parameters and microbial turnover processes in soil; on the other hand it shows that the plant–soil system can buffer against extreme drought events, at last for the period of investigation.     

Effects of plant species diversity, atmospheric [CO2], and N addition on gross rates of inorganic N release from soil organic matter

A significant challenge in predicting terrestrial ecosystem response to global changes comes from the relatively poor understanding of the processes that control pools and fluxes of plant nutrients in soil. In addition, individual global changes are often studied in isolation, despite the potential for interactive effects among them on ecosystem processes. We studied the response of gross N mineralization and microbial respiration after 6 years of application of three global change factors in a grassland field experiment in central Minnesota (the BioCON experiment). BioCON is a factorial manipulation of plant species diversity (1, 4, 9 and 16 prairie species), atmospheric [CO₂] (ambient and elevated: 560 μmol mol^?1), and N inputs (ambient and ambient +4 g N m^?2 yr^?1). We hypothesized that gross N mineralization would increase with increasing levels of all factors because of stimulated plant productivity and thus greater organic inputs to soils. However, we also hypothesized that N addition would enhance, while elevated [CO₂] and greater diversity would temper, gross N mineralization responses because of increased and reduced plant tissue N concentrations, respectively. In partial support of our hypothesis, gross N mineralization increased with greater diversity and N addition, but not with elevated [CO₂]. The ratio of gross N mineralization to microbial respiration (i.e. the ‘yield’ of inorganic N mineralized per unit C respired) declined with greater diversity and [CO₂] suggesting increasing limitation of microbial processes by N relative to C in these treatments. Based on these results, we conclude that the plant supply of organic matter primarily controls gross N mineralization and microbial respiration, but that the concentration of N in organic matter input secondarily influences these processes. Thus, in systems where N limits plant productivity these global change factors could cause different long‐term ecosystem trajectories because of divergent effects on soil N and C cycling.     

Mechanisms behind plant diversity effects on inorganic and organic N leaching from temperate grassland

Higher plant diversity reduces nitrate leaching by complementary resource use, while its relation to leaching of other N species is unclear. We determined the effects of plant species richness, functional group richness, and the presence of specific functional groups on ammonium, dissolved organic N (DON), and total dissolved N (TDN) leaching from grassland in the first 4 years after conversion from fertilized arable land to unfertilized grassland. On 62 experimental plots in Jena, Germany, with 1–60 plant species and 1–4 functional groups (legumes, grasses, tall herbs, small herbs), nitrate, ammonium, and TDN concentrations in soil solution (0–0.3 m soil layer) were measured fortnightly during 4 years. DON concentrations were calculated by subtracting inorganic N from TDN. Nitrogen concentrations were multiplied with modeled downward water fluxes to obtain N leaching. DON leaching contributed most to TDN leaching (64 ± SD 4% of TDN). Ammonium leaching was unaffected by plant diversity. Increasing species richness decreased DON leaching in the fourth year. We attribute this finding to enhanced use of DON as a C and N source and enhanced mineralization of DON by soil microorganisms. An increase of species richness decreased TDN leaching likely driven by the complementary use of nitrate by diverse mixtures. Legumes increased DON and TDN leaching likely because of their N\(_{2}\)-fixing ability and higher litter production. Grasses decreased TDN leaching because of more exhaustive use of nitrate and water. Our results demonstrate that increasing plant species richness decreases leaching of DON and TDN.     

Ecosystem functions are resistant to extreme changes to rainfall regimes in a mesotrophic grassland

Aims

Major changes to rainfall regimes are predicted for the future but the effect of such changes on terrestrial ecosystem function is largely unknown. We created a rainfall manipulation experiment to investigate the effects of extreme changes in rainfall regimes on ecosystem functioning in a grassland system.

Methods

We applied two rainfall regimes; a prolonged drought treatment (30 % reduction over spring and summer) and drought/downpour treatment (long periods of no rainfall interspersed with downpours), with an ambient control. Both rainfall manipulations included increased winter rainfall. We measured plant community composition, CO₂ fluxes and soil nutrient availability.

Results

Plant species richness and cover were lower in the drought/downpour treatment, and showed little recovery after the treatment ceased. Ecosystem processes were less affected, possibly due to winter rainfall additions buffering reduced summer rainfall, which saw relatively small soil moisture changes. However, soil extractable P and ecosystem respiration were significantly higher in rainfall change treatments than in the control.

Conclusions

This grassland appears fairly resistant, in the short term, to even the more extreme rainfall changes that are predicted for the region, although prolonged study is needed to measure longer-term impacts. Differences in ecosystem responses between the two treatments emphasise the variety of ecosystem responses to changes in both the size and frequency of rainfall events. Given that model predictions are inconsistent there is therefore a need to assess ecosystem function under a range of potential climate change scenarios.     

Is the relation between nutrient supply and biodiversity co‐determined by the type of nutrient limitation?

Correlative studies have shown a ‘hump‐backed’ relation between the vegetation N:P ratio and plant species diversity with the highest diversity at balanced N:P ratios (between 10 and 14). We tested the hypothesis that adding growth‐limiting nutrients to mesotrophic grasslands that were in shortage of either N (N:P ratio<10) or P (N:P ratio>14) would lead to an increase of plant diversity. Thereto, we studied the effects of long‐term (11 yr) experimentally increased N and/or P supply on soil nutrient pools, vegetation nutrient dynamics and biodiversity in a riverine grassland in the Netherlands with a low soil N:P ratio (N shortage) and a peat grassland with a high soil N:P ratio (P shortage), respectively. Eleven years of nutrient addition hardly had any effects on the total stocks of C, N and P in the soils of both sites, due to the large size of the soil nutrient pools already present and to the management at both sites (annual hay‐making and ‐removal). However, in the riverine grassland the treatments increased the cycling of the small pool of labile N and P compounds resulting in large increases in annual fluxes of especially N. In the unfertilised controls, species establishments balanced more or less species losses during an 11 year period, thus leading to a dynamic equilibrium of the species pool. However, contrary to our hypothesis, addition of the growth‐limiting nutrient led at both sites to a reduction of species diversity even when total biomass remained below critical levels. Species diversity and species evenness were strongly determined by N mineralisation and to a lesser extent by total soil N and extractable P, respectively. Total aboveground biomass of the vegetation was determined by total soil N. Our study shows that patterns found in correlative studies of the relation between plant diversity and soil and vegetation N:P ratio can not be translated into successful experimental manipulations to enhance biodiversity. The most likely explanation is that colonization limitation occurred in the fertilized plots and that not sufficient diaspores of potentially new species could reach and/or colonize the plots to compensate for the species extinctions as a result of increased nutrient supply.     

Priming effect and microbial diversity in ecosystem functioning and response to global change: a modeling approach using the SYMPHONY model

Integration of the priming effect (PE) in ecosystem models is crucial to better predict the consequences of global change on ecosystem carbon (C) dynamics and its feedbacks on climate. Over the last decade, many attempts have been made to model PE in soil. However, PE has not yet been incorporated into any ecosystem models. Here, we build plant/soil models to explore how PE and microbial diversity influence soil/plant interactions and ecosystem C and nitrogen (N) dynamics in response to global change (elevated CO₂ and atmospheric N depositions). Our results show that plant persistence, soil organic matter (SOM) accumulation, and low N leaching in undisturbed ecosystems relies on a fine adjustment of microbial N mineralization to plant N uptake. This adjustment can be modeled in the SYMPHONY model by considering the destruction of SOM through PE, and the interactions between two microbial functional groups: SOM decomposers and SOM builders. After estimation of parameters, SYMPHONY provided realistic predictions on forage production, soil C storage and N leaching for a permanent grassland. Consistent with recent observations, SYMPHONY predicted a CO₂‐induced modification of soil microbial communities leading to an intensification of SOM mineralization and a decrease in the soil C stock. SYMPHONY also indicated that atmospheric N deposition may promote SOM accumulation via changes in the structure and metabolic activities of microbial communities. Collectively, these results suggest that the PE and functional role of microbial diversity may be incorporated in ecosystem models with a few additional parameters, improving accuracy of predictions.     

黄土高原退耕还草地植物群落动态对生态系统碳储量的影响

退化农地通过植被恢复能够提高生态系统的固碳能力,但是植被恢复中植物群落特征如何影响生态系统碳储量仍存在不确定性。以农田为对照,选取自然恢复8、15、25、35 a草地为对象,探讨退耕还草地植物群落特征对生态系统碳储量的影响。结果表明：群落盖度随着恢复年限的增加而显著增加,恢复35 a时达到最大值（64.0%）,优势种从达乌里胡枝子、赖草、茵陈蒿演变为长芒草、铁杆蒿;禾草类、多年生草本和灌木逐渐成为优势种。Shannon-Weiner指数、Patrick指数均呈先上升后下降的趋势,均在第15年达到最大值。地上植被碳储量和地下植被碳储量在恢复期间呈直线增加的趋势,且均在35 a达最大值,分别为0.83 Mg C/hm²、1.49 Mg C/hm²,而凋落物碳储量在第25年达到最大值,为0.40 Mg C/hm²。土壤碳储量与有机碳含量总体呈先下降后上升的趋势,在第8年达到最低值,在第35年恢复到农田水平之上,占生态系统碳储量的93.3%-99.6%;表层0-10 cm土壤碳储量占0-30 cm碳储量的38.9%-50.3%,呈表聚现象。生态系统碳储量与土壤碳储量趋势一致,即恢复到第8年最低,为24.32 Mg C/hm²,恢复到第35年最高,为43.70 Mg C/hm²。群落盖度、地上生物量、凋落物生物量、禾草、豆科以及多年生植物的重要值与生态系统碳储量呈显著正相关（P<0.05）,杂草和一年生植物重要值与生态系统碳储量呈显著负相关（P<0.05）。研究表明植被群落组成的动态变化通过增加植被碳储量和土壤碳储量实现生态系统碳储量的增加,而多年生植物、杂草与禾草的重要值和地下生物量与凋落物生物量是影响生态系统碳储量的重要植被因子。     

Extreme weather events and plant–plant interactions: shifts between competition and facilitation among grassland species in the face of drought and heavy rainfall

Biotic interactions play an important role in ecosystem function and structure in the face of global climate change. We tested how plant–plant interactions, namely competition and facilitation among grassland species, respond to extreme drought and heavy rainfall events. We also examined how the functional composition (grasses, forbs, legumes) of grassland communities influenced the competition intensity for grass species when facing extreme events. We exposed experimental grassland communities of different functional compositions to either an extreme single drought event or to a prolonged heavy rainfall event. Relative neighbour effect, relative crowding and interaction strength were calculated for five widespread European grassland species to quantify competition. Single climatic extremes caused species specific shifts in plant–plant interactions from facilitation to competition or vice versa but the nature of the shifts varied depending on the community composition. Facilitation by neighbouring plants was observed for Arrhenatherum elatius when subjected to drought. Contrarily, the facilitative effect of neighbours on Lotus corniculatus was transformed into competition. Heavy rainfall increased the competitive effect of neighbours on Holcus lanatus and Lotus corniculatus in communities composed of three functional groups. Competitive pressure on Geranium pratense and Plantago lanceolata was not affected by extreme weather events. Neither heavy rainfall nor extreme drought altered the overall productivity of the grassland communities. The complementary responses in competition intensity experienced by grassland species under drought suggest biotic interactions as one stabilizing mechanism for overall community performance. Understanding competitive dynamics under fluctuating resources is important for assessing plant community shifts and degree of stability of ecosystem functions.     

Effects of nitrogen addition on vegetation and ecosystem carbon in a semi-arid grassland

To clarify responses of plant and soil carbon (C) and nitrogen (N) pools in grassland ecosystem to N addition, a field experiment was performed in a grassland in Keerqin Sandy Lands, Northeast China. We investigated vegetation composition and C and N pools of plant and soil (0–30 cm) after five consecutive years of N addition at a rate of 20 g N m^?2 y^?1. Vegetation composition and species diversity responded dramatically to N addition, as dominance by C₄ perennials was replaced with C₃ annuals. Carbon in aboveground pool increased significantly (over two-fold), mainly due to the increase of the C in aboveground living plants and surface litter, which increased by 98 and 134%, respectively. Although soil C did not change significantly, the root C pool decreased in response to 5 years of N addition. The total ecosystem C pool was not significantly impacted by N addition because the large soil pool did not respond to N addition, and the increase in aboveground C was offset by the decrease in root C pool. Moreover, N addition significantly increased the aboveground N pool, but had no significant effects on belowground and total ecosystem N pools. Our results suggest that in the mid-term N addition alters the C and N partitioning in above- and belowground pools, but has no significant effects on total ecosystem C and N pools in these N-limited grasslands.     

Functional diversity of leaf nitrogen concentrations drives grassland carbon fluxes

Little is known about the role of plant functional diversity for ecosystem‐level carbon (C) fluxes. To fill this knowledge gap, we translocated monoliths hosting communities with four and 16 sown species from a long‐term grassland biodiversity experiment (‘The Jena Experiment’) into a controlled environment facility for ecosystem research (Ecotron). This allowed quantifying the effects of plant diversity on ecosystem C fluxes as well as three parameters of C uptake efficiency (water and nitrogen use efficiencies and apparent quantum yield). By combining data on ecosystem C fluxes with vegetation structure and functional trait‐based predictors, we found that increasing plant species and functional diversity led to higher gross and net ecosystem C uptake rates. Path analyses and light response curves unravelled the diversity of leaf nitrogen concentration in the canopy as a key functional predictor of C fluxes, either directly or indirectly via LAI and aboveground biomass.     

Carbon and Nitrogen Decoupling Under an 11-Year Drought in the Shortgrass Steppe

The frequency and magnitude of drought is expected to increase in the US Great Plains under future climate regimes. Although semiarid systems are considered highly resistant to water limitation, novel drought events could alter linkages among biogeochemical processes, and result in new feedbacks that influence the timescale of ecosystem recovery. We examined changes in carbon and nitrogen cycling in the last 2 years of an 11-year drought manipulation in the shortgrass steppe, and under the first 2 years of recovery from drought. We measured plant production, plant tissue chemistry, soil trace gas flux, and soil inorganic nitrogen dynamics to test the extent that this magnitude of drought altered carbon and nitrogen fluxes and how these changes affected post-drought dynamics. We found that soil inorganic nitrogen was up to five times higher under severe drought than under control conditions, but that this nitrogen may not have been accessible to plants and microbial communities during drought due to diffusion limitations. Drought plots had higher N₂O flux when they received equal rainfall pulses, showing that this accumulated N may be vulnerable to loss. In addition, plants in drought plots had higher tissue nitrogen for 2 years following drought. These results show that decadal-length droughts that may occur under future precipitation regimes are likely to alter ecosystem properties through interactions among precipitation, vegetation, and N cycling. Shifts in plant N, vulnerability of nitrogen to loss, and rainfall use efficiency that we observed are likely to affect the recovery time of semiarid systems subject to droughts of this magnitude.     

Invader presence disrupts the stabilizing effect of species richness in plant community recovery after drought

Higher biodiversity can stabilize the productivity and functioning of grassland communities when subjected to extreme climatic events. The positive biodiversity–stability relationship emerges via increased resistance and/or recovery to these events. However, invader presence might disrupt this diversity–stability relationship by altering biotic interactions. Investigating such disruptions is important given that invasion by non‐native species and extreme climatic events are expected to increase in the future due to anthropogenic pressure. Here we present one of the first multisite invader × biodiversity × drought manipulation experiment to examine combined effects of biodiversity and invasion on drought resistance and recovery at three semi‐natural grassland sites across Europe. The stability of biomass production to an extreme drought manipulation (100% rainfall reduction; BE: 88 days, BG: 85 days, DE: 76 days) was quantified in field mesocosms with a richness gradient of 1, 3, and 6 species and three invasion treatments (no invader, Lupinus polyphyllus, Senecio inaequidens). Our results suggest that biodiversity stabilized community productivity by increasing the ability of native species to recover from extreme drought events. However, invader presence turned the positive and stabilizing effects of diversity on native species recovery into a neutral relationship. This effect was independent of the two invader's own capacity to recover from an extreme drought event. In summary, we found that invader presence may disrupt how native community interactions lead to stability of ecosystems in response to extreme climatic events. Consequently, the interaction of three global change drivers, climate extremes, diversity decline, and invasive species, may exacerbate their effects on ecosystem functioning.     

植被与土壤特征对青藏高原不同程度退化草地的响应

高寒草地作为青藏高原高寒生态系统的重要组分之一,其退化已严重影响到高原的可持续发展和草地恢复重建。搜集了2004—2022年间关于青藏高原高寒草地退化的64篇研究结果,包含土壤有机碳、生物量和多样性指数等16个指标的1403组数据,运用meta分析解析了草地退化对土壤理化性质、植被生产力和物种多样性的影响,并对重度退化草地的土壤理化性质和植物生物量进行线性回归分析。结果表明：随着草地退化的加剧,土壤有机碳、全氮、全磷、有效氮、有效磷、有效钾、土壤含水量、地上生物量、地下生物量和植被高度显著下降;土壤容重显著上升;土壤pH、全钾在各个退化阶段没有明显差异;Shannon多样性指数、Pielou均匀度指数和Margalef丰富度指数整体呈下降趋势。土壤有机碳、全氮、全磷、有效氮、有效磷、有效钾和土壤含水量与地上生物量、地下生物量存在显著的正相关;土壤容重与地上生物量、地下生物量呈显著的负相关;土壤pH与地上生物量、地下生物量呈负相关。因此,青藏高原高寒草地退化通过改变土壤理化性质而改变地上群落多样性和生物量,为阐明植被与土壤特征对草地退化的响应机制以及高寒退化草地的恢复提供了科学依据。     

Dynamics of vegetation and soil carbon and nitrogen accumulation over 26 years under controlled grazing in a desert shrubland

For decades, arid desert ecosystems in northwest China, covering one-fourth the country’s land surface, have experienced a rapid decline in plant species diversity, productivity and soil carbon stock owing to degradation by overgrazing. In this study, plant community composition, diversity and productivity, as well as soil carbon (C) and nitrogen (N) stocks, were monitored over 26 years from 1981 to 2006 in a severely degraded Haloxylon ammodendron-dominated shrubland where livestock densities were reduced from 4–5 to 1–2 dry sheep equivalent ha^-1. The objective was to assess long-term grazing effects on vegetation and soil C and N accumulation dynamics. Results showed that the reduction of grazing pressure significantly increased vegetation cover, plant diversity and productivity, resulting primarily from an increase in livestock-preferred species. Controlled grazing also led to marked increases in soil C and N stocks in the top 30 cm of soil. This increase was strongly associated with increased plant species richness, vegetation cover and biomass production. Averaged over 26 years, soil C and N accumulated at rates of 89.9 g?C and 8.4 g?N m^-2?year^-1, respectively, but rates of C and N accumulation varied greatly at different time periods. The greatest species regeneration occurred in the first 8 years, but the largest C and N accumulation took place during years 9–18, with a time-lag in response to changes in vegetation. Our results provide insights into the long-term recovery patterns of different ecosystem components from the influence of prolonged overgrazing disturbance that cannot be inferred from a short-term study. The findings are important for assessing the resilience of these livestock-disturbed desert ecosystems and developing a more effective strategy for the management of this important biome from a long-term perspective.     

Arctic browning: Impacts of extreme climatic events on heathland ecosystem CO2 fluxes

Extreme climatic events are among the drivers of recent declines in plant biomass and productivity observed across Arctic ecosystems, known as “Arctic browning.” These events can cause landscape‐scale vegetation damage and so are likely to have major impacts on ecosystem CO₂ balance. However, there is little understanding of the impacts on CO₂ fluxes, especially across the growing season. Furthermore, while widespread shoot mortality is commonly observed with browning events, recent observations show that shoot stress responses are also common, and manifest as high levels of persistent anthocyanin pigmentation. Whether or how this response impacts ecosystem CO₂ fluxes is not known. To address these research needs, a growing season assessment of browning impacts following frost drought and extreme winter warming (both extreme climatic events) on the key ecosystem CO₂ fluxes Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), ecosystem respiration (R_eco) and soil respiration (R_soil) was carried out in widespread sub‐Arctic dwarf shrub heathland, incorporating both mortality and stress responses. Browning (mortality and stress responses combined) caused considerable site‐level reductions in GPP and NEE (of up to 44%), with greatest impacts occurring at early and late season. Furthermore, impacts on CO₂ fluxes associated with stress often equalled or exceeded those resulting from vegetation mortality. This demonstrates that extreme events can have major impacts on ecosystem CO₂ balance, considerably reducing the carbon sink capacity of the ecosystem, even where vegetation is not killed. Structural Equation Modelling and additional measurements, including decomposition rates and leaf respiration, provided further insight into mechanisms underlying impacts of mortality and stress on CO₂ fluxes. The scale of reductions in ecosystem CO₂ uptake highlights the need for a process‐based understanding of Arctic browning in order to predict how vegetation and CO₂ balance will respond to continuing climate change.