The interaction between temperature and precipitation greatly affects plant phenology. However, these effects can vary between additive, antagonistic, and synergistic, and it is not clear what determines these different outcomes. One hypothesis is that the effect on plant phenology varies with aridity and plant functional group. Here, we find support for this hypothesis using a global meta-analysis on the timing of leaf-out (2178 values from 55 experimental sites) and flowering (4027 values from 117 experimental sites). We find that, globally, the onset of leaf-out is more influenced by water availability than by temperature, while first flowering is more affected by temperature than by precipitation. On its own, warming advances leaf-out for all functional groups (except in semi-humid regions), whereas warming combined with decreased precipitation delays leaf-out in semi-arid regions. Warming also advances flowering across all functional groups, regardless of changes in precipitation and aridity. We observe synergistic effects of warming and precipitation on leaf-out for forbs in semi-arid regions, while antagonistic effects occur for grasses and sedges except in arid regions. Our findings suggest that considering drought tolerance or resistance of plants across ambient climates is critical for improving our understanding and predictions of how plant phenology responds to climate change.

A widely accepted perspective posits that an extension of the growing season enhances plant growth by increasing the duration of favorable environmental conditions under warming, which is described as an ecological effect. However, changes in growing season length can also influence plant functional traits and physiological processes, as suggested by the "leaf economics spectrum" theory, a physiological aspect frequently overlooked. Disentangling the ecological and physiological effects of growing season length on plant growth remains challenging due to their co-variation with climate factors. Here we explored the physiological effect through common garden experiments on the Qinghai−Tibetan Plateau. Our findings revealed a trade-off between growing season length and plant growth under controlled climatic conditions, a pattern further corroborated by satellite-based observations across most regions of the plateau. This trade-off was driven by a negative correlation between growing season length and photosynthetic efficiency, suggesting that an extended growing season does not necessarily translate into enhanced carbon assimilation. However, state-of-the-art dynamic global vegetation models failed to adequately capture this trade-off, underscoring the need to integrate the physiological effects of growing season length into these model frameworks for improved predictions of plant growth under climate change.

[Objective] Under the background of climate warming and changing precipitation pattern, how to ensure the sustainable and stable function of grassland ecosystem and their ability to provide ecological services depends on comprehensive study of the relationship between biodiversity and productivity and its stability. This is also one of the central focus in the study of global change ecology. [Reviews] This paper systematically reviews the latest research progress in global change ecology at home and abroad, focusing on the following three aspects: First, we provide an overview of how species diversity affects productivity and its stability, as well as the potential mechanisms underlying these effects. Second, we provide a detailed overview of recent studies on the relationship between plant diversity and productivity and its stability in response to climate warming and changing precipitation, based on field experiments and other methods. Finally, we explore how dynamic changes in plant species composition play a crucial role in regulating the stability of community productivity. [Prospect] This paper puts forward some suggestions on the key issues and future research directions of plant community stability and its mechanism.

Isolated individual processes of ecosystem carbon (C) cycles have largely shaped our understanding of C cycle processes under environmental change. Yet, in reality, C cycle processes are inter-related and hierarchical. How these processes respond to warming and grazing has rarely been investigated in a single manipulative experiment. Moreover, biodiversity loss is a major driver of ecosystem change under environmental change, but whether these responses are mechanistically linked to biodiversity remains unclear. Here, we performed a 5-year field manipulative warming with seasonal grazing experiment in an alpine meadow on the Qinghai-Tibetan Plateau. Our results showed that both warming and moderate grazing decreased net ecosystem productivity (NEP) by 42.1% and 38.3%, and their interaction decreased it by 56.2% during the summer grazing period. However, they had no significant effects on NEP during the winter grazing period. Overall, annual gross primary productivity (GPP) and ecosystem respiration (Re) were mainly determined by aboveground rather than belowground processes, and Re variation which was mainly controlled by aboveground respiration explained 50% of the variation in annual NEP under warming and grazing. Moreover, lower species richness induced by warming and grazing caused smaller NEP with smaller net primary productivity and higher aboveground respiration. The responses of aboveground C cycle processes were greater than that of belowground C cycle processes, suggesting asymmetric above- and belowground responses to warming and grazing. Therefore, our findings suggested that there were higher GPP and Re with lower C sequestration (‘two high with one low patterns’) under warming and moderate grazing. Plant diversity modulated the responses of soil C sequestration to warming and grazing. It is essential to understand the underlying mechanisms of the effects of biodiversity on hierarchical C cycle processes under combined warming and grazing in the future.

Aim: Chronic directional climate changes in temperature and precipitation are predicted to increase the frequency of extreme climatic events (ECEs); however, their co-occurring effects on the temporal stability of community productivity (i.e. ANPP stability) are still unclear. Here, we evaluate whether the increased frequency of ECEs reduces ANPP stability, and how it modulates the effects of chronic directional climate factors on ANPP stability in natural grassland. Location: Twenty-two sites in Asia and 14 sites in North America. Time period: 1980s-2010s. Major taxa studied: Herbaceous plant. Methods: We collected 36 long-term observational and consecutive ANPP data (at least 10 years) and resampled yearly ANPP via a consecutive resampling method of nested time windows for each field. We used linear mixed-effect models, partial regression analysis and structure equation models to explore the interactive effects of three climatic factors on ANPP stability and their associated intermediate processes of sensitivity, asymmetry, resistance and resilience. Results: The increased frequency of ECEs was observed within the long-term rising temperature and elevating precipitation trend across sites in the past several decades. Elevating precipitation rather than rising temperature was the primary driver influencing ANPP stability. Elevating precipitation increased ANPP stability through increasing mean ANPP and decreasing the standard deviation (i.e. SD) of ANPP due to a decrease in sensitivity of ANPP to precipitation. The increased frequency of ECEs decreased ANPP stability mainly by increasing the SD of ANPP, and it reduced the positive effect of elevated precipitation on ANPP stability via a decrease in resilience. Main conclusion: Our results demonstrated that recurrent and discrete ECEs had cumulatively negative effects on ANPP stability, and the decreased resilience was identified as the primary factor reducing the grassland community stability under long-term climate change. This highlighted the potential risks of increased frequency of ECEs for grassland ecosystem functions.

Temperature and precipitation are the main factors determining plant community succession and aboveground net primary productivity (ANPP) in natural grasslands. However, most climate manipulative experiments have mainly focused on their impacts in isolation, especially in alpine regions. Here we explored the relative effects and interaction of warming and precipitation alteration on succession and ANPP using a 7-year experiment involving warming with precipitation alteration (increase (IP) or decrease (DP) in precipitation relative to ambient precipitation (AP)) on the Tibetan Plateau. Our results showed that warming and warm-wet conditions increased species richness, diversity index, height and cover of overall species. Conversely, decreasing precipitation reduced them, but increased S. purpurea and the rate of change in community composition. Importantly, warming mitigated the impacts of decreased precipitation on plant community composition, and the interactive effects of warming and altered precipitation on cover, height and plant ANPP varied with year and plant species. Generally, warming increased community ANPP through increases in forb ANPP or non-dominant species ANPP and biodiversity regardless of change in precipitation. However, decreased precipitation reduced community ANPP via decreases in the ANPP of sedges and forbs and biodiversity. Precipitation alteration affected the relationship between biodiversity and community ANPP regardless of warming (IP

It is still debated whether warming induces degradation of alpine grasslands and whether warming accelerates the restoration of degraded alpine grasslands. Here we conducted a warming experiment using open top chambers to explore warming effects on plant composition and diversity, aboveground net primary productivity (ANPP) and net ecosystem carbon exchange (NEE) in alpine grassland at three degradation levels (i.e., no-degradation, moderate degradation and desertification) on the Tibetan Plateau from 2014 to 2016. There were no interactive effects between warming and degradation level on the variables measured in our study, except for plant diversity. Warming enhanced plant species diversity at no-degradation sites but had no significant influence on species diversity at sites of moderate degradation and desertification. Generally, warming alone had no significant effects on ANPP or seasonal average NEE during the experimental period, but its effect on them varied with year. In addition, compared with non-degraded sites, heavy degradation decreased ANPP by 35.8%, 44.2% and 63.8%, and reduced NEE by 71.8%, 63.4% and 70.3% in 2014, 2015 and 2016, respectively. The effects of moderate degradation on ANPP and NEE varied with year, enhancing ANPP by 37.3% and 37.7% in 2015 and 2016, but moderate degradation had no remarkable influence on seasonal average NEE compared with non-degraded sites. These results suggest that warming neither accelerates degradation of alpine grasslands nor promotes restoration of degraded alpine grasslands on the Tibetan Plateau.

There is a debate about unmatched results between manipulative warming using constant warming rates every year (CW) and long-term observations warming affect temperature sensitivity of flowering phenology. This may be because long-term observations represent the actual yearly increase in temperature (i.e. a yearly stepwise warming rate per year, SW) which would differ from CW and their effects would be regulated by precipitation alteration. We conducted a warming experiment with CW (temperature increase by +1 degrees C and sustained this elevated temperature for the duration of the study) and SW (temperature increase by + 0.25 degrees C progressively each year) with precipitation addition in an alpine grassland for four years. Our results showed that neither warming rate affected community flowering phenology. However, precipitation addition advanced onsets of flowering for early-spring flowering (ESF) and mid-summer flowering (MSF) groups, and advanced the end date of flowering for ESF but delayed it for the MSF group. Therefore, flowering duration remained stable for the ESF group and prolonged for the MSF group, and further prolonging the flowering duration of the community. There were no interactions between warming rates and precipitation addition on the community's flowering phenology. A severe drought in a year significantly decreased the maximal number of community flowers in the following year. Therefore, a change in precipitation has a greater effect than warming on the community flowering phenology in the semi-arid alpine grassland.

Both warming and grazing already affect the reproductive phenology of alpine plants. However, their effects have mostly been studied in isolation, and their interaction is still unclear. In this study, an asymmetric warming (average + 1.2 °C during daytime and + 1.7 °C during nighttime and + 1.5 °C during summer and + 2.0 °C during winter) with moderate grazing experiment was conducted for four years to determine their individual and interactive effects on the onsets and durations of reproductive phenophases for fifteen alpine plant species on the Qinghai-Tibetan Plateau. Individual warming and grazing simultaneously advanced the average start dates and ending dates of budding, flowering and fruiting by 5.3-6.2 days, and further resulted in smaller effects on their durations for most plant species. The interactions between warming and grazing on them varied with plant species and year, which advanced by average 12.1 days for all plant species. The effects of grazing on the temperature sensitivity of the start dates of reproductive phenophases (average by -8.5 days °C-1) were greater than that of warming alone (average by -3.4 days °C-1) and warming with grazing (average by -5.5 days °C-1) for most of the alpine plant species. There were significant effects of the previous phenological events on subsequent reproductive phenophases. Therefore, our results suggested that both warming and grazing advanced reproductive phenophases through altered soil temperature and soil moisture and carry-over effects of previous phenological events on subsequent phenological events. Warming reduced the temperature sensitivity of the start dates of reproductive phenophases to grazing, suggesting that it depressed strength of selection pressure of grazing on the onsets of reproductive phenology in alpine plants.

Fine-root decomposition is a critical process regulating ecosystem carbon cycles and affecting nutrient cycling and soil fertility. However, whether interaction between warming and grazing affects fine-root decomposition is still under-researched in natural grasslands. A two-factorial experiment with asymmetric warming (i.e. daytime vs. nighttime and growing season vs. nongrowing season) and moderate grazing (i.e. about average 50% forage utilization rate) was conducted to explore whether warming and grazing affect fine-root decomposition and loss of nutrients during a 2-year decomposition period in an alpine meadow on the Tibetan Plateau. Both warming and grazing facilitated carbon cycling through increase in fine-root decomposition, and influenced element cycling which varies among elements. The effects of warming and grazing on fine-root decomposition and loss of nutrients were additive. Both warming and grazing significantly increased cumulative percentage mass loss and total organic carbon loss of fine roots during the 2-year experiment. Only warming with grazing treatment reduced percentage nitrogen loss, whereas warming, regardless of grazing, decreased percentage phosphorus loss. Warming and grazing alone increased percentage loss of potassium, sodium, calcium and magnesium compared with control. There were no interactions between warming and grazing on fine-root decomposition and loss of nutrients. There was greater temperature sensitivity of decreased phosphorus loss than that of decreased nitrogen loss. Different temperature sensitivities of percentage loss of nutrients from fine-root decomposition would alter ratios of the available nutrients in soils, and may further affect ecosystem structure and functions in future warming.

The impacts of human-driven environmental changes on the stability of natural grasslands have been assessed by comparing differences between manipulative warming and grazing plots and reference plots. However, little is known about whether or how ambient climate regulates the effects of manipulative treatments. A 36-year observational dataset shows that there is a nonlinear response of community stability to ambient climate. Manipulative warming and grazing decrease community stability with experiment duration through an increase in legume coverage and/or decrease in species asynchrony, due to exceeding the threshold of background annual mean air temperature with decreasing background annual mean air temperature through time during the 10-year experiment period. Moreover, the temperature sensitivity of community stability is more sensitive under the ambient treatment than under the manipulative treatments. Therefore, our study emphasizes the importance of the context dependency of the response of community stability to human-driven environmental changes.

The ongoing phenological changes in vegetation on the Qinghai–Tibetan Plateau could modify land surface and atmospheric processes. In this Review, we summarize these changes, their drivers and the resulting impacts. The start of the growing season advanced by 9.4 ± 2.2 days during 1982–1999 and 8.3 ± 2.0 days over 2000–2020, and the end of season delayed by 8.2 ± 1.9 days during 2000–2020. The main identified drivers of these changes are warming temperatures and increasing precipitation, but their impacts vary substantially across the Qinghai–Tibetan Plateau. Other factors, such as grazing and nitrogen deposition, also potentially influence phenological changes, but these relationships are poorly constrained. In manipulation experiments, grazing and nitrogen addition have no individual effects on most phenophase timings at the population level, but nitrogen addition markedly delays flowering. Additionally, there are carry-over effects between phenophases that control subsequent temperature and precipitation responses. Phenological changes in turn could alter species interactions, modulate carbon and water cycling, and affect Asian monsoons and spring rainfall over eastern China, but evidence of these interactions is limited. Harmonization of remote-sensing-based and in situ observations, and simultaneous testing of both biotic and abiotic factors, are needed for a mechanistic understanding of Qinghai–Tibetan Plateau phenology dynamics.

The pools of plant community carbon (C) and nitrogen (N) are important sources of soil organic matter in terrestrial ecosystems and directly affect soil C and N cycling. A large amount of studies were manipulated with multiple N levels on soil C and N pools. However, how and whether the frequency of N addition can affect the plant C and N pools is still unclear. In order to comprehensively understand the N addition effects (including frequencies and levels) on C and N pools of the plant community, we executed a randomized complete block experiment with the addition of five levels of N, including 0, 2, 10, 20 and 50 g N m−2 yr−1 (designated as N-0, N-2, N-10, N-20 and N-50) and two N addition frequencies (twice a year vs. monthly, F2, F12) in August of 2008. After 5 years of treatment, the physical-chemical properties of the plants and soil were measured in 2013. The results indicated that with increasing N addition levels, the C and N pools of the plant community significantly increased, while N addition frequency had no significant effects. Moreover, significant interactions between N addition levels and the frequencies on the C and N pools of the plant community were also found in this typical grassland. Under different frequencies of N addition treatment, the plant community C and N pools showed different response patterns along with N addition levels in plants aboveground and belowground, respectively. Under different frequencies of N addition, the changes in the C and N pools of the plant community caused by N addition were regulated by different environmental factors. We highlight that long-term N deposition could affect the plant community C and N pools and would influence C and N cycling of terrestrial ecosystems based on global climate change in the future.

Warming and grazing, and litter quality jointly determine litter decomposition and nutrient releases in grazing ecosystems. However, their effects have previously been studied in isolation. We conducted a two factorial experiment with asymmetric warming using infrared heaters and moderate grazing in an alpine meadow. Litter samples were collected from all plots in each treatment, among which some subsamples were placed in their original plots and other samples were translocated to other treatment plots to test the relative effects of each treatment on litter decomposition and nutrient releases. We found that warming rather than grazing alone significantly increased total losses of litter mass, total organic carbon, total nitrogen (TN) and total phosphorus (TP) per unit area due to increases in both mass loss rates and litter biomass. However, grazing with warming did not affect their total mass losses because increased mass loss was offset by decreased litter biomass compared with the control. Seasonal mean soil temperature better predicted litter decomposition than litter lignin content or carbon to nitrogen ratio. There were interactions between warming and grazing, but there were no interactions between them and litter quality on litter decomposition. The temperature sensitivity of TN loss was higher than that of TP loss per unit area. Our results suggest that increased temperature has a greater effect on litter decomposition and nutrient release than change in litter quality, and that more N release from litter could result in greater P deficiency in the alpine meadow.

Grasslands on the Qinghai–Tibetan Plateau (QTP) foster biodiversity, store carbon, maintain productivity and support pastoral livelihoods. These systems are being altered by climate change and anthropogenic activities, but the relative importance of these drivers are still debated. This Review examines QTP grassland changes since the 1980s and discusses the impacts of global change on plant communities and soil properties. The normalized difference vegetation index (NDVI) (which can be used to track vegetation greenness) has generally increased since the 1980s, but with substantial spatial variability and some local decreases. Rising temperatures were key in driving the NDVI increases, but also likely exacerbated water deficiency in areas with little precipitation (<100 mm year –1 ), accounting for some of the spatial variability in trends. Intense livestock grazing negatively affects vegetation and soil when stocking rates are higher than grassland carrying capacity, causing grassland degradation. Degraded grassland can be effectively restored by management policies that minimize or exclude grazing and by adaptive management; these practices became important drivers of net primary production increases after 2000. However, better management of grasslands under a future of increasing temperatures and settlement requires a deeper understanding of the large-scale plant species composition shifts and the combined effects of climate change and anthropogenic activities.

Leaf senescence is the final stage in the life cycle of leaves and is critical to plants' fitness as well as to ecosystem carbon and nutrient cycling. To date, most understanding about the responses of leaf senescence to environmental changes has derived from research in forests, but the topic has been relatively neglected, especially under grazing conditions, in natural grasslands. We conducted a 3-year manipulative asymmetric warming with moderate grazing experiment to explore the responses of leaf senescence of five main species in an alpine meadow on the Qinghai-Tibetan Plateau. We found that warming prolonged leaf longevity through earlier leaf-out and later leaf senescence, and grazing prolonged it through a greater advance in leaf-out than first leaf coloration for all plants. Warming did not affect leaf nitrogen (N) content or N resorption efficiency (NRE), but grazing increased N content in coloring leaves for P. anserine and P. nivea and decreased NRE for K. humilis, P. anserine under no-warming, and for P. nivea under warming. The interactive effects of warming and grazing on leaf phenology and leaf traits depended on species identity and year. There were positive relationships between leaf-out and leaf senescence mainly derived from grazing, and positive relationships between NRE from old leaves and leaf senescence for three out of five plant species. Therefore, our results indicated that earlier leaf-out could result in earlier leaf senescence only under grazing, but depending on plant species. Delayed leaf coloring increased NRE from old leaves for some plant species measured under warming and grazing. Our results suggested that alpine plants may develop strategies to adapt to warming and grazing to assimilate more carbon through prolonged leaf longevity rather than increased NRE through earlier leaf coloring in the alpine meadow.

The ongoing phenological changes in vegetation on the Qinghai–Tibetan Plateau could modify land surface and atmospheric processes. In this Review, we summarize these changes, their drivers and the resulting impacts. The start of the growing season advanced by 9.4 ± 2.2 days during 1982–1999 and 8.3 ± 2.0 days over 2000–2020, and the end of season delayed by 8.2 ± 1.9 days during 2000–2020. The main identified drivers of these changes are warming temperatures and increasing precipitation, but their impacts vary substantially across the Qinghai–Tibetan Plateau. Other factors, such as grazing and nitrogen deposition, also potentially influence phenological changes, but these relationships are poorly constrained. In manipulation experiments, grazing and nitrogen addition have no individual effects on most phenophase timings at the population level, but nitrogen addition markedly delays flowering. Additionally, there are carry-over effects between phenophases that control subsequent temperature and precipitation responses. Phenological changes in turn could alter species interactions, modulate carbon and water cycling, and affect Asian monsoons and spring rainfall over eastern China, but evidence of these interactions is limited. Harmonization of remote-sensing-based and in situ observations, and simultaneous testing of both biotic and abiotic factors, are needed for a mechanistic understanding of Qinghai–Tibetan Plateau phenology dynamics. Warming temperatures, shifting precipitation patterns and human activities on the Qinghai–Tibetan Plateau are impacting phenology, including advancing the start and delaying the end of the growing season. This Review examines the drivers, patterns and impacts of changing vegetation phenology across this region.

Changes in precipitation patterns and eutrophication can cause changes in plant traits and abundance, potentially affecting plant community structure and functions. Here, we studied responses of traits and abundance of Potentilla anserina to precipitation change and nitrogen (N) and phosphorus (P) additions, and the effect of traits on its abundance in an alpine meadow of the Qinghai-Tibet Plateau. We found that precipitation change and N and P additions significantly affected the mean value of traits such as specific leaf area (SLA), leaf dry matter content (LDMC), single leaf area, plant height and individual size, while only P addition significantly affected intraspecific variation of SLA and individual size. Increased precipitation and N and P additions shifted plant traits to more resource acquisitive, and increased plant abundance. Responses of plant traits to P addition were larger than that of N addition. Plant abundance was mainly affected by precipitation, and was limited by N or P dependent on precipitation conditions. In conclusions, our research shows that P. anserina can respond to environmental changes by changing its traits to improve its adaptability, potentially affecting community structure and ecosystem functions.

Changes in ecological processes over time in ambient treatments are often larger than the responses to manipulative treatments in climate change experiments. However, the impacts of human-driven environmental changes on the stability of natural grasslands have been typically assessed by comparing differences between manipulative plots and reference plots. Little is known about whether or how ambient climate regulates the effects of manipulative treatments and their underlying mechanisms. We collected two datasets, one a 36-year long-term observational dataset from 1983 to 2018, and the other a 10-year manipulative asymmetric warming and grazing experiment using infrared heaters with moderate grazing from 2006 to 2015 in an alpine meadow on the Tibetan Plateau. The 36-year observational dataset shows that there was a nonlinear response of community stability to ambient temperature with a positive relationship between them due to an increase in ambient temperature in the first 25 years and then a decrease in ambient temperature thereafter. Warming and grazing decreased community stability with experiment duration through an increase in legume cover and a decrease in species asynchrony, which was due to the decreasing background temperature through time during the 10-year experiment period. Moreover, the temperature sensitivity of community stability was higher under the ambient treatment than under the manipulative treatments. Therefore, our results suggested that ambient climate may control the directional trend of community stability while manipulative treatments may determine the temperature sensitivity of the response of community stability to climate relative to the ambient treatment. Our study emphasizes the importance of the context dependency of the response of community stability to human-driven environmental changes.