Permafrost and drought regulate vulnerability of Tibetan Plateau grasslands to warming

Warming positively promoted community appearance restoration of the degraded alpine meadow although accompanied by topsoil drying

On-going climate warming is threatening the ecological function of grassland ecosystems. However, whether warming has positive effects on community microhabitats and appearance, especially in degraded grasslands, remains elusive. To address this issue, we conducted a 2-year field experiment on the severely degraded alpine meadow and undegraded alpine meadow with no warming and warming treatments. Community coverage and height in degraded meadow significantly increased under warming, while these changes were not significant in undegraded meadow. Two-year warming increased the community height of degraded meadow and undegraded meadow by 56.55% and 10.99%, respectively. Warming also increased community coverage by 41.88% in degraded meadow and decreased community coverage by 3.01% in undegraded meadow. Moreover, the response of topsoil temperature to warming was stronger in degraded meadow (6.89%) than in undegraded meadow (- 0.26%), while the negative response of topsoil moisture to warming was weaker in degraded meadow (- 13.95%) than in undegraded meadow (- 20.00%). The SEMs further demonstrated that warming had positive effects on topsoil temperature and community height, while had negative effects on topsoil moisture both in degraded and undegraded meadows. Our results confirm that warming-induced soil drying is an important pathway affecting the community appearance in alpine meadows. These findings highlight that warming has positive effects on community height and coverage and is particularly effective in improving community coverage appearance in severely degraded alpine meadow with topsoil drying.

Cumulative effects of drought have an impact on net primary productivity stability in Central Asian grasslands

Global warming has exacerbated the threat of drought in Central Asia, amplifying its ecological implications within the region's grassland ecosystems. This has become an increasingly prominent issue that requires attention and action. The temporal link between grassland development and drought is asymmetric. However, a quantitative assessment of the temporal effects of multiscale drought on Central Asian grasslands has yet to be explored. Based on correlation analysis and the coefficient of variation method, this study analysed the cumulative and lag effects of multitimescale drought on grassland NPP (net primary productivity) under different climatic zones, altitudes and water availabilities in Central Asia from 1982 to 2018, and discussed the impact of temporal effects on grassland NPP stability. Our results on the cumulative effects of drought on grasslands indicate the 6.72 months preceding NPP measurement was the duration for which, on average, drought was most strongly correlated with NPP. Additionally, we found a mean lagged effect of 5.36 months, meaning that the monthly drought 5.36 months prior to NPP measurement was, on average, most strongly correlated with NPP. The degree to which grassland NPP was affected by cumulative drought at a given level of water availability was inversely proportional to the number of cumulative drought months. Under different water availabilities, the lagged effect of grassland NPP was stronger in dry areas than in wet areas, and the number of lag months tended to decrease and then increase as the water availability increased. The percentage of areas where grassland NPP was dominated by the cumulative and lagging effects of drought was 30.02% and 69.98%, respectively. The stability of grassland NPP was adversely affected by the drought accumulation effect. The findings of this study contribute to a deeper understanding of the long-term effects of drought on grassland ecosystems. Additionally, it will aid in the development of strategies for mitigating and adapting to drought events, thereby minimizing their negative impacts on agriculture, livestock, and ecosystems.

Predicting the Responses of Functional Leaf Traits to Global Warming: An In Situ Temperature Manipulation Design Using Iris pumila L

Phenotypic plasticity is widely acknowledged as one of the most common solutions for coping with novel environmental conditions following climate change. However, it is less known whether the current amounts of trait plasticity, which is sufficient for matching with the contemporary climate, will be adequate when global temperatures exceed historical levels. We addressed this issue by exploring the responses of functional and structural leaf traits in Iris pumila clonal individuals to experimentally increased temperatures (~1.5 °C) using an open top chamber (OTC) design. We determined the phenotypic values of the specific leaf area, leaf dry matter content, specific leaf water content, and leaf thickness in the leaves sampled from the same clone inside and outside of the OTC deployed on it, over seasons and years within two natural populations. We analyzed the data using a repeated multivariate analysis of variance, which primarily focusses on the profiles (reaction norms (RNs)) of a variable gathered from the same individual at several different time points. We found that the mean RNs of all analyzed traits were parallel regardless of experienced temperatures, but differed in the level and the shape. The populations RNs were similar as well. As the amount of plasticity in the analyzed leaf trait was adequate for coping with elevated temperatures inside the OTCs, we predict that it will be also sufficient for responding to increased temperatures if they exceed the 1.5 °C target.

Response of Aboveground Net Primary Production, Species and Phylogenetic Diversity to Warming and Increased Precipitation in an Alpine Meadow

The uncertain responses of aboveground net primary productivity (ANPP) and plant diversity to climate warming and increased precipitation will limit our ability to predict changes in vegetation productivity and plant diversity under future climate change and further constrain our ability to protect biodiversity and ecosystems. A long-term experiment was conducted to explore the responses of ANPP, plant species, phylogenetic α-diversity, and community composition to warming and increased precipitation in an alpine meadow of the Northern Tibet from 2014 to 2019. Coverage, height, and species name were obtained by conventional community investigation methods, and ANPP was obtained using observed height and coverage. Open-top chambers with two different heights were used to simulate low- and high-level climate warming. The low- and high-level increased precipitation treatments were achieved by using two kinds of surface area funnel devices. The high-level warming reduced sedge ANPP (ANPPsedge) by 62.81%, species richness (SR) by 21.05%, Shannon by 13.06%, and phylogenetic diversity (PD) by 14.48%, but increased forb ANPP (ANPPforb) by 56.65% and mean nearest taxon distance (MNTD) by 33.88%. Species richness, Shannon, and PD of the high-level warming were 19.64%, 9.67%, and 14.66% lower than those of the low-level warming, respectively. The high-level warming-induced dissimilarity magnitudes of species and phylogenetic composition were greater than those caused by low-level warming. The low- rather than high-level increased precipitation altered species and phylogenetic composition. There were significant inter-annual variations of ANPP, plant species, phylogenetic α-diversity and community composition. Therefore, climate warming and increased precipitation had non-linear effects on ANPP and plant diversity, which were due to non-linear changes in temperature, water availability, and/or soil nutrition caused by warming and increased precipitation. The inter-annual variations of ANPP and plant diversity were stronger than the effects of warming and especially increased precipitation on ANPP and plant diversity. In terms of plant diversity conservation and related policy formulation, we should pay more attention to regions with greater warming, at least for the northern Tibet grasslands. Besides paying attention to the responses of ANPP and plant diversity to climate change, the large inter-annual changes of ANPP and plant diversity should be given great attention because the large inter-annual variation indicates the low temporal stability of ANPP and plant diversity and thus produces great uncertainty for the development of animal husbandry.

Pan-Arctic soil moisture control on tundra carbon sequestration and plant productivity

Long-term atmospheric CO₂ concentration records have suggested a reduction in the positive effect of warming on high-latitude carbon uptake since the 1990s. A variety of mechanisms have been proposed to explain the reduced net carbon sink of northern ecosystems with increased air temperature, including water stress on vegetation and increased respiration over recent decades. However, the lack of consistent long-term carbon flux and in situ soil moisture data has severely limited our ability to identify the mechanisms responsible for the recent reduced carbon sink strength. In this study, we used a record of nearly 100 site-years of eddy covariance data from 11 continuous permafrost tundra sites distributed across the circumpolar Arctic to test the temperature (expressed as growing degree days, GDD) responses of gross primary production (GPP), net ecosystem exchange (NEE), and ecosystem respiration (ER) at different periods of the summer (early, peak, and late summer) including dominant tundra vegetation classes (graminoids and mosses, and shrubs). We further tested GPP, NEE, and ER relationships with soil moisture and vapor pressure deficit to identify potential moisture limitations on plant productivity and net carbon exchange. Our results show a decrease in GPP with rising GDD during the peak summer (July) for both vegetation classes, and a significant relationship between the peak summer GPP and soil moisture after statistically controlling for GDD in a partial correlation analysis. These results suggest that tundra ecosystems might not benefit from increased temperature as much as suggested by several terrestrial biosphere models, if decreased soil moisture limits the peak summer plant productivity, reducing the ability of these ecosystems to sequester carbon during the summer.

Warming enhances dominance of vascular plants over cryptogams across northern wetlands

Climate warming causes profound effects on structure and function of wetland ecosystem, thus affecting regional and global hydrological cycles and carbon budgets. However, how wetland plants respond to warming is still poorly understood. Here, we synthesized observations from 273 independent sites to explore responses of northern wetland plants to warming. Our results show that warming enhances biomass accumulation for vascular plants including shrubs and graminoids, whereas it reduces biomass accumulation for cryptogams including moss and lichen. This divergent response of vascular plants and cryptogams is particularly pronounced in the high latitudes where permafrost prevails. As warming continues, this divergent response is amplified, however, the reduction in cryptogams is more drastic. Warming leads to declined surface soil moisture and lowered water table, thereby shifting wetlands from a wet system dominated by cryptogams to a drier system with increased cover of vascular plants. Under a high-emission scenario of Shared Socioeconomic Pathways (SSP5), a 4.7-5.1°C mean global temperature rise will cause more than fivefold loss of cryptogams compared with current climate. As cryptogams are largely concentrated at northern high latitudes, where warming will likely be greater than the projected global mean, modification in wetland plant composition and major reduction in cryptogams are expected to occur even much earlier than 2100.

Biodiversity showed positive effects on resistance but mixed effects on resilience to climatic extremes in a long-term grassland experiment

Understanding the role of biodiversity in maintaining ecosystem functioning and stability under increasing frequency and magnitude of climatic extremes has fascinated ecologists for decades. Although growing evidence suggests that biodiversity affects ecosystem productivity and buffers ecosystem against climatic extremes, it remains unclear whether the stability of an ecosystem is caused by its resistance against disturbances or resilience towards perturbations or both. In attempting to explore how species richness affects resistance and resilience of above-ground net primary productivity (ANPP) against climatic extremes, we analyzed the grassland ANPP of the long-running (1997-2020) Bayreuth Biodiversity experiment in Germany. We used the Standardized Precipitation Evapotranspiration Index to identify climatic conditions based on 5- and 7-class classifications of climatic conditions. Mixed-effects models and post-hoc test show that ANPP varied significantly among different intensities (e.g. moderate or extreme) and directions (e.g. dry or wet) of climatic conditions, with the highest ANPP in extreme wet and the lowest in extreme dry conditions. Resistance and resilience of ANPP to climatic extremes in different intensities were examined by linear-mixed effects models and we found that species richness increased ecosystem resistance against all dry and wet climatic extremes, but decreased ecosystem resilience towards all dry climatic extremes. Species richness had no effects on ecosystem resilience towards wet climatic extremes. When the five level of species richness treatment (i.e., 1, 2, 4, 8, and 16 species) were considered, the relationships between species richness and resistance and resilience of ANPP under extreme wet and dry conditions remained similar. Our study emphasizes that plant communities with greater species richness need to be maintained to stabilize ecosystem productivity and increase resistance against different climatic extremes.

Driving Climatic Factors at Critical Plant Developmental Stages for Qinghai–Tibet Plateau Alpine Grassland Productivity

Determining the driving climatic factors at critical periods and potential legacy effects is crucial for grassland productivity predictions on the Qinghai–Tibet Plateau (QTP). However, studies with limited and ex situ ground samples from highly heterogeneous alpine meadows brought great uncertainties. This study determined the key climatic factors at critical plant developmental stages and the impact of previous plant growth status for interannual aboveground net primary productivity (ANPP) variations in different QTP grassland types. We hypothesize that the impact of climatic factors on grassland productivity varies in different periods and different vegetation types, while its legacy effects are not great. Pixel-based partial least squares regression was used to associate interannual ANPP with precipitation and air temperature at different developmental stages and prior-year ANPP from 2000 to 2019 using remote sensing techniques. Results indicated different findings from previous studies. Precipitation at the reproductive stage (July–August) was the most prominent controlling factor for ANPP which was also significantly affected by precipitation and temperature at the withering (September–October) and dormant stage (November–February), respectively. The influence of precipitation was more significant in alpine meadows than in alpine steppes, while the differentiated responses to climatic factors were attributed to differences in water consumption at different developmental stages induced by leaf area changes, bud sprouting, growth, and protection from frost damage. The prior-year ANPP showed a non-significant impact on ANPP of current year, except for alpine steppes, and this impact was much less than that of current-year climatic factors, which may be attributed to the reduced annual ANPP variations related to the inter-annual carbon circulation of alpine perennial herbaceous plants and diverse root/shoot ratios in different vegetation types. These findings can assist in improving the interannual ANPP predictions on the QTP under global climate change.

Vegetation green-up date is more sensitive to permafrost degradation than climate change in spring across the northern permafrost region

Global climate change substantially influences vegetation spring phenology, that is, green-up date (GUD), in the northern permafrost region. Changes in GUD regulate ecosystem carbon uptake, further feeding back to local and regional climate systems. Extant studies mainly focused on the direct effects of climate factors, such as temperature, precipitation, and insolation; however, the responses of GUD to permafrost degradation caused by warming (i.e., indirect effects) remain elusive yet. In this study, we examined the impacts of permafrost degradation on GUD by analyzing the long-term trend of satellite-based GUD in relation to permafrost degradation measured by the start of thaw (SOT) and active layer thickness (ALT). We found significant trends of advancing GUD, SOT, and thickening ALT (p < 0.05), with a spatially averaged slope of -2.1 days decade^-1 , -4.1 days decade^-1 , and +1.1 cm decade^-1 , respectively. Using partial correlation analyses, we found more than half of the regions with significantly negative correlations between spring temperature and GUD became nonsignificant after considering permafrost degradation. GUD exhibits dominant-positive (37.6% vs. 0.6%) and dominant-negative (1.8% vs. 35.1%) responses to SOT and ALT, respectively. Earlier SOT and thicker ALT would enhance soil water availability, thus alleviating water stress for vegetation green-up. Based on sensitivity analyses, permafrost degradation was the dominant factor controlling GUD variations in 41.7% of the regions, whereas only 19.6% of the regions were dominated by other climatic factors (i.e., temperature, precipitation, and insolation). Our results indicate that GUDs were more sensitive to permafrost degradation than direct climate change in spring among different vegetation types, especially in high latitudes. This study reveals the significant impacts of permafrost degradation on vegetation GUD and highlights the importance of permafrost status in better understanding spring phenological responses to future climate change.

Nutrient Analysis and Species Diversity of Alpine Grasslands: A Comparative Analysis of Less Studied Biodiversity Hotspots

The alpine grasslands of Kashmir Himalaya act as a treasure house of floristic biodiversity. They have remained largely unstudied because of their remoteness and inaccessibility. It is imperative to have quantitative studies of these areas to allow the long-term monitoring of flora in these fragile ecosystems. During the present study, nutrient analysis and species diversity of some alpine grasslands were investigated. Electroconductivity (EC) of the soils ranged between 0.12 and 0.33 (dSm−1). With an increase in altitude and precipitation and a decrease in temperature, soil pH and available macro-nutrients (OC, N, P, K) show a considerable decrease. Sixty-six plant species belonging to twenty-nine families and fifty-one genera were reported with members predominantly from the Asteraceae, Rosaceae and Plantaginaceae families. Seven species were common to all study areas and Renyi diversity profiles showed that Kongwattan was the most diverse followed by Poshpathri and Yousmarg. The results of the Sorensen β diversity index showed a relatively lower dissimilarity index among the three studied alpine sites. In the majority of the growth forms, growth initiation was recorded in April, whereas senescence occurred in September. The highest bloom was seen in June-July. The plant species exhibited a greater variability in their phenophases under different environmental conditions and altitudinal gradients. Plants were more vigorous at lower altitudes and showed rapid response to the prevailing conditions. Stoloniferous forbs and tussock forming graminoids such as Sibbaldia cuneata, Trifolium repens, Plantago major, Trifolium pratense, Poa compressa, Poa angustifolia, and Plantago lanceolata showed a greater importance value index (IVI). The sedentary system of livestock rearing at Yousmarg resulted in the decreased density of the palatable species. This study allowed us to conclude that direct knowledge of soil nutrient composition and species diversity in alpine ecosystems can enhance conservation and ensure better management practices over a period of time.

Plant uptake of CO2 outpaces losses from permafrost and plant respiration on the Tibetan Plateau

High-latitude and high-altitude regions contain vast stores of permafrost carbon. Climate warming may result in the release of CO2 from both the thawing of permafrost and accelerated autotrophic respiration, but it may also increase the fixation of CO2 by plants, which could relieve or even offset the CO2 losses. The Tibetan Plateau contains the largest area of alpine permafrost on Earth. However, the current status of the net CO2 balance and feedbacks to warming remain unclear, given that the region has recently experienced an atmospheric warming rate of over 0.3 °C decade-1 We examined 32 eddy covariance sites and found an unexpected net CO2 sink during 2002 to 2020 (26 of the sites yielded a net CO2 sink) that was four times the amount previously estimated. The CO2 sink peaked at an altitude of roughly 4,000 m, with the sink at lower and higher altitudes limited by a low carbon use efficiency and a cold, dry climate, respectively. The fixation of CO2 in summer is more dependent on temperature than the loss of CO2 than it is in the winter months, especially at higher altitudes. Consistently, 16 manipulative experiments and 18 model simulations showed that the fixation of CO2 by plants will outpace the loss of CO2 under a wetting-warming climate until the 2090s (178 to 318 Tg C y-1). We therefore suggest that there is a plant-dominated negative feedback to climate warming on the Tibetan Plateau.

Projections of surface air temperature required to sustain permafrost and importance of adaptation to climate change in the Daisetsu Mountains, Japan

Permafrost is known to occur in high mountainous areas such as the Daisetsu Mountains in Japan, which are located at the southernmost limit of the permafrost distribution in the world. In this study, areas with climatic conditions suitable for sustaining permafrost in the Daisetsu Mountains are projected using bias-corrected and downscaled climate model outputs and statistical relationships between surface air temperatures and permafrost areas. Using freezing and thawing indices, the size of the area in the Daisetsu Mountains where climatic conditions were suitable for permafrost were estimated to be approximately 150 km² in 2010. Under the RCP8.5 scenario, this area is projected to decrease to about 30 km² by 2050 and it is projected to disappear by around 2070. Under the RCP2.6 scenario, the area is projected to decrease to approximately 20 km² by 2100. The degradation of mountain permafrost could potentially affect the stability of trekking trails due to slope displacement, and it may also have deleterious effects on current alpine ecosystems. It is therefore important to accurately monitor changes in the mountain ecosystem environment and to implement measures to adapt to an environment that is projected to change significantly in the future.

Plants with lengthened phenophases increase their dominance under warming in an alpine plant community

Predicting how shifts in plant phenology affect species dominance remains challenging, because plant phenology and species dominance have been largely investigated independently. Moreover, most phenological research has primarily focused on phenological firsts (leaf-out and first flower dates), leading to a lack of representation of phenological lasts (leaf senescence and last flower) and full phenological periods (growing season length and flower duration). Here, we simultaneously investigated the effects of experimental warming on different phenological events of various species and species dominance in an alpine meadow on the Tibetan Plateau. Warming significantly advanced phenological firsts for most species but had variable effects on phenological lasts. As a result, warming tended to extend species' full phenological periods, although this trend was not significant for all species. Experimental warming reduced community evenness and differentially impacted species dominance. Shifts in full phenological periods, rather than a single shift in phenological firsts or phenological lasts, were associated with changes in species dominance. Species with lengthened full phenological periods under warming increased their dominance. Our results advance the understanding of how altered species-specific phenophases relate to changes in community structure in response to climate change.

Increasing sensitivity of alpine grasslands to climate variability along an elevational gradient on the Qinghai-Tibet Plateau

Monitoring and mapping the sensitivity of grassland ecosystems to climate change is crucial for developing sustainable local grassland management strategies. The sensitivity of alpine grasslands to climate change is considered to be high on the Qinghai-Tibet Plateau (QTP), yet little is known about its spatial pattern, and particularly the variations between different elevations. Here, based on the Normalized Difference Vegetation Index (NDVI) and three climate variables (air temperature, precipitation, and solar radiation), we modified a vegetation sensitivity index-approach to capture the relative sensitivity of alpine grassland productivity to climate variability on the QTP during 2000-2016. The results show that alpine grasslands on the southern QTP are more sensitive to climate variability overall, and that the climate factors driving alpine grassland dynamics are spatially heterogeneous. Alpine grasslands on the southern QTP are more sensitive to temperature variability, those on the northeastern QTP display strong responses to precipitation variability, and those on the central QTP are primarily influenced by a combination of radiation and temperature variability. The sensitivity of alpine grasslands to climate variability increases significantly along an elevational gradient, especially to temperature variability. This study underscores that alpine grasslands at higher elevations on the QTP are more sensitive to climate variability than those at lower elevations at the regional scale.

Response of plant production to growing/non-growing season asymmetric warming in an alpine meadow of the Northern Tibetan Plateau

A field growing/non-growing season asymmetric warming experiment (C: control, i.e., no warming in the entire year; GLNG: growing season warming lower than non-growing season warming; GHNG: growing season warming higher than non-growing season warming) was conducted in an alpine meadow of the Northern Tibetan Plateau in early June 2015. The effects of growing/non-growing season asymmetric warming on the normalized difference vegetation index (NDVI), soil adjusted vegetation index (SAVI), aboveground biomass (AGB) and gross primary production (GPP) in 2015-2017 were examined. The 'GLNG' and 'GHNG' treatments significantly increased the annual mean air temperature (T_a) by 2.95 °C and 2.76 °C, and the vapor pressure deficit (VPD) by 0.23 kPa and 0.28 kPa but significantly reduced the annual mean soil moisture (SM) by 0.02 m³ m^-3 and 0.02 m³ m^-3 respectively; however, changes in the annual mean T_a, VPD and SM were the same between the 'GLNG' and 'GHNG' treatments over the three years in 2015-2017. There were no significant differences in the SAVI and GPP among the 'C', 'GLNG' and 'GHNG' treatments over the three growing seasons in 2015-2017. The 'GLNG' and 'GHNG' treatments did not significantly affect the NDVI and AGB compared to 'C', whereas the NDVI and AGB under the 'GLNG' treatment were significantly greater than those under the 'GHNG' treatment over the three growing seasons in 2015-2017. The significant differences in NDVI and AGB between the 'GLNG' and 'GHNG' treatments may be attributed to the different effects under the 'GLNG' and 'GHNG' treatments on the non-growing season T_a, growing season water availability and soil nitrogen availability. Therefore, the non-growing season with a higher warming magnitude may have stronger effects on the aboveground plant production than did the growing season with a higher warming magnitude in the alpine meadow of the Northern Tibetan Plateau.

Warming and land use change concurrently erode ecosystem services in Tibet

Alpine meadows on the Tibetan Plateau comprise the largest alpine ecosystem in the world and provide critical ecosystem services, including forage production and carbon sequestration, on which people depend from local to global scales. However, the provision of these services may be threatened by climate warming combined with land use policies that are altering if and how pastoralists can continue to graze livestock, the dominant livelihood practice in this region for millennia. We synthesized findings from a climate warming and yak grazing experiment with landscape-level observations in central Tibet to gain insight into the trajectories of change that Tibet's alpine meadows will undergo in response to expected changes in climate and land use. We show that within 5 years, experimental warming drove an alpine community with intact, sedge-dominated turfs into a degraded state. With removal of livestock, consistent with policy intended to reverse degradation, a longer-term shift to a more shrub-dominated community will likely occur. Neither degraded nor shrub meadows produce forage or sequester carbon to the same degree as intact meadows, indicating that climate warming and drying will reduce the ability of Tibet's alpine meadows to provide key ecosystem services, and that livestock reduction policies intended to counteract trajectories of land degradation instead endanger contemporary livelihoods on the Tibetan Plateau.

Differential responses of carbon-degrading enzyme activities to warming: Implications for soil respiration

Extracellular enzymes catalyze rate-limiting steps in soil organic matter decomposition, and their activities (EEAs) play a key role in determining soil respiration (SR). Both EEAs and SR are highly sensitive to temperature, but their responses to climate warming remain poorly understood. Here, we present a meta-analysis on the response of soil cellulase and ligninase activities and SR to warming, synthesizing data from 56 studies. We found that warming significantly enhanced ligninase activity by 21.4% but had no effect on cellulase activity. Increases in ligninase activity were positively correlated with changes in SR, while no such relationship was found for cellulase. The warming response of ligninase activity was more closely related to the responses of SR than a wide range of environmental and experimental methodological factors. Furthermore, warming effects on ligninase activity increased with experiment duration. These results suggest that soil microorganisms sustain long-term increases in SR with warming by gradually increasing the degradation of the recalcitrant carbon pool.