High land-use intensity exacerbates shifts in grassland vegetation composition after severe experimental drought.

Climate change projections anticipate increased frequency and intensity of drought stress, but grassland responses to severe droughts and their potential to recover are poorly understood. In many grasslands, high land-use intensity has enhanced productivity and promoted resource-acquisitive species at the expense of resource-conservative ones. Such changes in plant functional composition could affect the resistance to drought and the recovery after drought of grassland ecosystems with consequences for feed productivity resilience and environmental stewardship. In a 12-site precipitation exclusion experiment in upland grassland ecosystems across Switzerland, we imposed severe edaphic drought in plots under rainout shelters and compared them with plots under ambient conditions. We used soil water potentials to scale drought stress across sites. Impacts of precipitation exclusion and drought legacy effects were examined along a gradient of land-use intensity to determine how grasslands resisted to, and recovered after drought. In the year of precipitation exclusion, aboveground net primary productivity (ANPP) in plots under rainout shelters was -15% to -56% lower than in control plots. Drought effects on ANPP increased with drought severity, specified as duration of topsoil water potential ψ < -100 kPa, irrespective of land-use intensity. In the year after drought, ANPP had completely recovered, but total species diversity had declined by -10%. Perennial species showed elevated mortality, but species richness of annuals showed a small increase due to enhanced recruitment. In general, the more resource-acquisitive grasses increased at the expense of the deeper-rooted forbs after drought, suggesting that community reorganization was driven by competition rather than plant mortality. The negative effects of precipitation exclusion on forbs increased with land-use intensity. Our study suggests a synergistic impact of land-use intensification and climate change on grassland vegetation composition, and implies that biomass recovery after drought may occur at the expense of biodiversity maintenance.

Stampfli A ，Bloor JMG ，Fischer M ，Zeiter M ... -  《-》

Lasting effects of climate disturbance on perennial grassland above-ground biomass production under two cutting frequencies.

Climate extremes can ultimately reshape grassland services such as forage production and change plant functional type composition. This 3-year field research studied resistance to dehydration and recovery after rehydration of plant community and plant functional types in an upland perennial grassland subjected to climate and cutting frequency (Cut+, Cut-) disturbances by measuring green tissue percentage and above-ground biomass production (ANPP). In year 1, a climate disturbance gradient was applied by co-manipulating temperature and precipitation. Four treatments were considered: control and warming-drought climatic treatment, with or without extreme summer event. In year 2, control and warming-drought treatments were maintained without extreme. In year 3, all treatments received ambient climatic conditions. We found that the grassland community was very sensitive to dehydration during the summer extreme: aerial senescence reached 80% when cumulated climatic water balance fell to -156 mm and biomass declined by 78% at the end of summer. In autumn, canopy greenness and biomass totally recovered in control but not in the warming-drought treatment. However ANPP decreased under both climatic treatments, but the effect was stronger on Cut+ (-24%) than Cut- (-15%). This decline was not compensated by the presence of three functional types because they were negatively affected by the climatic treatments, suggesting an absence of buffering effect on grassland production. In the following 2 years, lasting effects of climate disturbance on ANPP were observable. The unexpected stressful conditions of year 3 induced a decline in grassland production in the Cut+ control treatment. The fact that this treatment cumulated higher (45%) N export over the 3 years suggests that N plays a key role in ANPP stability. As ANPP in this mesic perennial grassland did not show engineering resilience, long-term experimental manipulation is needed. Infrequent mowing appears more appropriate for sustaining grassland ANPP under future climate extremes.

Zwicke M ，Alessio GA ，Thiery L ，Falcimagne R ，Baumont R ，Rossignol N ，Soussana JF ，Picon-Cochard C ... -  《-》

Plant diversity moderates drought stress in grasslands: Implications from a large real-world study on (13)C natural abundances.

Land-use change and intensification play a key role in the current biodiversity crisis. The resulting species loss can have severe effects on ecosystem functions and services, thereby increasing ecosystem vulnerability to climate change. We explored whether land-use intensification (i.e. fertilization intensity), plant diversity and other potentially confounding environmental factors may be significantly related to water use (i.e. drought stress) of grassland plants. Drought stress was assessed using δ(13)C abundances in aboveground plant biomass of 150 grassland plots across a gradient of land-use intensity. Under water shortage, plants are forced to increasingly take up the heavier (13)C due to closing stomata leading to an enrichment of (13)C in biomass. Plants were sampled at the community level and for single species, which belong to three different functional groups (one grass, one herb, two legumes). Results show that plant diversity was significantly related to the δ(13)C signal in community, grass and legume biomass indicating that drought stress was lower under higher diversity, although this relation was not significant for the herb species under study. Fertilization, in turn, mostly increased drought stress as indicated by more positive δ(13)C values. This effect was mostly indirect by decreasing plant diversity. In line with these results, we found similar patterns in the δ(13)C signal of the organic matter in the topsoil, indicating a long history of these processes. Our study provided strong indication for a positive biodiversity-ecosystem functioning relationship with reduced drought stress at higher plant diversity. However, it also underlined a negative reinforcing situation: as land-use intensification decreases plant diversity in grasslands, this might subsequently increases drought sensitivity. Vice-versa, enhancing plant diversity in species-poor agricultural grasslands may moderate negative effects of future climate change.

Klaus VH ，Hölzel N ，Prati D ，Schmitt B ，Schöning I ，Schrumpf M ，Solly EF ，Hänsel F ，Fischer M ，Kleinebecker T ... -  《-》

Drought intensity and post-drought precipitation determine vegetation recovery in a desert steppe in Inner Mongolia, China.

Extreme drought events are expected to increase in frequency and severity, posing significant threats to ecosystems worldwide. While considerable research has been concentrated on the effects of climate extremes on the stability of grasslands, the process by which grassland productivity may recover after extreme drought events are still not well understood. Here, we conducted a four-year (2019-2022) recovery investigation after four-year's (2015-2018) extreme drought treatments of different intensities (control, press and pulse) to explore the vegetation recovery of desert-grassland ecosystems Inner Mongolia, China. Press drought involved a 66 % reduction in natural precipitation from May to August, while pulse drought reduced it by 100 % during June and July. We found that both press and pulse droughts led to a sharp decrease in aboveground net primary productivity (ANPP) after four years, primarily due to reduced growth, density, and productivity of annual and perennial plants. However, ANPP under pulse drought could recover fully after four years of stopping of drought treatment, and it could not under press drought. Additionally, community structure (i.e., species richness, plant density, and height) fully recovered within 1 year after the end of the two extreme drought treatments. Both plant density and height contributed to the ANPP recovery after press and pulse droughts. Structural equation modeling (SEM) results further revealed that the reduction in ANPP during the extreme drought was primarily due to a decrease in plant density caused by reduced soil water content. The recovery of ANPP in pulse drought was directly caused by increased soil water content in the post-extreme drought. These results suggest that drought intensity and precipitation determine ANPP recovery in a degraded desert steppe. Our findings are crucial for deepening understanding of the processes and mechanisms of ecosystem recovery after extreme drought, as well as for the successful management and protection of grassland ecosystems.

Li X ，Song Z ，Hu Y ，Qiao J ，Chen Y ，Wang S ，Yue P ，Chen M ，Ke Y ，Xu C ，Yu Q ，Zuo X ... -  《-》

Responses of a semiarid grassland to recurrent drought are linked to community functional composition.

Recurrent droughts are an inevitable consequence of climate change, yet how grasslands respond to such events is unclear. We conducted a 6-year rainfall manipulation experiment in a semiarid grassland that consisted of an initial 2-year drought (2015-2016), followed by a recovery period (2017-2018) and, finally, a second 2-year drought (2019-2020). In each year, we estimated aboveground net primary productivity (ANPP), species richness, community-weighted mean (CWM) plant traits, and several indices of functional diversity. The initial drought led to reduced ANPP, which was primarily driven by limited growth of forbs in the first year and grasses in the second year. Total ANPP completely recovered as the rapid recovery of grass productivity compensated for the slow recovery of forb productivity. The subsequent drought led to a greater reduction in total ANPP than the initial drought due to the greater decline of both grass and forb productivity. The structural equation models revealed that soil moisture influenced ANPP responses directly during the initial drought, and indirectly during the subsequent drought by lowering functional diversity, which resulted in reduced total ANPP. Additionally, ANPP was positively influenced by CWM plant height and leaf nitrogen during the recovery period and recurrent drought, respectively. Overall, the greater impact of the second drought on ecosystem function than the initial drought, as well as the underlying differential mechanism, underscores the need for an understanding of how increased drought frequency may alter semiarid grassland functioning.

Luo W ，Muraina TO ，Griffin-Nolan RJ ，Ma W ，Song L ，Fu W ，Yu Q ，Knapp AK ，Wang Z ，Han X ，Collins SL ... -  《-》