Intraspecific trait variability mediates the response of subalpine grassland communities to extreme drought events

Climate change is expected to increase the magnitude and the frequency of extreme climatic events such as droughts. Better understanding how plant communities will respond to these droughts is a major challenge. We expect the response to be a shift in functional trait values resulting from both species turnover and intraspecific trait variability, but little research has addressed the relative contribution of both components. We analysed the short-term functional response of subalpine grassland communities to a simulated drought by focusing on four leaf traits (LDMC: leaf dry matter content, SLA: specific leaf area, LNC: leaf nitrogen concentration and LCC: leaf carbon concentration). After evaluating species turnover and intraspecific variability separately, we determined their relative contribution in the community functional response to drought, reflected by changes in community-weighted mean traits. We found significant species turnover and intraspecific variability, as well as significant changes in community-weighted mean for most of the traits. The relative contribution of intraspecific variability to the changes in community mean traits was more important (42-99%) than the relative contribution of species turnover (1-58%). Intraspecific variability either amplified (for LDMC, SLA and LCC) or dampened (for LNC) the community functional response mediated by species turnover. We demonstrated that the small contribution of species turnover to the changes in community mean LDMC and LCC was explained by a lack of covariation between species turnover and interspecific trait differences.Synthesis. These results highlight the need for a better consideration of intraspecific variability to understand and predict the effect of climate change on plant communities. While both species turnover and intraspecific variability can be expected following an extreme drought, we report new evidence that intraspecific variability can be a more important driver of the short-term functional response of plant communities.

Nutrient limitations induced by drought affect forage N and P differently in two permanent grasslands

Alexandre Buttler, Claire Emilie Deléglise, Pierre Rémi Mariotte, Amarante Vitra

Drought events can strongly affect ecosystem functioning by modifying relationship between plants, microbes and soil chemistry, with consequent impacts on nutrient cycling. However, the potential impacts of a soil moisture reduction on the nitrogen (N) and phosphorus (P) cycling in grasslands remain poorly understood, especially in regard to. forage production. To fill this knowledge gap, a drought experiment was carried out using rainout shelters in two permanent grasslands, characterized by similar vegetation communities but contrasted soil nutrient limitations. Drought treatments were applied during two months, either when plant growth was highest (Early-season drought) or after the peak of biomass production (Late-season drought). Dry matter production, forage N status (NNI) and P content as well as N and P contents in microbial biomass and soil were determined. Both early and late-season drought significantly reduced soil moisture during the vegetation growth period. Forage yield was also reduced by drought, but only when it occurred late in the season. Using a structural equation model, we showed that soil moisture reduction had a direct effect on forage N status, suggesting that water shortage induced lower transpiration and water fluxes. Soil moisture reduction also affected forage P by reducing the availability of soil P. However, other mechanisms played a larger role and were site-specific. At the more fertile site, reduction in soil moisture directly impaired forage P, suggesting that water stress mainly resulted in lower diffusion rates to roots, while at the less fertile site, an indirect reduction of forage P through a pathway implying microbes (decrease in microbial P) was detected. Our results suggest that the two grasslands suffered mainly from water shortage per se, but also from drought induced nutrient deficiency (mainly P), which amplified yield losses and further decreased forage quality. Overall, our findings emphasize the need for further research on the plant-soil-microbe system functioning, in order to secure a sustainable and resilient forage production in the context of climate change.

ELSEVIER2019

Effects of climate warming on Sphagnum photosynthesis in peatlands depend on peat moisture and species-specific anatomical traits

Vincent Eric Jules Jassey, Constant Signarbieux

Climate change will influence plant photosynthesis by altering patterns of temperature and precipitation, including their variability and seasonality. Both effects may be important for peatlands as the carbon (C) sink potential of these ecosystems depends on the balance between plant C uptake through photosynthesis and microbial decomposition. Here, we show that the effect of climate warming on Sphagnum community photosynthesis toggles from positive to negative as the peatland goes from rainy to dry periods during summer. More particularly, we show that mechanisms of compensation among the dominant Sphagnum species (Sphagnum fallax and Sphagnum medium) stabilize the average photosynthesis and productivity of the Sphagnum community during summer despite rising temperatures and frequent droughts. While warming had a negligible effect on S. medium photosynthetic capacity (A(max)) during rainy periods, A(max) of S. fallax increased by 40%. On the opposite, warming exacerbated the negative effects of droughts on S. fallax with an even sharper decrease of its A(max) while S. medium A(max) remained unchanged. S. medium showed a remarkable resistance to droughts due to anatomical traits favouring its water holding capacity. Our results show that different phenotypic plasticity among dominant Sphagnum species allow the community to cope with rising temperatures and repeated droughts, maintaining similar photosynthesis and productivity over summer in warmed and control conditions. These results are important because they provide information on how soil water content may modulate the effects of climate warming on Sphagnum productivity in boreal peatlands. It further confirms the transitory nature of warming-induced photosynthesis benefits in boreal systems and highlights the vulnerability of the ecosystem to excess warming and drying.

WILEY2019

Transplantation of subalpine wood-pasture turfs along a natural climatic gradient reveals lower resistance of unwooded pastures to climate change compared to wooded ones

Alexandre Buttler, Konstantin Svetlozarov Gavazov, Thomas Spiegelberger

Climate change could impact strongly on cold-adapted mountain ecosystems, but little is known about its interaction with traditional land-use practices. We used an altitudinal gradient to simulate a year-round warmer and drier climate for semi-natural subalpine grasslands across a landscape of contrasting land-use management. Turf mesocosms from three pasture-woodland land-use types-unwooded pasture, sparsely wooded pasture, and densely wooded pasture-spanning a gradient from high to low management intensity were transplanted downslope to test their resistance to two intensities of climate change. We found strong overall effects of intensive (+4 K) experimental climate change (i.e., warming and reduced precipitation) on plant community structure and function, while moderate (+2 K) climate change did not substantially affect the studied land-use types, thus indicating an ecosystem response threshold to moderate climate perturbation. The individual land-use types were affected differently under the +4 K scenario, with a 60 % decrease in aboveground biomass (AGB) in unwooded pasture turfs, a 40 % decrease in sparsely wooded pasture turfs, and none in densely wooded ones. Similarly, unwooded pasture turfs experienced a 30 % loss of species, advanced (by 30 days) phenological development, and a mid-season senescence due to drought stress, while no such effects were recorded for the other land-use types. The observed contrasting effects of climate change across the pasture-woodland landscape have important implications for future decades. The reduced impact of climate change on wooded pastures as compared to unwooded ones should promote the sustainable land use of wooded pastures by maintaining low management intensity and a sparse forest canopy, which buffer the immediate impacts of climate change on herbaceous vegetation.

Springer2014