Responses of plant leaf economic and hydraulic traits mediate the effects of early- and late-season drought on grassland productivity

Drought can occur at different times during the grassland growing season, likely having contrasting effects on forage production when happening early or later in the season. However, knowledge about the interacting effects of the timing of drought and the development stage of the vegetation during the growing season is still scarce, thus limiting our ability to accurately predict forage quantity losses. To investigate plant community responses to drought seasonality (early-vs. late-season), we established a drought experiment in two permanent grasslands of the Swiss Jura Mountains that are used for forage production. We measured three plant functional traits, including two leaf traits related to plant economics (specific leaf area, SLA; leaf dry matter content, LDMC) and one hydraulic trait related to physiological function (predicted percentage loss of hydraulic conductance, PLCp), of the most abundant species, and plant above-ground biomass production. Plant species composition was also determined to calculate community-weighted mean (CWM) traits. First, we observed that CWM trait values strongly varied during the growing season. Second, we found that late-season drought had stronger effects on CWM trait values than early-season drought and that the plant hydraulic trait was the most variable functional trait. Using a structural equation model, we also showed that reduction in soil moisture had no direct impacts on above-ground biomass production. Instead, we observed that the drought-induced decrease in above-ground biomass production was mediated by a higher CWM PLCp (i.e. higher risk of hydraulic failure) and lower CWM SLA under drought. Change in CWM SLA in response to drought was the best predictor of community above-ground biomass production. Our findings reveal the importance of drought timing together with the plant trait responses to assess drought impacts on grassland biomass production and suggest that incorporating these factors into mechanistic models could considerably improve predictions of climate change impacts.

Responses of permanent grasslands to drought during the plant growing season: combining agronomic, functional and ecophysiological approaches

Amarante Vitra

Extreme summer drought events are predicted to increase in intensity and frequency within the current century. The resulting water scarcity could dramatically impact human activities related to agriculture, due to the strong relationship between precipitations and plant biomass production. However, little is known about the interactions between timing of drought and vegetation dynamics along the growing season. In Switzerland, were grasslands are at the core of the agriculture, droughts could have severe economical consequences. Thus, farmers should be provided with insights on how to adapt and optimize their management to the changing climate. In order to further our understanding on the response of grasslands to different timing of drought and its interaction with other factors such as management intensity, productivity or soil characteristics, we developed the GRASSALT project -which was the context of this PhD- as a joint venture between AGROSCOPE, EPFL and WSL, with the aim to investigate this challenge through a multidisciplinary perspective at the crossroad between applied and fundamental research. The setup of a realistic drought experiment in three semi-natural grasslands in the Swiss Jura Mountains (even though this thesis is focusing mainly on two) where 70% of the precipitations were removed during two months at two different (during the peak of vegetation growing season vs after this peak later in the season) and two management intensities were applied, enabled us to investigate how grassland communities responded to drought at different timings and sites and investigate its interacted with management. To perform this investigation we adopted a combined agronomic, functional and ecophysiological approach that leads us to analyse the effect of drought from the plant community level to the physiological level and to the analysis of the aboveground-belowground interaction. The main findings of this thesis were the following: (i) a drought event occurring later in the growing season leads to stronger reduction in aboveground productivity than a drought event occurring during the peak of growing season. (ii) The decrease in aboveground productivity is mediated by an intraspecific variability in the functional traits of the most abundant plant species. (iii) the effect of drought is more deleterious under higher mowing intensity. (iv)The effect of drought on plant aboveground and belowground productivity is dependent on soil types and modulated through the effects on microbial communities in response to climate. Altogether the presented work has shown that timing of drought plays a big role on the impact of drought on the productivity of grasslands per se or by interacting with otherfactorsand that this requires further in-depth studies to offer efficient management strategies to farmers.

EPFL2019

Competitive advantage of Rumex obtusifolius L. might increase in intensively managed temperate grasslands under drier climate

Constant Signarbieux

Climate models predict decreasing amounts of precipitation for future summers in Switzerland. Since grasslands cover about one quarter of the area, severe consequences might be expected for Swiss agriculture, ranging from loss of grassland productivity to changes in vegetation composition. Since stressed ecosystems are also more susceptible to invasion, future drier conditions might favour the emergence of weeds. However, the response of temperate grasslands to drought has not been investigated in great detail so far. Using transparent rain shelters, we simulated extreme summer drought conditions in intensively managed temperate grassland in the Swiss lowlands at 400 m (Chamau, located near the city of Lucerne) and studied the drought response of Rumex obtusifolius, one of the most troublesome weeds for forage production. We quantified above-ground biomass and assessed the resource use in terms of carbon, nitrogen and water. R. obtusifolius increased its above-ground biomass production in response to drought, comprising up to 80% of the total community biomass in 2006. Within the drought plots, highest pre-dawn leaf water potentials, high values for midday leaf water potentials, stomatal conductance and assimilation clearly indicated that R. obtusifolius was much less affected by drought than other plant species. In general, no significant differences were found for these variables between drought and control R. obtusifolius plants, in contrast to the other plant species. Higher water use efficiency together with a change in N acquisition patterns resulted in a competitive advantage of R. obtusifolius over other species, favouring the spread of this weed. Thus, our results suggest a potential increase of weed pressure by R. obtusifolius under future climatic conditions, demanding additional management measures to limit its success. © 2009 Elsevier B.V. All rights reserved.

Elsevier2010

Subalpine Pasture-Woodlands in a Changing Climate

Konstantin Svetlozarov Gavazov

Climate change in temperate mountain systems and associated increase in temperature and decrease in precipitation are expected to have strong implications for vegetation productivity, species diversity and carbon turnover in subalpine grasslands. Little is known, however, about the interaction between the effects of climate change and those of local land use management and possible changes in landscape structure. Pasture woodlands in the Swiss Jura Mountains are a traditional landscape, resulting from a long-lived sustainable use of grasslands and woodlands, and as such provide a suite of important ecosystem services to human society. These range from carbon sequestration and biodiversity preservation, to provision of timber and forage for livestock, and last but not least an aesthetic value, much appreciated by tourism. In this thesis various aspects of ecosystem functioning have been studied, investigating the combined effects of experimental climate change and land use on structurally different wooded pastures. An altitudinal gradient method has been used to simulate future climate change conditions, by imposing warmer and drier climate on subalpine turfs transplanted at lower elevation. The resulting gradient in mean annual temperature and precipitation – ranging from cold and wet in the subalpine zone, to warm and dry in the colline zone – has allowed for the detection of tipping points and altered states of ecosystem functioning in response to the treatments. The method employed provided also the possibility for a direct comparison of three land use types: unwooded pastures, sparsely wooded pastures, and densely wooded pastures (the result of pasture management intensity), in their response to climate perturbation. During the four years of experimental work, a series of observations have been made at the plot scale (square metre) in terms of plant performance and biogeochemical cycles, as well as at the landscape scale (hectare) in terms of forage production. A general threshold level for ecosystem resistance to experimental climate change was detected between the moderate IPCC scenario (+2 K mean annual temperature; -20 % annual precipitation) and the intensive IPCC scenario (+4 K mean annual temperature; -40 % annual precipitation). A concomitant gradient in ecosystem response to climate change was observed across the three land use types. The intensively managed unwooded pasture type was consistently more affected by the experimental treatment and rarely exhibited signs of resistance, especially under the intense climate change scenario. A drastic loss of plant species diversity, reduction of herbaceous biomass, impaired litter decomposition and soil microbial metabolic activity have all contributed to the altered state of ecosystem functioning. In contrast, the two extensively managed wooded pasture types showed considerable resistance to climate perturbation in terms of both above and belowground ecosystem processes. The reported inter-annual variation in herbaceous diversity and biomass production within these land use types demonstrated their resilience (recovery) potential too. Using a modelling approach for upscaling these results to the heterogeneous landscape of pasture woodlands in the Swiss Jura Mountains, has proven that extensively used wooded pastures could grant sustainable ecosystem services in terms of forage provision for cattle under climate change. Considering that the two experimental climate change intensities implemented this study are the projected ‘best’ and ‘worst’ case scenarios for the coming decades, the reported resistance of wooded pastures to climate change has to be embraced, and sustainable land use set as a goal in high altitude mountain pastures.

EPFL2013