Charlotte Grossiord et al. have an interesting paper recently published online in Journal of Ecology, entitled ‘Precipitation, not air temperature, drives functional responses of trees in semi-arid ecosystems‘. The scientists from the Los Alamos National Laboratory provide evidences that trees in semi-arid systems may acclimate to reduced precipitation but not to temperature rise. Charlotte kindly agreed to share the main findings of the study through the blog post below. 

Pierre Mariotte
Blog Editor, Journal of Ecology

Charlotte Grossiord received her PhD from the University of Lorraine and INRA-Nancy (France), supervised by Damien Bonal, and co-supervised by Arthur Gessler from the WSL Institute (Switzerland). The main focus of her project was to determine how tree species diversity affects the water relations in European forest ecosystems, with a particular focus on how it affects the resistance of trees to drought stress. Charlotte conducted her PhD as part of a large FP7 EU project FundivEUROPE. After her such research, Charlotte moved to the United States where she is currently a Director’s postdoctoral fellow at the Los Alamos National Laboratory in New Mexico. Her research focuses on tree acclimation processes in response to the combinations of precipitation reduction and warming. Specifically, she is interested in physiological and structural adaptation of vegetation to climate change such as for instance responses in carbon assimilation rates, water and nitrogen use, growth allocation and water extraction depth.

Trees acclimate to reduced precipitation but not to temperature rise

Higher temperatures and altered precipitation regimes will affect the exchange of energy, carbon, water and nutrients between plants and the environment. Extreme events induced by climate change can have drastic consequences for forest functions and services and may result in extended die-off events. Scenarios of climate change predict that environmental stresses such as drought and warming will most likely occur simultaneously in most regions. However, although the responses of vegetation to drought and temperature increase have been individually studied in various ecosystems, scientists know little about plant responses to these two stresses acting at once.

Warming in combination with reduced precipitation could increase water loss from plants and soils and therefore amplify seasonal drought duration and intensity. This combination could lead to exacerbated consequences on plant functioning and survival compared to environmental stresses acting alone. To persist and thrive under projected climate change, plants will need to undergo rapid adjustments in their physiology and morphology. However, although the capacity of plants to modify their characteristics to changing environmental conditions is widely recognized among ecologists, climate-vegetation models rarely consider these acclimations, limiting our current ability to make long-term predictions.

Researcher from the Los Alamos National Laboratory studied the plastic changes trees undergo in their physiology and structure when they are exposed to warmer droughts. They used a unique experimental setup located in New Mexico (USA) where adult trees in natural conditions are exposed to simultaneous reduction in precipitation and increase in atmospheric temperature.

This study shows that physiological and morphological acclimation of trees to climate change are more driven, and seem to be determined mostly by reduced soil moisture rather than by increasing air temperature. Furthermore, Grossiord et al. showed that the combination of warming and drought does not exacerbate the physiological and morphological responses of trees compared to single stresses. Contrary to what is commonly assumed, atmospheric warming (here about 5°C, reflecting projections for 2100 under a business-as-usual scenario) did not induce a large increase in water loss from this semi-arid ecosystem and thus did not generate higher seasonal drought intensity and/or duration. Overall, this study shows that in semi-arid ecosystems where the functioning of plants is already highly limited by precipitation, warming as anticipated with climate change may have less impact on acclimation processes than previously thought.

The next step of this project is going to be an evaluation of the role of these acclamatory responses for the resistance and survival to climate change. We will also try to include these adaptive responses in climate-vegetation and mechanistic plant physiology models to improve model predictions. Most importantly, we feel that this work revealed that long-term experiments in natural conditions where climate is manipulated are extremely useful for anticipating the effects of climate change on vegetation. New experiments, in a large variety of climates, combining multiple global change stresses are urgently needed to help bring some light on the functioning and survival of plants under climate change and improve our predictive power.