Experiment: Separating the effects of above and belowground drought on grassland ecosystems

Severe droughts are expected to have increasingly large impacts on terrestrial ecoystems, and numerous experiments have attempted to examine how this will effect grassland diversity and productivity. However, experiments to date have generally only been able to study the impacts of soil drought, without consideration of potentially important effects of air drought. To overcome this limitation, we utilized the climate control capacities of the NPEC Ecotron facility to disentangle the effects of aboveground and underground drought on the aboveground and underground functional traits of grassland plants grown as mono- or polycultures. We hereby aim to better understand the differential impacts of aboveground and underground drought on plant physiology, competition, and facilitation. The results will not only advance our understanding of drought impacts on grassland ecosystems, but will also serve as a full test of the Ecotron capabilities, as we explore the range of ecological questions that can uniquely be addressed using this experimental platform.

Experiment Aim

Global biodiversity and ecosystem functions are suffering under rapid climate change and human activities. An important task in current ecology is to understand how changes in the external environment will affect these important ecosystem functions. To this end, numerous researchers have studied the impacts of drought on grassland ecosystems to understand how plant communities respond to drought and how plant traits and interactions are related to these responses. At the same time, previous research has shown that biodiversity can help grasslands better adapt to climate change. However, the relationship between grassland biodiversity and ecosystem functioning may have vastly different outcomes under different types of drought stress. To date, however, the vast majority of such experiments simply reduce the water supplied to drought-treated plots, which fails to account for the large declines in air humidity or changes in vapor pressure deficit (VPD) that often accompany drought events. Indeed, the majority of experimental studies fail to explain changes in productivity and diversity that can be observed in the field as related to actual drought events of precipitation gradients. It is known that different plant traits are involved in plant responses to above versus belowground drought, and these plant traits may therefore differentially affect both intra as well as intraspecific interactions to drought. For instance, higher air humidity may contribute to the positive impact of diversity on productivity when there is no water stress in the soil. However, high atmospheric humidity might also weaken the protective effects of species richness on some species under soil drought.

In order to tease apart the differential effects of air versus soil drought, we conducted an experiment in which above and belowground drought were regulated independently in comparison to a no-drought control. Ecotrons were planted with either one or four plant species, representing two grass and two forb species (Figure 1). Aboveground plant traits were monitored throughout the experiment, and a full range of above and belowground (root) traits were measured at the experiment’s conclusion. We hypothesized that: (1). Air and soil drought stresses impact different suites of plant traits, and (2). Plant species with more disparate root traits niches will have more complementary effects under drought stress.

This project is close linked to the UU BioCliVE research platform, in which the interactions between biodiversity and shifting precipitation patterns are studied in a large-scale mesocosm experimental setting. The results of our Ecotron experiment will help determine the role of different above and belowground plant traits in driving species performance, facilitation and drought responses within the larger BioCliVE platform.

This study also served as a test case to explore the capabilities of the Ecotron facility, including for the first time the ability to independently manipulate above versus belowground drought. It also allowed for extensive testing of the range of soil sensors and soil sampling options. The optimization of operation protocols and the development of user tools during this project will also facilitate future projects that seek to utilize the range of possibilities offered via the Ecotron module.

NPEC Usage

The NPEC Ecotron module was used for this experiment. We have set three drought levels (control, air drought, and soil drought), using a total of 24 Ecotron units, 8 full climate-controlled units, and 16 standard units. In the control group, the air humidity and soil moisture were 65% and 15%, respectively. The air drought treatment maintained the same soil moisture while reducing the air humidity to 35%. The treatment of soil drought maintained control levels of air humidity, while reducing soil moisture to 5% in soil drought treatment. We also divided each drought treatment into two diversity levels: (1) Monocultures in which each unit was planted with a single plant species; and (2) 4-Mixed multicultures in which all four species were planted together in each unit. All units maintained the same belowground temperature profile, mimicking field conditions.

Experiment Researchers

The project was initiated by the Ecology and Biodiversity group within the Utrecht University Department of Biology. The research team included George Kowalchuk, Kathryn Barry, Yann Hautier, and Yuheng Chen. The project also involved collaboration with the Copernicus Institute of Sustainable Development, Utrecht University, including Hugo de Boer, Jan Lankhorst, and Astrid Odé.