Increases and decreases in soil moisture in water-limited plant communities cause asymmetrical responses in biomass but not in diversity

Prepared by Liesbeth van den Brink, Rafaella Canessa, Pierre Liancourt, Harald Neidhardt, Lohengrin A. Cavieres, Yvonne Oelmann, Maaike Y. Bader and Katja Tielbörger

Changes in rainfall, especially changes in the number of times it rains during a certain period and the amount of rain that falls, are likely to affect plant communities. Predicted (and already occurring) climate extremes come with both severe drought periods and heavy rainfall. However, if severe drought has the opposite effect on plants than more rainfall is still unclear. In this paper we studied how the biomass production and biodiversity of water-limited plant communities responds to small and large changes of drier and wetter conditions.

To do this, we exchanged small patches of soil containing seeds between a dry semi-arid and wetter mediterranean climate to simulate large changes in water availability (which we call macroclimatic change). Because of the Earth’s inclination, sun-facing slopes receive more radiation and are generally a bit drier than the slopes that face the other way. We used this to our advantage to translocate soil between slopes and create small changes in water availability (we call this microclimatic change).

After two years, we counted and harvested the plants that grew in each soil patch. We found that biomass production had an unexpected response to climate change and was not higher when there was more water available. Instead, when the soil and seeds went from wet to dry, the wettest conditions showed the lowest biomass production. The biomass production was higher when this soil was placed in the driest conditions (large change) but was the highest after a small change to the dry slope in the wet site. When the change went the other way around, from dry to wet, we found almost no biomass production on the dry slope in the wet site. The diversity of the plants that germinated did not respond to small changes and was consistently highest in the driest climate.

Our results show that it is important to study different response factors that might be affected by climate change: species diversity, for example, responded the same to water availability changes, independently of where the soil was coming from. However, the biomass production response depended on the soil’s origin, which indicates that the drought might have an effect on biomass production that cannot be reversed easily by heavy rainfall, which is called hysteresis. Therefore, it is important to study climate change in both directions (simulating drought and extreme rainfall years) and also include small and large changes. Our study thus shows that if researchers only manipulate climate in one direction, they might not capture the full picture of how plant communities will respond to climate change.

This is a plain language summary of the paper of van den Brink et al. published in the Journal of Vegetation Science (https://doi.org/10.1111/jvs.13300)