The sun is the basis for photosynthesis, but not all plants thrive in strong sunlight. Strong sunlight even constrains plant diversity and plant biomass in the world's grasslands, a new study shows. Temperature, precipitation, and atmospheric nitrogen deposition have less impact on plant diversity.

These results were published yesterday in the journal PNAS, by a research team led by Marie Spohn from the Swedish University of Agricultural Sciences.

The steppes of North America, the Serengeti savanna, the Svalbard tundra and natural pastures in the Alps are examples of habitats that are described as grasslands, with the common feature that there are no trees and the vegetation is dominated by grasses and other herbaceous plants. The diversity of plant species in these grasslands varies considerably, but the question of what controls plant diversity has challenged researchers for decades.

Last year, in a study on grasslands, Marie Spohn from SLU and colleagues found that soil properties and climate factors, such as temperature, did not explain variations in plant diversity.

“This finding surprised me” says Marie Spohn. “And that's when I started wondering about the importance of sunlight for plant diversity in grasslands and decided to start a new project that would explore this relationship”.

Now, Marie Spohn and 34 colleagues from different parts of the world show that plant diversity in global grasslands was negatively correlated with sunlight: increased levels of sunlight generally mean a lower diversity. But it was not all solar radiation that was important. It was intense sunlight in the wavelengths used in photosynthesis that constrained the number of plant species. Neither UV-B radiation, temperature, precipitation, dryness index or atmospheric nitrogen deposition were as closely related with plant diversity as intense sunlight.

“We know that plants can be stressed by receiving more photosynthetically active radiation than the photosynthetic machinery can handle and that this reduces photosynthesis, but this is the first study to show that photosynthetically active radiation is a factor that constrains plant diversity in the world's grasslands,” says Marie Spohn.

The researchers also discovered that plant diversity was constrained by intense solar radiation particularly at higher elevations. The reason for this is most likely that at higher elevations, where the air pressure is lower, the air contains lower concentrations of atmospheric gases and aerosols that make solar radiation more diffuse.

The research also found that above 430 meters above sea level, not only plant diversity but also plant aboveground biomass was negatively correlated with strong sunlight.

“Our study is relevant regarding the question of what happens to plant communities when the climate gets warmer. It suggests that intense solar radiation may limit the ability of many plant species to migrate to higher elevations in response to climate warming. This supports the concern that such species are currently ‘riding the elevator towards extinction’,” says Marie Spohn.

In contrast to previous estimates, the new results suggest that it is the photosynthetically active radiation, rather than UV-B radiation, that limits plant migration upwards.

While total plant diversity was negatively correlated with intense sunshine, different groups of plants were affected to different degrees. For example, grass diversity was only negatively correlated with sunshine at higher elevations, and legume diversity was not correlated with sunshine at all.

The finding that photosynthetically active solar radiation constrains species richness and plant biomass in the world’s grasslands, was only possible due to the highly collaborative approach of the study. A team of scientists working on six continents collected standardized data on plant species richness and biomass in 5590 plots in natural and semi-natural grasslands. These data were then analysed together with satellite-derived data of solar radiation covering 22 years.