Intense solar radiation constrains plant species richness in global grasslands

The search for predictors of plant diversity has challenged scientists for decades. The purpose of this study was to understand the relationship between plant species richness and solar radiation in global grasslands.

We collected standardized plant species richness and plant aboveground biomass data at 150 natural and semi-natural grasslands on six continents and analyzed them together with different measures of photosynthetically active radiation (PAR) and UV-B radiation derived from satellite data (with a temporal resolution of three hours covering several decades) as well as other environmental variables.

We identified PAR as a major factor constraining plant species richness in global grasslands. We show that the strength of the negative relationship between species richness and PAR increases with increasing elevation and that species richness is more strongly correlated with intense PAR than with UV-B radiation, climate variables, and atmospheric nitrogen deposition. In addition to species richness, plant biomass was also negatively correlated with PAR at higher elevations, indicating that intense PAR also constrains plant biomass in montane grasslands.

Furthermore, we show that the decrease in plant species richness with increasing PAR is mainly caused by a decrease in species richness of forbs, sedges, and rushes. In contrast, species richness of grasses was only negatively correlated with PAR at high elevations, and species richness of legumes was not significantly correlated with PAR. The reason why species richness of grasses was not correlated with PAR across all sites is likely that grasses are better adapted to high PAR than plants of other plant functional groups. The leaf area of grasses is negatively correlated with PAR globally, and grasses accumulate larger amounts of silicon than forbs, legumes, and rushes. Silicon forms silicate minerals, so-called phytoliths, in grass leaves, which play an important role in diffusing solar radiation inside the leaves before it hits the photosystems.

Our results suggest that PAR constrains plant species richness in global grasslands and limits the extent to which plant species of specific functional groups can migrate uphill in response to climate warming.

 

Reference

Spohn et al.:  Intense solar radiation constrains plant species richness in global grasslands. PNAS (in press).