A new global study has found that intense sunlight rather than temperature, rainfall, or pollution is a major force reducing plant diversity and biomass in grasslands worldwide, raising fresh concerns about how ecosystems will cope with climate change.
The research, led by Marie Spohn of the Swedish University of Agricultural Sciences and published in the Proceedings of the National Academy of Sciences, shows that high levels of photosynthetically active radiation the wavelengths of sunlight used in photosynthesis can overwhelm plants, limiting both the number of species and the total plant growth in grassland ecosystems.
Grasslands, which include North American prairies, the Serengeti savanna, Alpine pastures, and Arctic tundra regions like Svalbard, cover vast areas of the planet and support biodiversity, livestock, and carbon storage. Yet scientists have long struggled to understand why some grasslands are rich in plant species while others are comparatively sparse.
Earlier research by Spohn and her colleagues had already shown that soil properties and common climate factors such as temperature and precipitation failed to explain these differences. That unexpected result led the team to look more closely at sunlight itself.
Their new analysis reveals a clear global pattern: as intense sunlight increases, plant species richness declines. Crucially, it is not ultraviolet radiation that drives this effect, but the same light plants rely on for photosynthesis. When plants receive more photosynthetically active radiation than their biological systems can process, photosynthesis becomes stressed rather than enhanced, reducing growth and survival for many species.
The findings are based on an unusually large and collaborative dataset. Scientists from six continents collected standardized information on plant diversity and aboveground biomass from 5,590 plots in natural and semi-natural grasslands. These data were combined with satellite-based measurements of solar radiation collected over 22 years, allowing researchers to assess long-term global trends.
The effects of intense sunlight were especially pronounced at higher elevations. Above 430 metres above sea level, both plant diversity and biomass declined sharply with stronger solar radiation. Researchers believe this happens because thinner air at higher altitudes contains fewer particles that scatter sunlight, making radiation more direct and intense.
This elevation-related pattern has serious implications in a warming world. As temperatures rise, many plant species are expected to migrate uphill in search of cooler conditions. The study suggests that intense solar radiation may block that escape route, preventing plants from establishing at higher elevations and increasing the risk of extinction for vulnerable species.
Spohn described this as reinforcing concerns that many plants are effectively being pushed “up the mountain” by climate change, only to find conditions they cannot tolerate at the top.
Not all plants are affected equally. The study found that grasses showed reduced diversity mainly at higher elevations, while legumes appeared largely unaffected by intense sunlight. This uneven sensitivity could reshape grassland ecosystems, altering species composition and ecological balance over time.
By identifying photosynthetically active radiation as a key global constraint on plant diversity, the study challenges long-held assumptions about how grasslands function and respond to environmental change. It also highlights that climate impacts are not driven by temperature alone, but by complex interactions between light, geography, and plant physiology.
As grasslands play a critical role in supporting food systems, wildlife, and carbon storage, the researchers say understanding these hidden pressures is essential for predicting—and possibly mitigating—the ecological consequences of climate change in the decades ahead.