Savannas are biomes defined by the coexistence of trees and grasses. Across climate types, savannas in humid environments have different environmental pressures to savannas in semi-arid environments. Changes in savanna tree species composition, both within landscapes and across climatic gradients, suggest that species differ in their ability to utilize resources and cope with grass competition. 

Researchers from Xishuangbanna Tropical Botanical Garden (XTBG) and their collaborators conducted a study to investigate how tree species differ in response to competition from grasses and nutrient supply.  

They also wanted to see whether the responses can be related to plant traits. 

The researchers compared seedlings of 40 tree species from humid and semi-arid savannas of three continents (Africa, Australia, and South America) with and without grass competition and with high and low nutrient supply.  

They then measured traits related to soil resource capture, light capture and growth. They then tested whether those traits were related to performance under the four treatments. 

They found that all tree species were suppressed by grass competition and most by nutrient limitation. Only species from humid savannas in Australia grew better under nutrient limitation than their semi‐arid counterparts. 

"Differences in tree species performance with and without grass competition suggest that savanna communities may develop different trajectories based on disturbance to the grass layer, with fast growing species advantaged in short grass patches and slow-growing species advantaged in tall grass patches", said Dr. Kyle W. Tomlinson, principal researcher of the study. 

They further found that traits that improved performance under grass competition differed by continent, which may relate to differences in leaf habit and constraints on seed size. 

The study entitled "Seedling growth of savanna tree species from three continents under grass competition and nutrient limitation in a greenhouse experiment" has been published online in Journal of Ecology.