Drought is rare in tropical rainforests yet it does occur, especially during El Niño Southern Oscillation years. Climate change also predicts an increase in the severity and frequency of these intermittent droughts. The impacts of intensified drought and the resilience of ecosystems will depend on tolerances of individual species but also of the community as a whole.

Michael O’Brien of the Consejo Superior de Investigaciones Científicas, Spain, and his team investigated the problem by simulating intermittent drought conditions using rainfall exclusion shelters over a two-year period. They tested the effect on both monocultures and mixtures of tropical trees in Malaysian Borneo, as they report in Nature Ecology & Evolution.

The researchers found the growth rates of species grown as a monoculture to be significantly lower in drought conditions compared with wet conditions. When these same species were grown in mixed communities, there was no difference in the growth rates between drought and wet conditions.

The findings indicate that diverse communities show greater resilience to drought. The researchers think this is because a community consisting of a mixture of species experiences less intense competition for water, allowing growth rates to be maintained.

The implications are two-fold. Forests with high diversity are more resistant to drought. Additionally, drought can increase diversity by reducing the success of individuals in communities that aren’t very diverse. Alternatively, a negative feedback loop can develop: as forests with low diversity are more susceptible to drought, exposure to drought can lead to a further loss of biodiversity.

“To prevent this reverse feedback loop, it is essential to conserve biodiversity in tropical forests – both by maintaining existing diverse forests and by using diverse seed mixtures when planting new forests during restoration,” said O’Brien. “Using such diversity strategies will improve forest resistance to climate change.”