Contrasting growth and mortality responses of different species lead to shifts in tropical montane tree community composition in a warmer climate

Abstract. The response of tropical trees and tree communities to climate change is crucial for the carbon storage and biodiversity of the terrestrial biosphere. Trees in tropical montane rainforests (TMFs) are considered particularly vulnerable to climate change, but this hypothesis remains poorly evaluated due to data scarcity. To reduce the knowledge gap on the response of TMFs trees to warming, we established a field experiment along a 1300–2400 m elevation gradient in Rwanda. Twenty tree species native to montane forests in East and Central Africa were planted in multispecies plots at three sites along the gradient. They have overlapping distributions but primarily occur in either transitional rainforest (1600–2000 m a.s.l) or mid elevation TMF (2000–3000 m a.s.l.), with both early- (ES) and late-successional (LS) species represented in each elevation origin group. Tree growth (diameter and height) and survival were monitored regularly over two years. We found that ES species, especially from lower elevations, grew faster at warmer sites while several of the LS species, especially from higher elevations, did not respond or grew slower. Moreover, a warmer climate increased tree mortality in LS species, but not much in ES species. ES species with transitional rainforest origin strongly increased in proportion of stand basal area at warmer sites, while TMF species declined, suggesting that lower-elevation ES species will have an advantage over higher-elevation species in a warming climate. The risk of higher-elevation and LS species to become outcompeted by lower-elevation and ES species in a warmer climate has important implications for biodiversity and carbon storage of Afromontane forests.