Speaker: Bonaventure Ntirugulirwa
Abstract: Knowledge on mortality responses of tropical trees and communities under projected warmer climate is limited. Trees in tropical montane forests (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 warming response of TMFs, we have established a field experiment along an elevation gradient ranging from 2400 m a.s.l. (15.2 °C mean temperature) to 1300 m a.s.l. (20.6 °C mean temperature) in Rwanda. Twenty tree species, native to East and Central Africa, from two forest types of origin (transitional rainforest at 1600 - 2000 m a.s.l, and TMF at 2000 - 3000 m a.s.l.) and two successional groups, early (ES) and late succession (LS), were planted in multispecies plots at three sites along the gradient. Tree growth and survival of 5400 trees was monitored regularly over two years. The results showed that ES trees 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, tree mortality increased in a warmer climate and this was more pronounced in high-elevation and LS species compared to lower-elevation and ES tree species. ES species with transitional rainforest origin strongly increased in proportion of stand basal area at warmer sites, while tropical montane forest species declined, suggesting that higher-elevation and LS species are at risk to be outcompeted by lower-elevation and ES species in a warmer climate. We conclude that tree mortality and growth responses combined may lead to modified tree community composition in a warmer climate, favouring lower-elevation and ES tree species. This has important implications for biodiversity and carbon storage of Afromontane forests.