Shade tolerance and increased temperature responses in tropical montane trees in Rwanda

Abstract

Tropical montane forests have a large influence on the rate of ongoing climate change, but they remain relatively understudied. This study investigated physiological and structural traits of selected tree species aiming to understand plant traits that control shade tolerance in tropical montane forest tree species and their response to increased temperature. Three climax and three pioneer species were planted in pots under canopies of three different light transmittance (open, sparse, and dense) in the Arboretum of Ruhande to study shade tolerance among species from March 2015 to May 2016 , while other three climax and three pioneer species were planted in pots at Sigira (high-elevation), Rubona (mid-elevation), and Makera (low-elevation) from December 2017 to December 2018 to study effects of temperature changes on growth. Results indicated that climax species had higher fractional biomass in leaves and branches while pioneer species invested more into stems and these differences were greater under low radiation. The relative growth rate of the studied species, except Harungana montana increased with temperature from low- to mid-elevation but dropped at the high-elevation site. Leaf temperatures in open plots were markedly higher in climax species due to their low transpiration rates and large leaf sizes. Climax species demonstrated low thermal acclimation capacity with low growth in the low elevation site, thus confirming that they are less flexible in their responses to rising temperature. The results of this study confirm that tropical climax species are susceptible to global warming and suggest that interspecific variation in shade tolerance of tropical trees is controlled by species differences in whole-plant biomass allocation strateg