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African tropical rainforests are commonly found at higher altitudes than tropical forests in South America and Asia. Lower temperatures in these African forests have been suggested to explain why their carbon sink is more resilient to climate change. However, higher-altitude tree communities may be subjected to changes in species composition when temperatures increase, in turn affecting both carbon storage and biodiversity. Namely, lower-altitude and early successional species with faster growth and shorter life cycles may be at an advantage against higher-altitude and late successional species, leading to thermophilisation of tree communities.

We suggest that tree water-use strategies control how growth and survival of different species respond to heat and drought and that hydraulic traits can be used to characterize these strategies.

We studied young plants of 20 tropical forest tree species grown at three different altitudes in Rwanda differing in temperature and precipitation. The tree species were chosen to represent common montane and transitional rainforest species with different successional strategies. We studied the following hydraulic traits: stomatal conductance (gs), minimum conductance of leaves (gmin), plant hydraulic conductance (Kplant), leaf osmolality and defoliation during drought. We also related these traits to growth and mortality data.

We found contrasting water-use responses of early and late successional tree species to high temperature and drought. Late successional species with low photosynthesis, gs and gmin were at a higher risk for mortality at the warmer sites. This may be due to low transpiration causing high leaf temperatures and pronounced photosynthetic heat stress in these species. On the other hand, early successional species were more likely to shed leaves at the occurrence of a drought stress, preventing excessive loss of water. This strategy lead to lower mortality and more pronounced growth enhancement at warm sites in our study, but may be disadvantageous for larger trees or when droughts are longer or more severe.

We show that African tropical tree species follow distinctly different strategies to deal with heat and drought. Late successional species experience more heat stress and may be at a disadvantage when temperatures increase, though this may be different for older and larger trees. Early successional species may be better suited for a warmer climate as long as defoliation can bridge drought periods. The relative decline in late successional species in a warmer climate observed here may lead to biodiversity loss of plant and animal species associated with the affected tree species.

Keywords:

Climate change, defoliation, hydraulic conductance, minimum conductance, osmolality, stomatal conductance

Maria Wittemann, Myriam Mujawamariya, Bonaventure Ntirugulirwa, Etienne Zibera, Donat Nsabimana, Göran Wallin, Johan Uddling

Presentation within symposium:

S-19 Tropical forest response to temperature: a pantropical synthesis of elevation gradients, and leaf thermoregulation studies

Water-use strategies under heat stress and their impact on growth and mortality of Rwandan tropical trees

-Review-