Ecophysiological assessment of drought vulnerability of the African tropical tree species Maesopsis eminii Engl

Abstract

Africa is endowed with the second largest block of nature-engineered machinery to sequester carbon: forests. Sadly, the functional traits and responses of this machinery are poorly understood, under non-drought, current drought and projected drought. Controversy surrounds the response of Africa’s forest tree species to drought. Moreover, little is known on the mechanisms or strategies they evoke to cope with drought. In this PhD study, the drought coping strategies and mechanisms of an African pioneer semi-deciduous tree species Maesopsis eminii Engl. (M. eminii) is investigated. A review of published and unpublished data provided insights in the species’ associated functional traits, phenology and provenance. The species’ occurrence niche was then characterized by combining this information and occurrence data of M. eminii in Africa with data on climate, soils and vegetation. It was found that M. eminii established itself in different ecosystems with diverse soil types and precipitation amounts. However, M. eminii mostly preferred the tropical rainforest region with fertile soils and annual precipitation exceeding 1000 mm yr-1. M. eminii’s presence in areas with pronounced seasonal precipitation prompted ecophysiological studies under natural and greenhouse settings. The aim was to determine additional traits used by this species to cope with drought. Different from most other plants, M. eminii seedlings continued to grow for a few days after the onset of drought and were also found to have nocturnal sap flow, because of low transpirational control. On the one hand, this trait seemed useful during non-drought conditions, but accelerated dehydration during drought and the eventual death of the seedlings. Unexpectedly, fast growth of M. eminii was not linked to its photosynthetic rate, as this was rather low. Instead, autoradiographs indicated a leaf role partition with older leaves actively loading sugars into the phloem compared to the expected passive loading in younger ones. Regardless of the loading mechanism, severe drought halved M. eminii’s photosynthetic rate but increased relative leaf respiration compared to a non-drought situation. This might explain why its growth eventually declined after a few days into drought. The hydraulic conduits of M. eminii were wide, which increased the likelihood of airseeding and may make this species’ xylem inherently vulnerable to drought-induced cavitation. But M. eminii was found to additionally have a considerable amount of water in its wood structure as evidenced by its low wood density, high volumetric water content and substantial hydraulic capacitance. This water probably contributed to the observed hydraulic redistribution between leaves, by which M. eminii could cope with drought for a couple of more days. During this hydraulic redistribution, a few leaves are shed while others remained hydrated, which reduced water loss but maintained limited carbon fixation. We also observed that M. eminii has low hydraulic conduits connection, which may limit the spread of drought-induced cavitation.

This research has revealed different novel mechanisms (i.e., nocturnal sap flow, active sugar loading into the phloem in adult leaves, and hydraulic redistribution in the leaves) that have substantially increased the knowledge on how M. eminii copes with drought. It also shows that more attention must be paid to understand how African trees and eventually forests will deal with drought, if we aspire to more accurately predict the impact of climate change on this terrestrial ecosystem.