The role of economic, venation and morphological leaf traits in plant and ecosystem function along forest-savanna gradients in the tropics

Predictions about the impacts of current and projected anthropogenic climate change on tropical forest ecosystems require a better understanding of sensitivity and variability of plant attributes to biotic and abiotic factors. Leaf functional traits are particularly informative because they determine carbon and water fluxes between plants and atmosphere. However, due to the lack of data in the tropics, especially, in Africa, many uncertainties surround the interpretations of trait-environment relationships as well as trait-trait trade-offs and their importance on plant and ecosystem functioning. Therefore, the principal aim of the thesis was to better understand, and test widely held assumptions surrounding the functional significance of key leaf economic, morphological and venation traits in the tropical forest ecosystems. The data was collected for 90 angiosperm adult tree species along a strong environmental gradient (rainfall, fertility, midday temperature and vegetation) in Ghana, West Africa and presented in four data chapters. The first data chapter of the thesis (Chapter 4) focuses on the links between leaf photosynthetic capacity and key nutrients along a vegetation gradient from forest to savanna. This is important because these relationships are employed in the modelling of global primary productivity, with great uncertainties evident across the tropical forest-savanna boundaries. To allow for better interpretation of the results, this chapter includes an additional data set of neotropical forest-savanna boundary in Cerrado, Brazil. Special attention is paid towards patterns of nitrogen allocation between leaf structure and photosynthetic apparatus. In contrary to the expectations, the key findings of this chapter suggest that studied species do not maximise photosynthetic capacity per available foliar nitrogen, phosphorus or potassium due to adaptation to intermittent water availability in these strongly seasonal environments. The two following data chapters explore morphological adaptations of leaves to the extremes of the studied environmental gradient. Chapter 5 examines the drivers and functions of leaf morphology, specifically, leaf shape. Based on the leaf energy budget theory, the findings of this chapter demonstrate the importance of narrow and elongated leaves in thermoregulation and maintenance of photosynthetic rates in the savanna study plot, which is characteristic of high irradiance, midday temperatures and seasonal water deficits. Additionally, this chapter suggests that narrow and elongated leaf blades are also important for optimising canopy light capture in shaded canopies of wet evergreen forests. Including leaf shape metrics in plant performance modelling can improve understanding of ecosystem response to projected increases in and extreme temperatures. The penultimate data chapter of the thesis (Chapter 6) explores the role and diversity of functional traits that describe leaf venation network. The data presented in this chapter test and support the hypothesis that minor vein traits play an important hydraulic function that underlies plants adaptation/acclimation to intermittent water availability. This is seen in denser and more reticulated networks present at the driest end of the gradient and strong links between minor veins and potassium (important nutrient for osmoregulation), leaf size, leaf deciduousness and wood density. These relationships were shown to be affected more significantly by environmental filtering than by phylogenetic relatedness among species. The final data chapter (Chapter 7) joins the recurring themes of the thesis by assessing the relationships between studied leaf economic, morphological and minor venation trait dimensions and plant and community ecological strategies. Leaf functional traits clustered along three significant ecophysiological axes of variability related to gas exchange, structure and water transport. Additionally, the findings highlight the role that leaf functional traits play in the ecosystem functioning as leaf trait dimensions relate to aboveground net primary productivity and biomass. Finally, this chapter emphasises the need for more large-scale studies linking functional traits with plant and ecosystem function at different scales of tropical forests. Such studies would enable the development of a significant mechanistic understanding of what creates and drives functional trait variability across tropical forests and in modelling their effects on water and carbon fluxes in tropical forest ecosystems.