Disentangling tropical forest stand characteristics in terms of cation availability and functional traits

Abstract

Nutrient dynamics underpin the ecological processes in tropical forests. The soils in which these forests are based on are held as being generally poor given their weathered nature and therefore, plants evolved a diversity of ecological strategies to cope with these nutritional limitations. The woody tissues of trees are important in this sense as they reflect the life history of trees as well as their performance. Furthermore, wood is the dominant forest biomass component and harbours the majority of the nutrient stocks present in a forest ecosystem. This thesis investigates the causes of nutrient variation, not only at the tree level but also across species and spatial scales to understand how these chemical portfolios of plants are associated. I start by investigating how nutrient concentrations in plant tissues are associated across spatial scales and evaluated the environmental underpinnings of nutrient tissue variation (Chapter 2). I detected strong influences of climatic and edaphic properties on the variation of nutrient concentration in leaf and woody tissues. Wood density emerged as an important trait associated with nutrient content in wood as well as in leaves. The link between specific nutrients across the spatial scale stimulated the question of how nutrients varied along the whole plant, thus scaling down to the individual level (Chapter 3). Nutritional profiles of entire trees were created and tree size, wood density as well as differences between sites were taken into consideration for this assessment. Once again, wood density varied along with specific nutrients in stems and branches. The patterns found in the first two analytical chapters motivated the examination on how nutrients and water content were associated in the woody tissues and how these relationships varied across environment and species (Chapter 4). This time I scaled down to the tissue level and contextualized these relationships into the ecosystem level, thus establishing a link between the individual’s traits and forest stands. Here I have demonstrated that the wood of these tropical trees not only stores a large amount of nutrients, but that wood cation composition varies according to plot location and species identity. It also correlates with other wood traits such as water storage capacity which was found to be strongly related to increased K concentrations and with denser woods storing less water in their tissues, thus requiring less potassium per unit volume.