Phylogenetic diversity and ecosystem services of urban forests

Abstract

Urbanization is one of the most increasing kinds of land-use conversion, leading to strong modifications to the landscape structure and ecosystem functioning. Forests within urban areas are vulnerable to altered environmental conditions such as higher temperature and light intensity, water deficit, and increased CO2 availability, besides air and soil pollution. These environmental filters may favor certain species traits or functional groups, leading to the removal of entire lineages and causing impacts on their ecosystem functions. Although urban forests can still contribute to the overall carbon storage and sequestration, anthropogenic land-use changes along with stand age have been shown to set tropical tree communities back to an earlier successional stage causing a reduction in standing biomass. The aim of this thesis was to investigate how phylogenetic diversity and composition of tree species are affected by urbanization itself (urban forest fragments derived directly from intact forest) and land-use history (urban forests regenerated from cropland or otherwise denuded landscapes) (chapter 1). Also, in chapter 2 we aimed at investigating how aboveground biomass (AGB) is affected by urbanization and land-use history and to explore the contribution of three important biomass drivers (niche complementarity, mass-ratio and soil fertility hypothesis). Our results suggest that species richness, rarefied species richness and phylogenetic diversity are all affected by the land-use history of urban forests and that urban forests without previous land use can house substantial amounts of angiosperm evolutionary diversity, which highlights the importance of preserving natural forest fragments as cities expand. Besides, high aboveground biomass in urban forests is greatly dependent on their land-use history. We found that the mass-ratio hypothesis (wood density) is of great importance for driving high aboveground biomass in secondary urban forests, but that aboveground biomass is greater for the groups of phylogenetically distant species only for old-growth forests. For the other forest categories, especially urban forests with cropland and denudation land-use history, high aboveground biomass is related to species being phylogenetically close.