Investigating the Influence of Tropical Urban Climates on Vegetation Phenology

Abstract

Urbanisation poses a major challenge for global sustainability because urban areas drive environmental change and the global population of urban dwellers is expected to continue to grow rapidly. Vegetation phenology, which is the study of the timing and duration of plant development phases, is important for monitoring the effects of urbanisation and the spatial variability of urban vegetation ecosystem and provisioning functions. Moreover, studies on the impact of the urban heat island effect on phenology can be a useful way for understanding the impacts of climate change. However, although the drivers of phenology in cities in temperate climates are well understood, vegetation phenology in the tropical city context is less studied and poorly resolved. Phenology in tropical cities would particularly be useful because urban residents rely strongly on vegetation for ecosystem and provisioning functions such as urban agriculture and regulation of all year round high temperatures. Linked to this is the fact that tropical cities are expected to have a high risk of exposure to the impacts of climate change which vegetation cover could minimise. However, tropical developing cities are experiencing the fastest rates of urbanisation around the world characterised by high losses of vegetation cover. This creates an urgent need for empirical studies on vegetation dynamics in tropical cities that cover a wide range of tropical climate types and urbanisation practices, including sub-Saharan Africa that is largely underrepresented in the urban ecology literature. This study quantifies the effects of urbanisation and urban climate on vegetation phenology in the tropical city of Kampala, Uganda (sub-Saharan African) using a wholly empirical approach that assesses phenology at the landscape and tree species levels. The specific objectives of the study include the following: (1) examining the influence of the urban heat island effect on landscape phenology; (2) investigating the spatiotemporal dynamics of urban climate during the wet-dry season transition; (3) investigating the sensitivity of canopy phenology to local environmental settings. This study showed that increase in surface temperature associated with the Urban Heat Island (UHI) resulted in shorter vegetation growing seasons at the landscape scale. Also, the magnitude of spatial variability in the UHI and Urban Dryness Island (UDI) was dependent on moisture availability and increased with the advancement of the dry season. Therefore, the influence of land cover composition on the UHI and UDI varied between seasons and was greatest during the dry season. With regard to the phenology of individual trees, a stronger UHI resulted in high leaf loss which equates to shorter growing seasons observed at the landscape scale. Moreover, the dynamics of canopy cover change varied between species. These findings about the effect of the UHI in tropical cities contrasts with the established knowledge in temperate cities were stronger UHIs lead to longer growing season lengths. Therefore, urban planning policies geared towards lengthening the vegetation growing season by minimising the UHI and UDI intensity ought to be adopted to maximise the benefits of vegetation for the attainment of sustainable urban development.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Sarah Lindley (Supervisor) & Angela Harris (Supervisor)