Managing mature trees for biodiversity conservation in urban environments

Abstract

Biodiversity is in decline globally with over 400 vertebrate species becoming extinct in the 21st century. Land use change is one of the key contributors, with substantial areas of important habitat still being cleared. A key driver of land use change is the rapid rate of urbanisation. Few studies have examined the contribution of mature trees to biodiversity conservation in urban environments. My research quantifies mature tree loss in urban landscapes, explores the impact on biodiversity, and determines retention strategies. The specific aims are to: a) determine mature tree loss rates due to urban development; b) explore relationships between mature trees and fauna in urban greenspace; c) quantify the risks associated with retaining mature trees in urban areas; and d) identify how residential developments can minimize impact on wildlife dependent on mature trees. This research was conducted across Australia's east coast, focusing on South East Queensland (SEQ), Canberra, and greater Sydney. My thesis comprises four chapters prepared as manuscripts, including one published chapter and three submitted to international journals. A final chapter serves to synthesize the chapters and prescribe management actions to implement the findings of my research. In my first manuscript I investigate the impact of greenfield residential development on mature trees. Here I quantify the reduction in native canopy cover, identify where trees are retained, and evaluate the effectiveness of legal protections. Key findings include: greenfield residential development in reduces native canopy cover by 49%; mature trees are more likely to be retained in remnant vegetation than in urban greenspace or residential blocks; and legislative protections only marginally improve the retention of mature trees. In the next two manuscripts I explore the role of large trees as keystone structures for birds and microbats in urban environments. We conducted bird surveys and deployed ultrasonic bat detectors at 83 sites across Canberra and SEQ. I found positive relationships between the number of large trees and several bird community metrics (species richness, Chao's S, and abundance). I found strongest associations between bird community metrics and numbers of trees >70cm diameter at breast height (DBH) in the ACT, and trees >40cm DBH in SEQ. Of the bird species with strong positive associations with tree size, 73% were hollow dependent. Additionally, I found positive associations between five bat community metrics and the number of large trees >50cm DBH. I also provide evidence that large trees offering equal to greater biodiversity value in landscapes with lower woody vegetation cover. In the fourth manuscript I quantify the risk associated with retaining mature trees in urban environments. I developed models predicting the probability of branch fall in eucalypts to then inform an adjusted version of the QTRA to apply to eucalypts in urban greenspace, including the development of a risk matrix. I demonstrate that even senescing trees can be safely retained in urban areas in dedicated greenspace with low pedestrian occupancy, and away from infrastructure. Overall, my research provides a multi-faceted view of the ecological and practical issues related to retaining mature trees in urban areas. My research reveals that most of the trees retained during residential development are retained in patches of remnant vegetation, rather than as scattered trees. However, I also show that mature trees in isolation have equal if not greater value for biodiversity. This loss of important habitat is contributing to biodiversity loss in urban areas as well as reducing people's access to nature by removing trees from within neighbourhoods. To reverse this pattern of loss, we need stronger regulations to protect mature trees, in combination with better planning systems, ensuring that retention of mature trees is prioritised from the early stages of development.