Understanding the environmental functions of urban forests in Sydney, using remote technologies

With global warming and rapid urbanisation, improved urban forest management is essential and, while the importance of green infrastructure is generally acknowledged, surveys of Australian urban forests have identified several major challenges. These include: better protection for existing forest assets and active rehabilitation of degraded areas; increased local government area (LGA) data collection, and vegetation plan development; improved co-ordination between LGAs to resolve forest disputes or priorities; and more comprehensively incorporating ecosystem service benefits into land-use instruments (Chap. 2). To investigate the general research question of how to integrate remotely sensed data to rapidly assess resources in more detail for better management outcomes, two major transport corridors in the Sydney Metropolitan Area (SMA) were selected for detailed studies. These sites are examples of the linear or dendritic forms of urban forest commonly resulting from urban sprawl. Among residual ecological corridors, roadside forests have become more important due to the high density of road networks. In these SMA studies two remote sensing technologies (LiDAR and aerial hyperspectral imaging) were integrated and results post-analysed using geographic information systems (GIS). The surveys (see Chap.3) demonstrated: unexpected tree diversity on both Parramatta Road and the Pacific Highway; limited and patchy roadside forest distribution, especially along Parramatta Road; the prime role of local government in maintenance and management; how urban vegetation policy had changed and become connected with many other issues; the variability in strategies or management between LGAs and other factors that influence the status and composition of roadside forest. Urban forests modify microclimates by intercepting radiation and levels of solar radiation are a prime determinant of how much energy urban surfaces absorb and temperature distribution patterns. So understanding shading impacts of trees on buildings is essential to estimating energy use or potential energy conservation. Remotely sensed data sets were used to model direct and diffuse radiation received by building roofs by season, then, radiation profiles were related to tree features at plot level. Even the limited street trees present reduce radiation received by adjacent building roofs by up to 14% (Chap.4).om To extend understanding of urban trees on received solar radiation in urban environments, further information on individual tree characteristics was needed. Local maxima algorithms identified tree tops, while watershed segmentation was used to delineate individual tree crowns. These attributes were then related to modelled shadows and solar radiation on building roofs. Analyses showed that tree height and canopy area were inversely related to solar radiation, emphasising the importance of keeping taller trees for radiation reductions. Higher ratios between tree and building features (tree height : building height; crown area : building roof area) were significantly related to lower radiation, suggesting that larger trees are more effective in areas with larger commercial or industrial buildings; broad-leaved deciduous species may be more appropriate in these areas, permitting maximal use of lowered radiation in winter. In residential areas with smaller buildings, smaller trees are recommended (Chap.5). Urban forests create management problems that require active managerial attention. Roadside stands form very extensive networks, comprising a major component of the urban forest complex. The choice of tree species and sizes should be optimised and tailored to different land uses with different building and ground characteristics. The value of existing mature trees should be fully recognised and these larger trees preserved when possible, taking cultural and medical issues into account. To compensate for continuous canopy loss, on-going planting strategies are essential. Aside from monitoring existing forest, integrating remote sensing and GIS can assist decisions for new plantings (Chap.6). Clearly remote sensing technologies can assess tree characteristics with precision, and model tree environmental impacts and interrelationships. These data can then be used to assist planning for shade provision, improving management and connectivity of all urban forest sub-systems. The basic management framework proposed permits incorporation of diverse data from multiple resources to enhance decision-making.