74 Pearce et al.: Using Size Class Distributions of Species for the Private Urban Forest Arboriculture & Urban Forestry 2013. 39(2): 74–84 Using Size Class Distributions of Species to Deduce the Dynamics of the Private Urban Forest Lilian M. Pearce, James B. Kirkpatrick, and Aidan Davison Abstract. Urban governance in Western societies is increasingly shaped by awareness of the importance of trees in maintaining the environmental function and social livability of cities. Records of change in urban forest composition on public land are generally good. However, a great proportion of trees in western cities occur on private land, where such changes are poorly-documented. The study authors trialed the use of size class analysis, a technique widely used to deduce the dynamics of natural forests, to determine change in the private urban forest. From a sample of blocks in ten suburbs of the Australian cities of Melbourne and Hobart, in which most dwellings have front and back gardens, researchers assessed the implica- tions of changes for the functionality of the urban forest. The height class distributions of a large number of front garden tree taxa were classified. Although the factors affecting height class distributions differ between a natural and an urban forest, those distributions found for most species were so extreme that there was little doubt in interpretation. Tree species that can grow to a large height were under-represented in the smaller height classes, indicating their future decline in the private tree estate. Individuals of glossy-leaved small tree species were over-represented in the smaller height classes, indicating a recent increase in their popularity. The shift toward smaller, denser trees on private land has implications for the functions of the urban forest. A higher level of large tree protection on private land and compensation through planting on public land could mitigate impacts. Key Words. Garden Tree; Species Composition; Species Preference; Street Tree; Suburban Residents; Tree Management; Urban Forest; Urban Plants; Urban Vegetation. Urban trees help maintain the environmental function and livability of cities by providing social (Sullivan et al. 2004; Elmendorf 2008), psychological (Milligan and Bingley 2007; Nordh et al. 2011), health (Lovasi et al. 2008; Morris and O’Brien 2011), economic (Donovan and Butry 2011; Mill- ward and Sabir 2011), biophysical (Dobbs et al. 2011; Pataki et al. 2011), and biological (MacGregor-Fors 2008; Goddard et al. 2010) benefits. These benefits have motivated efforts to manage cities as coupled social-ecological systems (Alberti 2009; McGrath and Pickett 2011; Pickett et al. 2011), par- ticularly in relation to a recent planning focus on green in- frastructure (Wright 2011; Young 2011) and requirements for development contributions towards green space and com- pensation for tree amenity loss (City of Melbourne 2011). Urban forestry has only recently gained ground in Austra- lia, despite the pioneering advocacy of John French (1975). A notable step toward embedding urban forestry in munici- pal land management came in 2003 with the adoption of an urban forest policy by the Local Government Association of New South Wales, although this arguably failed to lead to ini- tiatives on the ground. More recent practical steps include the publication of urban forest strategies by North Sydney Coun- cil (2011) and the City of Melbourne (2011). These initiatives reflect considerable, if largely undocumented, advances in the professionalization of arboriculture in Australia over the past twenty years. Amongst the drivers behind this transformation of urban tree management, concern about drought, climate ©2013 International Society of Arboriculture change, the urban heat island effect, and urban sustainability have been prominent (Shears 2009; City of Melbourne 2011). Tree species differ markedly in their suitability for these and other purposes; for example, native bird species conservation is encouraged by both native trees and a subset of exotic trees (Daniels and Kirkpatrick 2006b), and large deciduous trees are particularly useful in-house temperature regulation, while often not being particularly frugal with water consumption. Municipal land managers employ increasingly sophisticated tools for urban forest mapping, monitoring, and valuation to achieve their goals of increasing the size and health of the urban forest (Myeong et al. 2001; McPherson et al. 2005; Soares et al. 2011; Wu et al. 2008). Knowledge of compositional changes in the public tree estate can usually be expected as a product of this planning and management process. The complementary private tree estate is substantial in many urban areas (Clark et al. 1997; Heynen and Perkins 2005). Knowledge of compositional change in the private urban forest is more difficult to obtain. There is some scope to monitor change using historic and con- temporary remote sensing imagery, but discrimination between trees is generally only possible at the life form, rather than at generic or specific levels (Kirkpatrick et al. 2011). The dynam- ics of natural forests have long been widely deduced using size class analysis (Colinvaux 1993). The size of all individuals of all tree species at a site is measured. If a histogram for a particu- lar species showing number of individuals in each class on the y axis and class on the x axis is reverse-J-shaped (i.e., number
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