60 Staudhammer et al.: Change and Hurricane Impacts to Houston’s Urban Forest Structure Arboriculture & Urban Forestry 2011. 37(2): 60–66 Rapid Assessment of Change and Hurricane Impacts to Houston’s Urban Forest Structure Christina Staudhammer, Francisco Escobedo, Alicia Lawrence, Mary Duryea, Pete Smith, and Mickey Merritt Abstract. A subsample of 332, 0.06-hectare plots measured during 2001–2002 in Houston, TX, U.S., were relocated and measured in 2008 following Hurricane Ike. These 37 re-measured plots provide a unique opportunity to explore the effects of urbanization and hurricanes on the forest structure of coastal urban forests. Statistical analyses of growth, mortality, and in-growth were conducted using plot- and tree-level factors. In total, 305 trees were re-measured, of which 195 (63.9%) still remained on-site and 110 (36.1%) had been removed. Ninety-seven (31.8%) of these trees were deter- mined to be removed due to urbanization and 13 trees (4.3%) were removed due to hurricane impacts. Results show an overall annual net loss in tree numbers and an increase in tree density during the analysis period. Average annual mortality and in-growth rates were 3.9% and 5.3%, respectively. Growth rates were significantly influenced by land cover type, tree stem diameter, crown width, and percent dieback (P < 0.05). Overall, Hurricane Ike resulted in the removal of 4.3% of all trees measured, with removal occurring on six (16%) of the 37 re-measured plots. These initial findings could be used to understand changes in forest structure in coastal urban areas, improve estimates of carbon sequestration, and develop management goals. Key Words. Emergency Management; Hurricane Damage; Urban Forest Growth; Urban Forest Mortality. Urban forests are subject to various factors that alter the amount and type of structure and produce changes in the provision of ecosystem services such as aesthetics, shading, pollution re- moval, and carbon sequestration (Nowak et al. 1990; Kuo 2003; Nowak et al. 2004; Texas Forest Service 2005; Escobedo et al. 2010). While urbanization and land cover change often produce gradual changes, natural disturbance events, such as hurricanes and ice storms, cause immediate damage, resulting in dramatic changes to the urban ecosystem. While both types of change of- ten give rise to long-term modifications in forest structure and composition through mortality and species replacement, hur- ricanes detrimentally affect urban forest structure (Zhao et al. 2010), have immediate and substantial effects to infrastructure (e.g., debris removal, hazards to property and human life, res- toration needs), and contribute to emissions of carbon through degradation of downed trees and debris (McNulty 2002; Dur- yea et al. 2007a; Duryea et al. 2007b; Escobedo et al. 2009). Annual urban tree mortality has been the subject of relatively few scientific studies in North America. In a limited number of studies tree mortality has been shown to be related to tree condi- tion, size, and management practices. Street tree size and condi- tion in Syracuse, NY, U.S., were found to influence annual mor- tality rates with higher mortality in large trees (5.4%) and trees with crown deterioration (6.4%) (Nowak 1986). Another street tree study in Boston, MA, U.S., reported that mortality aver- aged 9% over a ten-year period and depended on tree planting methods (Foster and Blaine 1978). In Oakland, CA, U.S., an- nual mortality of newly planted street trees averaged 19% over a two-year period (Nowak et al. 1990), with lower tree mortal- ity next to single family housing and rapid transit stations and ©2011 International Society of Arboriculture higher mortality in proximity to apartments, green spaces, and areas with low socio-economic status and high unemployment. Water and nutrient stress, and soil properties are also a common factor in urban tree mortality (Gilbertson and Bradshaw 1985). Previous studies have used re-measurement of permanent plots to understand changes in urban forest structure (Jo and McPherson 1995; Nowak et al. 2004). In Baltimore, MD, U.S., two-year plot re-measurements yielded annual tree mortality and net change in the number of live trees of 6.6% and -4.2%, respectively, with the lowest mortality reported in medium to low-density residential land use areas and the highest mortal- ity on industrial lands (Nowak et al. 2004). Tree health, size, and species also affected tree mortality; smaller tree sizes were more significantly related to mortality than was poor tree health. Tree growth in urban areas is variable even among the same tree types due to different land uses, cultural practices, soil prop- erties, site conditions, and disturbances (Nowak et al. 2004). In Chicago, IL, U.S., growth rates for urban trees in residential ar- eas were determined using annual diameter increment measure- ments from core samples and were found to be 1.09 cm per year for hardwood trees and 0.51 cm per year for softwood trees (Jo and McPherson 1995). Comparable growth rates for forest-grown hardwood trees in Indiana and Illinois averaged 0.38 cm per year (Smith and Shifley 1984). Park trees in New York City were reported to have annual growth rates of 0.61 cm/year (deVries 1987). Although Jo and McPherson (1995) reported that individ- ual urban tree growth in their study was affected by poor rooting conditions, air pollution, heat, and severe pruning, their annual growth rates were nearly twice as those reported by Smith and Shifley (1984) for forest-grown hardwood species. As mentioned
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