Arboriculture & Urban Forestry 37(2): March 2011 earlier, this discrepancy in hardwood tree growth rates might be a result of the spatial heterogeneity of urban forest structure and open-grown conditions of urban trees (Zhao et al. 2010). In the southeastern U.S., urban tree growth has been studied on a species level with emphasis on live oaks (Quercus virgin- iana Mill.), a historically important common species and a large component of total biomass by species in the region. In a study of parking lots in Florida, Grabosky and Gilman (2004) found that growth, in terms of canopy radius and height, declined as nonpaved surface area was reduced for Chinese elm (Ulmus parvifolia Jacq.), Sycamore (Platanus occidentalis L.), Shu- mard oak (Quercus shumardii Britton), and laurel oak (Quercus laurifolia Michx.); but not live oak. Templeton and Putz (2003) report that laurel oak growth rates in the Gainesville, FL area were 1.3 cm/yr. Information on tree growth and mortality is commonly being used for estimates of urban forest carbon se- questration (Nowak and Crane 1998; Escobedo et al. 2010). In 2008, following Hurricane Ike in Houston, TX, a subsam- ple of 332, 0.06-hectare circular plots originally measured during 2001–2002, were relocated and measured. In total, 37 permanent urban forest plots in Houston were re-measured to analyze the ef- fects of Hurricane Ike on urban forest cover, mortality, and debris generation from urban trees, and to examine urban forest structure change over time in terms of growth and in-growth rates. These permanent plots, originally measured during November 2001 through May 2002, were assessed for hurricane debris generation but also provide a unique opportunity to explore changes in the city’s urban forest structure over a seven-year period and assess the influences of Hurricane Ike and urbanization on both tree and plot level characteristics. The study authors define urbanization as processes (e.g. increased impervious surfaces, new buildings, roads, and other human influences) associated with urban devel- opment and land cover/use change (Texas Forest Service 2005). Specifically, this study: 1) evaluated the effects of urbanization on tree numbers, basal area, mortality, and in-growth; 2) examined the impacts of Hurricane Ike on urban forest structure; and 3) de- termined annual growth rates for the most common species by di- ameter size class found in Houston, TX. Results can be used to un- derstand temporal changes in urban forest structure and to estimate growth and mortality rates used increasingly in urban forest carbon sequestration assessments in the Gulf Coast of the United States. MATERIAL AND METHODS Plot Selection In 2001, the Texas Forest Service established 332 plots using a systematic, random sampling design across an eight county re- gion in the Houston area. During the 2001-2002 analysis, 2,010 trees were measured on 162 (49%) plots; the remaining plots contained no trees. On September 13, 2008, Hurricane Ike made landfall on Galveston Island at 2:10 a.m. Central Daylight Time as a strong category two hurricane on the Saffir-Simpson Hur- ricane Scale with 177 kph winds and a central pressure of 952 mb (Berg 2009). Its broad size and long fetch across the Gulf of Mexico created one of the most devastating storm surges to affect the coasts of upper Texas and Louisiana in the last 150 years (Berg 2009). Following Hurricane Ike, a subsample of 37 plots or, 23% of the original 2001 plots with trees, were re- measured to assess the effects of Hurricane Ike on urban for- 61 est structure and tree debris generation. Plots were selected for re-measurement using the following criteria (Figure 1): 1. Plots with at least one tree taller than 6.09 meters and in close proximity to residential buildings; 2. Plots with at least one tree taller than 6.09 meters, and clos- est and farthest from the storm track; 3. Plots severely affected by storm surge were eliminated from the sample; and 4. Plots with no access or safety concerns, and approved ac- cess by the Texas Forest Service and homeowners. Figure 1. Distribution of plots measured in 2001 in Houston, TX, highlighting plots re-measured in 2008 and their proximity to the track of Hurricane Ike. Field Measurements Modified Urban Forest Effects (UFORE/ECO) field measure- ment methods from Nowak and Crane (1998), Zhao et al. (2010), and data from the 2001 sample were used along with additional measurements for hurricane damage and debris generation to re-measure the 37 selected re-measurement plots. Using origi- nal data and maps from the 2001–2002 UFORE/ECO sampling, each plot center was re-located using GPS coordinates and ref- erence object distance and direction measurements. Once the perimeter for the 0.06 hectare plot was established, individual trees with DBH greater than 12.7 cm at the time of the origi- nal measurement in 2001-2002 were re-located and re-mea- sured with the aid of original field data measurements, site and aerial photos from 2001, and Google Earth® maps from 2008. New trees with DBH greater than 12.7 cm were also measured. The following data were collected for each tree on the plot: species (DBH; cm), total and crown base height (m), crown width in two directions (m), percent dieback, and crown light exposure using methods outlined in Bechtold (2003), Nowak et al. (2004), ©2011 International Society of Arboriculture
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