Arboriculture & Urban Forestry 37(2): March 2011 63 Figure 2. Comparison of average percent change in basal area (m2/ ha), trees per hectare (# trees/ha), and tree cover from 2001 to 2008 for all 37 matched plots following Hurricane Ike in Houston, TX. mortality rates per hectare are shown in Figure 4 by land cover. Nonhurricane changes were mostly in the DO land cover category, while hurricane-induced mortality was largest in DL land cover. Two competing mortality models are shown in Table 1. These models indicate that land cover and trees per hectare significantly influenced mortality at P < 0.05. The model with trees per acre had a lower AICC, indicating more evidence that the data arose from this model; the competing model is of in- terest since it does not rely on field data. The highest mortal- ity occurred in DO land covers, followed by WW, then DL and DH. The only significant differences between land cov- ers were found between the DO and DH land cover cat- egories. Mortality increased as trees per hectare increased. Only land cover was found to be significant in pre- dicting in-growth of trees in Houston. As in the mortal- ity model, more new trees occurred in the DO land cover. This was followed by DL, WW, then DH land covers. As with the mortality model, the only significant differences were found between the DO and DH land cover categories. Due to the limited amount of tree mortality that could be at- tributed to Hurricane Ike, only a descriptive analysis of hurri- cane-caused tree mortality was possible. While developed low intensity (DL) land covers had more hurricane losses on a tree per hectare basis, 7 of the 13 hurricane removed trees were found in the WW land cover type. Figure 4 indicates—with the excep- tion of WW—hurricane mortality increased with increasing rates of development. Only 29 trees located on 10 plots had trees with more than 25% of the tree crown damaged and tree crown de- foliation was estimated to average 25% of total crown density across all measured trees. Only three plots had visual evidence of direct damage to residential or commercial buildings from trees. Growth rates were influenced by land cover type, initial Figure 3. Percent average annual change in basal area, tree densi- ty, and tree cover by land use/cover type in Houston, TX, between 2001 and 2008. DBH, crown width, and percent dieback (P < 0.05; Table 1). The highest growth rates were found in DO, then DL and DH land cover types, while lower growth rates were found in WW plots. Growth was significantly lower in WW as compared to all other land cover categories. Growth rates increased as crown width increased, and decreased as DBH and dieback increased. The top four species in terms of growth were live oak, south- ern red oak (Quercus falcata Michx.), loblolly pine (Pinus taeda L.), and American elm (Ulmus americana L.) (Table 2). These were not, however, the most prevalent species; loblolly pine, water oak (Quercus nigra L.), sweet gum (Liquidambar styraci- flua L.), and Chinese tallow (Triadica sebifera L.) were the most prevalent, accounting for 63% of the trees in the sampled plots. DISCUSSION Figure 4. Average nonhurricane and hurricane mortality rates for 37 re-measured plots by land cover type in Houston, TX, between 2001 and 2008. Mortality, In-growth, and Growth Models The overall average annual mortality and in-growth rates were 4.7% and 2.4%, respectively. Average non-hurricane and hurricane Mortality and overall net loss of trees during the analysis peri- od were less than reported in a similar study in Baltimore, MD (Nowak et al. 2004). In addition, the 4.3% tree mortality from Hurricane Ike was much lower than would be predicted for the minimum percent urban forest loss, as defined by Duryea et al. (2007a), using the polynomial relationship reported from eight hurricanes across several wind speeds (Duryea et al. 2007a; Dur- yea et al. 2007b). For example, according to this relationship, Hurricane Georges with the same maximum sustained winds as Ike (177 km/hr) resulted in 13% tree mortality in Puerto Rico (Duryea et al. 2007b). Differences might be due to different climate, a comparatively smaller sample size in this study and/ ©2011 International Society of Arboriculture
March 2011
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