64 Staudhammer et al.: Change and Hurricane Impacts to Houston’s Urban Forest Structure Table 1. Tests of fixed effects for growth, mortality and in-growth models for Houston, TX’s urban forest. Annual Growth Rate Model Effect Land cover DBH 2001 Crown width % Dieback Land cover Trees per hectare In-growth Model Effect Land cover Num DF Den DF 3 1 1 1 Mortality Model 1 (AICC= 141)z Effect Mortality Model 2 (AICC= 132) Effect 187 187 187 187 Num DF Den DF 3 33 Num DF Den DF 1 35 Num DF Den DF 3 33 z Low AICC indicates better model; DF, Degrees of Freedom. Table 2. Top four species ranked by highest growth rates and frequency in Houston, TX. Rank Common name Species By Growth Rate 1 2 3 4 By Frequency 1 Live oak Southern red oak Loblolly pine American elm 2 Water oak 3 4 Loblolly pine Sweet gum Chinese tallow Quercus virginiana Quercus falcata Pinus taeda Ulmus americana Pinus taeda Quercus nigra Liquidamber styraciflua Triadica sebifera Average growth SE rates (cm/yr) 1.22 0.94 0.90 0.81 0.90 0.57 0.44 0.65 or the selection criteria used to measure the 2008 plots. Most importantly, Duryea et al. (2007a) only sampled trees along rights-of-way (e.g., transportation land cover/use), whereas the authors of the current study sampled several land cover/uses. The highest growth rates found in the new study were on devel- oped open space land cover. But this rate was not significantly dif- ferent from the other developed land cover categories (DL and DH) in the study. However, significantly lower growth rates were found in WW. Also, measures of tree density decreased in WW and in- creased in DL during the analysis period. These results might have been influenced by a single plot in the WW land cover category where a loss of all 22 trees occurred due to land clearing activi- 0.25 0.44 0.18 0.15 0.18 0.09 0.10 0.21 F value 6.66 6.49 8.35 9.47 F value 3.79 F value 16.3 F value 3.45 Pr > F 0.0003 0.0116 0.0043 0.0024 Pr > F 0.0193 Pr > F 0.0003 Pr > F 0.0277 ties in 2008. Greater sampling intensity would likely have improved growth, mortality, and in-growth models using tree- and plot-level characteristics. However, due to post- hurricane access and safety issues, this was not possible. A study limitation is the time lag in tree mortal- ity measurements immediately following Hurricane Ike, which more than likely affected the results. It is also probable that trees might have been removed in anticipa- tion of hurricane landfall. Thus, the study authors could not determine if a tree was removed as a direct result of hurricane impacts or removed for other reasons. Un- fortunately, safety, access, homeowner privacy, and lo- gistical concerns following the hurricane delayed sam- pling. Recent tree removals and damaged trees were determined, however, and were thus associated with pre and post-Hurricane Ike impacts with relative certainty. Despite this, in general the authors found that with the exception of WW hurricane mortality increased with in- creasing urbanization as defined by comparing WW to ur- ban DL and DH land covers. Greater hurricane severity was experienced on the eastern portion of Hurricane Ike as ex- pected (Berg 2009), and resulted in greater damage and de- bris in the WW plots located in this area. In addition, debris on WW plots was likely not removed during post-hurricane debris removal activities. Similar findings of increased tree mortality with increasing development and hurricane sever- ity have been reported by Duryea et al. (2007a) and Escobe- do et al. (2009), who found increased tree damage and de- bris in areas characterized by open-grown, single trees and communities with low tree density. Observations indicated that severe tree damage to standing trees was not common, finding only three plots that indicated visible evidence of di- rect tree damage to buildings on or near the measured plots. Results are within the range of other studies of mortality and growth of urban forests. To compare this study’s results to existing studies, mortality and growth data were arranged according to the diameter size classes reported in Nowak et al.(2004) (Table 3). Nowak et al.’s (2004) mortality rates were determined similarly by the re- measurement of 200 permanent plots sampled randomly across different land cover types in Baltimore, MD. Table 3 can be used to facilitate comparisons with existing growth, mortality, and biomass values used in the various urban for- est structure-function models such as the UFORE/ECO model that is increasingly being used to assess carbon sequestra- tion by southeatern U.S urban forests (Escobedo et al. 2010). Table 3. Percent annual mortality and annual growth rates by diameter size class. DBH (cm) Houston 2001–2008 mortality and growth Percent annual mortality (N) 0–7.6 7.7–15.2 15.3–30.5 30.6–45.7 45.8–61.0 61.1–76.2 >76.2 N, sample size ©2011 International Society of Arboriculture 0 (0) 12 (21) 5.1 (48) 6.8 (28) 6.7 (11) 4.8 (2) 0 (0) Average annual growth rate (cm/yr) (N) 0 (0) 1.01 (14) 1.03 (110) 0.43 (46) 0.62 (19) 0.47 (6) 0 (0) Nowak et al. (2004) mortality Percent annual mortality (N) 9 (528) 6.4 (267) 4.3 (201) 0.5 (109) 3.3 (62) 1.8 (28) 3.1 (33)
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