Arboriculture & Urban Forestry 39(2): March 2013 71 Table 2. Trees in central and northern New Jersey at the 95th percentile (maximum observed), DBH values including average DBH (cm) and maximum DBH (cm) observed. Letters within row were deemed statistically significant at alpha = 0.05. Species Acer platanoides Acer rubrum Acer sacchariunum Acer saccharum Gleditsia triacanthos Platanus × acerifolia Pyrus calleryana Quercus palustris Quercus rubra Tilia cordata Zelkova serrata Pit Max 42 27.5 33.5 36z 15.5 42 21.5 45.5z 34 27.5 9.5 Average 27.34a 11.9a 33.5a 32.1ab 15.5a 39.3a 18.1ab 33.7a 33.5a 24.5a 9.5a Planting strip Max 27.5 41 49 34 27.5 49 24 44 50 39.5 22 z All trees in category were observed to be recommended for removal on condition appraisal. Table 3. Central and northern New Jersey maximum size ex- pectations. General Linear Model, DBH (cm) versus planting site type and planting tree size and comparisons of means. Factor Type Levels Values Site typology fixed Species size fixed Source DF Site typology 2 Species size Interaction 2 4 3 3 F 26.63 non-limited, planting strip, tree pit large, medium, small P 187.00 <0.001 2.81 <0.001 0.024 Table 4. Comparison of DBH means (cm) in species size ver- sus site typology with Bonferroni protection in pre-planned contrasts. Letters in column denote statistical significant at alpha = 0.05. Site typology Non-limited Planting strip Tree pit Species size Small 26.0 (b) 19.5 (a) 13.8 (a) Medium 57.8 (b) 33.7 (a) 37.8 (a) Large 94.2 (b) 79.5 (a) 71.4 (a) urban forest. In traditional forest management, harvesting or harvest intervals are in part determined by the maximum size a particular species can reach in a forest, or how long it takes to reach a targeted merchantable size based on its maximum poten- tial. Although, there are many factors that could possibly affect a tree’s maximum size in the urban forest, this study focused on tree selection on a site-specific level with the criterion of appar- ent available soil. Although age is relevant to the urban forest, a more practical criterion for evaluation of maturity is the expected maximum size, and this can break into rapidly defined site types in which the tree species grows as a proxy variable. Most inven- tories already supply the metrics used in this study and offer the ability to apply the approach to other tree species and regions. Trees in the 95th percentile reflect when the tree can become a liability to the urban forest as seen in the comments by the ar- borist who consulted on the project and collected the inventories. For example (Table 1), Acer platanoides, Acer saccharum, and Quercus palustris, trees that were included in the 95th percentile (the maximum size trees) in these species were recommended for immediate removal due to poor condition. This suggests that their useful service life had already been exceeded. The data also illustrated pulses of planting activity (data not shown) as clusters of size, in which a large numbers of trees at a specific DBH were observed. This suggests large planting initiatives that planted cer- tain species at the same time at a common purchased size. These trees will all reach a relative maturity at the same time within a site type, potentially causing an entire portion of the urban forest to be removed for risk at the same time. The urban forest needs to be managed for the future to avoid an even-aged population in which all trees would be dying at the same time. In traditional silvicul- tural forest management, even-age stands can be useful as some species regenerate better this way. As the urban forest is typi- cally manually planted, it may be wiser to move towards stands of uneven-age for a given species to protect against loss of a certain proportion of the canopy cover due to when the species reaches a maximum DBH. Knowing the maximum DBH for a given spe- cies would allow urban foresters to estimate how rapidly they are approaching this potential end point for a species and begin selec- tive harvesting and replanting over time with the goal of moving toward a mixed-aged stand. As such, the urban forest should be managed on a schedule in which trees are planted in multiple years to enable a cropping rotation to determine an uneven-aged stand management. For a street renovation planning sequence, there is a possible benefit in planning harvest intervals on a small street-level scale rather than over the entire management zone. Treated as an aggregated general population in the general lin- ear model, DBH is assumed to be associated with canopy size. Natural form suggests that tree canopy volume is proportional to natural height and DBH. Regardless of the size class of the tree species, reduced planting space resulted in reduced maxi- mum DBH. As trunk size is related to canopy volume, it stands to reason that reduced planting spaces result in reduced canopy volume. For design, problem solving, or planning in the manage- ment of the urban canopy and trying to determine the service life of trees, plotting DBH versus tree height or canopy can provide urban forest managers a reasonable estimation of size expectation. Although this study does not definitively identify the cause of re- duced growth in urban trees, it does offer a degree of explanation. Care in the interpretation of results is warranted. While some of the species have been actively planted over the past 30 years (e.g., Pyrus calleryana, Zelkova serrata), their full life expectancy will likely not have been met. Therefore, maximum size criterion will likely need to be adjusted over time until there is a proven stability over multiple inventory updates within a given community, as the ©2013 International Society of Arboriculture Average 27.3a 31b 40.6b 29.4a 19.9a 38.4a 17.4a 36.0b 38.4b 31.4b 16.9b Non-limited Max 43.5 46 56 45 45 61 25 80 71 45 37.3 Average 30.7b 32.3b 45.7c 34.9b 32.1b 41.6a 21.5b 41.0c 48.2c 36.9c 28.7c
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