Arboriculture & Urban Forestry 41(3): May 2015 tance for street trees are those values reported in Peper et al. (2008), calculated as the mean of the relativized frequency, leaf area, and canopy cover estimate for each species. Leaf area and canopy cover data were not available for historical and recent natural area trees, and so importance was calculated by adding frequency and relativ- ized mean DBH for each species and dividing by two. The two methods for calculating impor- tance are not directly comparable but provide a measure of which trees are most predominant in terms of both number and size in each data set. The distribution of trees among size classes for the most common species was graphed to compare forest composition through time for all three data sources. This approach, analyzing tree size based on size class ranges rather than individual tree sizes, reduces some sources of error potentially associ- ated with individual diameter values recorded in the General Land Office Surveys (see discussion in Schulte and Mladendoff 2001). Size implies age of tree and reproductive health of populations. RESULTS Comparing and Historical and Recent Trees The last 200 years have seen a change in tree composition in Indianapolis as the city has de- veloped. Although all but five species (occurring in frequencies of less than 1%) identified during historical land surveys are still found in remnant natural areas in Indianapolis, the frequencies of some of the most common species have shiſted significantly (Table 2). The most common species in presettlement forests, American beech (Fagus grandifolia), declined as a percentage of total trees by 95%. Two species, each representing 11% of the original forest, now have divergent patterns: sugar maple (Acer saccharum) has more than doubled in frequency, while white ash (Fraxinus americana) has declined to less than 1% of trees present. White oak (Quercus alba) has declined from 6% to 1%. Native elms (Ulmus spp.) have increased the most through time as a percentage of all trees, increasing almost five-fold from 5% of witness trees to 23% of trees in current natural areas. Boxelder (Acer negundo) and hickories (Carya spp.) changed in composition by less than 3% between the record- 139 ed periods. Twenty-two other species were pres- ent in very low numbers at both points in time. The largest trees, based on mean DBH in the presettlement forests, were tulip popular (Lirio- dendron tulipifera), sycamore (Platanus occiden- talis), various oak species, black walnut (Juglans nigra), and blue ash (Fraxinus quadrangulata). Tulip poplar, sycamore, and oak continue to be among the largest trees in Indianapolis’ forests, while the mean size of black walnut and blue ash has declined. Cottonwood (Populus spp.) is the only species to nearly double in mean size (Table 2). Street Trees Street trees differ in composition from those in presettlement forests and in current natural areas in Indianapolis. All species recorded in historical surveys are present in street tree inventories, ex- cept one, black willow (Salix nigra). The difference comes with the addition of species not found in presettlement forests. Eight species of non-native broadleaf trees are among the most common in number on city streets (Table 2). Only one, tree of heaven (Ailanthus altissima), occurred in recent samples from natural areas. An additional group of non-native trees, evergreen gymnosperms, ac- count for 10% of all street trees. The most common native street trees were silver maple, sugar maple, hackberry (Celtis occidentalis), and white ash. The most numerous non-natives were crabapple (Malus spp.) and white mulberry (Morus alba). Similarities of Species Composition in Data Sources Based on the species presented in Table 2 (all spe- cies in historical and recent studies and street trees present at 5% frequency or greater), Jaccard’s simi- larity of species composition between trees recently surveyed in remnants and those from the historical record is 66%, while street trees are only 28% similar to trees from the historical record, highlighting the difference in street tree composition compared with trees historically present in Indianapolis. Trees in remnants have a similarity of 41% with street trees. Importance Importance values document the decline of American beech and the increase of sugar maple ©2015 International Society of Arboriculture
May 2015
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