16 branched profusely at dbh; in those cases, diameters were measured at ground line just above the root col- lar (drc) and adjusted to dbh as described below. All nonoverlapping crown cover and all per- hectare calculations were based on GIS base maps for lot areas using scanned copies of original architec- tural redevelopment plans on file with the City of Falls Church. Because the city right-of-way was not always labeled clearly on redevelopment plans, cal- culated lot areas differed slightly (maximum about ± 8%) from those supplied by city property tax records; for consistency, our calculated lot area was used. Diameter was measured at dbh for single-stemmed trees but was calculated as for multiple -stemmed trees from individual-stem diameters (di )(Batcheler 1985). Because all analyses were done at dbh, a drc-to-dbh conversion model was needed for 56 trees (on 14 lots) only measured at drc (Chojnacky and Rogers 1999); the genera of these trees were Acer, Amelanchier, Cercis, Lagerstroemia, Magnolia, Taxus, Picea, Prunus, and Ulmus. This need was anticipated; subsamples for all tree sizes and spe- cies were measured at both drc and dbh. A model was constructed from data from 163 subsampled trees from the needed genera (dbh = -0.8399 + 0.8244 drc + 1.7648 IL for Lagerstroemia, 0 otherwise; IP = 1 for Prunus, 0 otherwise; (R2 , c2 + 1.0336 IP; where diameters in cm, IL -statistic = 0.93; data limit dbh < 40 cm). = 1 Crown area at Time2 was computed as a circle by using crown diameter calculated as the geometric mean of crown diameter (c1 ) measurements . Crown Modeling Crown diameter was modeled for 217 cut and 125 preserved trees in initial Time0 and Time1 inventories because redevelopment plans lacked crown diameter measurements. A separate crown diameter (crndia) model was developed from Time2 inventory data for each of the 21 lots, which averaged approximately 25 trees per lot (lncrndia = ß0 Ieh and Ic + ß1lndbh + ß0 Iehß0Ic; where are indicator [0,1] variables for evergreen hardwood and conifer species respectively; R2 -statistics = 0.80–0.99, median = 0.94). To avoid illogical extrapolations when all Time2 inventory trees for a given lot were considerably smaller than initial inven- tory cut trees (as was the case for 10 lots), a few large ©2020 International Society of Arboriculture Chojnacky et al: Redevelopment Effects on Urban Forest trees from neighboring lots that matched the species in question were included in the estimation. Calculations of crown diameter worked well in later analysis after including a modification, moti- vated by some cases where calculated canopy growth for preserved trees was negative (particularly when the interval between Time1 and Time2 was less than 10 years). The modification used an adjustment ratio based on regression residuals from the crown diame- ter model. For each preserved tree, actual measured crown diameter at Time2 was divided by a model estimate of crown diameter at Time2; the Time1 model- estimate of crown diameter was then multiplied by this ratio. If the Time2 ratio was less than one, then the model predicted high, and the ratio multiplication reduced the Time1 estimate; similarly if the ratio was greater than one, the model predicted low, and Time1 crown diameter was adjusted upward by the ratio. The ratios ranged from 0.4 to 1.9, but most (25th to 75th percentiles) ranged from 0.91 to 1.12. Canopy Cover Calculations We calculated nonoverlapping canopy cover on each lot from individual geographically located tree crown areas using a series of ArcMap™ geoprocessing tools—Buffer, Dissolve, Union, and Clip (i.e., within a lot boundary, half the area of overlap from the union of circles corresponding to the crowns was excluded). The total nonoverlapping cover within a lot was divided by the total lot area (total area of open space and nonoverlapping canopy) with no exclusions for the house footprint and expressed as percent canopy cover. Cover from trees spreading into neighbor lots was excluded, as was cover from neighbors’ trees or street trees extending into the sampled lot; this was consis- tent with the canopy cover definition used by the City of Falls Church and appeared reasonable. A paired t-test using data from 3 of our 21 sampled lots showed no significant difference between nonoverlapping cover from within-lot trees that extended over a neighbor’s lot and that from neighbor trees that extended into the sampled lot. Only 3 lots were ana- lyzed because comparison was limited to lots where all neighbor trees extending into a sampled lot had both measured field data and geolocated coordinates; these were difficult to obtain because access permis- sion was required from all surrounding neighbors while in the field.
January 2020
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