24 Catton et al.: Evaluation of Bur Oak Decline ment Canada (2002) and collected at Winnipeg International Airport (49°55N 97°14W, elevation 239 m [789 ft]) from 1938 to 2002. RESULTS General Tree and Environmental Information Variables were compared between the two groups using Stu- dent’s t-test (0.05) for continuous variables or 2 analy- sis ( 0.05) for ordinal variables unless otherwise noted. The overall mean tree age and standard deviation for all trees, excluding hollow specimens for which age data were not available, was 102.3 ± 31.4 years (Table 1). Healthy trees were significantly younger than declined trees (94.6 ± 30.9 and 112.1 ± 29.4 years, respectively), although there were no differences in dbh between the two groups (data not shown). A comparison of wound ratings for the 120 trees showed there was a significant difference in the proportions of trees with different wound ratings between the health categories (Table 1). The healthy category was dominated by trees rated as having 5% to 10% of their trunk circumference affected by wounding and relatively few with 10% to 25% of damaged trunk circumference. The distribution in the declined cat- egory was significantly different with a smaller proportion of trees rated as slightly wounded (5% to 10% wounding) and a higher number with the most severely wounded rating. In short, the declined category had a higher percentage of trees in the extreme wound categories than the healthy category. Declined trees had significantly more competition from surrounding trees than healthy trees (Table 1). Bur oak was by far the most abundant competing tree species surrounding sample trees, accounting for over 50% of average total com- petition, and appeared to account for most of the differences in total competition between the health categories. Bur oak was followed in order of decreasing abundance by elm, ash, maple, basswood, poplar, and spruce species, although none of these other species differed in abundance between the health categories (data not shown). Many variables recorded were not significantly different between the two health categories. These included the pres- ence or absence of tree buttresses (a total of 66% of the 120 sample trees had visible buttresses) and all foliar nutrients measured when considered both separately in Student’s t-test and together in multiple discriminant analysis (percentages of N, P, K, S, Mg, Ca, and Na; data not shown). Overall, 46% of the 120 sample trees had space character- istic of urban areas (concrete, covered soil, unavailable soil space from building basements) within a radius of the tree’s height, but there was no significant difference between the categories. Analysis of soil variables also revealed no signifi- cant difference between soil collected around the subsamples of healthy and declined trees for bulk density, percent organic matter, electrical conductivity (EC), pH, and percentages of sand, silt, and clay. The only soil variable that varied signifi- cantly was total exchangeable cations, which was higher in soil surrounding healthy trees. Of the extractable soil ions— NO3 −, PO43−,K+, SO42−, Ca2+, Mg2+, and Na+—the only significant difference between trees in the two health classes investigated was that Mg2+ was higher around healthy trees than declined trees (data not shown). Levels of all ions were highly variable among sites, especially Na+. Tree Ring Data Ring width series for individual trees were not processed to remove age- or size-related trends, because this could have masked the decline signal in the series. However, because tree age is known to directly influence annual growth ring widths (Fritts 1976), trees with no rings before 1930 were excluded from this comparison so that the average ages of both groups of remaining trees (healthy n 47, declined n48) were not significantly different. Average ring widths for all years together and each individual year were compared between the healthy and declined groups using Student’s t- tests ( 0.05). Only values from the years 1900 to 2001 were included to ensure sample sizes for each year were reasonably large. Table 1. A comparison of various potential decline indicator variables for 120 bur oak trees in Winnipeg, Manitoba, Canada, visually rated as healthy (n = 68) or declined (n = 52) based on crown dieback levelsz . Tree type Decline indicator Age (years since breast height)y Wounding (visual rating based on trunk circumference wounding)x Total tree competition (cm2 competing tree trunk basal area/m2 Total competition from oaks (cm2 growing area) competing tree trunk basal area/m2 growing area) Healthy 032%, 153%, 215% a 19.2 ± 12.2 a 9.9 ± 9.9 a Declined 044%, 131%, 225% b 27.4 ± 14.7 b 19.8 ± 15.6 b Total 94.6 ± 30.9 a 112.1 ± 29.4 b 102.3 ± 31.4 038%, 143%, 219% 22.7 ± 13.9 14.2 ± 13.6 zValues for variables with no letters in common were statistically different (P < 0.05), according to Student’s t-test, unless otherwise noted. ySample numbers lower as a result of several hollow trees: healthy n 63, declined n 49. xComparison made with 2 analysis test of independence. Visual wound ratings correspond to the percentage of the trunk circumference visibly affected by wounds or scars: 0 <5%, 1 5–10%, 2 10–25%. ©2007 International Society of Arboriculture
January 2007
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