Arboriculture & Urban Forestry 33(1): January 2007 27 dovar and Somogyi 1998) or less vigorous (Amorini et al. 1996; LeBlanc 1998; Pedersen 1998) than healthy trees in the early parts of their lives and thus have different energy de- mands. Instead, the observation in the current study suggests that something about the now-declined trees or their environ- ments may have changed to cause the decrease in growth. A comparable pattern of similar growth followed by divergent growth between visually classified healthy and declined oaks was observed in the southeastern United States (Tainter et al. 1990). The divergence was attributed to a series of short-term droughts, although it was not clear why certain trees became declined, whereas others remained healthy. Widespread symptoms of bur oak decline in Winnipeg were first noted in the 1980s (Allen and Kuta 1994); however, the observed ring width patterns indicate that the decline actually began in the 1940s, much earlier than estimated by preliminary investiga- tions (Allen and Kuta 1994; Allen 2000). This finding is consistent with reports that a decrease in annual ring widths in declining trees is evident much sooner than the onset of visual symptoms (Kenk 1983; Hornbeck and Smith 1985; Greve et al. 1986) and that declining oaks grow slower than their healthy counterparts for decades before death (Jenkins and Pallardy 1995; Pedersen 1998). The divergence in growth rate between healthy and de- clined trees corresponded with a period of intense urbaniza- tion in Winnipeg, where many neighborhoods were con- structed in a short time in previously undeveloped areas to accommodate the demand for housing after World War II (Dafoe 1998). Urbanization may have positive (i.e., possibly from removal of competing trees) or negative (i.e., soil com- paction, wounding) effects on bur oak growth (Catton 2005). It appears that overall, healthy oaks were unaffected or posi- tively affected by surrounding development as demonstrated by their increased growth rate over time. This may be a result of healthy trees being younger than the declined trees at the time of urban disturbance, because established oaks are well known for their poor adaptability to the changes caused by urbanization (Ware 1970; Allen and Kuta 1994). In some cases, urban disturbances may have been beneficial around healthy trees as demonstrated by their lower levels of sur- rounding competing trees. In addition to having high levels of competition, some now-declined specimens may have been exposed to more severe or recent disturbances as indicated by the more severe trunk wounding observed in the declined group of trees. A change in natural drainage patterns is often an unfortu- nate outcome of urban development, and oaks in formerly well-drained areas that are suddenly left in standing water from high precipitation levels suffer greatly from the lack of soil oxygen (Ware and Howe 1974), a known causal factor in the decline of oak (Gaertig et al. 2002) and other tree species (Dyer and Mader 1986). In this study, the association of wet years with poor growth of now-declined trees, but not healthy trees during the critical period, suggests the possibility that trees in the two categories were being exposed to different levels and/or durations of soil moisture, presumably a com- bination of high precipitation levels and impeded drainage around now-declined trees. Considering the low tolerance of bur oak to flooding (Johnson 1990), and the poor internal drainage of Winnipeg’s heavy clay soils, this potential change would have been a major stress for now-declined sample trees. Mature bur oak trees have been observed as being able to survive up to 30 consecutive days of saturated soil during the growing season, compared with more flood tolerant species such as cottonwood (Populus deltoides) or white ash (Fraxinus americana) that can survive an entire growing season under deep flooding (Whitlow and Harris 1979). The damaging effects of urbanization to bur oaks may stretch far beyond immediate physical impacts at the site of development, particularly in naturally forested areas where no manmade drainage systems exist. This idea may be sup- ported by the increased competition levels surrounding de- clined trees as opposed to healthy trees (Table 1), which may have been in more “street-like” environments with more im- mediate urban disturbance but better drainage. Impeded drainage was thought to be the main cause of a decline in a stand of bur oak in southern Manitoba near a newly con- structed road (Boone 2003) and several stands of oak forests in Winnipeg with known drainage changes from recent de- velopment (Catton 2005). It should be noted that neither physical nor chemical soil variables nor leaf nutrient levels differed between healthy and declined trees in this study. Contrary to many studies on oak decline (Tainter et al. 1990; Pedersen 1998), drought did not appear to be a major causal factor in the decline of the bur oaks in this study. Although declined trees were generally less vigorous than healthy trees during the critical period (Figure 1), the groups behaved most similarly in dry years, which may not be sur- prising considering that bur oak is known to be a relatively drought-tolerant species but vulnerable to flooding (Johnson 1990). However, the fact that now-declined bur oaks were less able to capitalize on their growing environments com- pared with healthy trees, even in years when poor soil aera- tion was presumably not a factor, supported the idea that now-declined trees likely suffered from root damage (Mc- Clenahen and Dochinger 1985; Innes 1990). Whether this damage was a direct (i.e., root severance) or indirect (i.e., changed drainage patterns) result of urbanization or other factors is not clear and probably varied with specific site conditions. Although drought was not shown to be a major inciting factor in bur oak decline in Winnipeg, it is not pos- sible to dismiss its potential importance in individual cases depending on the severity of the stress and the health condi- tion of the trees. ©2007 International Society of Arboriculture
January 2007
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