. Critical trends assessment Program 2003-04 report. Critical Trends Assessment Project; Ecosystem management; Environmental monitoring; Natural resources surveys. Critical Trends Assessment Program 2003-2004 Report If an established species is not regenerating well the diameter class distribution may look like Figure 3. Most of the individuals are in the larger size classes with little or no recruitment in the smaller classes. Something like this typically happens with oaks and hickories in forests undergoing maple Figure 3 Maple takeover is one side of a two-sided problem. Maple t
. Critical trends assessment Program 2003-04 report. Critical Trends Assessment Project; Ecosystem management; Environmental monitoring; Natural resources surveys. Critical Trends Assessment Program 2003-2004 Report If an established species is not regenerating well the diameter class distribution may look like Figure 3. Most of the individuals are in the larger size classes with little or no recruitment in the smaller classes. Something like this typically happens with oaks and hickories in forests undergoing maple Figure 3 Maple takeover is one side of a two-sided problem. Maple takeover deals with the increasing abundance of sugar maple in the smaller size classes. If sugar maples greatly outnumber oaks and hickories in the smaller classes, it is in a position to fill any canopy gaps that occur, thereby replacing the other species. The other side of the problem is the lack of oaks and hickories in the smaller classes. This is not primarily the result of an increased amount of small maples. Rather, it deals with overall decreasing light levels on the forest floor as a result of an ever-closing canopy and sub-canopy, which inevitably occurs in forests as they age without disturbance. Much like takeover, decreasing light levels is the result of decreasing fire frequency and other changes to the landscape. Most oak and hickory species are shade intolerant, though a few hickories may be classified as moderately intolerant in Illinois. When the understory is heavily shaded, oak and hickory seedlings and saplings cannot survive (IDENR 1994). So if a canopy gap is created, there may not be any oaks or hickories in a position to fill it. Quite often, sugar maples are on hand to exploit the opportunity. In summary then, sites suffering from maple takeover are upland forest sites dominated by oak-hickory in the large tree classes, with maple dominant in the small classes and mostly missing from the largest classes. Based on these somewhat subjective criteria, 32
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