Aspen Bibliography


Neighborhood Effects, Disturbance, and Succession in Forests of the Western Great Lakes Region

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Neighborhood effects that enhance conspecific replacement at the time of canopy-tree death were examined in 3 cold-temperate forest types, using spatial analyses and historical reconsauction on mapped plots. Important neighborhood effects in these forests include overstory-understory effects whereby a species enhances likelihood of self replacement by influencing the local understory, and disturbance-activated effects that switch on after stand-killing disturbance. Neighborhood analyses (radius 9 m) show that understory composition is significantly related @ < 0.001) to overstory composition in a hardwood-hemlock forest, but not in white pine or near-boreal jack pine forests. Jack pine has strong disturbance-activatedeffects in the form of abundant serotinous seedfall after fire. White pine apparently has no signif~cant neighborhood effects of either type. Other species are likely to replace white pine in treefall gaps, and after fue it must reinvade a given neighborhood by means of outside seed sources. In the forest types with strong neighborhood effects a change in the usual disturbance regime may occur that overwhelms the self-replacement mechanisms. Such events would include lack of fue in near-boreal jack pine forest, which causes succession to a spruce-fu-birch-cedarmixture, and intense

f m after heavy windfall in hardwood-hemlock forest. which can convert the forest to paper birch and aspen. From the point of view of species compositional stability, these changes in disturbance regime are aue catastrophes, unlike stand-killing fires in jack pine or complete canopy windthrow in hardwood-hemlock. These compositional catastrophes operate in a punctuated-stability context, and initiate episodes of succession, which are relatively short periods of rapid change in species composition. We put forth the neighborhood-effect hypothesis of forest dynamics. This hypothesis predicts that forest types with weak neighborhood effects have great temporal instability of composition, with recovery from disturbance and related successional sequences nearly always in progress, whereas species composition may be generally stable with sudden alternation between different states in forests with strong neighborhood effects.