Effects of crown expansion into gaps on evaluation of disturbance intensity in northern hardwood forests

Crown encroachment by trees surrounding newly formed gaps reduces their size, a fact that may cause error in area-based analyses of rate of gap formation and disturbance intensity. Generally, an estimate of proportional area occupied by age classes with a stand of trees can be obtained by determining the age of a selected canopy tree nearby each of randomly or uniformly located sample points. Possible methods of selecting nearby canopy trees include: 1) nearest-tree selection method, in which the canopy tree with its stem closest to the sample point is chosen; and 2) overhead-crown selection method, in which the canopy tree whose crown is directly above the sample point is chosen. These 2 methods differ greatly in their sensitivity to crown encroachment. In sugar maple Acer saccharum stands, the nearest-tree selection method leads to better estimates of the area originally occupied by a cohort of trees. The overhead-crown selection method leads to an unbiased estimate of area currently occupied by a cohort. -Authors