A Half-Century of Changes in the Herb Layer of a Climax Deciduous Forest in Michigan

Abstract

(1) Recent assessments of species-abundance at Warren Woods, a climax beechsugar maple forest in Michigan, were compared with data from 1919-21 and 1933. The results indicated that the abundance of several herbaceous species had declined over the past half-century. A few, mostly species already abundant in 1919-21, had increased in abundance. (2) Most of the declining species were among the ecologically more-generalized forest herbs, but the list of currently-abundant species included species having both narrow and broad patterns of community-occurrence in Wisconsin. (3) Seasonally-low light flux seemed to be a major factor in the changes. Over half of the currently-abundant species were spring ephemerals, but only one-tenth of the species which had decreased were spring ephemerals. Physiological adaptation to shade alone seemed rarely to be a successful strategy in the undisturbed forest. (4) It is suggested that the decreases were related to the continued maintenance of dense shade, and also to its intensification as sugar maple and beech increased their dominance. The loss of larger individuals of late-leafing, shade-intolerant trees eliminated patches where herbs other than spring ephemerals could make substantial growth before the canopy closed. (5) The study supported the view that recurrent disturbance tends to preserve diversity, but gap-phase regeneration did not seem to be important in the process. (6) A decreasing rate of change suggested that the forest might be approaching a steady state in herbaceous composition, but at a lower diversity than is usual in the climax stage of such forests.