Demographic Responses of Herb Layer Species to Experimental Canopy Gaps in a Northern Hardwoods Forest

Abstract

(1) To determine whether herbs respond to the mean (single-tree) and median (multiple-tree) sizes of gaps in a 70-year old hardwoods forest on the Allegheny Plateau of Pennsylvania, U.S.A., herb growth and reproduction were monitored for one season before and for three years after gap creation. Gaps were created by cutting canopy and understorey trees at 1 m above ground, and comparisons were made between plots inside the gaps and beneath the adjacent intact canopy. There was no soil turnover associated with the experimental gaps. (2) There were no significant changes of minimum daily air temperature, soil temperature and moisture, or inorganic nitrogen content of soil beneath the gaps. Maximum daily air temperature was higher in gaps than in surrounding closed forest. Midday insolation on clear days increased significantly, with maximum illumination north of gap centre. (3) Analysis of vegetation in 1008 permanent 0 25-mi2 plots found no consistent zonation of cover response around the gaps that would indicate herb layer response to the open canopy. (4) Erythronium americanum beneath singleand multi-tree gaps had greater leaf area than beneath closed forest; however, most of the variation in Erythronium population and plant size was independent of plant location. Flowering was greater than expected in single-tree, but not in multiple-tree gaps. (5) Prunus serotina, the summer dominant of the understorey and potential competitor with herbaceous species, was examined to help understand herb response. Height of Prunus seedlings increased each year from 1981 to 1984, while the number of plants declined. (6) Number and size of fern leaves, and number of minor herbs as a group, increased during this study; growth was not statistically greater or lower beneath gaps.