Response of Herb Layer Cover to Experimental Canopy Gaps

To monitor response of herb layer species to canopy openings, we cre- ated three single-tree (33-36 m2) and three multitree (51-151 m2) gaps in a northern hardwoods stand. Percent cover of herbs and tree seedlings beneath the gaps and ad- jacent closed canopy was estimated in late May and July during 1 season before, and 3 seasons after, creation of the gaps. Environmental factors (e.g., light, soil moisture and temperature, and air temperature) were measured beneath gap and canopy. An outbreak of Hydria prunivorata in the 1st 2 postgap seasons opened the remaining can- opy during July and August and potentially influenced growth of summergreen understory species. Aster acuminatus, not present on the study area before gap crea- tion, established in two multitree sites by the 3rd postgap season. Early summer herbs, such as Maianthemum canadense and Oxalis acetosella, increased cover over the study period. However, response of these herbs occurred in both gap and canopy lo- cations. Additionally, cover of no herb species correlated significantly with the gap environment. Cover of Prunus serotina seedlings was primarily influenced by moth her- bivory in the 1st 2 postgap seasons; however, recovery in the 3rd season was greatest in the largest multitree gap. These results (1) indicate the importance of co-occurring disturbances in the forest and (2) suggest that small gaps are below the threshold for pronounced herb layer response on the Allegheny Plateau.