We investigated the occurrence of nonnative invasive plants on approximately 175 ha comprising a long-term, 60-year-old U.S. Forest Service silvicultural experiment and old-field stands in the Penobscot Experimental Forest (PEF) in central Maine. Stands in the silvicultural experiment were never cleared for agriculture, but have been repeatedly partially cut. Our objectives were to determine the extent of nonnative invasive plant populations in the PEF, and to relate invasive plant abundance and distribution to management history and environmental factors (overstory composition and basal area, canopy openness, and soil characteristics). We found ten invasive plant species in the study area. Very few occurrences of these were in the silvicultural experiment; where present, invasive plants there appear to be associated with proximity to seed source, and a greater degree of recreational or silvicultural disturbance. Ordination showed that the environmental variables which were associated with invasive species in the old fields were not associated with the presence of invasives in the silvicultural treatments. In the old-field stands, invasive plant cover was positively related to exposed mineral soil and negatively related to organic horizon thickness; invasive plant richness was negatively related to hardwood litter cover. Frangula alnus was the most frequent invasive plant species in both the old-field stands and silvicultural experiment; its distribution was not correlated with observed environmental variables. Control measures are recommended to prevent further encroachment of invasive plants into the silvicultural experiment.
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