Influence of Silvicultural Treatment, Site Characteristics, and Land Use History on Native and Nonnative Forest Understory Plant Composition on the Penobscot Experimental Forest in Maine
This study investigates forest understory plant diversity and composition in managed and unmanaged stands within the context of a long-term silvicultural experiment in the Acadian Forest of Maine. I examined the effects of silvicultural intensity and past land use on understory plant species diversity and composition. Silvicultural treatments include three variants of the selection system, three variants of the shelterwood system, modified and fixed diameter-limit cutting, and an unregulated harvest. Two types of unmanaged stands were studied: a continuously forested natural area and secondary forest stands on old fields. Chapter 1 presents analysis of understory plant diversity and composition in managed and unmanaged stands; patterns were examined in relation to site history, current management or use, and environmental factors such as overstory composition, basal area, canopy openness, and soil drainage. A total of 234 species were found in 317 plots. The old field stands had a richer and more diverse understory than all other treatments. In continuously forested managed and unmanaged treatments, understory species richness and diversity generally declined with decreasing silvicultural intensity. Stands without an agricultural history were more similar in understory composition than old field stands. Differences in diversity and composition of understory plants appear to be related to canopy composition and forest floor disturbance. Old field stands were characterized by an overstory dominated by hardwoods and had greater mineral soil cover, while all other treatments were conifer-dominated and had greater basal area and more softwood litter cover. Softwood basal area was the best predictor of understory species diversity and richness in the continuously forested areas of the PEF. All continuously forested stands, including those treated with silviculture, were composed of native forest plant species typical of the Acadian Forest, though plots in the natural area and unregulated harvest treatment included a few normative invasive plant seedlings. The understory composition of the old fields contained 13 normative species, nine normative invasive species, and a greater component of early successional ruderals than the continuously forested stands. While silvicultural treatments are associated with understory plant compositional changes, these differences are slight in comparison to the effects of an agricultural past. Continued monitoring of the understory vegetation is needed to understand the short- and long-term responses of understory plant populations to silvicultural treatment. In Chapter 2, I further explore the pattern of normative invasive plant abundance and distribution on the PEF. Multivariate ordination of data from the old field stands revealed positive associations between invasive plants and exposed mineral soil and percent hardwood basal area. Spearman correlation analysis indicated the percent cover of invasive plants was negatively correlated with distance from a roadside, hardwood litter cover, and organic horizon thickness. Glossy buckthorn (Frangula alnus) was the most frequent invasive species in the old field stands, and its distribution was not correlated with any of the observed environmental variables. An investigation of invasive plant occurrences in the silvicultural experiment area of the PEF assessed invasive plant encroachment. Meander surveys revealed that invasive plants were infrequent and were most often found close to woods roads and trails. Frangula alnus was the most frequent invasive plant in the silvicultural experiment area. The majority of invasive plant occurrences were in two locations: one replicate of the unregulated harvest and the natural area. These two areas are in close proximity to large invasive seed sources, and both areas have a greater degree of recreational or silvicultural disturbance, which is associated with invasive plant presence. Monitoring of the normative invasive plants will yield needed information about their patterns of establishment in a conifer-dominated Acadian Forest. The prevalence of invasive species in the old fields warrants immediate action to prevent their spread into the managed areas of the PEF. An invasive species management plan should be implemented to protect the integrity of the long-term experiment and biodiversity at the PEF. A successful and cost-effective control strategy can only occur if applied while invasive plant populations are still small and sparse
