Patterns of Plant Diversity in Overbrowsed Primary and Mature Secondary Hemlock-Northern Hardwood Forest Stands

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Many studies report that herbaceous species diversity higher in primary forest than mature secondary forests, but this pattern has not yet been demonstrated in forests overbrowsed by white-tailed deer. We compared the diversity of herbaceous and tree seedling species in 5 primary and 5 mature secondary riparian zone forest stands in northwestern Pennsylvania, a region which has been overpopulated with white-tailed deer for the past 65 years. We sampled study plots with 16 1-m2 quadrats and recorded herb and tree seedling species presence and abundance. At the 1-m2 scale, primary forest sites had significantly more tree seedling species per quadrat but the number of herb species per quadrat in primary a secondary forest sites was not significantly different. At the 2.25 ha scale, primary forest sites had significant higher herb and tree seedling richness than secondary sites. Herbs and tree seedlings also exhibited significantly greater evenness in primary sites than secondary sites. There was considerable species composition overlap between primary and secondary sites. Tsuga canadensis, Fagus grandifolia, and Oxalis acetosella had a significantly higher frequency of occurrence and significantly greater abundance in primary forest sites, whereas Prunus serotina and Erythronium americanum had a significantly higher frequency of occurrence and significantly greater abundance in secondary forest sites. Acer rubrum was also significantly more abundant in secondary growth sites. We hypothesize that the trend towards greater diversity in primary forest plots is the result of the stands experiencing levels of disturbance closer to the historic disturbance regime.

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