Interactions between climbing vines and forest edges influence tree mortality in Mid-Atlantic forests
Vines are an integral component of forest ecosystems. They compete with trees for above and below ground resources and can have strong influences on forest composition and structure, carbon sequestration, and the availability of food resources for wildlife. Vine abundance appears to be increasing in many tropical forests, perhaps as a result of increasingly disturbed and fragmented forest ecosystems or in response to elevated CO2. Vine research in the temperate zone has been more limited, but recent studies in the eastern U.S. have suggested a similar increase in vine abundance. The National Park Service’s National Capital Region Inventory and Monitoring Network monitors forest vegetation at >400 permanent plots in the Washington, D.C. region. Using these data, we asked the following questions: (1) In Mid-Atlantic forests, is abundance of climbing vines increasing? And is vine abundance greater near forest edges? (2) Are vines more likely to spread to trees located near forest edges (i.e., are edge trees with no climbing vines at T1 more likely to have vines at T2 than trees located in forest interiors?) (3) Does the presence of climbing vines on trees affect tree growth and mortality?
Based on a negative binomial mixed effects model, vine abundance is greatest near forest edges and is increasing over time in Mid-Atlantic forests. Based on a mixed effects binomial model, climbing vines are more likely to spread to trees that are closer to forest edges than to trees that are in forest interiors. While the presence of climbing vines does not affect tree growth rates, the probability of mortality increases significantly for trees that have vines in their crown. Based on a mixed effects binomial model, the probability of mortality for trees with vines in the crown was twice that for trees without vines in their crown. Additionally, there was a significant interaction between distance from edge and probability of mortality: the effect of vines in the crown was greater for trees located near a forest edge. These results suggest that vines play an important role in determining tree survivorship in temperate ecosystems, particularly near forest edges. Given land-use patterns in the mid-Atlantic region, the influence of vines may become increasingly important as remnant forest patches are further fragmented.