Vines on Trees at Forest Edges

Climbing vines are increasing and tree mortality is higher at forest edges. Over time, this combination means we may see forest edges recede and forest patches shrink. This creates a grave threat for small urban forests, and the life that depends on them. Managing vines in trees along forest edges can help reduce tree mortality.

Vines climb the forest edge at Rock Creek’s Barnard Hill Park.

NPS/Nortrup

Climbing vines on trees are abundant and increasing in National Capital Region (NCR) park forests (Matthews 2014). Many tropical forests face similar vine increases due to forest fragmentation and the creation of more forest edge habitat. This is important for both NCR and tropical areas since climbing vines have been shown to change forest structure and composition.

To learn what kinds of changes vines are making in our forests, we looked at data around tree growth rates and mortality, forest edges, and the ecological differences between native and non-native vine species.

Vine Questions & "Treesults"

Are climbing vines on trees becoming more common? Is there a difference between native and non-native vine species?
From 2010 to 2014, the rate of trees with vines rose from 21.5 to 25.7%. Of those vines, the percent of that were non-native invasives increased from 33 to 44%.

Figure 1. Vine recruitment on the forest edge.

NPS

Are climbing vines more likely to spread to new trees if the tree is located near a forest edge?

Climbing vines on trees occur in the highest numbers near forest edges. Figure 1 shows the odds of a vine climbing onto a tree, based on the distance of the tree from the forest edge. Trees within 50 meters of a forest edge are more vulnerable to vine infestation. Non-native vines on trees are constrained to a narrower zone near the forest edge.

Figure 2. Tree growth rates depend on presence of vines in the tree's crown.

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Do climbing vines on trees affect tree growth and mortality?
Most vines climb up trees to get more sunlight, closer to the canopy of trees. Once a vine reaches the crown of a tree, the tree's growth slows. Figure 2 shows that trees without vines in their topmost branches (the crown) generally grew at faster rates than those with vines in the crown, regardless of location.

Trees have different growth patterns based on where they grow, either in the forest interior or near a forest edge. Trees located in the increased light of forest edges grow at faster rates while young, but slow down once they reach a certain size. Trees located in the forest interior grow slowly in their early years and then speed up once they reach larger sizes – growing even faster than the young trees on the forest edge.

Figure 3. Tree mortality from vines in crowns.

NPS

Tree mortality risk is also influenced by vines and proximity to a forest edge. At the forest edge, trees are more vulnerable to a the stressful effects of more wind and sun exposure. So the risk of mortality rises the closer the tree is to a forest edge, and the presence of a vine in the tree’s crown amplifies this risk (Figure 3).

Virginia creeper (Parthenocissus quinquefolia) climbing up a tree at C&O Canal National Historical Park.

NPS/Coriell

Monitoring

To learn how vines, trees and forest edges interact, we analyzed forest vegetation data collected in National Capital Region parks. We visited 403 forest plots twice between 2007 and 2014. We measured growth rates of trees larger than 10 cm, whether trees were living or dead, any vine species climbing the trunk, and the presence of vines in the tree’s crown.

Further Reading

This material was originally presented in a 2016 resource brief and is based on the article, Climbing vines and forest edges affect tree growth and mortality in temperate forests of the U.S. Mid-Atlantic States.

To learn more about how National Park Service scientists are monitoring the health of parks in the National Capital Region, visit the NCRN I&M website.