Vines on trees at forest edges raise tree mortality in National Capital Region parks

Climbing vines are increasing in National Capital Region (NCR) park forests, and tree mortality is higher at forest edges. Together they may make edges recede and forest patches shrink, a grave threat to small urban forests and the life that depends on them. Managing vines in trees along forest edges can help reduce tree mortality. The findings come from a 2016 Park Service resource brief on vines and forest edges.

Vines climb the forest edge at Rock Creek's Barnard Hill Park. NPS photo.

Many tropical forests face similar increases because of forest fragmentation and the creation of more forest edge, and vines have been shown to change forest structure and composition.

What the monitoring asked

NPS scientists analyzed forest vegetation data from NCR parks: 403 forest plots visited twice between 2007 and 2014, recording the growth rate of trees larger than 10 cm, whether each was alive or dead, any vine species climbing the trunk and the presence of vines in the crown. They looked at growth and mortality, forest edges, and differences between native and non-native vines.

Findings

Are vines becoming more common? From 2010 to 2014 the share of trees with vines rose from 21.5 to 25.7 percent. Among those vines, the share that were non-native invasives rose from 33 to 44 percent.

Do vines spread to trees near edges? Vines on trees are most numerous near forest edges. Trees within 50 meters of an edge are more vulnerable to infestation, and non-native vines are confined to a narrower zone near the edge.

Vine recruitment at the forest edge. Figure from the National Park Service's page.

Do vines affect growth and mortality? Most vines climb for more sunlight, toward the canopy. Once a vine reaches a tree's crown, the tree's growth slows: trees without vines in the crown generally grew faster than those with vines, wherever they stood.

Tree growth rates depend on whether vines are in the crown. Figure from the National Park Service's page.

Growth also depends on location. In the extra light at forest edges, young trees grow faster but slow down once they reach a certain size, while trees in the forest interior start slowly and then speed up, eventually outgrowing the edge trees.

Mortality rises with closeness to the edge, where trees face more wind and sun, and a vine in the crown amplifies that risk.

Tree mortality from vines in the crown. Figure from the National Park Service's page.

Virginia creeper (Parthenocissus quinquefolia) climbing a tree at Chesapeake & Ohio Canal National Historical Park. NPS photo.

Sources

• National Park Service, "Vines on Trees at Forest Edges" (nps.gov article), based on a 2016 NPS resource brief and the article "Climbing vines and forest edges affect tree growth and mortality in temperate forests of the U.S. Mid-Atlantic States"; rewritten in hubnx's own words.