Impact of the invasive Amur honeysuckle (Lonicera maackii) on stand transpiration in a wetland forest

Background/Question/Methods   Amur honeysuckle is an invasive shrub found in the Ohio River Valley that excludes native plants from areas that it invades. It also has an extended leaf-out period and thus has the potential to increase transpiration in invaded forest stands. Its effect on water use by a wetland forest in Kentucky near the Ohio River was determined during the summer and fall of 2009. An old-growth stand was compared with a second-growth stand on the St. Anne Wetlands Research and Educational Center in Melbourne, KY. While the old-growth stand had a very sparse shrub canopy, the second-growth stand had a dense cover of Amur honeysuckle. Shrub basal area was more than 5 times greater in the second-growth stand, and >85% was honeysuckle. Sapflow rates in trees and large shrubs were measured with Granier sapflow probes, while rates in small shrubs were determined with heat balance sensors. Sapwood areas were then used to calculate transpiration rates.

Results/Conclusions   Transpiration rates from trees were similar in the two stands. Shrub transpiration from the old-growth stand was only 1.4% of the tree transpiration (0.9% due to honeysuckle) but 6.7% (5.5% due to honeysuckle) from the second-growth stand. Shrub transpiration was dominated by honeysuckle in the second-growth stand. Because of its extended leaf-out period, honeysuckle continued to transpire late in the fall, when tree and native shrub transpiration has ceased. Amur honeysuckle transpired the equivalent of ~7 mm of rainfall in the second-growth site over the monitored period, whereas it transpired the equivalent of ~ 1.3 mm in the old-growth site, a greater than 5-fold increase. The additional transpiration caused by Amur honeysuckle may shorten the lives of ephemeral ponds and streams in wetlands, with adverse impacts on organisms, such as amphibian larvae, that require them.