Amur honeysuckle (Lonicera maackii) is a non-native shrub that has invaded the forest understory in the Ohio River Valley, where it often displaces native shrubs such as paw-paw (Asimina triloba) and spicebush (Lindera benzoin). This may be due in part to differences in physiology among these species. The purpose of this study was to examine water use by these three species in relationship to soil water availability (volumetric water content or VWC) and vapor pressure deficit (VPD). Data were collected from a stand in a wetland forest in Kentucky near the Ohio River during the 2011 growing season. Transpiration rates were calculated from sap flow rates and sapwood areas, and VPD and VWC were measured and recorded by data loggers. Multivariate ARIMA models, using daily transpiration as the dependent variable and VPD and VWC as independent variables, were tested on each species. A photosynthesis system was also used to periodically measure gas exchange parameters under ambient light and VPD conditions.
Transpiration time series showed strong serial dependence, usually with a 1-day lag, and models explained 35-60% of total variance. The three species responded to VPD and VWC somewhat differently. Amur honeysuckle transpiration had a greater response to same-day VPD than to same-day VWC. Paw-paw responded to VWC in a more complex (though generally positive) manner, while both same- and previous-day VPD increased transpiration. Spicebush responded positively to same-day VPD while being affected by both same- and previous-day VWC. Amur honeysuckle transpiration was much less responsive to increases in VPD and VWC than the other two species. Gas exchange measurements mirrored these findings, with transpiration usually responding positively to VPD and VWC in all species. Amur honeysuckle appears to be the most drought-resistant of these three shrubs. This, combined with its longer growing season, may contribute to its success in both wetland and upland environments.