Assessing the influence of riparian invasion by the shrub Lonicera maackii on terrestrial subsidies to headwater streams

Abstract

Invasion of Amur honeysuckle (Lonicera maackii) into riparian areas of headwater streams creates strong potential for alterations of terrestrial subsidies that may drive bottom-up effects on aquatic biota. In this study, we analyzed effects of L. maackii on terrestrial subsidies in stream sites that represented a gradient of invasion intensity in temperate deciduous forests of southwestern Ohio (USA). Leaf litter biomass, terrestrial and aquatic fine woody debris (0.5–9.9 cm diameter) volume and count, and terrestrial and aquatic coarse woody debris (>9.9 cm diameter) volume were analyzed. Based on previous research, we hypothesized that sites with a higher density of L. maackii would have decreased native species subsidies due to the dense overarching structure of the invasive shrub preventing materials from entering the stream. Over the course of the study, we found no evidence of differences in native leaf biomass across the invasion gradient. There were marginally significant effects of invasion level on fine woody debris, and regression analysis revealed a significant (R-squared 0.11; P = 0.043) negative relationship between L. maackii stem density and fine woody debris volume. Coarse woody debris volume was lower in sites with heavier invasion although no statistically discernible effects were detected. Regression analysis indicated a statistically significant negative relationship between L. maackii basal area and coarse woody debris volume (R-squared 0.14; P = 0.026). These results indicate linkage between invasion intensity and terrestrial subsidies mediated through the unique physiognomy of the invasive shrub. Riparian invasion of L. maackii may alter the fundamental biology of streams through alterations to deposition of terrestrial organic matter that serves as a structuring component of smaller waterways.