Riparian forest invasion by a terrestrial shrub (Lonicera maackii) impacts aquatic organic matter processing and biota in headwater streams

Background/Question/Methods

Lonicera maackii, an invasive dominant shrub in riparian zones, has adverse affects on terrestrial insects and plant survivorship, growth, and reproduction.  Lonicera maackii grows abundantly along riparian corridors, creating an overarching canopy which deposits substantial volumes of leaf litter into stream systems each autumn.  The influence of these leaf litter inputs on aquatic communities and organic matter processing is currently unknown.  We investigated linkages between this terrestrial invader and the aquatic macroinvertebrate community and organic matter processes via a (1) senesced leaf pack breakdown experiment and (2) riparian zone restoration experiment. The leaf breakdown experiment was conducted in three 3^rd order headwater streams in southwestern Ohio, and included three leaf pack treatments: L. maackii (invasive), Fraxinus spp. and Plantanus occidentalis (native), and a native-invasive species mix.  Leaf packs were deployed 15 December 2009 and collected weekly for 53 d to calculate leaf breakdown rates for each treatment and to characterize the colonizing macroinvertebrate community in terms of taxon density and functional feeding group (FFG) relative abundance.  In the riparian restoration project, all woody invasive flora was removed in August 2010 from a 1600 m²riparian buffer.  Autumnal terrestrial organic litter inputs and macroinvertebrate communities were monitored for approximately one year. 

Results/Conclusions

Results for the leaf breakdown experiment indicated that L. maackii leaf pack breakdown was significantly faster compared to native and mix-species leaf packs within all stream sites (F = 20.46, P < 0.001).  Lonicera maackii leaf packs also supported a macroinvertebrate community that was significantly dominated by the gathering-collector FFG, which primarily consisted of Chironomidae (F = 73, P <0.01).  Invasive riparian plant removal significantly decreased L. maackii and increased native litter inputs compared to the upstream control reach (P <0.05).   Removal of riparian invasives also substantially influenced leaf deposition of specific taxa.  For instance, L. maackii significantly dominated the in-stream leaf litter community in the upstream control reach on days 7 and 14 while P. occidentalis significantly dominated the community within the removal reach on those days (P <0.05).  Hydropsychidae and Chironomidae densities were greater in the removal reach and represented a community dominated by collector-gatherers and filterers (P <0.05).  These results suggest the presence of L. maackii in the riparian zone acts as a filter for native organic matter.  Removal of this invasive species significantly increases native organic matter input to the stream system affecting macroinvertebrate communities, thereby, affecting ecosystem processes in headwater streams.