Removal of the invasive shrub, Lonicera maackii, from riparian forests influences headwater stream biota and ecosystem function

Background/Question/Methods

Lonicera maackii, an invasive dominant shrub in riparian zones, has adverse effects 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 biota and ecosystem 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 m² riparian buffer.  Autumnal, in-stream leaf litter was assessed over 75d, while macroinvertebrate density and algal biomass was measured for three years and a nutrient limitation study was conducted seasonally.  

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 comprised of the gathering-collector FFG, which primarily consisted of Chironomidae (F = 73, P <0.01).  Invasive honeysuckle removal significantly reduced canopy cover, light availability, and nitrogen and differentially influenced the timing and abundance of leaf litter genera within the stream (all P < 0.01). For example, Platanus spp. contributed the most organic matter within the removal reach (35-40%) but was mainly absent in the control reach.  Honeysuckle leaf litter consistently contributed ~25% of in-stream leaf litter in the removal reach, but was mostly absent in the removal reach.  Macroinvertebrate density significantly increased one year after invasive removal, and was primarily driven by Simuliium sp.  These findings suggest removal of a dominant invasive shrub substantially impacts terrestrial organic matter and nutrient subsidies into headwater streams, influencing the timing and abundance of leaf litter habitat and food resources for aquatic macroinvertebrates.