Riparian overstory-understory interactions and their potential implications for forest-stream linkages

Despite growing recognition that understory vegetation is an essential component of forest ecosystems, research on forest-stream linkages and the management of riparian zones continues to focus on overstory trees. Streamside herbs, grasses and shrubs have the potential to fulfill several functions upon which adjacent stream ecosystems rely, such as providing detritus and prey to aquatic communities. Here we investigated patterns of riparian understory vegetation and the associated invertebrate community along headwater streams in deciduous broadleaf forest managed under traditional silvicultural practices. We studied eleven sites along a large gradient of overstory canopy openness to assess whether and how resource competition with canopy trees limits the growth of riparian understory vegetation and affects the abundance and community composition of understory invertebrates. We found that the height and above ground biomass of understory vegetation increased steadily with decreasing overstory canopy cover, suggesting that the structure of riparian zones is contingent upon antagonistic interactions between overstory and understory plants. We also observed that invertebrate abundance did not track changes in overstory canopy cover and understory plant biomass although some numerically important invertebrate taxa displayed clear preferences to either closed or open-canopy riparian forest. Our findings demonstrate that riparian understory vegetation can achieve fairly high biomass and, therefore, should help maintain inputs of plant litter to aquatic ecosystems in harvested or naturally disturbed riparian forest. Because understory invertebrates may not fully compensate for reduced inputs of canopy invertebrates to streams, retaining trees along streams may still be the most effective management option for headwater streams.