Landscape and successional influences on plant invader success in a primary successional riparian zone

Background/Question/Methods

The Zoar Valley Canyon of 6^th-order Cattaraugus Creek in western New York State contains a pristine riparian zone that encompasses >11 river km and 500 ha of publically protected old- and first-growth hardwood forest, with stand ages from <10 to >300 years. We explore here the roles of disturbance and diversity in invasibility of riparian successional forest at the landscape scale (~100ha). Specifically, we are addressing the following: 1) Does invader success (e.g. bush honeysuckle, multiflora rose, black locust) change over successional time? 2) Are there biotic and/or abiotic stand characteristics associated with recipient community invasibility? 3) What are the influences of Eltonian ecological resistance vs. invasive species access via the river corridor? In addition to distribution/abundance of major invasives, recipient ecosystem variables such as stand age and composition and landform history were quantified on four raised terraces (understory reinitiation/multi-aged), six lower terraces (stem exclusion/understory reinitiation), and seven active channel margins/mid-channel islands/abandoned channels (stand initiation).

Results/Conclusions

Invasives, even black locust, were rarely found on landforms >80 years old (i.e. stand ages >60 years), which may reflect both a successional threshold and/or the historical introduction of the invaders. In contrast, active channel margin and mid-channel island stands <30 years old were extensively invaded, with aerial coverage by bush honeysuckle in particular exceeding 30%. Minimally wooded old channels (40 – 60 years old) were also heavily invaded, whereas mature-forest canopy gaps were not. Multi-aged (150 – 300+ years) stands were uniformly free of invasives, except at very localized natural disturbances such as alluvial fans and seepage-created nascent wetlands, where isolated individual clumps of honeysuckle and multiflora rose have colonized. Results thus far suggest distinct, perhaps even punctuated, changes in invasibility over successional time, with mid- and late-seral stands and their canopy gaps uninvaded. The youngest and most invaded landforms/stands were subject to the highest impact of hydrologic disturbance, whereas the least invaded successional stages actually displayed the highest native species diversity.