The Invasion Ecology of Ivy (Hedera spp.) in Portland's Forest Park

Abstract

Invasive ivy (Hedera spp.) has extensive impacts on Pacific Northwest urban forests, many of which are not yet fully understood. In this study of Forest Park, Portland, Oregon, I evaluated several environmental variables obtained or derived from monitoring datasets at three spatial scales to determine the following: how ivy is spatially distributed; what factors are most correlated with ivy abundance; and how ivy abundance influences shrub community composition. I found that ivy is significantly clustered at all scales with multiple apparent epicenters along the park's urban periphery. Using NMDS ordination, I determined that ivy is a significant factor in the ecosystem in general and the shrub community in particular at all scales. Random Forest regression found different sets of important environmental predictors and shrub associations at each scale, but spatial relatedness and the abundance of Mahonia nervosa consistently emerged from predictor and shrub models, respectively. All this suggests that while ivy has complicated and often site-and scale-specific interactions with its environment, its clustered dispersal pattern may be at least as important as conditions in the ecosystems it invades. However, each of the monitoring datasets had significant limitations. Future research with refined data methods could be used to infer causal relationships, measure changes over time, and model ivy's ability to spread through the Forest Park landscape. My thanks go first and foremost to my adviser, Dr. Jeff Gerwing, and to my committee members, Dr. Marion Dresner and Dr. Andrés Holz. I owe an equally great debt to Marshall Johnson, ecologist at Portland Parks and Recreation, who is (among other things) the reason this study exists. This thesis bears the marks of many other hands who helped with its creation: Carole Hardy, whose good cheer, gift for collaboration, and organizational brilliance have helped keep me on the path on many occasions; Dr. Cat de Rivera, who provided me with the opportunity and guidance to learn and share more about ivy than I even knew there was to know;