Mechanisms for invasion by non-native trees: Facilitation, ecosytem engineering, and escape from belowground enemies

Invasive species are one of the greatest threats to global biodiversity. Numerous hypotheses have been proposed to explain the invasive success of non-native species and the invasibility of natural systems; however, invaders are neither predictable nor mechanistically similar. My research explores facilitation and ecosystem engineering as important processes affecting the invasion of Norway maple (Acer platanoides). In addition, this research investigates whether the invasive success of black cherry ( Prunus serotina), box-elder (Acer negundo), and A. platanoides in their non-native ranges relates to escape from belowground enemies found within their native ranges.; In riparian stands, correlative and experimental results suggest that A. platanoides has a negative effect on native species and a positive effect on conspecifics. Also, these effects are dependent upon the level of invasion suggesting that Acer trees experience a “lag phase” before spreading and severely impacting the native community. In coniferous forests, A. platanoides seedlings had higher survival and experienced less physiological stress in a conspecific patch than in an adjacent conifer forest. Substantial demographic, physiological, and environmental differences between sites appeared to be due to ecosystem engineering by mature A. platanoides. I propose that Acer trees produce a more mesic environment by modifying the structure and phenology of the forest canopy and by altering the timing of transpirational water loss. Overall, these results indicate that environmental modification by Acer trees may facilitate conspecifics in their understories and suppress native species.; Recent research has suggested that invasive plants may also be released from their natural soil pathogens; however, there have been no tests comparing the sensitivity of invasive plants to resident soil biota both in their native and non-native ranges. Here we provide the first evidence of invasive trees (Acer spp. and Prunus) being facilitated by the soil biota in their non-native ranges, whereas the soil biota decreases survival and growth in their native ranges. In the newly invaded forests, where the soil biota effect is slightly positive, Acer and Prunus trees are closely spaced relative to the negative soil biota effect and more widely dispersed trees in their native ranges. In total, this research suggests that several interactions (facilitation, ecosystem engineering, and plant-soil interactions) not currently considered in invasive theory may facilitate invaders and impact natural systems.