Evaluating and monitoring invasive plant processes

Abstract

Non-indigenous plant species (NIS) are a concern for both natural area land managers and the public. These species are perceived negatively despite the fact that the processes and impacts related to NIS are not fully understood. Also, control is generally conducted without understanding NIS population ecology. As a result, few NIS populations have been successfully controlled. We studied the population ecology of NIS in natural areas. First, Linaria vulgaris was monitored at different spatial scales over three years to determine its invasiveness, and also to determine which life history states drove this invasiveness. An invasive population was one that increased in density and/or spatial extent over time. We found that not all study populations were invasive, that invasiveness varied across spatial scales and habitats, and that invasiveness at one spatial scale did not necessarily correspond with invasiveness at another. This invasiveness was driven primarily by vegetative spread. Relative population invasiveness was then evaluated using an invasiveness index, a tool for prioritizing management. Next, we explored the contribution of NIS propagule pressure with distance from a road to NIS colonization of a natural grassland. Roads are the major vector for NIS propagule movement into natural areas, and NIS occurrence decreases rapidly with distance from roads. Two species with different colonization strategies, Cirsium arvense, and Bromus inermis were studied. Bromus inermis seed rain and seed bank decreased both with distance from the road and established patches. Cirsium arvense seed rain decreased with distance from established patches, but not the road. Soil water and conditions for emergence were studied using surrogate NIS to see if emergence varied with distance from the road. Soil water only varied with distance from the road in June; and soils were otherwise uniformly moist or dry. Surrogate NIS emergence did not vary with distance from the road or varying soil water. Finally, seed predation was studied to determine if whether it varied with distance from the road or between four different species. Seed predation varied with distance from the road and also between species for two of the four species.