DIFFERENT FACTORS DRIVE EMERGENCE AND PERSISTENCE IN AN INVASIVE GRASS

Premise of research. Hypotheses to explain nonnative plant invasions often suggest that propagule pressure promotes invasion success. However, interactions between propagule pressure and abiotic or biotic habitat characteristics also may regulate establishment and persistence of invaders, but they are not often tested. Additionally, results from the few published studies considering interactions among factors are conflicting, with some studies finding that propagule pressure overwhelms abiotic and biotic conditions and others suggesting that environmental factors play a key role in determining invasion success independent of propagule pressure. Methodology. We conducted a field study to evaluate the roles played by propagule pressure (1-, 4-, or 16-rhizome segments), disturbance (disturbed or undisturbed, to manipulate environmental resistance), and their interaction in the emergence (initial ramet production) and persistence (presence at the beginning of the second growing season) of highly invasive cogongrass (Imperata cylindrica). To determine whether elevated propagule pressure or disturbance aids in overcoming environmental resistance in shaded habitats, we conducted the experiment in both open field (full sun) and forest understory (deep shade) habitats. Pivotal results. Cogongrass emergence was driven by both propagule pressure and disturbance, which enhanced emergence probability by threefold and more than twofold, respectively. However, persistence occurred only in open, full sun habitats, with no tillers surviving in the deeply shaded forest plots (average photosynthetically active radiation was 93% lower in forest than in open field plots) in the second growing season. Conclusions. Our results demonstrate that the emergence, persistence, and spread of plant invaders can be driven by different abiotic and biotic factors in the recipient habitat. Furthermore, these results were generated by monitoring an experimental invasion process for longer than a single season, providing a realistic and robust evaluation of the drivers of invasive plant success and suggesting a general need for longer-term establishment studies in invasive plant ecology.