Disturbance impacts on non-native plant colonization in black spruce forests of interior Alaska

Abstract

While boreal forest habitats have historically been relatively free from invasive plants, there have been recent increases in the diversity and range of invasive plants in Alaska. It is critical that we understand how disturbances influence invasibility in northern boreal forests, to avoid the economic damage other regions have experienced from invasive plants. Black spruce (Picea mariana) is the dominant forest type in interior Alaska, and wildfire is the dominant disturbance in these forests. Furthermore, disturbances in the form of management for fire suppression are common in forests close to urban areas. I surveyed recently burned, managed, and undisturbed black spruce forests for invasive plants to determine if fire and management facilitate invasive plant colonization. I also conducted an experimental seeding trial with three invasive plants common to Alaska (bird vetch (Vicia cracca), common dandelion (Taraxacum officinale), and white sweetclover (Melilotus officinalis) in burned and mature black spruce forest to determine if fire facilitates invasive plant germination. To determine the effect of substrate type on invasive plant germination, I planted seeds on a variety of substrates in the burned forest. Results indicate that fire and fire suppression treatments promote invasive plant colonization, as invasive plants were observed in burned and managed areas, but not in mature stands. Analysis of environmental data taken at survey sites indicate that fire mediates invasibility through its effects on substrate quality. In burned stands, invasive plants are most likely to colonize areas of shallow post-fire organic depth. Results from the seeding trials were consistent with results from invasive plant surveys, with reduced germination in mature compared to burned forest, and no germination on the residual organic layer in the burned forest. The highest germination occurred on mineral soil in burned forest, indicating that severe fires that combust the organic layer are likely to increase invasibility. The results of this study suggest that invasive species control efforts should be prioritized to disturbed forests, particularly areas where the disturbance has exposed mineral soil.