| The spread of introduced species is one of the most widespread and important human influences on the biosphere, and with sustained introductions, novel ecosystems dominated by introduced species are rapidly emerging worldwide. Because the abundance of novel ecosystems is growing, we will increasingly depend on them for the basic services of nature, yet novel ecosystems are often ignored in ecological thought and they have been poorly studied. In this dissertation, I explore the community- and ecosystem-level properties of novel forest ecosystems in Wisconsin and Hawai'i in comparison to co-occurring native forests that prevailed historically I address key questions concerning the differences in composition, structure, and function between novel and native ecosystems, focusing specifically on whether novel ecosystems provide habitat for native tree species, and whether novel ecosystems provide the basic services of nature, such as production of biomass, nutrient cycling, and storage of carbon. Specifically, I documented large increases in stem densities in ecosystems dominated by introduced Rhamnus cathartica (common buckthorn) compared to native forests in Wisconsin (Chapter 1). I surveyed novel forests and abandoned plantings of introduced trees in Hawai'i and found that native tree species accounted for a small portion of basal area and stem densities, although two species were maintained at low abundances in limited areas (Chapters 2 and 3). In a natural experiment where novel and native forests co-occur on parallel substrate age gradients in Hawai'i, I found that local tree diversity is higher in novel compared to native Hawaiian forests, and that aboveground net primary productivity, belowground carbon storage, and nutrient turnover rates met or exceeded levels found in native forests (Chapter 4). I show that large increases in productivity are due to the presence of N2-fixing tree species, a novel functional type that thrives on Hawaii's N-limited lava flows (Chapter 5). Finally, I demonstrate that while novel forests dominated by N2-fixing tree species have higher belowground storage of carbon and nitrogen, these ecosystem pools of carbon and nitrogen remain limited by abiotic controlling factors: namely parent material type and age (Chapter 6). |
