Forest restoration and alternate stable states on Hawaii island
The Hawaiian islands have one of the most unique species assemblages on the planet but ongoing habitat loss and the introduction of non-native species has caused precipitous declines or extinctions of many Hawaiian island endemics. On the big island of Hawaii, mid to high elevation forest has been severely impacted through clearing for ungulate grazing and the subsequent introduction of non-native plants that outcompete native trees. Efforts are now underway to restore native plant communities with the goal of improving overall ecosystem functioning and providing important habitats for many of the endangered native bird species.
I am currently a post-doc working with Drs. Carla D'Antonio (University of California Santa Barbara), Stephanie Yelenik (USGS) and a team of collaborators to understand why past efforts to restore mid-elevation Hawaiian forests have fallen short of their goals. We are specifically testing how the presence on non-native grasses inhibits the recruitment of native plants.
Threshold dynamics to recruitment
We are combining grass removal with seed addition to experimentally test at which threshold(s) recruitment of native plant species may occur. It is possible that simply removing grass will allow native plants to establish. However, it is also possible that limited seed dispersal due to the loss of avian seed dispersers also inhibits natural recruitment.
We are creating a gradient of grass removal and adding various amounts to seeds to plots ranging from zero seeds to several hundred times the natural seed rain amount. This information will be used to guide future management efforts as well as ask important ecological questions associated with ecological thresholds
Tree architecture and native plant recruitment
The thick layer of grass within our restoration forests likely limits plant recruitment in several ways. One way for a native seed to germinate and grow is to simply find a place above the grass layer to establish. We are investigating how tree architecture may benefit recruitment of additional understory species. We are specifically interested in how the shape of tree bases, available horizontal area of above-ground roots and overall tree architecture may promote recruitment of additional individuals.
We have been sampling 'Ōhi'a trees throughout our study area and finding that tree architecture varies widely. We suspect that the understory recruiting below these focal trees will also greatly vary in the number of stems and the species composition based on the architecture of the focal tree itself.
Freezing temperatures limit recruitment of species in certain restoration sites
Frost is thought to potentially restrict native woody plant recruitment in open grasslands. If periodic freezing events kill native woody plants, then succession may be slowed or stopped. We are taking a community approach to understand how freezing temperatures limit species distributions within our study area and prevent recruitment in the harsher microclimates experienced in the open grasslands.
By using simple freezing assays on leaves, we are able to determine the species most likely to establish in open grasslands where low temperatures are more extreme and stressful. These data can guide which species should be used for restoration in certain habitat types.