PARENT SESSION
Oral Session # 86: Invasive Species VII: Forests.
Presiding: C Huebner
Thursday, August 7. 1:30 PM to 5:00 PM, SITCC Meeting Room 204.
Invasion by the N-fixing tree, Falcataria moluccana, alters ecosystem processes in wet forests of Hawaii.
Hughes, R. Flint*,1, Denslow, Julie1, 1 Institute of Pacific Islands Forestry, USDA Forest Service, Hilo, Hawaii
ABSTRACT- Invasive species pose major threats to the integrity and functioning of ecosystems. Previous studies have shown that when invasive species introduce new biological processes into an ecosystem, they can change the fundamental rules by which other species survive and reproduce. Here we describe the effects of an invasive alien N-fixing tree, Falcataria moluccana, on ecosystem processes and stand structure of wet lowland forests on recent volcanic substrates of Hawaii. Substrates ranged in age from 50 to 300 yr and, as such, supported a gradient of developing stands undergoing primary succession. On each of 3 substrates, study sites were established in stands either invaded by, or free of, Falcataria. Inputs of N via litter fall were 4 to 55 times greater, and P inputs 2 to 28 times greater, in forests invaded by Falcataria relative to native-dominated forests on comparable substrates. Decomposition rates differed substantially in response to Falcataria as well. Decomposition rate constants (k) of Falcataria leaf litter were 3 times greater than those of leaf litter of the dominant native species, Metrosideros polymorpha, and rate constants of leaf litter were 2 to 7 times greater in Falcataria-invaded sites compared to native forest sites. Further, the presence of Falcataria led to significant increases in plant-available forms of N and P; combined values of NO3-N and NH4-N captured by ion exchange resins were 12 to 46 times greater,and PO4-P were 2 to 57 times greater, in Falcataria-dominated sites compared to native-dominated sites. These Falcataria-mediated changes in ecosystem processes were accompanied by alterations in forest structure and species composition as well. While Metrosideros dominated the native forests, almost 100% of Metrosideros suffered mortality following Falcataria invasion. In contrast, the alien tree Psidium cattleianum was absent from native-dominated sites, but reached densities as high as 14,000 stems ha-1within Falcataria-invaded sites. By transforming ecosystem functions, Falcataria has facilitated the invasion of species that would otherwise be scarce in these early successional forest systems.
Key words: wet lowland forests, facilitation, invasive species, ecosystem processes