Working with the Hawai‘i Division of Forestry and Wildlife (DOFAW), we have built a partnership that allows Kapi‘olani Community College (KCC) students to work alongside community members restoring a two hectare exclosure in Wailupe Valley constructed by DOFAW in November 2013.
This student opportunity allowed for first-hand experience with establishing a restoration site in its beginning stages and the complex factors involved with field work ecology. Our goals are to establish a transect grid, document current forest composition as a baseline for comparison prior to invasive species removal and establish removal comparison plots, to determine the most efficient restoration techniques. The primary target monitoring species was Strawberry Guava (Psidium cattleianum), but all native and introduced species were recorded. Removal of invasive species was initiated through the help of community partnerships. Creating opportunities for the community to volunteer is vital for the contingency of Wailupe and is becoming one of the projects’ focuses in 2018. These efforts are to restore native plant species and restore natural habitat for the endangered native bird the ‘Elepaio.
We want to know if using just hand-tools is an efficient method to remove invasive species in order to increase the populations of native tree species. Over time this will provide the ‘Elepaio with a native tree canopy that is a natural and sustainable habitat.
Photo of 'Elepaio taken in the Wailupe area. Map and location of the Wailupe study site.
The Wailupe Valley exclosure is located in East O'ahu, Hawaii in mesic forest. In 2015, KCC students established a grid of 10 x 10 meter plots marked in the field with poles made from pvc pipe. In 2018, to provide a more accurate representation of the forest structure of young and older trees, we updated our methodology. We looked for adjacent plots with similar topography and species so that we have control plots to compare later removal efforts to. We monitored and recorded the tree species within the 10x10m plots.
We differentiated between adult trees (> 1 meter in height) and seedling/saplings (< 1 meter in height). We measured the number and the diameter at breast height (DBH) of tree species >1 meter in height. For individuals < 1m we recorded a count only. We then calculated the basal area [BA=π(DBH/2)²] of the recorded flora > 1m and summed the total basal area for each species.
We removed introduced species using hand-removal techniques and weed wrenches and then monitored the plots again for a pre and post removal assessment. Comparing the basal area of all species pre-removal with post -emoval basal area helps us determine if hand pulling introduced species alone is an efficient reforestation technique for the exclosure.
During the Spring 2018 semester (January-April) three plots were established, pre monitored, removed in, and post monitored. We analyzed data on the three most common introduced tree species: Strawberry Guava Psidium cattleianu, Cook Pine Araucaria columnaris, and Christmas Berry Schinus terebinthifolius and two native tree species: Lama Diospyros sandwicensis and Alahe'e Psydrax odorata.
We found that for the seedling/sapling category (< 1m in height) pre-removal, 80% of the total count of individuals were A. columnaris and only 9% were P. cattleianum (Figure 1). For the adult trees (> 1 m in height), out of the five most common species in three plots, P. cattleianum had the largest total basal area at 69% while A. columnaris was only 0.2% (Figure 2). It is notable, that there are far more P. cattleianum >1m than A. columnaris >1m, but more A. columnaris <1m than P. cattleianum <1m.
Post-removal, seedling/saplings (<1 m in height) showed some slight regeneration, except for of S.terebinthifolius which did not show any regeneration, (and those S.terebinthifolius >1 m declined in basal area). For the native species (D. sandwicensis and P. odorata) the number of seedlings/saplings slightly increased in the post monitor data and there was little to no change in the basal area for adults ( >1m in height).
The total adult basal area P. cattleianum pre-removal (5746cm2) decreased post removal (4111cm2) which tells us that only 30% of all adult P. cattleianum were able to fit into our weed wrenches to remove the roots (Figure 2).
Our efforts at shifting the balance of forest tree species towards native species has shown some positive trends and identified some potential issues. The primary biomass by basal area in the plots was P. cattleianum, however we were only able to remove 30% of the P. cattleianum > 1m in height, so alternative methods will be needed to remove the remaining P. cattleianum. Even though the plots that were established and monitored in this semester had substantially larger basal area for P. cattleianum than A. columnaris, there were eight times more A. columnaris seedlings/saplings than P. cattleianum in the inital monitoring. Our removal efforts were able to reduce the number of A. columnaris seedlings but we suspect that neighboring A. columnaris outside of the study plots must be dropping their seeds which then germinate and reduce the ground area available for native species. This suggests that areas like Wailupe with large A. columnaris will take more resources to keep recruitment numbers down, even after initial manual removal.
The slight increase in the native species, D. sandwicensis and P. odorata, seedlings/saplings in the post monitor data is encouraging. Removal of invasive saplings has likely relieved pressure on native tree species, allowing the native species to benefit from more available resources. Future research will continue to monitor pre and post-removal plots. We will be monitoring these plots again in Fall 2018 and we expect to see an increase in the number of native seedlings. Over the next year, we also plan to involve more community members and school groups to participate in the removal process.