Assessing the impacts of invasive alien tree species in lowland forest ecosystem in order to improve
restoration strategies in Samoa(復元計画改善を目的としたサモア低地林における侵略的外来木本種の影響評価 )Hokkaido University, Graduate School of Environmental Science, Division of Environmental Science
Development, Course in Global Environmental Management
Josef T. Pisi
Biodiversity in the Pacific Islands has faced great threats by invasive alien species (IAS), yet most studies
failed in assessing the spatio-temporal changes. The evaluation of IAS impacts is complicated and isusually perceived differently by stakeholders. Understanding species attributes in different habitats isnecessary for the restoration of ecosystem structures and functions. Recently, IAS is becoming
conspicuous in the forest ecosystems of Samoa. This study assessed the spatio-temporal changes in theimpacts of IAS on Mount Vaea Nature Reserve in Samoa. In addition, the recovery of species diversity(resilience) after natural disturbances is investigated by analyzing normalized difference vegetation index(NDVI). Transplantation experiments were conducted by using seedlings of native species that have highgrowth and survival potentials.
The initial forest survey conducted in 34 transects (50 m × 10 m) in 2007 showed that 62% of stems
were IAS. Three problematic species, i.e., F. elastica, C. elastica and S. campanulata, which accounted
for 57% of total stems, impacted the most to the declining abundance, richness and diversity of nativeplants. IAS dominated in various strata of the forests from the floor to crown. They were ubiquitous in
all size classes with C. elastica and F. elastica dominating small to medium size classes, and S.
campanulata on large size class. C. elastica had the highest presence of 92% followed by F. elastica 77%
in the site. The dispersion parameters for C. elastica and F. elastica were lower than S. campanulata,indicating a clustered to a random distribution. Species diversity and richness were high in less-impacted
transects but low in highly-degraded transects, signifying the important roles of native species infacilitating forests dynamics. NDVI analysis showed highly-degraded sites to be susceptible to theimpacts of tropical cyclones (TC), with >80% of damaged stems being IAS. The field observations
confirmed that IAS tended to uproot more. The leaf area index averaged 3.1 and canopy openness
averaged 11% implied a poor and vulnerable forest structure to natural calamities such as TC. After themajor TC Evan that occurred in 2012, a stagnant disproportional abundance (70-90%) was recorded forIAS. Overall, IAS had transitioned from an aggregated to a contiguous distribution where native species
were suppressed. In contrast, the abundance of native species remained below 25% after 2013, due
probably to their slow growth. Native species also had lower basal area (median < 5000m2). Qualitative
observations on the forest floor showed native species appealing more at the seedling and sapling stages.
However, native species were more tolerant to TC than IAS. Only 15% of native species were damaged
in 2013, by bending and/or snapping, with less uproots. Two native species P. samoensis and F.
godeffroyi out of 22 monitored tree species in the natural forest had high resilience since 2013, while fivenative species R. taitensis, S. fanaiho, P. pinnata, , T. richii, and C. vitiense, out of 12 showed highsurvival and growth rates in the experimental plots.
The impacts of IAS increased with time and were likely to be the strongest drivers in the declines andextinctions of native plant and animal populations. Understanding the factors affecting the establishmentof IAS is crucial to develop adequate plans to reduce biological invasions, including the restoration ofecosystems. The present study showed that the threshold of concerns has been breached and a reactive
adaptive management strategy is essential. In conclusion, improvement in restoration must be conductedin two steps: firstly IAS, in particular, F. elastica, C. elastica and S. campanulata, should be prioritizedfor management and reduced to below 50% abundance, focusing on the removal of mature individuals;
and secondly a few native species (7 species) recommended by the experiments are to be transplanted
closely in management zones.

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