We describe compositional variation among forest fragments on Tongatapu in terms of successional status and site conditions. We then examine two factors that directly influence the potential for long-term persistence of these fragments – tree regeneration and alien invasion.
Tongatapu is the largest (261 km2), most densely inhabited (population 67,000) island in the Kingdom of Tonga, western Polynesia. Inhabited for 3000 years, it is now more than 90% deforested.
We mapped land cover from 1990 aerial photographs and sampled forest fragments and selected regenerating (late bush fallow) stands. Composition data from forest fragments were analysed with cluster analysis and ordination, and then an environmental framework was constructed to allow floristic comparisons between these and late bush fallow stands. We analysed size-structures of tree populations to evaluate regeneration. Distribution patterns of aliens introduced by Polynesian settlers (ancient introductions) and after European contact (modern introductions), were compared. Growth forms were compared to determine whether aliens are qualitatively distinct from the native forest fragment flora. Correlation analysis and hierarchical regression determined whether factors such as disturbance, soil fertility, species richness, fragment size, and distance to forest margin, which influence invasion patterns elsewhere, were important on Tongatapu.
Forest fragments occupy 3.2% of the land area; regenerating `bush fallow' stands occupy 7.9%. We recorded 209 vascular plant species, half of which were trees or shrubs; other growth forms were, in decreasing order of importance, forbs, lianes and vines, graminoids and ferns. Eight per cent of the species were ancient introductions and 33% were modern introductions. Forest fragments were classed into four types: interior, coastal, Excoecaria coastal swamp and Inocarpus coastal swamp – the latter two unknown elsewhere in Tonga. Successional tree species are important components of interior and coastal forests. Most frequent species in forest fragments also occur in late bush fallow plots. In forest fragments, six of the nine most frequent tree species have abundant juveniles available to replace adults; three are successional and their regeneration was not observed under closed canopies. Most (thirty-three of forty-five) less common tree species were more frequent as juveniles than adults. Invasion patterns differ among forest types. Late bush fallow stands support the highest alien cover, mostly trees and shrubs. Among forest fragments, interior forest is the most invaded and Excoecaria swamp forest is least invaded.
Forest fragments on Tongatapu are small, highly disturbed and spatially disjunct. Composition relates, at least in part, to proximity to the coast, soil drainage and soil type and composition of most non-successional tree species appears to be being maintained. Compositional integrity is threatened by selective harvesting and invasive aliens. Modern introductions are more abundant than ancient introductions in late bush fallow areas and may pre-empt their establishment. Significantly, even small, degraded forest patches harbour an important component of native biodiversity.