The role of advance growth in upland rainforest restoration, Pohnpei, Federated States of Micronesia

摘要

Upland tropical rainforest on Pohnpei island, Federated States of Micronesia, declined from 42 % of land area in 1975 to 15 % in 1995, largely through conversion to sakau (Piper methysticum G. Forst.) cultivation. As part of a research programme aimed at restoring degraded forest, I hypothesised that forest succession is retarded in abandoned sakau plots because of a loss of advance growth after forest clearance and weeding. This would exclude advance growth from playing a major role in regeneration and prevent species relying upon advance growth from establishing in sakau plots. I compared regeneration in abandoned sakau plots with regeneration following tree-fall disturbance in intact forest. All available tree and shrub species were sampled for composition, density, height, and health. The area covered by ferns, grasses and lianas by species was scored. Light and litter were measured in the smallest sample units for analyses independent of plot types. Samples were remeasured over 15 months to track the growth and survival of the initial cohort and newly recruited seedlings. Small single-tree falls brought only subtle changes to the forest; light varied more in gaps without necessarily producing significant shifts in mean responses. Recruitment of the pioneer canopy tree Campnosperma brevipetiolata Volkens and understorey tree Aglaia ponapensis Kaneh. was significantly greater in gaps than in mature forest. With the exception of C. brevipetiolata, both the initial and newly recruited mature and gap populations were dominated by heavy-seeded species. Height growth was greater in gaps. In contrast to the forest, sakau plots were characterised by: open canopies; shallow litter depth; wide coverage of ferns, grasses and lianas; and few trees and shrubs, predominantly light-seeded small-tree and shrub species. In 15 months, seedlings and saplings grew rapidly, litter depth and fern cover increased and grass cover decreased markedly. At a broad disturbance scale (quadrat data from all plot types combined), light was correlated with species responses such as seedling abundance and height growth; however, light was not very influential at the narrower scale of small-gap dynamics. Larger and older seedlings survived longer, but new recruits initially grew faster. Results suggested that tree species that are better adapted for survival in Pohnpei's mature- and gap-phase forest, typically those with relatively heavy seeds, established from advance growth and/or seed rain. The prevalence of advance growth in Pohnpei's upland forest meant that mature-forest species were self-perpetuating into the gap phase. In contrast, species composition and abundance in sakau-plot regeneration reflected a greater reliance upon propagule input from the soil seed bank and seed rain. In abandoned sakau plots, the breakdown of advance growth, combined with a seed rain deficient in heavy-seeded species (Winthrop, 1998) and competition from ferns and grasses, excluded from establishment many species normally common in Pohnpei's forest, thereby impairing forest recovery. I examined the implications of this for restoration strategies.