Composition and Structure of Lowland Rain-Forest Tree Communities on Ta'u, American Samoa

Abstract

This study examined the composition and structure of tropical rain forest in four permanent plots on the island of Ta'u, American Samoa. Two 1-ha plots were established in coastal forest, one in an abandoned coastal cultivation site (a ''plantation'' ca. 17 yr post-abandonment) and another in a Dysoxylum samoense-dominated coastal forest. Two 2-ha plots were established in lowland forest at 200-250 m elevation, one in an abandoned plantation (ca. 13 yr postabandonment) and the other in less-disturbed mid-to late-secondary mixed lowland forest. In the total 6 ha, we encountered 54 tree species, with Dysoxylum samoense the most dominant species overall and abundant in all four plots. The upper forest plot was the most diverse plot and exhibited low similarity with any of the other three plots. This plot exhibited a rarefaction curve similar to those of four lowland hill forest plots on Tutuila but was most similar in composition to regenerating disturbed forest on Tutuila rather than mature, less-disturbed forest. Low similarity was found between the two Dysoxylum-dominated coastal forests and the Dysoxylum-Pometia-dominated Ottoville lava flow forest on Tutuila. By examining the populations of D. samoense across the four plots we found an impact of agriculture and cyclones on height structure of the forest. Examination of species' diameter class distributions allowed us to propose several hypotheses related to the life histories of several tree species. We documented the natural establishment of the introduced species Flueggea flexuosa into both abandoned plantations and natural forest. Monitoring these plots over time will allow us to better understand successional processes in natural and human-impacted forest including changes in composition, structure, relative abundance of nonnative species, as well as the impact of cyclones on different forest types.Forest covers more than half the surface of the five main islands of American Samoa.Lowland rain forest, occurring from sea level to 350-500 m elevation, is the most widespread forest type on the Islands, differentiated into several habitat types including coastal, ridge, valley, slope, and lava flow forests (Whistler 2002). These habitat types are a function of topographical position , with the exception of lava flow forest, which is formed on relatively flat, young lava flows (Whistler 1980(Whistler , 2002.The island of Ta'u is located approximately 100 km east of Tutuila, at 169 28 0 W, 14 14 0 S. It is a small (39 km 2 ) shield volcano formed over a geologic hot spot that formed the entire Samoan Archipelago. Ta'u is the youngest of the high islands of the Samoan Island chain, being formed approximately 300,000 yr B.P. (Nunn 1998 activity of one large volcano (Lata Volcano), two smaller lava volcanoes, Tunoa Volcano and Luatele Volcano, contributed lava flows to the northwestern and northeastern corners of the island. On the south side of the island, steep cliffs from Lata summit (960 m above sea level) to the sea are the inner walls of the caldera...

Factors Affecting Tropical Tree Damage and Survival after Catastrophic Wind Disturbance

“…Ta'u was formed approximately 300,000 yr BP (Nunn 1998) as a shield volcano. Slopes above the escarpment average 12° (Webb et al 2006), and soils are moderately deep, well-drained stony clay loams (Typic Dystrandepts) formed in volcanic ash and underlain with lava (Natural Resources Conservation Service 2004). Slopes above the escarpment average 12° (Webb et al 2006), and soils are moderately deep, well-drained stony clay loams (Typic Dystrandepts) formed in volcanic ash and underlain with lava (Natural Resources Conservation Service 2004).…”

“…Lowland rain forest of Ta'u is dominated by the large, longlived tree species Dysoxylum samoense (Whistler 1992, Webb et al 2006. In Ta'u coastal lowland forest, Dysoxylum is found in association with Diospyros samoensis, Sterculia fanaiho, and Pisonia umbellifera, whereas above the escarpment (to about 250 m asl) Hibiscus tiliaceus, Syzygium inophylloides, Rhus taitensis, and Alphitonia zizyphoides are important associates (Webb et al 2006).…”

Forest Plant and Bird Communities in the Lau Group, Fiji

“…Such a plea for new field work may be time-limited; although the forests here still consist primarily of native species, without the problem of invasive trees as in parts of Samoa [28] and large islands of western Fiji [78], the appearance of invasive non-native trees could occur at any time.…”

Recovery of native forest after removal of an invasive tree, Falcataria moluccana, in American Samoa

“…Additionally, Samoa experiences more frequent but less cataclysmic disturbances in the form of cyclone ''near misses'', tropical storms, and tropical depressions; together these create what has been termed a 'chronic disturbance' regime (Webb et al , 2011. In total, disturbances from cyclones, whether large or small, have played a potent evolutionary role in shaping the composition, structure, and function of Samoa's native forests (Webb and Fa'aumu 1999;Webb et al 2006).…”