VEGETATION CHANGES ALONG GRADIENTS OF LONG-TERM SOIL DEVELOPMENT IN THE HAWAIIAN MONTANE RAIN-FOREST ZONE

The development of the Hawaiian montane rainforest was investigated al ong a 4.1-million-year soil age gradient at 1200 m elevation under two levels of precipitation, the mesic (c. 2500 mm annual rainfall) vs. w et (> 4000 mm) age gradient. Earlier analyses suggested that soil fert ility and foliar nutrient concentrations of common canopy species chan ged unimodally on the same gradients, with peak values at the 20,000-1 50,000 yr old sites, and that foliar concentrations were consistently lower under the wet than under the mesic conditions. Our objectives we re to assay the influences of soil aging and moisture on forest develo pment using the patterns and rates of species displacements. The canop ies at all sites were dominated by Metrosideros polymorpha. Mean heigh t and dbh of upper canopy Metrosideros trees increased from the younge st site to peak values at the 2100-9000 yr sites, and successively dec lined to older sites. A detrended correspondence analysis applied to m ean species cover values revealed that significant variation among sit es occurred only on one axis (axis 1), for both soil-age gradients. Sa mple scores along axis 1 were perfectly correlated with soil age on th e mesic gradient, and significantly correlated on the wet gradient. Hi gher rainfall appeared to be responsible for the higher rates of speci es turnover on the wet gradient probably through faster rock weatherin g and greater leaching of soil elements. We concluded that the changes in species cover values and size of the canopy species was a reflecti on of the changing pattern of nutrient availability associated with so il aging.