Elevational and latitudinal changes in tree associations and diversity in the Eastern Arc mountains of Tanzania

A total of 200 variable-area plots covering 20.3 ha containing 4000 trees with a basal area of 921.4 m(2) were assessed over a combined elevational range of 410-2180 m in the moist forests of three Tanzanian Eastern Are mountains: West Usambara, Nguru and Udzungwa. Plot data were ordinated on the basis of species presence/absence, frequency and basal area. Axis 1 of ordinations based on species presence/absence are correlated with elevation. Axis 1 of the frequency-weighted ordination was correlated with elevation in the Nguru and Udzungwa mountains, but plots from the West Usambara showed a rainfall-related discontinuity. Axis 1 of the West Usambara basal area-weighted ordination showed evidence of long-term dynamics of Ocotea usambarensis and in the Udzungwa mountains was determined by presence of Parinari excelsa. Plot diversity was not correlated with elevation or latitude, but was lower in disturbed, low rainfall or more seasonal forest. Stem density was positively correlated with elevation and was greater on ridge tops than valley sides and valley bottoms.