Changes in tropical forest vegetation along an altitudinal gradient in the Udzungwa Mountains National Park, Tanzania

This study describes changes in woody vegetation in the Mwanihana forest, Udzungwa Mountains National Park, Tanzania, over an altitude range of 470-1700 m. Two methods, fixed- and variable-area plots, are compared to elucidate altitudinal variation in tropical forest structure, diversity and community composition. Six 25 m x 100 m fixed area plots recorded a total of 2143 woody stems of >= 3 cm d.b.h. from 204 species. The 78 variable-area plots recorded the nearest twenty trees of >= 20 cm d.b.h. to an objectively chosen point, giving a total of 1560 stems in 9.1 ha from 156 species. A linear trend of increasing stem density with altitude was seen for variable-area plots. Species diversity is highest at high elevations. There was no clear zonation of elevational vegetation types. Restricted range taxa occur at all altitudes sampled. The study also revealed some methodological considerations. Bias in sample size and plot area can be tested by employing two sampling methods. Of the two methods used, fixed area plots are preferred as variable area plots are impractical in tangled understorey. Plot size must be controlled for in order to make reliable observations of diversity. Sampling along a continuous or near-continuous altitudinal gradient with sufficient replication is also important.