Small-Scale Altitudinal Variation in Lowland Wet Tropical Forest Vegetation

Abstract

All trees and lianas (.gtoreq. 10 cm dbh) on 12.4 ha of primary lowland wet tropical forest at La Selva, Costa Rica, were enumerated. A total of 5530 live stems were encountered, representing 269 spp. Palms comprised 25.5% of stems, lianas 2.4%, tree ferns 0.1% and true trees 72.0%. The most abundant species, Pentaclethra macroloba (Mimosaceae), accounted for 13.6% of stems. Species richness ranged from 79-107 spp. per ha. Species composition varies continuously with altitude over a range of 39 m, as shown by a detrended correspondence analysis ordination of data from 0.04-ha subplots. The pool of available species at any altitude is large, and floristic variation between sites at the same altitude is influenced by chance. Seasonally-flooded sites are floristically distinct from sites on higher ground, and show great variation in species composition over short distances. Microtopography and drainage features in a 1.8-ha swamp forest were mapped. There is a dramatic reduction of stem density and species richness with increasingly poor drainage. This lower number of species reflects the exclusion of species intolerant of waterlogged soils.