Broadleaved tree diversity in very small, natural riparian forest fragments in Central America

Broadleaved tree diversity was measured in a network of very small gallery forests within the Mountain Pine Ridge savanna in Belize. Research focussed on forest patches smaller than 1 hectare in size (micro-forests) and linear strips of trees along creeks lacking interior core zones with low understorey light levels (tree thickets). Twenty-five micro-forests and 51 tree thicket sites were sampled throughout the savanna. More detailed enumerations were conducted in 15 micro-forests and 6 tree thicket sites. A total of 144 morphospecies ≥5 cm dbh were found, which represents 1/5 of the tree species in Belize. Most of the species encountered are typically found in tropical rain forests and few are restricted to savanna or riparian environments. There was a high rate of species turnover from site to site in both sampling units, and a few species exhibited distributions that were spatially aggregated. Although watershed and geology were found to be significantly correlated to landscape-scale patterns of diversity, neither were able to account for much of the variation in the data. The data consistently revealed substantial differences in broadleaved tree diversity among tree thickets and micro-forests. The results also indicated that the changes in species composition from one sampling unit to the other followed a predictable pattern with certain species more likely to be present in one unit or the other. Changes in species assemblages among only micro-forests were not as structured, however, but some species did exhibit significant preferences for the core and edge zones. Unlike in tree thickets, the diversity in micro-forests did not appear to depend on mass effects from larger gallery and continuous forest. However, genetic analysis of analysis of one tree species determined that populations in micro-forests had, on average, lower genetic diversity than populations in continuous forest. The data also raised the possibility that the genetic consequences depended on the amount of local forest cover in proximity to the fragmented populations. The results indicate that networks of very small forest patches can contain high numbers of species and could therefore contribute to the maintenance of regional biodiversity.