MONOSPECIFIC DOMINANCE OF A TROPICAL DECIDUOUS FOREST IN MEXICO

The typical diversity of lowland tropical forests is contradicted by e xamples of forests dominated by a single species of canopy tree (disre garding early successional sites or waterlogged sites). On the Pacific slope of Mexico, Celaenodendron mexicanum Standley (Euphorbiaceae) fo rms such forests. Natural history study shows that this species is dio ecious, wind-pollinated, autochorous, deciduous but drought-resistant, slow-growing, VA mycorrhizal, rich in secondary metabolites, and has both specialist parasites and generalist predators. Monodominant fores ts of Celaenodendron mexicanum (CF) occur as discontinous patches rang ing in size from < 0.1 ha to > 1000 ha. Celaenodendron mexicanum is es sentially absent outside of these patches, which are surrounded by for est with a high diversity of tree species (mixed forest, MF). Five sam ples, three from CF and two from MF, were compared regarding structure and floristics in the woody component with DBH greater-than-or-equal- to 2.5 cm. The percentage of canopy trees with DBH greater-than-or-equ al-to 10 cm attributable to the most common species was high in two CF samples (63% Celaenodendron mexicanum) compared with MF samples (19% Caesalpinia eriostachys Benth.). Diversity was correspondingly low in these two CF samples relative to MF samples (Inverse of Simpson's inde x = 4 and 13, respectively; Exponential Shannon-Wiener index = 6 and 1 6). The third sample of CF had an intermediate value for dominance (35 %) and diversity indices similar to MF samples (7 and 12). Discriminan t analysis showed three groups with floristic richness as the principa l distinguishing attribute. The structural characteristics of overall density and basal area did not differentiate monodominant and mixed sa mples. The trees and lianas occuring in CF were also found in MF; Cela enodendron mexicanum itself was the only species exclusive to its mono dominant patches. Celaenodendron mexicanum also dominates the smaller size classes (DBH < 10 cm) in all three CF samples. These individuals may represent recruitment and suggest that CF is persistent, and that CF3 is an intermediate phase of MF changing into CF.