Decoupling of tropical forest structure and diversity: Stand characteristics, growth, and mortality in wet and dry Hawaiian forests and global comparisons

Background/Question/Methods

The biogeographic isolation of Hawai‘i resulted in pre-contact forest ecosystems with dramatically fewer species than continental tropical forests at comparable latitudes. The low species richness of Hawaiian forests makes them a model for testing conflicting theories for how diversity corresponds with tropical forest structure and function. Theories have been developed for predicting either positive or negative correlations of species diversity with forest structure (i.e., stem density, basal area, biomass, and size class distributions) and forest dynamics (growth and mortality), whereas other theories predict that structure and function of tropical forests should be independent of diversity, or linked to diversity indirectly through climate. We established 4-ha forest dynamics plots (FDPs) in Hawaiian montane wet (MWF) and Hawaiian lowland dry forest (LDF), which have mean annual temperatures of respectively 16°C and 20°C, and mean annual precipitation of 3440 mm and 835 mm. We used Center for Tropical Forest Science (CTFS) methodology, tagging and measuring all stems ≥ 1 cm diameter, to describe the structure and dynamics of low diversity Hawaiian tropical forests, and to compare across CTFS sites varying widely in species diversity.

Results/Conclusions

The Hawaiian FDPs have the lowest species richness in the global CTFS network (21 MWF, 15 LDF), consistent with Hawai‘i’s young age and isolation. The MWF and LDF contrasted strongly in composition and structure. The MWF had greater evenness and species diversity (H’= 1.82 MWF, 1.00 LDF), higher basal area (67.3 m²/ha MWF, 8.6 m²/ha LDF), and biomass (249 Mg/ha MWF, 29.4 Mg/ha LDF), but lower stem density (3078/ha MWF, 3486/ha LDF). The MWF stem density, basal area, and size class distributions were well within the range of other tropical FDPs, but the LDF had low basal area in comparison to other tropical FDPs, consistent with being the driest FDP in the CTFS network. The diameter growth in the MWF ranged from -0.06-0.71 cm yr^-1 and ranged from -0.02-0.04 cm yr^-1 in the LDF. Species-specific mortality rates were 0.6-5.1% yr^-1 in the MWF and 0.0-31.0% yr^-1 in the LDF. Only seven species in the MWF and two species in the LDF recruited. Despite very low species diversity and slow growth rates, Hawaiian forests are structurally similar to other tropical forests, and consistent with global trends for relationship of forest structure to climate. These results support the hypothesis that tropical forest structure and function are more strongly linked with climate than with species diversity.