RELATIONSHIP BETWEEN FOREST ABOVEGROUND BIOMASS GROWTH AND TREE SPECIES RICHNESS, IDENTITY, AND STRUCTURE
Authors: Santosh Ojha and Luben Dimov
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The effects of stand structure and species composition on the forest diversity-productivity relationship have received much attention during the last few decades. The objective of this study was to compare dry live aboveground biomass (AGB) growth among species structures (pure hardwood, mixtures of hardwoods, mixtures of pines, mixtures of pines with hardwoods, and pure pine) for different stand age, stand stocking, and site productivity classes. We also examined the relationship between AGB growth and the predictors species richness, quadratic mean diameter, height, compacted crown ratio, relative density, slope, aspect, precipitation, and elevation. We used data from 2554 USDA Forest Inventory and Analysis (FIA) plots in Alabama. Tukey-Kramer procedure with multiplicity adjustment was applied to compare AGB growth among the five species structures. Species richness was 1–17 and AGB growth was 0.01–15.68 Mg ha-1 yr-1 across the plots. For the pooled data, there was a weak overall correlation between AGB growth and species richness (r=0.07, p=0.001). The AGB growth of pure and mixed stands was statistically the same. In high-stocked stands productivity (AGB growth) was statistically the same in the different structures. There was no difference between the AGB growth rate of pure pine stands and mixtures that contained pine. However, pure pine stands outperformed mixed stands that did not contain pines in the mix. Species identity was an important factor for AGB growth in pure and mixed stands. As stocking increased, species structure became less important for the AGB growth rate. Among stands with high-stocking, pure stands were as productive as mixtures with no overall growth benefit of monocultures. If high or medium stocking level is maintained, there is no obvious growth benefit from monocultures or mixtures. In stands with low-stocking however, there appeared to be a benefit to having multiple species in the composition.