Optimizing Forest Structural Integrity through Tree Diversity: Evidence from Tropical Diversity Experiments

The structure of tropical forests has been identified as one of the key determinants of the effects of diversity loss on productivity, carbon storage, and the long-term stability of restored tropical forest stands. However, the emergence of complex structure in mixed-species stands relative to the structural conditions of mature natural forests depends on how species interact with each other and with the abiotic components, which vary across environmental conditions. Yet we still lack a clear understanding of how the relationship between tree diversity and forest structural complexity is shaped by biotic and abiotic factors that exert regional control, as well as the mechanisms underlying these influences.

Using high-resolution three-dimensional models of forest structure generated from terrestrial laser scanning across species-diversity gradients in ten biodiversity experiments, we assessed how diversity influences structural integrity relative to natural forests. We also quantified how changes in tree diversity affect structural complexity within and across tropical regions. The experiments spanned 5 climate zones and 10 soil types with distinct taxonomic compositions and a diverse range of tree species richness, providing a robust range of conditions to support generalizable inferences.

We found that increasing species diversity in mixtures led to greater stand structural complexity, but the strength of the diversity effect differed markedly across regions, with greater variability between than within climate zones. Increasing species diversity also contributed to greater structural integrity, and this pattern was linked primarily to increased vertical stratification of tree crowns. We found no clear evidence that the impact of tree species on structural complexity is significantly related to regional context, but our results showed that the overall diversity effect varied in magnitude and importance depending on region-specific differences in water availability and stand age. These findings provide further evidence of the value of stand structure in determining the consequences of biodiversity loss, which is relevant for designing restoration strategies in the tropical region.