In the large-scale logging experimental site in Paracou, French Guiana we showed that functional dispersion, particularly based on traits related to shade tolerance, increased biomass gains through canopy packing in undisturbed Amazonian forests. However, this effect was absent in previously logged stands, where forest structural diversity did not fully recover even ca. 40 years after logging activities subsided, a pattern observed in previous studies, but reported here for the first time in the Amazon. Our findings indicate that the loss of the largest trees from the logging activities drastically changed the ecological process driving biomass productivity, altering light partitioning among species and disrupting the functional link between species composition, canopy packing, and biomass productivity.
To achieve objects 1, our results showed that undisturbed forests with higher multivariate functional dispersion showed higher Shannon PAD evenness (more homogeneous vertical distribution of plant material). Therefore, our findings confirm that higher canopy packing is achieved in communities characterized by higher functional complementarity (represented in this study by functional dispersion), where shade-tolerant co-exist with light-demanding and pioneer species, fostering the formation of functionally diverse tree species communities in terms of physiological adaptation to shade. Following our causal conceptual model, we found that an increase in Shannon PAD evenness led to an increase in biomass gain. More homogeneous occupancy of the canopy space is related to improved niche partitioning due to more efficient allocation of space and light resources. In older forest stands, the gaps created by large trees falling are more frequent, allowing the existence of niches that can be filled again by small trees . Those small trees do not directly compete with the largest trees due to vertical and horizontal stratification and their different sizes and shapes, but they can contribute to additional productivity to the whole stand due to a more complete use of resources. Indeed, the canopy packing effect was strongly related to tree recruitment. These findings indicate that niche complementarity (possibly related to gap dynamics) is an important determinant of forest productivity.
To achieve object 2, we found that the effect of functional dispersion on canopy packing is context-dependent, with functional dispersion not increasing Shannon PAD evenness in disturbed plots. In disturbed plots, Shannon PAD evenness showed no significant effect on biomass gain.
The project results show that the relationship between diversity, canopy packing, and productivity is strongly context dependent, and that, while enhancing biodiversity or canopy complexity may be beneficial in some circumstances, generalizations of nature-based initiatives—especially those aiming to boost biomass productivity—can be misleading, as biomass dynamics in tropical forests are governed by multiple interacting processes and mostly by the different land-use histories of secondary forests.
