Species composition and structure of certified açaí harvesting forests
In total, we sampled 131 tree species at the certified sites represented by 36 botanical families and 109 genera (Table S2). The number of families and tree species accounted for 50% and 18% of the MAUA database (control group) that includes floristic data from forests across Amazonia. On average, certified açaí managed forests harbored a greater number of tree species than non-certified açaí (Fig. 2a). The number of tree species was significantly higher (P = 0.00634) in certified harvesting areas and 50% higher than non-certified groves, on average (Table 1). In contrast, both certified and non-certified forests are less diverse in terms of species richness in comparison to the control plots (P = 1.0087e−9, P = 1.654e−8 for both pairwise Wilcoxon tests).
In terms of forest structure, certified açaí forests had trees with greater values of basal area per hectare (P = 0.01567e−6), Fig. 2). On average, basal area values were ca. 150% higher in certified forests (Table 1). The largest individual trees were from the species Ceiba pentandra (L.) Gaertn., Spondias monbin L. and Mora paraensis (Ducke) Ducke with diameters over 120 cm at the breast height (DBH). All certified studied plots obtained basal area values (26.9–91.7 m2.ha−1) within the range of variation comparable to intact forests (Table 1, Fig. 2).
Although species richness and basal area significantly differed between certified and non-certified plots, we found no significant difference between açaí abundance when both groups are compared (Fig. 2c, P = 0.8781). Therefore, certified areas were richer in terms of floristic composition, had a larger forest stature (higher basal area), and possessed comparable açaí density as non-certified forests.
Relationship between richness and açaí clump density
We found a negative relationship between the density of açaí clumps and the species richness in açaí managed forests. The higher the açaí clump density is, the lower the diversity of tree species is in both certified and non-certified harvesting areas (Fig. 3). However, certified data showed a trend with a lower slope than non-certified data (slopecertified = − 0.01, R2certified = 0.215; slopenon-certified = − 22.75, R2non-certified = 0.677, respectively). Even in sites with a high density of açaí clumps, certified harvesting areas tend to have a more species rich composition and a larger tree basal area than non-certified açaí groves (Table 1, Fig. 3).
The relationship between the density of açaí clumps and the species richness is still significant when the explained variation is partitioned using the db-RDA analysis. Despite the climatic distinction over a large spatial range among the studied plots (Figure S1A), we did not find supportive evidence that species richness could be associated with the climatic variation over the space. We did find a partial association between richness and the spatial distances among the plots (Figure S1B), possibly due to the overwhelming number of control plots from the PELD-MAUA research network that are located in western Amazonia. Even though the spatial correlation is present, the number of species among certified, non-certified, and control plots was still significantly explained by the density of açaí clumps in the studied managed forests (Figure S1B).
Flora of certified açaí managed forests
The most dominant botanical families found in the certified plots were Fabaceae (27 spp., 303 individuals), followed by Myristicaceae (2 spp., 95 individuals), Malvaceae (9 spp., 78 individuals), Anacardiaceae (2 spp., 64 individuals), and Euphorbiaceae (4 spp. 49 individuals). These families together account for more than 50% of the total number of tree species sampled in certified managed sites (Table S2). Virola surinamensis, the most representative tree species, and Minquartia guianensis are registered in the IUCN red list as endangered taxa. Some popular agroforestry species, including non-natives, were also found amid certified forests (e.g. Mangifera indica, Theobroma cacao, Spondias mombin, Musa spp.).