Tropical cloud forest recovery: the role of seed banks in pastures dominated by an exotic grass

Soil seed banks may contribute to forest recovery, but the presence of exotic grasses may represent a constraint for the natural regeneration process. This study aims to evaluate the composition of seed banks in three successional stages of the lower montane peri-urban forest in Veracruz, Mexico, and experimentally determine the role of an exotic grass (Cynodon plectostachyus) in suppressing seed bank expression. For this, soil samples were collected from an abandoned pasture (P), a young secondary forest (SF), and an old secondary forest (F), and grass was experimentally removed in P. Seed density (seeds/m²) was higher in P (12,189 ± 2421), followed by SF (2825 ± 347) and F (3552 ± 865) with 76, 75 and 78 species, respectively. Tree seeds accounted for 1689 (P), 126 (SF) and 265 (F) emerged seedlings of 6, 8 and 6 tree species, respectively. The highest percentage of pioneer tree species was recorded in P seed banks (99 %). Similarity in tree species composition of the seed banks was higher between P and SF (Jaccard = 0.8) than between P and F (0.5). Similarity between soil seed bank tree species and tree standing vegetation was higher in P than in SF and F. Aboveground (1810 g/m²) and belowground (508 g/m²) biomass of C. plectostachyus is suppressing the emergence of individuals from the seed bank: when the grass was removed, nine woody species became established. Exotic grass removal in small nuclei may trigger early forest recovery through allowing the establishment of the tree species already present in the soil seed bank.