The influence of seed functional traits and anthropogenic disturbances on persistence and size of the soil seed bank from dry subtropical forest species

Abstract

Soil seed banks (SSB) are reservoirs of viable mature seeds that play a crucial role in the dynamics and recovery of vegetation following disturbances. Despite its importance, there is still limited understanding of their characteristics of SSB in semiarid subtropical ecosystems. Our objective was to evaluate the effects of wildfire and roller-chopping on the size and persistence of the SSB of six native woody species, considering the influence of seed functional traits (FT). This study was conducted in dry subtropical forests of the western Argentine Chaco region. We evaluated SSB of six woody species: Aspidosperma quebracho-blanco (Aqb); Schinopsis lorentzii (Sl); Sarcomphalus mistol (Sm); Senegalia gilliesii (Sg) and Vachellia aroma (Va) in experimental plots located in two sites (forest plots of ca. 5 ha): forests without disturbance within the last four decades (conserved forest - CF) and forests subjected to fires and mechanical treatments for the partial removal of the shrub layer (roller-chopping and burned forest -RBF). Samples of the SSB were taken under the canopy of six focal individuals of each species in two reproductive seasons between 2016 and 2018. Seeds from ten mature trees of each species were collected to characterize their functional traits (shape and seed mass) related to their aptitude to form the SSB. Three groups of species were identified based on FT analysis: I-large, compressed seeds (Sl, Aqb and Sg); II- medium-size subspherical seeds (Va and Nn); and III- large spherical seeds (Sm). Aqb, Sl, Nn, and Va presented significant differences in the size of their SSB when comparing between sites and sampling years (p = 0.0013; p = 0.0006; p = 0.0020; p = 0.0151, respectively), but no clear patterns emerged among them. The SSB from Aqb and Va were greater in RBF than in CF, only during the second sampling season (105.3 ± 35.2; 352.6 ± 133.4 seeds.m−2, respectively). Sl also exhibited the highest SSB (97.7 ± 30.5) in the second year of sampling, while Nn showed the largest SSB in the first year, (1569.0 ± 847.0 seeds.m−2), both within CF. Finally, Sm and Sg did not present significant differences in their SBB between sites and years. We concluded that all studied species form transient SSB and that the shape index type II favors seed burial, so increasing SSB size and viability. Forest disturbances differentially affect the SSB of woody species in the dry subtropical Chaco because some species decreased, increased or showed no effects during different reproductive seasons. Thus, the potential long-term influence on the dynamics and resilience of dry forests in the subtropics needs to be monitored in the long term.