Edaphic and seasonal heterogeneity of seed banks in agricultural fields of a tropical dry forest region in southern Mexico

Abstract

The slash-and-burn agriculture practiced across tropical dry regions results in the elimination of native vegetation. Upon fi eld abandonment, the seed bank becomes a potentially important mechanism of natural regeneration at early successional stages. Soil properties and climate seasonality may affect seed bank characteristics, thus we analyzed the effects of these two factors on seed bank density and composition in agricultural fi elds of a seasonally dry tropical region of southern Mexico. Soil cores were collected for the rainy and the dry seasons in order to assess changes occurring in the seed bank from the time of harvest to the moment when succession could potentially start (the next rainy season). The 12 studied fi elds comprised three different soil types recognized by local inhabitants: sandy and stony, silty, and clayey soils, locally known as cascajo, black soil, and red soil, respectively. At each fi eld 20 soil cores (8 cm diameter, 4.5 cm depth) were collected and mixed to form four pooled samples, which were placed in a greenhouse to induce germination. A total of 4,422 seedlings (2,291 seeds m-2) representing 40 species were recorded. The most abundant species were, in decreasing order, Melanthera nivea, Rhynchelytrum repens, Waltheria indica, Amaranthus scariosus, Digitaria bicornis, and Cenchrus pilosus. Herbs were the prevailing growth form (> 80% of total richness). No clear pattern was observed in the seed bank related to soil type; however, seed bank characteristics tended to be associated with the time of use of the agricultural fi elds, a variable that was not controlled in the study. Seed bank species richness was signifi cantly larger in the dry season, and although seed density showed a similar trend, it was not signifi cant. The studied seed banks contain no elements of the regional primary tropical dry forest, which suggests that seed banks in deforested areas cannot guarantee their maintenance beyond forested areas.