Small seed bank in grasslands and tree plantations in former grassland sites in the South Brazilian highlands

The soil seed bank can be an important source for vegetation regeneration, and data on the similarity between aboveground vegetation and the seed bank can provide information about successional pathways after disturbances or land-use change. We conducted this study in natural grasslands in the subtropical highland region in southern Brazil. We evaluated the effect of silviculture on richness, density, and composition of the seed bank at former grassland sites converted to pine plantations 25 years ago. We worked at six grassland sites and three pine plantation sites and used the seedling emergence method. Seed bank density and richness in grasslands was lower than those reported in similar environments in other regions. Species richness and density varied considerably within each vegetation type therefore, richness and density were not statistically significant, while composition varied among vegetation types. In terms of species, the pine plantation seed bank was a small subset of the grassland seed bank. Seeds of typical grassland species were missing in the pine plantation, but also had only low abundances in the grassland, and similarity of seed bank and vegetation was low (less than 20%). The low seed density found in this study, including in grasslands areas, indicates that regeneration of species from the soil seed bank likely is of a limited role for the maintenance of plant populations after disturbances in this system. Our data further suggest that natural regeneration after tree planting in grasslands is reduced due to seed limitation.

土壤种子库（soil seed bank）是植被自然更新的重要来源，而地上植被与种子库间的相似性数据，可揭示干扰或土地利用变化后的群落演替路径。本研究于巴西南部亚热带高原地区的天然草原中开展。我们针对25年前由草原转为松林人工林的原草地样地，评估了营林活动对种子库的物种丰富度、密度及群落组成的影响。本研究共设置6个草原样地与3个松林人工林样地，并采用幼苗萌发法（seedling emergence method）开展实验。草原样地的种子库密度与物种丰富度均低于其他地区同类生境的已有报道结果。各植被类型内部的物种丰富度与密度波动幅度较大，但二者均未达到统计学显著性水平；而不同植被类型间的种子库组成则存在显著分化。从物种组成来看，松林人工林的种子库仅为草原种子库的一个小型子集。典型草原物种的种子在松林人工林样地中完全缺失，但在草原样地中的丰度也极低；且种子库与地上植被的相似性较低（不足20%）。本研究中（包括草原样地在内）测得的种子密度偏低，这表明在该生态系统的干扰后，依托土壤种子库实现的物种更新，对植物种群维持的作用十分有限。本研究数据还表明，草原区域开展造林后，因种子限制效应，其自然更新过程会受到抑制。