Legacy effects after seven years of afforestation with Pinus taeda in a natural grassland

The replacement of grassy vegetation by tree monocultures has become an emerging global activity. In South America, about half a million hectares of new plantations are incorporated each year, typically conducted with fast-growing exotic species, like Pinus and Eucalyptus species. However, information about the potential for spontaneous recovery of these grasslands is almost nonexistent. We assessed the biotic and abiotic legacies left by a 7-year plantation of Pinus taeda on a natural temperate sub-humid grassland of Uruguay. In a paired sampling design, comparing a clear-cut P. taeda plantation and a natural grassland, we analyzed the vegetation richness, cover and composition, the seed bank size and composition, and chemical soil parameters. Afforestation reduced species richness and vegetation cover by 30 and 58%, respectively. Also, species composition differed considerably between sites. Afforestation had a strong negative effect on grass species, excluding the shade-intolerant C-4 grasses that dominate in the grassland. Seed-bank richness did not differ between sites, but the total seedling density was significantly higher in the plantation compared to the grassland (6,620 and 4,420 seeds/m(2), respectively), mainly due to the contribution of forbs. Only 30% of the seed bank species were shared between the two sites. In terms of soil chemical parameters, afforestation induced soil acidification (deviation by 0.15-0.20 pH units) and reduction in soil carbon and cation (Ca and K) concentrations (deviation by 30, 20, and 40%, respectively). Our study showed limited possibilities for passive restoration of a natural grassland of Uruguay after a Pinus afforestation event.