XVI International Botanical Congess

Seedling emergence was evaluated for 47 species of Tamaulipan thornscrub in Northeastern Mexico under closed thornscrub and under direct sunlight. The trial was replicated in spring and autumn. There were three replicates per treatment with 20 seeds in each treatment per species. Seeds were planted in containers in soil from the area. All containers were kept inside cages to stop seed predators. Most species had higher germination rates at one of the seasons. Most of these had more seeds germinated in autumn. Almost half of the species had a higher germination under the sunlight than under the shaded condition. The association of most species with autumn germination is interpreted as an adaptation to avoid the hot dry summer spell. The fact that only one species had higher germination percentages in the shaded environment seems to support the theory of tree encroachment in a former savanna-type ecosystem as has been studied for Texas.