Using the small-scale gradients in light and fertility imposed by the presence of trees on Mediterranean open oak woodlands, we studied differences in the ecology, floristic diversity and species composition of an annual grassland, and modeled plant relationships with soil properties and radiation using generalized linear models. The strong intra-annual variability in the annual grassland greatly affected overall floristic diversity estimates. Therefore, sampling at least twice during the annual growing period was important to avoid underestimating floristic composition. As has been found in previous studies, different groups of grasses were clearly shaped as a response to the ecological gradient created by the influence of the tree. However, when analyzing specific relationships, only a few individual species directly responded to the studied soil and light availability factors analyzed. Among them, those species considered 'ruderal' such as Carduus tenuiflorus, Echium plantagineum, Lolium rigidum, Rumex bucephalophorus, and Tolpis barbata showed a clear response to some nutrients and to light availability but displayed no relationship to N soil content. Our results suggest that individual species are responding to a combination of ecological factors. Therefore, some of the generally accepted assumptions regarding the ecology of specific taxa may not be as straightforward as traditionally considered.