Some savanna areas in the Orinoco Llanos region in Venezuela are characterized by the abundance of small forest groves, resulting in a parkland landscape. We hypothesized that forest groves are the result of colonization of the open savanna by forest species, facilitated by the activity of leaf-cutting ants. In this paper we present results from a field study conducted on an ecotonal belt between a deciduous gallery forest and an open savanna in the western Llanos of Venezuela. In seven transects, trees, groves and leaf-cutting ant nests were counted, size parameters evaluated and distance from grove to forest measured. Soil chemical analysis and microclimatic measurements were performed in the forest, grassland and groves. Floristic composition of the woodland component of the savanna (scattered trees) was very dissimilar from that of the forest, but the composition of the groves was essentially a mixture of both woodland and forest species. Woodland trees were present in all groves, whereas forest trees were present only in larger groves. The size structures of populations of the two types of trees differed significandy according to the size of the grove; also, woodland tree population structure differed significantly between open savanna and groves. The frequency of groves with Atta laevigata nests increased with the size of the grove, and groves with forest trees were positively associated with the presence of nests. A. laevigata nests improved the soil of groves, increasing N, Mg, Ca and organic carbon, but other soil properties were not modified. Temperatures at ground level in the dry season were ameliorated in the groves compared with the grassland, and this effect increased with the size of the grove. We concluded that a dynamic process of grove formation is facilitated by the aggregation of a few woodland trees, followed by the successful invasion of forest trees and a leaf-cutting ant nest. The results did not show any clear sequence of arrival of the ants or the forest trees, but it seems clear that these two would act synergistically, improving conditions for each other.