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Plant spatial patterns in semiarid environments are often driven by a patchy availability of resources. We examined here drivers of forest patterns in a landscape where tree and shrub patches coexist in an herbaceous matrix on fog-inundated coastal hilltops in semiarid Chile, aiming to identify factors that set off forest patch formation and development. We hypothesized that a successional process driven by plant-plant interactions would start when tree seedlings establish in shrub patches, eventually leading to resource accumulation and increased diversity. Within shrub patches, soil moisture increased with increasing cover of a common shrub, Baccharis vernalis, unlike another shrub species, Griselinia scandens. Soil nutrients increased with patch size, but soil moisture remained similar among patches. Forest patches showed differences regarding soil moisture between wet windward and dry leeward edges, and these were associated with differences in community composition. While Baccharis patches were similar to windward forest edges, leeward edges were more similar to open areas. Patterns of species richness and diversity showed a shift in plant-plant interactions from positive to negative along successional transition from Baccharis to forest patches. Outside forest patches, tree species are rare but become established and survive under Baccharis vernalis, most likely thanks to a combination of fog-interception capacity, soil nutrient availability and low competition. Therefore, forest patch formation is strongly dependent on fog water interception and community composition. While some shrubs like Baccharis vernalis facilitate tree establishment and eventually initiate new forest patches, others compete with tree seedlings preventing establishment. Hence, a facilitation-competition balance between shrubs and trees explains the spatial arrangement of the forest-shrub patch mosaic in this semiarid environment.