The small scale spatial pattern of C3 and C4 grasses depends on shrub distribution

At micro-site scale, the spatial pattern of a plant species depends on several factors including interactions with neighbours. It has been seen that unfavourable effects generate a negative association between plants, while beneficial effects generate a positive association. In grasslands, the presence of shrubby species promotes a particular microenvironment beneath their canopy that could affect differently the spatial distribution of plants with different tolerance to abiotic conditions. We measured photosynthetic active radiation, air temperature and wind speed under shrub canopies and in adjacent open sites and analysed the spatial distribution of four grass species (two C-3 and two C-4) in relation to shrub canopy in a grazed sub-humid natural grassland in southern Uruguay. Radiation, air temperature and wind speed were lower under shrubs than in adjacent open sites. The spatial distribution of grasses relative to the shrub canopy varied depending on the photosynthetic metabolism of grasses. C-4 grasses showed a negative association or no correlation with the shrubs, whereas C-3 grasses showed a positive association. Our results highlight the importance of the photosynthetic metabolism of the grasses in the final outcome of interactions between grasses and shrubs. Micro-environmental conditions generated underneath shrubs create a more suitable site for the establishment of C-3 than for C-4 grasses. These results show that facilitation could be more important than previously thought in sub-humid grasslands.