The small scale spatial pattern of C 3 and C 4 grasses depends on shrub distribution GASTÓN FERNÁNDEZ, 1 * MARCOS TEXEIRA 2 AND ALICE ALTESOR 1 1 Instituto de Ecología y Ciencias Ambientales, Facultad de Ciencias, Universidad de la República, Iguá 4225 (PC 11400), Montevideo, Uruguay (Email: gfernandez@fcien.edu.uy); and 2 LART-IFEVA y Cátedra de Métodos Cuantitativos y Sistemas de Información, Facultad de Agronomía, Universidad de Buenos Aires – CONICET, Buenos Aires, Argentina Abstract At micro-site scale, the spatial pattern of a plant species depends on several factors including interac- tions 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 C3 and two C4) 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. C4 grasses showed a negative association or no correlation with the shrubs, whereas C3 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 C3 than for C4 grasses.These results show that facilitation could be more important than previously thought in sub-humid grasslands. Key words: C3 and C4 grasses, negative interactions, positive interactions, shrubs, South America, sub-humid grassland, Uruguay. INTRODUCTION In a given community, individuals of different plant species can exhibit a random or non-random spatial pattern (Fortin & Dale 2005). A large number of factors may regulate the spatial pattern of plants, and their relative importance varies with the scale of analysis (Greig-Smith 1983). Over small spatial scales (micro-site), species interactions are among the most important determinants of plant distribution (Fidelis et al. 2009). Such interactions comprise a set of nega- tive and positive effects that plants exert on each other (Bertness & Callaway 1994). Negative effects due to competition for light, water and soil nutrients or the release of allelopathic compounds generate repul- sion among plants (Rejmánek & Lepš 1996; Kikvidze et al. 2005). Positive effects (facilitation) due to the amelioration of harsh abiotic conditions or protection from herbivores generate attraction among plants (Oesterheld & Oyarzabal 2004; Tirado & Pugnaire 2005). It has been hypothesized that the relative importance of competition and facilitation may vary inversely along a gradient of abiotic stress (Bertness & Callaway 1994). The importance of competition is predicted to increase toward the most benign end of the abiotic gradient, while facilitation is predicted to decrease. Thus, most studies related to facilitation have been conducted in stressful environments, such as water limited ecosystems (Graff et al. 2007; Gómez-Aparicio 2009; Holmgren et al. 2012). In plants, photosynthetic metabolism affects the tol- erance of individuals to abiotic conditions (Lambers et al. 2008). As in the C 4 metabolism photorespiration is virtually eliminated, C4 plants have higher photosyn- thetic rates than C3 plants at high temperatures. Moreover, C4 plants have higher water use efficiency than C3 plants because at a given photosynthetic rate they have lower stomatal conductance (Sage 2004). The evolutionary emergence of the C4 pathway in grasses (Poaceae) has been identified as an adaptation to drier conditions (Pagani et al. 1999; Edwards & Still 2008). C4 grasses rarely colonize forest habitats where less light and lower temperatures prevail, while C3 grasses are favoured in shady habitats as they tolerate low light intensities (Sage et al. 1999). Many studies *Corresponding author. Accepted for publication November 2013. Austral Ecology (2014) ••, ••–•• © 2014 The Authors doi:10.1111/aec.12113 Austral Ecology © 2014 Ecological Society of Australia