Is the balance between competition and facilitation a driver of the patch dynamics in arid vegetation mosaics?

In most arid ecosystems, the vegetation is organized into two-phase mosaics, where high-cover vegetation patches are interspersed in a matrix of low plant cover. We studied the role of the biotic interaction balance (competition/facilitation) between shrubs and grasses as a driver of patch dynamics and maintenance of two-phase vegetation mosaics. Following Watt's seminal model, we conducted two field experiments in which we manipulated different vegetation patches to obtain the different stages along the building and degradation dynamics of high-cover patches. In addition we applied two possible belowground competition treatments (natural and experimentally reduced). We measured plant variables (emergence, survival, height, flower culms) on grass seedlings and transplants. We integrated all plant measurements into a single positive and negative component, to calculate the net balance along three stages of the patch dynamics proposed. The net biotic interaction balance was negative during the early and mature stages of high-cover patches because the average standardized effect from the negative component was below -0.44, while the positive component was not different from zero. However, the net biotic interaction balance was positive during the degraded stages of high-cover patches because the negative average component was -0.37, while the positive component reached 0.58. The negative net effects during early and mature stages of high-cover patches can be explained by the occurrence of wet years, because high rainfalls hide the aboveground facilitation. Our findings point out the importance of complementary mechanisms to the interaction balance in the mosaic maintenance (e.g. trapping of seeds by shrubs) according to the inter-annual rainfall variability.