Coexistence beyond equilibria: testing the ecological buffering mechanisms theory in mixed communities

<i>Aims. </i>Coexistence of woody and herbaceous species in mixed communities has been an ecological conundrum for several decades. The general conclusion is that no single mechanism can explain coexistence. Jeltsch et al. (2000) proposed a unifying theory predicting that, despite of states of equilibrium or domains of attraction, the mixed condition results from buffering mechanisms acting at the species level as the community approaches the limits of its existence (i.e., dominated by a single life-form). Here we experimentally tested, for the first time, these buffering mechanisms in grass and shrub species at the scale of a patch mosaic in a temperate shrub-grass steppe.<i>Location. </i>Shrub–grass Patagonian steppes, Argentina<i> </i>(45°24′S, 70°17′W)<i>Methods. </i>In six grazed and ungrazed plots large enough to represent accurately the vegetation mosaic of patches (121 m²), we created experimental communities exclusively dominated by grasses or shrubs. In addition, we established two control treatments: one with intact vegetation and another completely cleared. We constructed integral projection model for populations of grasses and shrubs, and evaluated the action of buffering mechanisms using the population growth rate over the transient period (λ_g).<i>Results. </i>We highlight three main results. First, we found that λ_g of the reduced life-form increased when it was experimentally reduced. Second, there were species-specific differences that module the population response to dominance and disturbance (i.e., physiognomy and grazing, respectively). Third, the vital rate driving the population response of both grasses and shrubs, was fecundity. Moderate grazing did not alter these results.<i>Conclusions.</i> Buffering mechanisms play a critical role in the persistence of the co-dominated state of the shrub-grass community. We argue that buffering mechanisms act through processes promoting the fecundity of species reduced at the extremes by (i) negative intra-dominant life-form interactions (competition), (ii) niche partitioning that reduces inter-life form niche overlap, and (iii) positive inter-life form interactions (facilitation).