Are irrigation and grazing effects transferred, accumulated, or counteracted during plant recruitment?

Abstract

How do effects from perturbations such as irrigation and grazing that have an impact at one stage of the recruitment process (e.g., seedling) affect performance at later stages (e.g., adult)? Such effects may be transferred to later stages without any further change (transferred effects), reinforced by a similar effect so that their importance increases (accumulative effect), or counteracted at later stages by an opposite effect (counteractive effect). We analysed the predominance of transferred, accumulative, and counteractive effects depending on (1) organization level (community, functional group, and species levels), (2) life cycle stage (seeds, seedlings, and adults), (3) grass/forb functional group (forbs versus grasses), and (4) seed mass (small-seeded species, medium-seeded species, and large-seeded species). The study was conducted in Alcala de Henares, central Spain. During one annual cycle, we counted the number of readily germinable seeds (3,909 seeds: 2,156 forbs and 1,753 grasses), emerged seedlings (2,126 seedlings: 1,238 forbs and 888 grasses), and surviving seedlings up to reproductive adult status (917 adults: 217 forbs and 700 grasses) of all species (74 species) in a factorial field experiment under three different simulated rainfall and three seasonal sheep grazing regimes in a natural Mediterranean annual plant community composed of forbs (dicotyledonous herbs) and grasses (monocotyledonous herbs). Our main results were: (1) the proportion of transferred and counteractive effects was similar between the three studied organization levels (community, functional group, and species levels); (2) we detected many more counteractive effects in the seedling stage than in the adult stage; (3) we did not find more counteractive effects in forbs versus grasses, but transferred effects were more frequent in grasses; and (4) we found more counteractive effects with decreasing seed mass and more transferred effects with increasing seed mass. Our main conclusions were that the prevalence of counteractive effects reduces the sensitivity of the whole recruitment process and enhances the relative importance of the final stage (adults). The prevalence of transferred effects makes the recruitment process more predictable and more dependent on the early stages (seeds and seedlings).