As succession proceeds in Mediterranean grasslands, total soil nitrogen increases, with a concomitant turnover in species. Yet, the role of nitrogen limitation on plant competition, allocation and fitness in species differing in successional status is still unclear. Here we use a nitrogen addition experiment to evaluate changes in plant allocation to leaves, stems, roots and reproductive parts in 18 species distributed evenly among early, intermediate, and late successional types. We combine this data with a physiologically-based model of plant competition for nitrogen and space to better understand the mechanisms shaping plant responses to nitrogen addition. In particular, we include in the model the nitrogen contents of these different compartments to evaluate differences in nitrogen uptake between species from different successional stages.
Across successional stage, nitrogen additions increase plant biomass, however this benefit does not always increase the reproductive output or fitness of the plant. Nitrogen additions has no effect on reproduction for many species, despite increasing plant biomass overall. We find contrasting patterns at the root and reproductive levels: late successional species decrease root biomass and increase reproductive biomass, while species from early and intermediate successional stages show the opposite pattern. Combining biomass and nitrogen contents of plant organs in our model, we evaluate the role of species differences in nitrogen uptake on these experimental outcomes. Our results suggest that species ecology influences variation in plant allocation in response to nitrogen addition. Our study thus provides a better understanding of the mechanisms shaping plant allocation response to limiting resources in Mediterranean grasslands.