Insights into the role of phosphorus partitioning in plant species coexistence

Background/Question/Methods Resource partitioning is one of the recognized mechanisms by which species differ their niche differences, increasing their probability of coexistence. Plants are limited by just a few resources, including phosphorus. Thus, specialization on different chemical forms of the same limiting phosphorus (P) resource could be an important source of niche differentiation between species. In order to understand the role of P partitioning in driving species coexistence, we asked: a) do potentially coexisting species differ in their P uptake patterns and b) do these differences relate to species’ ability to coexist in the field? Our study system is an acidic mountain grassland with P limited soil, from which we selected eight plant species. In a greenhouse experiment, we evaluated species biomass and P content growing in soil from the field site, when fertilized with 5 different organic and inorganic forms of P. Additionally, we set up a pairwise competition field experiment, with every species growing alone and experiencing intra- and interspecific competition at different competitor densities. Interaction coefficients between species pairs were estimated fitting non-linear models using competitor densities to predict biomass of focal species after three months of growth.

Results/Conclusions Principle components analysis of species biomass responses to P addition in the greenhouse experiment revealed that species differed primarily in their ability to mobilize organic P (axis1 = 60%) and aluminium and iron phosphate (axis2= 30%). Analysis of P content of the leaves confirmed our assumption that species were P limited. In the field, most species pairs showed positive interactions and the pairwise interspecific interaction coefficients did not relate to species differences in P uptake. This indicated that facilitation between species played a larger role than competition in determining plant establishment at the field site. Nonetheless, we also found that species’ ability to use inorganic phosphate was positively associate with their first-year biomass in the field with no competitors (R² _adj = 0.58 p=0.01), one component of competitive ability. Therefore, we conclude that phosphorus specialization may not relate to species’ niche differences but may underlie competitive dominance.