Does environmental heterogeneity affect the ability of plastic trait expression to mediate plant competition?

Background/Question/Methods

Intraspecific variation in functional traits can affect the strength of species interactions and may ultimately affect processes of community assembly. The role of trait plasticity in mediating competition may depend on how trait expression responds to environmental conditions. Separate studies have shown that plant community composition is related to heterogeneity in light availability and that soil origin affects the strength of competition and expression of functional traits. We conducted a greenhouse competition experiment in northeastern Ohio in which we manipulated both the light and soil environments to determine 1) whether heterogeneity affected competition strength in two congeneric pairs of perennial plants and 2) whether plastic trait expression might mediate competition in heterogeneous or homogeneous environments. We used spectral filters to create heterogeneous and homogeneous light environments. We collected field soil from the zone of root influence of our focal species and filled half of a pot with soil from each plant of a congeneric species pair in the heterogeneous treatment or mixed the two soil types in the homogeneous treatment. We crossed fully heterogeneous and fully homogeneous treatments with three levels of competition: individuals grown alone, with a conspecific, and with a congener.

Results/Conclusions

Relative interaction intensity (RII), measured as the change in biomass when a plant was grown with an interactor compared to grown alone, was significantly negative, indicating that plants grown together experienced competition. For three of the four focal species, RII did not differ in heterogeneous vs. homogeneous environments or for competition between conspecifics vs. congeners. Species, environmental heterogeneity, and competitor treatment interacted to affect RII for Plantago congeners, such that P. major experienced stronger competition with conspecifics than with congeners in the homogeneous environment. Specific leaf area (SLA) and root:shoot ratio differed when plants were grown in competition compared to grown alone, and SLA was significantly lower when plants competed with a conspecific than when plants competed with a congener. Root:shoot ratio was significantly lower in homogeneous than heterogeneous treatments, and also varied by soil origin within the heterogeneous treatment. Trait expression of Plantago congeners responded differently to heterogeneity, such that the greatest difference in mean trait values between P. lanceolata and P. major occurred in the homogeneous environment. Our results thus suggest that plastic trait expression may mediate competition between plants, but this role for plasticity may be influenced by environmental conditions that affect trait expression.