Soil type and species diversity influence selection on physiology in Panicum virgatum

Species diversity influences the productivity and stability of plant communities, but its effect on the evolution of species within those communities is poorly understood. In this study, we tested whether species diversity and soil type influence selection on physiology in switchgrass (Panicum virgatum). Plants were grown in 0.33–0.55 ha plots in eight full-factorial treatment combinations: four diversity treatments (1 species—switchgrass monoculture; 5 species—a mix of C4 grasses; 16 species—a mix of grasses, forbs, and legumes; 32 species—a mix of grasses, forbs, legumes, and sedges) and two soil types (Waukee loam “loam” and Spillville–Coland alluvial complex “clay”). We measured selection on photosynthetic rate, chlorophyll concentration, and specific leaf area in each treatment combination and compared the strength of selection between soil types and diversity treatments. When significant, selection favored increased photosynthesis, increased chlorophyll concentration, and decreased specific leaf area in all treatment combinations. Selection for these attributes was stronger in the faster-draining loam soil than the slower-draining clay soil. Selection rarely differed significantly between diversity treatments; however, most instances in which selection differed significantly between soil types occurred in the high-diversity mixes suggesting that diversity alters the impact of soil type as an agent of selection. Selection may have been stronger in the loam soil because of its lower available water capacity. There was a lengthy summer drought during our experiment. Under these conditions, plants with high photosynthesis and chlorophyll concentration would have more resources to invest in their root system for water uptake. Increased capacity for water uptake would benefit plants in both soil types during drought but would have greater adaptive significance in the faster-draining loam soil. Our results suggest that species diversity is a weak agent of selection and only influences physiological evolution by modifying the pressures exerted by other environmental factors.