| In plants, physiological traits control the uptake, use and allocation of resources, which ultimately determines growth and reproductive output. Consequently, most physiological variation is hypothesized to be adaptive. Comparative studies support this hypothesis by showing that physiological variation is correlated with environmental resource gradients; however, this approach fails to identify the agents and targets of selection. In this thesis, I used phenotypic selection and reciprocal transplant approaches to directly assess whether biotic and abiotic soil factors influence selection on photosynthetic traits in Bromus inermis Leyss.;To test whether these three populations are adapted to their local soil, I performed a greenhouse reciprocal transplant. Also, to test whether adaptation is driven by selection from abiotic or biotic factors, I grew plants in sterilized and non-sterilized (living) soil. Plants only displayed a home site advantage in living soil, suggesting that the biotic factors promote adaptation. Because most reciprocal transplants focus exclusively on abiotic factors, the environmental gradients driving plant adaptation may often be misinterpreted.;To assess whether nutrient availability and biotic soil factors influence selection on photosynthesis, I conducted a greenhouse phenotypic selection study. To isolate these selection pressures, I grew plants in high versus low nutrient field soil that was either sterilized or not (living). Selection for high early photosynthesis and accelerated growth was stronger in living soil and low nutrient soil. Soil biota may select for rapid growth so plants can escape from fungal parasitism.;Divergent selection between naturalized populations of Bromus inermis supports that high photosynthesis in nutrient-rich soil is adaptive; however, biotic soil factors alter nutrient-derived selection pressures and promote local adaptation. Therefore, it is the combined selection from both soil components that maintains physiological variation.;To test whether high metabolic activity and rapid growth is adaptive in nutrient rich soil, I measured selection in three naturalized populations with contrasting nitrogen and phosphorus availability, over three growing seasons. Selection for increased photosynthesis was stronger at sites with higher phosphorus availability, and in 2007, the driest year. Phosphorus-rich soil may select for high carbon uptake because of increased competition for light, water, and nutrients. |
