Variation in photosynthetic response to temperature in a guild of winter annual plants

How species respond to environmental variation can have important consequences for population and community dynamics. Temperature, in particular, is one source of variation expected to strongly influence plant performance. Here, we compared photosynthetic responses to temperature across a guild of winter annual plants. Previous work in this system identified a trade-off between relative growth rate (RGR) and water-use efficiency (WUE) that predicts species differences in population dynamics over time, which then contribute to long-term species coexistence. Interestingly, species with high WUE invest in photosynthetic processes that appear to maximize carbon assimilation, while high-RGR species appear to maximize carbon gain by increasing leaf area for photosynthesis. In high-WUE species, higher rates of carbon acquisition were associated with increased investment into light-driven electron transport (J_max). We tested whether such allocation allows these plants to have greater photosynthetic performance at lower temperatures by comparing the temperature sensitivity of photosynthesis across species in the community. Six species were grown in buried pots in the field, allowing them to experience natural changes in seasonal temperature. Plants were taken from the field and placed in growth chambers where photosynthetic performance was measured following short-term exposure to a wide range of temperatures. These measurements were repeated throughout the season. Our results suggest that high-WUE species are more efficient at processing incoming light, as measured by chlorophyll fluorescence, and exhibit higher net photosynthetic rates (A_net) than high-RGR species, and these advantages are greatest at low temperatures. Sampling date differentially affected fluorescence across species, while species had similar seasonal changes in A_net. Our results suggest that species-specific responses to temperature contribute to the WUE–RGR trade-off that has been shown to promote coexistence in this community. These differential responses to environmental conditions can have important effects on fitness, population dynamics, and community structure.