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A mechanistic understanding of population dynamics requires close examination of species’ differences in how physiological traits interact with environmental variation and translate into demographic variation. We focused on two co-occurring winter annual species (Pectocarya recurvata and Plantago insularis) that differ in photosynthetic resource-use efficiency and demographic responses to environmental variation and covariation between temperature and water availability. Previous work showed that Pectocarya has higher water-use efficiency and nitrogen allocation to light-driven dynamics of the Calvin cycle (J(max):V-Cmax) than Plantago, which is often associated with enhanced electron transport capacity at low temperatures and better light harvesting capacity. These traits could enhance Pectocarya photosynthesis during reliably moist but cool, cloudy periods following precipitation. We acclimated plants to low and high temperatures and then measured gas exchange across a 30 degrees C temperature range. As predicted, optimal temperatures of photosynthesis were lower for Pectocarya than Plantago. Additionally, Pectocarya experienced greater respiratory carbon loss than Plantago at higher temperatures (every 1 degrees C increase beyond 24 degrees C increased the ratio of carbon loss to gain 9% and 27% in cold and warm-acclimated plants, respectively). These differential patterns of photosynthetic optimization and assimilation in response to differing rainfall distributions may have important implications for population dynamic differences and species coexistence. (C) 2013 Elsevier Ltd. All rights reserved.