COS 157-5 - Variation in germination responses to temperature across elevation suggest adaptation to seasonal timing and temperature

Abstract: Germination is a critical transition in the life cycle of plants and one of the most vulnerable life stages. Temperature is one of the most important drivers of germination timing and the thermal germination niche of populations describes the range of temperatures over which germination is possible. The thermal germination niche can be quantified by estimating cardinal temperatures, including the minimum (T_b), optimal (T_o), and maximum (T_c) temperatures at which germination can occur. The hot-cold trade-off hypothesis suggests that cardinal temperatures should decrease with increasing elevation. In contrast, the climate variability hypothesis suggests that thermal niche breadth (T_c – T_b) should be greater in populations with greater temperature variability. To test these hypotheses, we performed thermal germination experiments for 11 populations of Streptanthus tortuosus spanning 2,000 m in elevation and asked two main questions, (1) How do cardinal temperatures compare among populations across elevation? and (2) Does thermal germination niche breadth vary across elevation and if so, is this variation driven by temperature variability? We fit thermal-time models to estimate cardinal temperatures and then tested for relationships between elevation, cardinal temperatures, and niche breadth. We found evidence of a hot-cold trade-off but not in the expected direction. Values of T_b and T_c were actually higher at higher elevations, whereas T_o had no relationship with elevation. This result may be due to variation in the timing of germination among populations of S. tortuosus, in which germination occurs for low elevation populations during cooler, fall months while germination for high elevation populations occurs during warmer, spring months. Thus, higher elevation populations may be thermally adapted to warmer germination temperatures. We found that all populations had similar thermal germination niche breadths across elevation, with no support for the climate variability hypothesis. Biochemical constraints are thought to drive similarity in thermal germination niche breadths, even though populations vary in their T_b and T_c values. Overall, we found evidence of thermal adaptation among populations for T_b and T_c thermal germination traits and suggest that these adaptations to both low and high temperatures have constrained the width of the thermal germination niche among populations across elevation.