The Climatic Variability Hypothesis and trade-offs in thermal performance in coastal and inland populations of Mimulus guttatus.

Ecologists and evolutionary biologists have long predicted that organisms in more climatically variable environments should be adapted to handle a wider range of conditions. This intuitive idea, known as the Climatic Variability Hypothesis (CVH), has gained mixed support from empirical studies. We tested the CVH in a novel system by comparing the thermal breadth of coastal and inland populations of Mimulus guttatus. To quantify thermal breadth, we performed a thermal performance experiment and built performance curves. Using these performance curves, we also evaluated evidence for a breadth-performance trade-off and the Hotter-is-Better hypothesis. We did not find support for the CVH; coastal and inland populations did not differ in thermal breadth. However, we found evidence for a breadth-performance trade-off and the Hotter-is-Better hypothesis. Surprisingly, the two most inland populations differed the most in the thermal performance traits we evaluated. Our results highlight the importance of explicitly measuring thermal performance to test explanations of species distribution patterns and the need to examine alternative mechanisms by which organisms occupy different climatic regimes.

Variation in the seasonal germination niche across an elevational gradient: the role of germination cueing in current and future climates.

PREMISE: The timing of germination has profound impacts on fitness, population dynamics, and species ranges. Many plants have evolved responses to seasonal environmental cues to time germination with favorable conditions; these responses interact with temporal variation in local climate to drive the seasonal climate niche and may reflect local adaptation. Here, we examined germination responses to temperature cues in Streptanthus tortuosus populations across an elevational gradient. METHODS: Using common garden experiments, we evaluated differences among populations in response to cold stratification (chilling) and germination temperature and related them to observed germination phenology in the field. We then explored how these responses relate to past climate at each site and the implications of those patterns under future climate change. RESULTS: Populations from high elevations had stronger stratification requirements for germination and narrower temperature ranges for germination without stratification. Differences in germination responses corresponded with elevation and variability in seasonal temperature and precipitation across populations. Further, they corresponded with germination phenology in the field; low-elevation populations germinated in the fall without chilling, whereas high-elevation populations germinated after winter chilling and snowmelt in spring and summer. Climate-change forecasts indicate increasing temperatures and decreasing snowpack, which will likely alter germination cues and timing, particularly for high-elevation populations. CONCLUSIONS: The seasonal germination niche for S. tortuosus is highly influenced by temperature and varies across the elevational gradient. Climate change will likely affect germination timing, which may cascade to influence trait expression, fitness, and population persistence.