With concern about global warming rising, assessing how increases in temperature impact plant performance is crucial in understanding how plant communities may be affected. Genetic diversity, the movement of genes through populations (gene flow), and variation in environmental response may interact with the direct effects of climate to determine how plants fare in a warmer world, but these interactions are poorly understood. This study measured how the climate of different elevations affects population growth of the high-elevation plant mountain monkeyflower, Mimulus tilingii, at Mt. Rainier National Park. Utilizing data from the past three years, we parameterized matrix population models to explore how climate influences fitness and population growth rates of different populations, thereby assessing how environmental characteristics of a warming world will affect population success. We also used matrix models to determine the extent of this impact on different life stages and how genetic diversity alleviates the impact of environmental change on populations.
We found that reproductive success (assessed using seed production) of plants in 2014 did not vary by environment (high vs. low elevation). However, plants that resulted from crosses between populations produced twice as many seeds as plants that resulted from within-population crosses (i.e. inbreeding depression). This result suggests that genetic diversity is an important primary driver of performance of young plants and that climate change might favor the intermixing of genes across elevations. When combining effects of the environment and genetic diversity on all three vital rates (growth, survival, and reproductive success) across 3 years, we found that asymptotic population growth rates (λ) varied considerably between sites. Specifically, λ values at low elevation were almost double their value at high elevation (native range of M. tilingii), suggesting that a warmer climate may actually benefit M. tilingii by boosting population fitness. Thus, despite strong effects of genetic diversity on seed production in 2014, environmental conditions were a stronger predictor of population success than cross type across all 3 years. Through these analyses, our understanding of population response due to warming will help us make better ecological inferences of what to expect in the future.