A resurrection study reveals limited evolution of thermal performance in response to recent climate change across the geographic range of the scarlet monkeyflower

Evolutionary rescue can prevent populations from declining under climate change, and should be more likely at high-latitude, “leading” edges of species’ ranges due to greater temperature anomalies and gene flow from warm-adapted populations. Using a resurrection study with seeds collected before and after a seven-year period of record warming, we tested for thermal adaptation in the scarlet monkeyflower Mimulus cardinalis. We grew ancestors and descendants from northern-edge, central, and southern-edge populations across eight temperatures. Despite recent climate anomalies, populations showed limited evolution of thermal performance curves. However, one southern population evolved a narrower thermal performance breadth by 1.31 °C, which matches the direction and magnitude of the average decrease in seasonality experienced. Consistent with the climate variability hypothesis, thermal performance breadth increased with temperature seasonality across the species’ geographic range. Inconsistent with performance trade-offs between low and high temperatures across populations, we did not detect a positive relationship between thermal optimum and mean temperature. These findings fail to support the hypothesis that evolutionary response to climate change is greatest at the leading edge, and suggest that the evolution of thermal performance is unlikely to rescue most populations from the detrimental effects of rapidly changing climate.

Methods
Refresher generation

For a refresher generation, we germinated field-collected seeds in pots filled with Fafard 4P Mix (Sun Gro Horticulture, Agawam, MA, USA) and topped with a thin layer of Sunshine Grow Mix (Sun Gro Horticulture, Agawam, MA, USA) in May 2018. Pots were placed into sub-irrigated trays in a greenhouse at North Carolina State University, watered daily, and fertilized weekly with Blossom Booster fertilizer (JR Peters Inc., Allentown, PA, USA) or Cal-Mag fertilizer (Everris, Dublin, OH, USA). Starting at ~4 months after planting, we performed crosses to produce experimental seed families, which were subsequently harvested in September 2018 through January 2019.

Resurrection experiment

Beginning in February 2019, we planted seeds into plug trays. Within 24 hours prior to planting, plug trays were filled with Fafard 4P Mix (Sun Gro Horticulture, Agawam, MA, USA), topped with a thin layer of Sunshine Grow Mix (Sun Gro Horticulture, Agawam, MA, USA), and thoroughly misted with water. We planted each family into a separate cell, such that each plug tray contained all ancestral and descendent families of one region (northern, central, or southern) to minimize competition due to size differences among regions. Each set of six trays that eventually went into each growth chamber run contained two representative trays for each region. Due to space limitations, we staggered planting, which occurred through May 2019. Because a previous trial showed that northern populations germinate slower than central and southern populations, we planted the two northern trays in each chamber run one week earlier than the two central and two southern trays to standardize plant size prior to growth chamber runs. Trays were sub-irrigated with a general nutrient solution (containing NPK + micronutrients) once daily. We rotated trays three times per week to reduce positional effects, and emptied water out of trays three times per week to reduce growth of fungus gnat larvae and algae in the soil.

Growth chamber runs occurred from April through July of 2019. After five (for central and southern populations) to six (for northern populations) weeks in the grow room, we put each tray set into growth chambers, with 16 chamber runs total. We randomized the order of temperature regimes and randomly assigned regimes to one of the four growth chambers. During all growth chamber runs, we sub-irrigated trays with reverse osmosis water (as opposed to nutrient solution, which may have confounded any effect of temperature if higher temperatures increased plant water use) twice daily, and randomized trays within each growth chamber twice per week to minimize positional effects.