LB 9-99 - Geographically based variation in red maple (Acer rubrum) spring phenology responses to soil warming: Implications for carbon sequestration and the Drawdown Georgia Project

Background/Question/Methods:

Drawdown Georgia (https://www.drawdownga.org/) is one of the first subnational efforts to reduce carbon emissions/increase carbon sequestration by year 2030 (Brown et al. 2021a,b). Temperate Forests and Afforestation/Silvopasture are two of our top 20 solutions. Red maple (Acer rubrum), an abundant tree species in Georgia and across eastern North America, serves as a model for phenological responses to warming and the potential for geographic genetic variation in response. Species with broad geographical ranges are often adapted to local conditions, and populations may diverge in response to warming. Climate warming has shifted species distributions and advanced spring phenology worldwide, but questions remain about how responses to warming differ among regions across a species’ natural range. In this two-year study, we examined intraspecific variation in survival, growth, and spring leaf-out phenology of red maple seedlings in response to experimental forest soil warming. In April 2020, two-year-old red maple seedlings sourced as seeds from three physiographic provinces or regions across the Southeast (Coastal Plain, Piedmont, and Mountains) were transplanted into experimental forest soil warming chambers located in the Georgia Piedmont. During these years soils were maintained at ambient temperatures, “warm” conditions (+1.5^oC) or “hot” conditions (+2.4^oC). We investigated spring leaf-out timing in 2021.

Results/Conclusions:

Overall, provenances/regions showed diverging responses in timing of spring bud break with increased temperature, indicating a Region-by-Temperature interaction (p< 0.01). With moderate warming (from Ambient to +1.5°C), each of the provenance/region’s leaf-out date tended to advance by 1-4 days. However, with more extreme warming (+2.4°C), the regions each responded differently. Coastal Plain seedlings leafed out later under hot (+2.4^oC) than under warm (+1.5) conditions (p< 0.05). Thus, at extreme +2.4^oC warming the leaf-out timing was similar to that at ambient temperatures. Mountain seedlings did not further advance their mean leaf-out date with extreme warming, such that leaf-out timing was similar between +1.5°C and +2.4°C treatments. Only the Piedmont seedlings advanced their leaf-out date by an additional 3 days with more extreme warming (p< 0.01). Changes in leaf-out timing affect the growing season length and the carbon budget, potentially affecting carbon sequestration rates in Temperate Forests and Silvopastures. These findings support the premise that locally adapted populations respond differently to changes in temperature, thus climate warming experiments, models and projections need to consider geographic genetic variation when forecasting the response of a species.