Physiologic adaptations to climate in red spruce (Picea rubens Sarg.)

John R. Butnor, Southern Research Station, U.S. Forest Service, Burlington, VT, Brittany M. Verrico and Stephen R. Keller, Dept. of Plant Biology, University of Vermont, Burlington, VT, Chris A. Maier, Southern Research Station, USDA Forest Service, Research Triangle Park, NC

Background/Question/Methods
Red spruce is a montane species that thrives cool, humid environments along the Appalachian Mountains in eastern North America. Across its range, it exists in places with mean annual temperature (MAT) of roughly 2-12° C but buffered from extended periods of high temperatures. This wide temperature range is a strong selective force and has likely led to adaptations in physiology and phenology. The species has been projected to suffer range contraction with warming climate, threatening its role as a foundational species. Using common gardens we sought to identify differences in specific leaf area (SLA), photosynthesis, photosynthetic capacity, and growth that could linked to the climate the populations were adapted to. This information could be applied to predict plant performance in future climates and advise conservation efforts.
We collected seeds (half-sib families) from both a broad scale; longitudinal range (lat. 39-46N) and a fine scale; elevation range (425-1129m) from a single mountain in Vermont (Mount Mansfield). Seedlings were planted at two elevations that differed in MAT: low (425m, 6.9° C) and mid (700m 4.9° C) within the range of the collections (2.6 to 6.9° C). At the end of their second field season in September 2021, ACi curves were measured to predict photosynthetic capacity along with SLA, height, biomass and detergent fiber analysis on foliage. Departure of family origin MAT from common garden MAT (MATD) was used quantify the climatic transfer.
Results/Conclusions
Our combination of planting sites and family origin MATs yielded a MATD range from -2 to +4.3° C. As MATD increased, SLA declined indicating that moving families to colder climes decreased leaf density and movement to warmer climes increased it. Similarly, Rubisco activity (Vcmax) and the maximum rate of photosynthetic electron transport (Jmax) also followed the same pattern, increasing directly with transfers to warmer climates. Photosynthetic rate (corrected to 25° C) increased 25% across the range of MATD indicating that cold-adapted families exhibited greater carbon assimilation when planted in warmer environments. These results demonstrate that there is indeed physiological adaptation to climate in red spruce along both broad and fine geographical scales. We are mindful that moving families operating near their northerly/cold limit to a more favorable environment is not the same as moving them to a deleteriously warm environment. Incorporation of height, biomass and detergent fiber analysis are ongoing and will pair the enhancement of photosynthetic capacity with investment in biomass and its metabolic cost.