MORPHOLOGICAL, PHENOLOGICAL, AND TRANSPIRATIONAL DIFFERENTIATION IN POPULATIONS OF QUERCUS MACROCARPA AND QUERCUS RUBRA

Abstract

The characteristics and consequences of climate change are unfolding with unprecedented speed worldwide, and few places are changing faster than northern Minnesota. This study takes a unique approach to quantifying arboreal variation in physical plant traits and plant water use in the context of climate change. Two populations, northern and southern, of two species, Quercus macrocarpa and Quercus rubra, were grown in a common garden experiment at a northern field site and compared for climatic suitability. Traits that are known to have adaptive value, such as growth, leaf attributes, and phenological traits, were measured and compared. In addition, the transpiration rates of these populations were captured via portable rapid evapotranspiration chamber for comparison. Trees in a forested watershed can be responsible for up to 70% of the ecosystem’s water loss. Population-level differentiation in transpiration has ramifications for water cycling and may be an important but understudied consideration in climate-forward forest management practices. Rather than struggling in an adverse environment, these two species maintained their cohorts well, indicating that climate shifts have created hospitable long-term conditions for these species. There was genetic differentiation between the two populations for each species for traits that are important for climate adaptation. These differences were especially strong for fall phenology traits and notable for physiological traits linked to water use adaptation, such as specific leaf area. Importantly, measured transpiration varied at a population level for both species. In late August 2020 and 2021, Q. macrocarpa populations significantly differed in their transpiration rate and therefore water use. Furthermore, in a year with more water stress, Q. rubra also demonstrated significant population differentiation in midsummer transpiration. This suggests true population differences, a pattern that may become more evident over time or if additional sites from the original experiment are measured. A model constructed using field precipitation, discharge, and transpiration measurements demonstrated that each population for each species would have a unique impact on the discharge of the Stewart River. This study is one of the known few to demonstrate population differentiation in tree transpiration rates, particularly in Q. macrocarpa and Q. rubra. Taken altogether, this study demonstrates that northern populations may be falling out of alignment with local climate, at the cost of lost growth opportunities and greater winter stresses at the margins of the growing season, and that tree cover can have distinct impacts on ecosystem water balances even at a population scale. This presents an opportunity to counteract the desynchronization of climate shifts with local adaptations, especially in heavily forested regions such as northern Minnesota, and highlights the need for further investigation of water use impacts of trees chosen for planting.