Population-level Variation of Fraxinus Americana L. Is Influenced by Climate Differences across the Native Range

—This paper reviews population variation in Fraxinus americana (white ash) to determine whether intraspecific variation in various physiological traits affects growth and survival in F. americana and to correlate any intraspecific variation in F. americana with temperature or precipitation differences throughout the species range. Beginning in the 1980s, results from common garden studies indicated that differences in growth and survival in F. americana are the result of ecotypic variation in the species. Recently, further insights into the mechanisms controlling these ecotypic differences in growth were made in a common garden at the edge of the species range. This project in northeastern Kansas found significant differences in morphology, phenology, and stomatal regulation among 44 F. americana populations. Even after 30 years of growth in the common garden, these population differences were well-correlated with climatic differences (temperature and precipitation) at the location of population origin. Temperature requirements for leaf emergence of F. americana populations in the common garden were positively correlated with the latitude of population origin. Differences in water use efficiency among other F. americana populations were observed and may be caused by morphological traits, such as variation in leaf mass per area and foliar nitrogen concentration. Thus, the original observations of differences in growth among F. americana ecotypes are likely caused by adaptive differentiation among populations.