Leaf shape and size variation in bur oaks: an empirical study and simulation of sampling strategies

Authors: DESMOND, SARA - Morton Arboretum; GARNER, MIRA - Morton Arboretum; FLANNERY, SEAMUS - Morton Arboretum; Whittemore, Alan; HIPP, ANDREW - Morton Arboretum

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2021
Publication Date: 1/29/2021
Citation: Desmond, S.C., Garner, M., Flannery, S., Whittemore, A.T., Hipp, A.L. 2021. Leaf shape and size variation in bur oaks: an empirical study and simulation of sampling strategies. American Journal of Botany. 108(8):1540-1554. https://doi.org/10.1002/ajb2.1705.
DOI: https://doi.org/10.1002/ajb2.1705

Interpretive Summary: The morphology of plant leaves, including leaf size, shape, and anatomy, play a key role in how plants adapt to their environment. Morphology provides the easiest and least expensive markers for genetic studies, since morphological data can be collected for analysis without special equipment. However, morphological characters are subject to variation due to environmental differences, and the sampling and analysis of these characters must be designed to account for this variation. Oaks have long been noted for their particularly variable morphology, even between trees of the same species. Scientists from ARS and the Morton Arboretum investigated the geographical distribution of leaf size and shape characters in bur oak, based on ten measurements and eight ratios, and the relationship of leaf variation to the climate and latitude where the trees originated. They devised a technique using statistical qualities of the characters, estimated from initial samples, to determine the most efficient sampling scheme. Use of this sampling approach will allow estimates of genetic variation based on morphological characters to be made efficiently, so they yield the maximum information for the time and resources expended.

Technical Abstract: Oaks are notoriously variable in leaf morphology, but little is known regarding the relative contributions of climate, population, and individual tree to total variation in leaf morphology. Is variation within populations comparable to variation among populations? If so, it will be difficult to quantify differences among populations with confidence. This study examines the contributions of within-tree, among-tree, and among-site variation to the total variation in leaf morphology in bur oak (Quercus macrocarpa), one of North America’s most geographically widespread species, as well as the effects of latitude on leaf traits. Samples were collected from four sites each at northern, central, and southern latitudes of the bur oak range. Ten leaf size traits were measured, and variance in these traits and eight ratios were partitioned into tree, population, and latitude components. Leaf blade length and petiole width were the most responsive traits to changes in latitude, and site contributed more than tree to total variation in leaf morphology. Using empirical data, we parameterized a simulation of leaf collections that vary in number of leaves per tree and number of trees per site, accounting for covariance among leaves on trees and trees at sites based on the covariance we observed in our collections. Simulations suggest that power to detect among-site variance in leaf morphology increases with either increases in leaves per tree (10-11 leaves from each of 5 trees) or trees per site (5 leaves from each of 10+ trees). Power estimates and code for conducting simulations are provided to help researchers plan sampling strategies to maximize the ability to detect among-site variance in leaf morphology.