Does Foliar Trichome Abundance, Morphology and Plasticity Vary with Climate in a Deciduous Oak?

Berenice Badillo, Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, CA, USA

Marissa Ochoa, Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, CA, USA

Jessica W. Wright, USDA Forest Service, Pacific Southwest Research Station, Davis, CA, USA

Victoria L. Sork, Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, CA, USA; Institute of Society and Genetics, University of California Los Angeles, Los Angeles, CA, USA

As global climate continues to warm dramatically, it is important to identify which traits affect a tree’s ability to respond to warmer climates and drought, an increasingly frequent occurrence in California’s Mediterranean climate. Foliar trichomes are one such trait because by increasing the thickness of the boundary layer that surrounds leaves, they can reduce excess transpiration. Utilizing an established common garden experiment in Chico and Placerville, California, we are assessing whether valley oak (Quercus lobata) trees are genetically differentiated and/or phenotypically plastic with respect to foliar trichome morphology and abundance, and, if so, assess if these differences correlate with climate variables of the maternal site. We sampled fully developed leaves from 9-year-old trees representing 74 families across valley oak’s native range and dried them in coin envelopes before taking pictures of the abaxial side of the leaf at 1500x magnification. To estimate trichome abundance, pictures are divided into 4 quadrants by superimposing a grid in ImageJ and counting the number of hairs crossing the grid lines. Trichome morphologies are categorized as either simple (one hair) bifurcate (two branches) or stellate (3 or more branches). Our findings indicate trichome morphology is more branched and trichome abundance is greater in trees from warmer, drier southern seed sources compared to cooler, wetter northern seed sources. We also find that plasticity in these traits is associated with warmer and more seasonal climates. Understanding which traits enhance the ability of trees to respond to rapid climate change helps inform the future management of oak populations.