Associate Professor Santa Clara University, United States
Plants in urban environments experience very different microclimatic conditions compared to nearby wild populations. Cities are typically warmer than surrounding areas, a pattern called the urban heat island effect. This can have marked effects on plant phenology and the expression of functional phenotypes. These, in turn, can have strong effects on ecosystem function. Here, we focus on functional traits of two widespread exotic grasses in the San Francisco Bay Area, comparing urban and wildland populations. Our study species are Bromus diandrus and Avena barbata. We have studied wild populations of these species throughout the Bay Area since 2017, and here pair these with new measurements of urban populations in Santa Clara, California. We hypothesize that, due to warmer temperatures during their winter growing season, urban populations will grow taller, produce larger leaves with higher specific leaf area and produce larger-diameter stems with lower specific stem length. We collected urban individuals of the two species near the end of their growing season (between February and April 2023). For each individual plant, we collected the length, width, and thickness of the healthiest leaf, the length and diameter of a portion of the stem, and the height of the plant measured from the base to the highest leaf. We took pictures of each leaf to measure the surface area using the software ImageJ. Specific leaf area is then calculated as a ratio of the leaf area to dry mass, and specific stem length is the ratio of stem length to dry mass. Urban Bromus diandrus are shorter (P = 0.001), longer-leaved (P < 0.001), wider-leaved (P < 0.001) and have lower specific stem length (P < 0.001) and thicker stems (P < 0.001). There is no significant difference in leaf thickness. Urban populations of Avena barbata have thicker stems (P = 0.006) and leaves (P = 0.005) than wild populations, but do not differ in other traits. Differences in phenotypes between urban and wildland populations likely reflect some mixture of phenotypic plasticity and evolutionary responses, and our future work will assess these mechanisms through common garden studies.
