Associate Professor of Organismal Biology Kennesaw State University, United States
Abstract: Due to differences in land cover and energy use, urban areas are often warmer than surrounding rural areas causing a phenomenon known as the Urban Heat Island (UHI) effect. Previous studies have found that UHIs influence fundamental processes in plants and increase plant stress, as indicated by chlorophyll fluorescence and photosynthetic rate. While these responses vary among plant species, it is not well understood how changes caused by UHIs differ between native plants and non-native plants. Non-native species frequently outcompete native species, and their competitive advantage may be further increased if they possess a greater tolerance to thermal stress. This study explores changes in plant stress due to increased temperature by comparing two deciduous, woody shrub species that co-occur in Georgia’s mesic Piedmont forests: native spicebush (Lindera benzoin) and non-native Chinese privet (Ligustrum sinense). Photosynthesis rate was measured in each species at three rural sites and three urban sites from June through August 2022. Plant cuttings were taken from the same individuals, and additional measurements were made while in an environmental chamber at temperatures ranging from 15 °C to 30 °C. To determine temperature optima among species and treatments nonlinear models were fit for both photosynthesis and chlorophyll fluorescence. Unexpectedly, field measures indicate the temperature optima of native spicebush declined 7.2 °C from rural to urban, while Chinese privet declined 3 °C in the same manner. Optima estimates from environmental chambers showed a smaller but opposing trend, with Chinese privet shifting by 2.6 °C compared to 0.5 °C in spicebush. Differences between field and chamber results could come from variation in plasticity among individuals, and a more limited range of temperatures in field observations. More consistent differences may be present during spring when photosynthetic rate may be higher due to a more open canopy, and wider temperature fluctuations.
