Trace element (TE) contamination of soil is a pervasive global problem, posing health and ecological risks in both urban and agricultural soils. There is limited research on the effects of TE soil contamination on photosynthesis in field settings where plants have been exposed to elevated TE for a prolonged period of time. Assessing TE effects on photosynthesis under field conditions will improve our understanding of plant physiological responses to abiotic soil stress and the limitations anthropogenic soils pose on productivity. This study focused on Betula populifolia trees (> 10 years old) which self-seeded in an urban brownfield with elevated concentrations of As, Cr, Cu, Pb, and Zn. It addressed two hypotheses: 1) high soil TE concentrations negatively affect photosynthetic parameters; and 2) high soil TE will accentuate the effects of a heat wave on photosynthesis. Photosynthesis parameters (e.g. dark respiration) were determined from light and A-Ci curve measurements made with a portable gas exchange system which varied leaf exposure to irradiance and ambient CO2 concentrations, respectively. Measurements were made monthly from May to August 2014 and 2015 in four study plots – two with a high concentration of TE (High) and two with lower concentrations (Low).
Soil TE alone was not a sufficient predictor of any tested photosynthesis parameter. However significant interactive effects between measurement month/year and TE were observed for maximum carboxylation rate (Vcmax), maximum electron transport rate (Jmax), and intrinsic water use efficiency (iWUE). These interactions indicate some seasonal effects of TE. High TE trees tended to have lower Vcmax, Jmax, maximum photosynthetic assimilation (Amax), light compensation point (LCP), and dark respiration (Rdark) along with higher CO2 compensation point (CCP) and iWUE, though these differences were small. After a heat wave in July 2015, iWUE was significantly higher in High TE trees indicating they are able to maintain higher photosynthetic rates in spite of low conductance. While Amax, Vcmax, transpiration, stomatal conductance, and Rdark all decreased dramatically following the heat wave, no other interacting TE effects were observed. These results suggest the trees’ photosynthetic rates are fairly robust in spite of elevated TE which is contrary to the outcomes of lab based research on the subject. It highlights the importance of observing plant stress responses under field conditions where factors such as soil nutrients, water availability and mycorrhizal communities may play important roles in ameliorating abiotic stresses.