Large-scale forest die off is a multifaceted problem that is becoming increasingly pressing in a hotter, drier climate. Among the factors implicated in contributing to tree death is a recently proposed mechanism by which hygroscopic aerosol ‘films’ form on living foliage due to increased levels of anthropogenically-generated pollutants. These films may be capable of driving excess evaporation of apoplastic water into the atmosphere with consequent dehydration of foliage. Few studies have investigated the extent of this phenomenon in urban forests. However, there exists great potential for it to negatively impact urban trees and the goods and services they provide to human communities. Cuticular conductance measurements quantify the water lost through the cuticle and imperfectly closed stomatal pores, and may provide an excellent proxy for identifying trees affected by anthropogenic aerosol pollution. This experiment was devised to compare cuticular conductance rates and photosynthetic performance between Giant sequoia (Sequoiadendron giganteum) trees in areas with high and low exposure to roadside pollution within Portland, OR.
Results/Conclusions
Our results showed an 86% increase in cuticular conductance rates of trees at the polluted site compared to the control site (1.147 mmol/cm² vs. 0.615 mmol/cm²; P < 0.001). Preliminary estimates of the effects of these elevated minimum conductance rates on whole tree water use suggest that this amounts to > 3000 gallons of additional water lost per year when stomata are theoretically “closed”. In addition, photosynthetic gas exchange was limited in polluted trees as evidenced by lower quantum efficiency and maximum photosynthetic rates determined from light response curves (P < 0.05). These results suggest that accumulation of air pollution on leaves is a potentially large driver of plant-atmosphere water and carbon flux that should be taken into consideration when estimating the effects of future climate-induced warming and drying trends on urban forests.