Seasonal variability of leaf water capacity and wettability under the influence of pollution in different city zones

Can dust and tarry substances that are rich in polycyclic aromatic hydrocarbons (PAHs), which are deposited on leaves, cause changes in the water retention of tree crowns? It is hypothesized that the contact angle between droplets and a leaf's surface and water capacity changes because of the content of PAHs. This angle is treated as a bioindicator of environmental pollution. The goal of this study was to analyse the relationship between rainfall water capacity and wettability of small-leaved lime and poplar in different zones in a city centre during the period from April through November. We implemented a series of simulated rainfall and angle measurements under laboratory conditions of tree twigs collected at three locations inside the city centre and one from an area outside the city. The background for the water capacity values were results acquired from selected PAHs contents determined in leaves using high-performance liquid chromatography (HPLC), and images of the leaves' surface were acquired via Scanning Electron Microscope (SEM). Based on the obtained results, we concluded that there were significant differences in water capacity between the areas for each month. In the city centre, water capacity from April to July was lower than that in the forested area. From July to the end of the growing season, the water capacity was lower in the city compared to the forest area. The contact angle was strongly correlated with water capacity. With a decreasing contact angle, the raindrops increasingly adhered to the leaf surface, and water capacity increased. It was found that the effects of pollution on water capacity cannot be ignored in developing forecasts or in models describing ecosystem and hydrological changes in natural and urbanized environments.