Trichloroacetic acid as a phytotoxic air pollutant and the dose-response relationship for defoliation of Scots pine

Various ubiquitous volatile organic air pollutants (VOCs), especially C2-halocarbons, may be converted to secondary air pollutants that are phytotoxic and known as herbicides. One of these is trichloroacetic acid (TCA), found in concentrations ranging from 10 to 130 ng/g in the foliage of forest trees in northern Finland. TCA has been used as a herbicide against monocotyledonous weeds. It has formative effects, inhibits growth, and induces chlorosis and necrosis of light-exposed leaves, including those of woody plant species. Twenty Scots pine trees in an experimental stand 50 km southeast of Rovaniemi were sampled for correlation of TCA levels and needle loss. The trees located at the northwesterly edge of the stand could be divided into two groups, one more resistant to the phytotoxicant TCA than the second. The range of TCA concentrations in needles was 8–65 ng/g. The extent of defoliation (range 25–90%) was lower in the TCA-resistant group, with a gradient of 0.32% defoliation per unit TCA concentration; in the sensitive group, the correlation line had a steeper slope of 0.78% defoliation per unit TCA concentration. These two groups serve as a basis for further studies on morphological, anatomical, and biochemical parameters.