Trentepohlia umbrina on Scots pine as a bioindicator of alkaline dust pollution

The present study was driven by the wish to broaden the selection of ecological indicators for estimating the alkaline dust pollution. The abundance of the algal species Trentepohlia umbrina was studied on Pinus sylvestris trunks, an acidic substrate that it normally does not occupy. The study was carried out in northern Estonia in the surroundings of four major limestone quarries, considerable local-level sources of dust pollution. The cover of T. umbrina on tree trunks was clearly higher near the quarries, evidently due to the elevated bark pH, its maximum values reaching ca. 30% (mean of five trees studied per sample plot). The cover of the species decreased steeply at the distance of 800–900 m from the quarries; further than 1000 m from the quarries the maximum cover was already less than 4%, and further than 2000 m less than 1%. The correlation between the cover of T. umbrina and measured bark pH values indicated a steep increase in cover at about pH 3.8–3.9; the cover values varied between 0 and 4% below and between 10 and 31% above that pH. These results confirm that T. umbrina could be used as an indicator species of alkaline dust pollution. The species forms reddish powdery coating on tree trunks, and in case of high abundance can be very conspicuous looking, being easily detectable at field.