Foliage responses of spruce trees to long-term low-grade sulfur dioxide deposition Original Research Article

Foliage on spruce trees (Picea rubens Sarg.) growing on dry SO2 deposition zones (dry SO2 deposition ranging from 0•5 and 8•5 S kg ha−1 year−1) downwind from a SO2 emission source was analyzed to assess chronic effects of long-term low-grade SO2 deposition on net photosynthesis, stomatal conductance, dark respiration, stomatal antechamber wax structures, elemental concentrations in and on foliage (bulk and surficial concentrations), and types of epiphytic fungi that reside in the phylloplane. Elemental distributions on stomatal antechambers, on fungal colonies, and on smooth surfaces between stomates and fungus colonies were determined with a scanning electronic microscope (SEM) by way of X-ray scanning. It was found that net photosynthesis of newly developed spruce foliage (current-year, and 1-year-old) was not significantly affected by the local SO2 deposition rates. Sulfur dioxide deposition, however, may have contributed to the gradual decrease in net photosynthesis with increasing needle age. Dark respiration rates were significantly higher on foliage taken from high SO2 deposition zones. Stomatal rod-web structures deteriorated to flakes with increasing needle age and increasing SO2 deposition. Further inspection of the needle surfaces revealed an increasing abundance of fungal colonies with increasing needle age. Many fungal taxa were isolated and identified. It was found that black yeasts responded positively, and Xylohypha pinicola responded negatively to high rates of SO2 deposition. Surficial concentrations of elements such as P, S, K, Cl, Ca were about 10 times higher on fungal colonies than on smooth needle surfaces. Surficial Ca contents on 4 or 5-year-old needles decreased with increasing SO2 deposition, but surficial S concentrations remained the same. In contrast, bulk foliar Ca and S concentrations increased with increasing SO2 deposition.