Microbial populations on Douglas fir needle surfaces

The surface microflora on Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) foliage from old-growth trees in western Oregon has been examined by epifluorescence and scanning electron microscopy. Colonies of microorganisms on both upper and lower surfaces of 1-, 3-, 5-, and 8-year-old needles from three heights in the canopy of a single tree have been counted in belt transects, and the relative abundance of various categories of microorganisms has been computed. Aggregations of microbial cells are prevalent in the midrib depression along the upper surface and in stomatal cavities and gutters between rows of epidermal cells on the lower surface. Darkly pigmented hyphae and clumps of cells occur, in general, more frequently on the upper needle surface, a habitat more subject to desiccation and UV exposure.Protococcus colonies become abundant on both upper and lower surfaces of older needles. Microbial cover was found to be significantly higher on the bottom of the needle than on the top. The factors involved in this effect are considered and discussed.