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 beenexamined by epifluorescence and scanning electron microscopy. Colonies ofmicroorganisms on both upper and lower surfaces of 1-, 3-, 5-, and 8-year-oldneedles from three heights in the canopy of a single tree have been counted inbelt transects, and the relative abundance of various categories of microorga-nisms has been computed. Aggregations of microbial cells are prevalent in themidrib depression along the upper surface and in stomatal cavities and guttersbetween rows of epidermal cells on the lower surface. Darkly pigmentedhyphae and clumps of cells occur, in general, more frequently on the upperneedle surface, a habitat more subject to desiccation and UV exposure.Protococcus colonies become abundant on both upper and lower surfaces ofolder needles. Microbial cover was found to be significantly higher on thebottom of the needle than on the top. The factors involved in this effect areconsidered and discussed.