SPATIAL EXTENT OF IMPACT OF RED ALDER ON SOIL CHEMISTRY OF ADJACENT CONIFER STANDS

We examined patterns in soil N availability and pH along transects extending from mixed stands of conifers (mostly Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)) and red alder (Alnus rubra Bong.) to pure conifer stands at two locations. At the relatively infertile Wind River site, increased N availability was apparent for about 8-12 m downslope of the alder-conifer stand, but no effect was apparent upslope. At the fertile Cascade Head site, no trend was apparent in N availability across the stand boundaries, but soil pH in the conifer stand was depressed for about 5 m from the alder-conifer stand. Overall, the effects of alder on soil chemistry appeared limited to a distance of less than half the height of the trees.