REGULATING COMPETITION ON CONIFER PLANTATIONS WITH PRESCRIBED CATTLE GRAZING

On conifer plantations, competitive understory vegetation often retard s growth and establishment of tree seedlings. Livestock grazing exempl ifies a method of controlling the understory vegetation and increasing the availability of site resources to tree seedlings. We hypothesized that prescribed cattle grazing ameliorates water stress of young tree seedlings by reducing root growth of competing understory species. On a Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) and ponderosa pi ne (Pinus ponderosa Dougl.) plantation in southwest Oregon planted in 1986, seedling water stress was evaluated with the pressure chamber te chnique and supplemented with gravimetric soil water determinations in 1986-1989. Root growth of orchardgrass (Dactylis glomerata L.), the m ajor understory competing species, was quantified in 1988 and 1989 wit h the root periscope/mini-rhizotron technique. Seedling water stress l evels during spring and summer were similar in a cattle-grazed vs. ung razed area in 1986 through 1988, but in summer 1989, water stress was reduced significantly in the grazed area. Soil water content was highe r in the grazed area in 1989, especially at the 10-20 cm soil depth. E nd of season (July) orchardgrass root growth was reduced 18% and 15% w ith grazing in 1988 and 1989, respectively. We conclude that repeated cattle grazing of orchardgrass reduced transpirational surface area an d root growth sufficiently to increase soil water availability to seed lings. Thus, prescribed cattle grazing on conifer plantations can enha nce seedling physiological status by acting as a regulator of above- a nd belowground competition.