Facilitation among woody plants establishing in an old field

Trees and shrubs establishing in grasslands frequently occur in clumps asso ciated with increased soil moisture and N availability. We tested whether t he presence of conspecifics or enhanced soil resources increased the growth and survivorship of a shrub (Symphoricarpos occidentalis) and a tree (Pice a glauca) in the presence of the perennial grass (Bromus inermis). Rhizomes of Symphoricarpos and seedlings of Picea were transplanted into plots eith er singly or with four conspecific neighbors, and with grass neighbors pres ent or removed. Half the plots received additional water, N, and shade to s imulate a forest environment. Roots and shoots of transplants were harveste d after two growing seasons. Soil moisture and available N were lowest at h igh transplant density, and highest in the simulated-forest environment and in treatments where grass was removed. Transplant survivorship was general ly enhanced in plots where grasses were removed and in the simulated-forest environment. Picea survivorship was reduced by grasses in plots without re source manipulations but was unaffected by grasses in the simulated-Forest environment, suggesting that the simulated-forest environment enhanced the ability of Picea to compete with grasses. Picea growth, however, was reduce d by conspecific neighbors in all cases. For Symphoricarpos, conspecific ne ighbors significantly decreased growth rates with grasses absent but signif icantly increased growth rates with grasses present. Thus the direct effect of high density in monoculture was to reduce Symphoricarpos growth, but th e indirect effect of high density in the presence of grass was to facilitat e Symphoricarpos growth. In summary, conspecific neighbors increased the gr owth rate of Symphoricarpos in competition with grass, and a simulated-fore st environment enhanced the ability of Picea to survive in the presence of grasses. The results support the idea that contagious distributions of wood y species invading grasslands enhance wooly plant establishment.