RESPONSE OF LOBLOLLY-PINE AND SWEETGUM TO INTRA-SPECIFIC AND INTER-SPECIFIC COMPETITION AND INFLUENCE OF SOIL ORGANIC-MATTER

Loblolly pine (Pinus taeda L.) seedlings and sweetgum (Liquidamber sty raciflua L.) sprouts were grown together for three growing seasons in a factorial combination of densities (additive series) on a formerly c ultivated field that varied in organic matter (OM) in the upper 15 cm of the soil. Mean aboveground biomass per plant of each species at the end of each growing season was examined using a nonlinear yield-densi ty model, incorporating soil OM as an index of soil productivity. In a ll years, competitive effects of sweetgum on itself and on loblolly pi ne were more than three times greater than effects of loblolly pine. H owever, the competitive effects of sweetgum relative to loblolly pine on response of both species decreased with time. Effects of density of one species declined with the increasing density of the other species . The effect of soil OM on biomass was positive for both species, bein g greater for sweetgum than for loblolly pine. The proportion of total variation in response explained by soil OM, after considering effects of plant density, decreased with age, becoming nonsignificant for lob lolly pine by age 3. The magnitude of increases in mean plant biomass with increasing soil OM was density dependent, being greater at low pl ant densities.