Influence of plant density and soil organic matter on the first-year growth of loblolly pine and sweetgum

Abstract

Loblolly pine (Pinus taeda L.) and sweetgum (Liquidambar styraciflua L.) seedlings were planted at a range of densities on a formerly cultivated field which varied in soil organic matter (soil OM) in the upper 15 cm of the soil. Mean aboveground biomass per plant at the end of one growing season was examined to see how it varied as a function of plant density and soil OM. Biomass per plant as a function of density, holding soil OM constant, was described well by a reparameterized form of a standard yield density equation. A simple linear equation suitably described per plant biomass as a function of soil OM, holding density constant. Effects of soil OM on biomass were positive for both species, with sweetgum exhibiting a stronger response than loblolly pine. Based on the results of fitting equations to single factors, an equation considering both density and soil OM was developed and fitted using all data for each species. The equations explained 85% of the variation in response for both species. The inclusion of soil OM was more important in describing response of sweetgum than for loblolly pine. Inclusion of both density and soil OM in a single equation to describe mean response of plants in a monoculture might be extended to a mixture of species and used to examine intraspecific and interspecific competitive effects of competition and their relationship with productivity.