Phytosociological Changes after Timber Harvest in a Southern Pine Ecosystem

Abstract

Changes in the plant community following a selection timber harvest in an all—aged pine—hardwood ecosystem were studied for 11 yr on 218 hectares of rolling uplands in central Louisiana, USA. Pine and selected hardwoods were harvested. Prior to the study, stands were harvested at 10—yr intervals by the single—tree selection method. Fire had been excluded for 30 yr. Principal tree species were Pinus taeda, Pinus echinata, Quercus alba, Quercus falcata, Nyssa sylvatica, and Liquidambar styraciflua. In the shrub layer pertinent species were Callicarpa americana, Vitis rotundifolia, Acer rubrum, Vaccinium elliottii, and Ostrya virginiana in association with reproduction of the principal tree species. In the absence of fire and hardwood arrest, periodic selection harvest, primarily of pine, was enhancing a definite shift from pine to hardwood. In the postharvest stand hardwoods averaged 84% of the tree density and 49% of the basal area. Residual pine in the dominant and codominant canopies will be harvested at the next cutting with hardwoods largely being potential sucessors. The density of young pine < 21.72 cm dbh (diameter at breast height) in the transgressive layers averaged 67.0%. Of the major hardwood species 93.5% of the average density consisted of young replacement trees < 21.72 cm. Among trees < 36.96 cm dbh, the percentage importance of hardwoods, as a group, was higher than the value for pine. In the shrub layer the relative dominances of pine regeneration progressively increased for 9 yr then declined appreciably as the species became intolerant to the high level of community competition. The net primary productivity (NPP) of herbaceous and woody species in the herb and shrub layer crested 2 yr after timber harvest. At this time NPP of the relatively sparse herbaceous dry matter averaged 111.9 kg/ha and that of woody species was 327.6 kg. Thereafter, NPP decreased progressively as many stems of tree reproduction grew above the shrub layer and the density and basal area of the tree stratum increased. Within 11 yr after the stand disturbance herbaceous growth was very sparse, with a NPP of 21.0 kg dry matter/ha. NPP of woody species declined to 116.6 kg dry matter/ha. The vegetative response among woody species in the herb and shrub layer after logging was primarily a general growth decline with increasing competition and not an elimination of established species nor an introduction of new species.