No abstract

Legume species distribution and abundance and selected environmental variables were quantified across a complex gradient (varying in both water-holding capacity and fertility) for frequently burned longleaf pine (Pinus palustris)-wiregrass (Aristida stricta) ecosystems. Legumes were present in all months; however, abundance peaked in June and was minimal after killing frosts in October. Legume species were prominent in the flora (43 species encountered) ubiquitous (94% of 2-m(2) subplots had at least one legume species), and abundant (nearly 120 000 stems/ha). Although most species were widely distributed throughout the gradient, Lespedeza angustifolia was distinctly associated with the more hydric end of the gradient, while both Petalostemon pinnatum and Galactia microphylla were located in the more xeric extreme. The percentage variation in species that could be accounted for by environmental variation was low (27%). Of the variation that could be accounted for, a number of environmental variables were important, including soil moisture, pine basal area (i.e., light), and bivalent base cations (e.g., Ca(2+)). Although gradients in resource availability among sites did not affect the distribution of species or abundance of legumes strongly, variation in resources are likely to regulate N(2)-fixation rates of the various native legume species, and thereby affect ecological functions such as maintenance of N capital and productivity.

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Pinus taeda L. plantations in the United States are typically established using rows that are spaced 3 to 4 m apart. Although one company now plants pines in 6.1 m rows, reports on performance using this row spacing are rare. This paper provides a case study (established at the Solon Dixon Forestry Education Center) that compares two densities [672 vs 1344 seedlings per ha (SPH)] when fixing the between-row distance to 6.1 m and using either 2.44 m or 1.22 m within-row distances between the planted trees. At age 13-years, the aboveground biomass mean annual increment on this old-orchard site was 13.5 green Mg/ha/yr. Planting half as many pine seedlings as typical (for this region) did not reduce dominant height (P=0.22), basal area/ha (P=0.58) or total merchantable tonnes/ha (P=0.67). As expected, the higher density produced trees that were smaller in mean diameter at breast height DBH (25 vs 22 cm) and had more pulpwood than 672 SPH plots. However, planting pines 1.2 m apart within the row reduced survival, soon after two severe summer droughts. Apparently this stress increased the risk of attracting bark beetles. An economic analysis indicates that 672 SPH produced more valuable timber (at age 13 years) and had the highest Net Present Value (NPV). In contrast, the Ptaeda3 model indicated 1344 SPH would have the greater NPV. The conflicting results are related to Ptaeda3 predicting one sawtimber-sized tree while 156 were present in the 672 SPH plots. Although these findings should not be extrapolated to cutover sites, it appears there may be several advantages to planting pines in wider than traditional rows. The reluctance to plant Pinus taeda in 6.1 rows on flat, agricultural lands might be due to a lack of field data and/or a reliance on output from growth and yield programs that were developed using data from cutover sites.

No abstract