No abstract

No abstract

The rising costs and social concerns over fossil fuels have resulted in increased interest in and opportunities for biofuels. Biomass in the form of coarse woody residues remaining after traditional timber harvest in the southeastern USA is a potentially significant source of biomass for bioenergy. Questions remain regarding whether the removal of this material would constitute a sustainable silvicultural practice given the potential impact on soil nutrient cycling and other ecosystem functions. Our objective is to review existing studies to estimate quantities of residual materials on southern pine forests that may be available, potential nutrient removals, and potential replacement with fertilizer. Regionally, it is estimated that 32 million Mg year −1 of dry harvest residues may be available as a feedstock. At the stand level, between 50 and 85 Mg ha −1 of material is left on site after typical stem-only harvests, of which half could be removed using chippers at the landing. Based on these estimates, increase in midrotation fertilization rates of 45% to 60% may be needed on some sites to fully replace the nutrients from harvesting residues removed for bioenergy. Field experiments suggest that residue removals do not degrade forest productivity in many cases, but more data are needed to assess the effects of frequent removals (i.e., from short-rotation systems) over longer periods and identify sites that may be particularly sensitive to the practice. A benefit of developing markets for previously nonmerchantable materials may create incentives for improved forest management by landowners.

Fertilization commonly increases biomass production in loblolly pine ( Pinus taeda L.). However, the sequence of short-term physiological adjustments allowing for the establishment of leaf area and enhanced growth is not well understood. The effects of fertilization on photosynthetic parameters, root respiration, and growth for over 200 d following the application of diammonium phosphate were intensively investigated in an effort to establish a relative sequence of events associated with improved growth. Root respiration, foliar nitrogen concentration [N] f , and lightsaturated net photosynthesis ( A sat ) temporarily increased following fertilization. A sat was correlated positively with [N] f when non-fertilized and fertilized treatments were pooled ( R 2 = = = = 0.47). Increased photosynthetic capacity following fertilization was due to both improved photochemical efficiency and capacity and enhanced carboxylation capacity of Rubisco. Positive effects of fertilization on growth were observed shortly after A sat increased. Fertilized seedlings had 36.5% more leaf area and 36.5% greater total dry weight biomass at 211 d following fertilization. It is concluded that fertilization temporarily increased photosynthetic capacity, which resulted in a pool of photo-assimilate used to build leaf area. The N from fertilizer initially invested in photosynthetic structures and enzymes probably re-translocated to newly developing foliage, explaining the reduction in [N] f and A sat that was observed after peak levels were achieved following fertilization.

To determine the effects of shade on biomass, carbon allocation patterns and photosynthetic response, seedlings of loblolly pine (Pinus taeda L.), white pine (Pinus strobus L.), red maple (Acer rubrum L.), and yellow-poplar (Liriodendron tulipifera L.) were grown without shade or in shade treatments providing a 79 or 89% reduction of full sunlight for two growing seasons. The shade treatments resulted in less total biomass for all species, with loblolly pine showing the greatest shade-induced growth reduction. Yellow-poplar was the only species to show increased stem height growth in the 89% shade treatment. The shade treatments increased specific leaf area of all species. Quantum efficiency, dark respiration and light compensation point were generally not affected by the shade treatments. Quantum efficiency, dark respiration, maximum photosynthesis and light compensation point did not change consistently between the first and second growing seasons. We conclude that differences in shade tolerance among these species are not the result of changes in the photosynthetic mechanism in response to shade.