Above- and below-stump biomass and nutrient content were estimated for a mature loblolly pine (Pinustaeda L.) plantation on an eroded site in the upper Piedmont of South Carolina. Pine above-stump biomass was 144.9 t•ha−1; below-stump biomass was 36.0 t•ha−1, 20% of total pine biomass. Total pine biomass was partitioned as 17% crown, 63% stem, and 20% roots. About 55% of below-stump biomass was in taproots and 45% in lateral roots. Dominant and codominant trees had a greater proportion of below-stump biomass in lateral roots, indicating that larger trees absorb a disproportionate quantity of the site's moisture and nutrients. Lateral roots contained 66 to 75% of below-stump nutrients. Fine roots (<0.6 cm diameter) accounted for 11% of below-stump biomass, but contained 24 to 30% of below-stump nutrients. Nutrient content (kg•ha−1) of above-stump biomass ranked as follows: N 164.7, Ca 154.2, K 78.0, and P 14.0. Nitrogen was also the most abundant nutrient in below-stump biomass (60.2 kg•ha−1), followed by Ca (48.9 kg•ha−1), K (41.2 kg•ha−1), Mg (11.1 kg•ha−1), and P (7.5 kg•ha−1). Below-stump biomass contained 27, 35, 35, and 24% of total pine N, P, K, and Ca, respectively. Fine roots and foliage, only 4% of total stand biomass, had about one-fourth of the stand's N and P.
A technique was developed for estimating below-stump biomass of mature loblolly pine (Pinustaeda L.). The technique required complete excavation of only three trees and partial excavation (taproot only) of 12 others. A regression methodology for roots > 0.6 cm diameter and traditional soil-core sampling for roots < 0.6 cm diameter formed the main elements of the technique. Strong relationships were found (1) between biomass of three diameter classes of lateral roots and their proximal end diameter and (2) between diameter at breast height and estimated below-stump biomass for roots > 0.6 cm diameter. This technique should be useful for estimating root biomass of plantations of loblolly pine and may be effective for large trees of other species.
Decaying root systems of harvested trees can be a significant component of belowground carbon storage, especially in intensively managed forests where harvest occurs repeatedly in relatively short rotations. Based on destructive sampling of root systems of harvested loblolly pine trees, we estimated that root systems contained about 32% (17.2 Mg ha -1 ) at the time of harvest, and about 13% (6.1 Mg ha -1 ) of the soil organic carbon 10 years later. Based on the published roundwood output data, we estimated belowground biomass at the time of harvest for loblolly-shortleaf pine forests harvested between 1995 and 2005 in South Carolina. We then calculated C that remained in the decomposing root systems in 2005 using the decay function developed for loblolly pine. Our calculations indicate that the amount of C stored in decaying roots of loblolly-shortleaf pine forests harvested between 1995 and 2005 in South Carolina was 7.1 Tg. Using a simple extrapolation method, we estimated 331.8 Tg C stored in the decomposing roots due to timber harvest from 1995 to 2005 in the conterminous USA. To fully account for the C stored in the decomposing roots of the US forests, future studies need (1) to quantify decay rates of coarse roots for major tree species in different regions, and (2) to develop a methodology that can determine C stock in decomposing roots resulting from natural mortality.
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