Compacting Coastal Plain soils changes midrotation loblolly pine allometry by reducing root biomass

Factorial combinations of soil compaction and organic matter removal were replicated at the Long Term Site Productivity study in the Croatan National Forest, near New Bern, North Carolina, USA. Ten years after planting, 18 preselected loblolly pine (Pinus taeda L.) trees were destructively harvested to quantify treatment effects on total aboveand below-ground tree biomass and to detect any changes in the absolute and relative allocation patterns. Stem volume at year 10 was not affected by compaction treatments, even though the ultisols on these sites continued to have higher bulk densities than noncompacted plots. However, even when site preparation treatments were undetectable aboveground, the treatments significantly altered absolute root growth and tree biomass allocation patterns. Soil compaction decreased taproot production and significantly increased the ratio of aboveground to belowground biomass. Decreased root production will decrease carbon and nutrient stores belowground, which may impact future site productivity. Résumé : Des combinaisons factorielles de traitements de compaction du sol et d’enlèvement de la matière organique ont été répétées dans le cadre de l’étude à long terme sur la productivité des stations dans la forêt nationale de Croatan, près de Bern en Caroline du Nord, aux États-Unis d’Amérique. Dix ans après avoir été plantés, 18 pins à encens (Pinus taeda L.) présélectionnés ont été récoltés pour quantifier l’effet des traitements sur les biomasses souterraine et aérienne totales et pour déceler les changements dans les profils d’allocation relative et absolue. Le volume de la tige à 10 ans n’a pas été affecté par les traitements de compaction, bien que les ultisols présents sur ces stations continuaient d’avoir une densité apparente plus élevée que dans les parcelles non compactées. Cependant, même lorsque les traitements de préparation de terrain n’avaient pas d’effets mesurables sur la partie aérienne des arbres, ils ont significativement altéré la croissance racinaire absolue et les profils d’allocation de la biomasse. La compaction du sol a causé une diminution de la croissance de la racine pivotante et significativement augmenté le rapport entre la biomasse aérienne et la biomasse souterraine. La diminution de la production de racines entraı̂nera une diminution des réserves de carbone et de nutriments dans le sol, ce qui pourrait dans l’avenir avoir une impact sur la productivité de la station. [Traduit par la Rédaction] Introduction Soil compaction and slash removal during harvest have immediate and potentially long-term effects on soil properties (Greacen and Sands 1980). Soil physical properties directly affect water and air movement through the profile and have immediate impacts on water and nitrogen availability to root systems (Rygiewiez et al. 2004). However, the extent to which root growth patterns and potential are directly affected by soil physical conditions and the duration of the effect are debatable. Soil organic matter characteristics similarly affect nutrient budgets, nutrient availability, and soil physical properties and, thus, can significantly affect root growth rate. Nutrient and water availability can also affect aboveground tree growth and vigor. Soil compaction of fine-textured or wet soils typically affects soil physical characteristics by increasing soil bulk density; decreasing soil porosity, aeration and infiltration capacity; and increasing soil strength and, potentially, water runoff and soil erosion. Data from various sources indicate that increases in bulk density of coarse-textured soils have little or no effect on plant growth (Ares et al. 2005; Powers et al. 2005) but that comparable increases in bulk density of fine-textured soils have substantial effects on plant growth (Helms 1983; Alexander and Poff 1985; Powers et al. 2005). Other studies suggest that, even without significant differences in bulk density, important changes in macroporosity will affect water and air movement through soil, thus potentially impacting root proliferation and distribution (Aust et al. 1995). The amount of slash left on site or incorporated into the soil will moderate soil temperature and moisture, and impact nutrient storage and availability. Increases in soil temperature and moisture after canopy removal, in addition to the mixing of litter and debris with mineral soil, increase biological activity and nutrient immobilization (Eisenbies et al. 2005). Any management practice that decreases long-term nutrient storage pools on a site has the potential to impact future site productivity. Because root systems from mature loblolly pines (Pinus taeda L.) can persist for more than 60 years after harvest (Ludovici et al. 2002a), tree roots Received 17 September 2007. Accepted 15 April 2008. Published on the NRC Research Press Web site at cjfr.nrc.ca on 30 June 2008. K.H. Ludovici. USDA Forest Service, Southeastern Research Station, 3041 Cornwallis Road, Research Triangle Park, NC 27709, USA (e-mail: [email protected]). 2169 Can. J. For. Res. 38: 2169–2176 (2008) doi:10.1139/X08-060 # 2008 NRC Canada have the potential to impart long-term influences on soil properties and processes. To that end, silvicultural impacts on root growth and biomass distribution should be better understood. The purpose of this experiment is to quantify total aboveand below-ground tree biomass at age 10 and determine to what extent the absolute and relative allocations shift when site quality changes because of soil compaction and organic matter removal. The specific objectives are to (i) quantify total tree allocation patterns and (ii) assess biomass partitioning for a sample of normally developing 10-year-old trees in response to soil compaction and organic matter (OM) removal treatments. Materials and methods Site and treatment description In 1991, researchers with the USDA Forest Service Southern Research Station cleared a 60-year-old natural pine–hardwood stand on the Croatan National Forest in Craven County, North Carolina, and designated three 4 ha blocks (NC1, NC2, and NC3) for a Long Term Site Productivity (LTSP) study (Powers et al. 1989).1 The site was blocked to account for soil drainage characteristics. The soil in NC1 is primarily a moderately well-drained Goldsboro soil (fine-loamy, siliceous, thermic aquic Paleudults), whereas the soils of NC2 and NC3 are somewhat poorly drained Lynchburg soils (fine-loamy, siliceous, thermic aeric Paleaqualts according to the USDA classification; Gleyic Acrisol in FAO soil unit). Additionally, the soil of NC3 is more poorly drained than that of NC2. Nine treatment plots were established in each block where three levels each of soil compaction and OM removal were applied in a 3 3 factorial design (Duarte 2002; Sanchez et al. 2006a). Precipitation and temperature data were collected from a weather station at the LTSP site. Information from the nearby Cherry Point Marine Base was used to supplement on-site climate data (www.cherrypoint.usmc.mil/weather/, accessed 2005). Experimental LTSP treatments were imposed on 0.4 ha measurement plots using three levels of soil compaction (C0, none; C1, intermediate; C2, severe) and three levels of OM removal (OM0, stem only; OM1, whole tree; OM2, whole tree plus forest floor). Treatment plots were further split into those treated with herbicides (U–) to achieve a complete and continual competition-free condition or untreated (U+) to achieve no competition control. Plots receiving the C0 treatment were not driven on during either harvesting or site preparation. The C2 treatment was intended to be within 20% of the approximate growthlimiting bulk density (Daddow and Warrington 1983). The C1 treatment levels were designed to be near the midpoint between the C0 and C2 treatment levels. Tree boles were removed by cranes on the C0 plots, and the increased levels of OM removal on the other C0 treatment combinations were accomplished by hand. On the C1 plots, tree boles were removed with skidders and any additional OM removal on the other C1 combination plots was done by hand. A bulldozer equipped with a shear blade removed the forest floor on the C2 plots. To facilitate severe compaction on the OM0 and OM1 plots, branches, foliage, and other forest floor material were removed prior to compaction. Mineral soil was compacted with a vibrating drum roller, without vibration on the C1 plots and with full vibration on the C2 plots. Branches, foliage, and other material removed prior to compaction were redistributed by hand on OM0 and OM1 plots. The U– plots were kept competition free using the herbicides Accord, Arsenal, and Oust. Brushsaws were used to remove volunteer pines and residual hardwoods. Collection of soil bulk density samples were made by depth from mineral soil surface (0–10, 10–20, and 20– 30 cm) using fixed volume soil rings (Blake and Hartge 1986). Pretreatment measurements included three randomly located cores per treatment plot and depth, whereas six cores per treatment plot per depth were collected to determine posttreatment soil bulk density values. All soil within that fixed volume was oven-dried and weighed. Calculations of soil bulk density used soil dry mass divided by the fixed volume. A mixture of 1–0 half-sib loblolly pine families were hand planted at a 3 m 3 m spacing. Every plot contained 242 seedlings such that the interior measurement plot had 80 trees, and each side of each plot was bordered by three rows of trees. Seedlings were planted after the silvicultural treatFig. 1. Soil bulk density on the Long Term Site Productivity study area averaged over the three blocks and measured: before treatment installation, immediately after installation, and 10 years later. Symbols show the different depths (*, 0–10 cm; &, 10–20 cm; ~, 20– 30 cm), and lines show differences in soil compaction level (broken, C0 (no compaction); solid, C2 (severe compaction). 1 R.F. Powers, G.A. Ruark, A.E. Tiarks, C.B. Goudey, J.F. Ragus, and W.E. Russell. 1989. Study plan for evaluating timber management impacts on long-term site productivity: a research and national forest system cooperative study. Unpublished study on file with the USDA Forest Service, Washington, D.C. 2170 Can. J. For. Res. Vol. 38, 2008