Published August 18, 2014
North American Forest Soils Conference Proceedings
Biomass Harvesting and Soil Productivity: Is the Science Meeting our Policy Needs? Eric D. Vance*
National Council for Air and Stream Improvement, Inc. P.O. Box 13318 Research Triangle Park, NC 27709
W. Michael Aust Brian D. Strahm
Dep. of Forest Resources and Environmental Conservation Virginia Tech 228 Cheatham Hall (0234) Blacksburg, VA 24061
Robert E. Froese
School of Forest Resources and Environmental Science Michigan Technological Univ. 1400 Townsend Dr. Houghton, MI 49931
Robert B. Harrison
School of Environmental and Forest Sciences University of Washington Box 352100 Seattle, WA 98195
Larry A. Morris
Warnell School of Forestry and Natural Resources Univ. of Georgia 180 E. Green Street Athens, GA 30602
Biomass harvesting and associated management practices increase the availability of forest-based feedstocks for emerging bioproduct and energy markets. Concerns about the sustainability of these practices have led to the development of biomass harvesting guidelines (BHGs) by state, national, and international agencies and nongovernmental organizations. Site productivity BHG provisions include retaining specific proportions of harvested residues and restricting biomass harvesting on some sites. Field experiments have shown that forest responses to biomass harvesting vary widely and are often counterintuitive. With site-specific data lacking, BHGs tend to rely on default assumptions supported by best professional judgment. These include (i) the natural or unmanaged state is an ideal frame of reference, (ii) conventional harvesting retains and distributes most residues across the site, (iii) biomass harvesting removes virtually all residues, (iv) decomposing residues always enhance soil C and site productivity, (v) biomass harvesting is conducted in the absence of operational practices that alleviate site deficiencies and sustain productivity, and (vi) changes in forest state are equivalent to changes in forest function. Effective BHGs are science based, operationally feasible, and protect values of interest while allowing managers the flexibility to prevent or mitigate potential impacts within constraints imposed by best management practices and forest certification provisions. While harvesting-induced nutrient deficiencies can be prevented or corrected with fertilizers or other soil amendments, soil disturbance and exposure may warrant greater attention. Policy-relevant field studies should incorporate operational practices, examine linkages between indicators and values of interest, and evaluate and improve prevention and mitigation options. Abbreviations: BHG, biomass harvesting guideline; BMP, best management practice; LTSP, Long-Term Soil Productivity; WTH, whole-tree harvesting.
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enewable energy markets are providing incentives for landowners to manage for and harvest previously non-commercial biomass. In addition to the benefits associated with a renewable source of fiber and energy, harvesting residues and other lower value biomass can reduce the costs associated with site operability, site preparation, and stand regeneration and improve forest health by reducing the risk of wildfire, insects, and disease (Gan and Smith, 2007; U.S. Department of Energy, 2011). In fact, the harvesting of lower value biomass and other forest fuel reduction treatments are needed in many regions for their forest health benefits alone (Agee and Skinner, 2005; Skog and Barbour, 2006). Questions have been raised about the potential impacts of more intensive and frequent harvesting of tree components with disproportionately high nutrient concentrations and associated site disturbance on soil productivity. Evidence for the impacts of biomass harvesting (i.e., whole-tree harvesting) on forest productivity is mixed. In the U.S. Forest Service Long-Term Soil Productivity (LTSP) study conducted This work was presented at the 12th North America Forest Soils Conference, Whitefish, MT, 16–20 June 2013, in the Linking Science to Forest Policy session. Soil Sci. Soc. Am. J. 78:S95–S104 doi:10.2136/sssaj2013.08.0323nafsc Received 1 Aug. 2013. *Corresponding author (
[email protected]). © Soil Science Society of America, 5585 Guilford Rd., Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Soil Science Society of America Journal
over 10 yr at 45 installations across North America, the effects of residue and litter removals and soil physical alterations (compaction) were evaluated. Overall, LTSP investigators concluded that whole-tree harvesting (WTH) had “little consistent effect on any response variable,” which included tree growth (Ponder et al., 2012). Recent findings from a long-term study in Pacific Northwest Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] likewise found no effects of intensive biomass removal on tree growth over 10 yr (Holub et al., 2013). However, negative effects of biomass harvesting on tree growth have been documented and are most often attributed to harvest-induced nutrient removals, particularly in boreal forests (Proe et al., 1996; Jacobson et al., 2000; Smith et al., 2000; Nord-Larsen, 2002; Akselsson et al., 2007; Walmsley et al., 2009; Thiffault et al., 2010; Egnell, 2011; Helmisaari et al., 2011). Concerns about biomass harvesting impacts on forest environmental values and site productivity have led states and other entities to develop biomass harvesting guidelines (BHGs) that restrict biomass harvesting and associated practices. These BHGs, which are intended to complement forestry best management practices (BMPs) and certification requirements, include an array of provisions to protect soil productivity, water quality, and biodiversity (Evans et al., 2013; Tables 1 and 2). Among the provisions intended to protect soil productivity are retention of specific quantities or proportions of biomass on site and restriction of biomass harvesting to soil types that are considered resilient. Unfortunately, while there is a general conceptual basis for the existing guidelines, scientific justification for many specific provisions such as those in Table 1 are lacking. Because restrictive or operationally complex provisions could have unintended consequences for forest managers and operations, the science supporting BHGs should continue to be examined to ensure that they are both effective and operationally feasible. Table 1. Example biomass harvesting guideline provisions designed to protect soil productivity. Provision
Source
Avoid biomass harvesting on aspen or hardwood cover types on shallow soils (20 cm [8 inches] or less) over bedrock
Minnesota Forest Resources Avoid biomass harvesting on organic soils deeper than Council 61 cm (24 inches) that are ombrotrophic (2007) Avoid additional biomass harvesting on erosion-prone sites (e.g., those sites on steep slopes of 35% or more) Retain and scatter tops and limbs from 10% of trees in the general harvest area Herrick et al. (2009) Do not harvest fine woody material on dry nutrientpoor sandy soils When 1/3 of the basal area is being removed on a 15to 20-yr cutting cycle, retain 1/4 to 1/3 of the slash, Forest Guild tops, and limbs from harvest Biomass As harvest intensity increases, more slash, tops, and Working limbs from harvests should be left on site Group (2010) Avoid harvesting on low-nutrient sites or adjust retention of tops, branches, needles, and leaves
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Frames of Reference The “natural” or unmanaged forest state is often explicitly or implicitly used as the frame of reference to assess the impacts of biomass harvesting and other practices on forest structure and function (e.g., Minnesota Forest Resources Council, 2007; Forest Guild Biomass Working Group, 2010; Forest Guild Southeast Biomass Working Group, 2012; Forest Guild Pacific Northwest Biomass Working Group, 2013). There are several limitations to this benchmark. One is that a reference state is only as valid as the environmental values to which it is linked, and the default assumption that the environmental characteristics of unmanaged forests are always superior to managed forests is not well documented. For example, the lack of specific data has led to provisions based on professional judgment or natural stand characteristics rather than specific environmental outcomes (Forest Guild Biomass Working Group, 2010; Forest Guild Southeast Biomass Working Group, 2012). In addition, aesthetically pleasing forests can sometimes be used as a false proxy for “naturalness” (Whitney, 1987), and unmanaged reference sites not impacted by past agriculture, logging, or other land uses and anthropogenic disturbances are rare (Cairns, 1987). Documentation supporting the specific quantities of residues needed to sustain soil productivity, biodiversity, and other forest values is particularly deficient, in part because the support functions residues provide vary substantially across sites. Conventional harvesting, commonly known as stem-only or bole-only harvesting, where tops and branches are left on site, is another common frame of reference used to assess the impacts of biomass harvesting. In practice, however, residues left following the harvest of managed forests are commonly piled and windrowed and were traditionally burned to provide access for site preparation and planting operations rather than distributed in a manner that may consistently benefit soil properties and productivity (Fig. 1; Morris et al., 1983; Morris and Lowery, 1988; Eisenbies et al., 2006; Knapp et al., 2008; Boateng et al., 2010). Conversely, the assumption that operational biomass harvesting removes essentially all residues does not appear accurate in most situations. Biomass harvesting field experiments often involve the removal of residues with extreme care, sometimes by hand, to provide clear treatment contrasts and reduce variability rather than to necessarily mimic operational practices (Powers, 2006; Table 2. The USDA–NRCS soil suitability criteria incorporated into South Carolina’s best management practices (South Carolina Forestry Commission, 2013). Soil or site property Water table depth, cm Restriction depth, cm Sand, % Available water capacity, cm Kw (soil erodibility factor)
Slightly or Not moderately limited limited >60 >100 18
