Visually Determined Soil Disturbance Classes Used as Indices of Forest Harvesting Disturbance

Visual estimates of soil and site disturbances are used by foresters, soil scientists, logging supervisors . and machinery operators to minimize harvest dis turbances to forest s i tes , to evaIuate compliance with forestry Best Management Pract ices (BMPs), and to determine the need for ameliorative pract ices such as tnechanical s i te preparat ion. Al though visual est imates are commonly used by f ie ld personnel , the actual relationships of visually determined soil disturbance classes to various soil physical properties and site characteristics have not been determined The purpose of this investigation was to evaluate if visually derermined soil disturbance classes ore related to quantitative soil end site properties that are known to in f luence soi l product iv i ty and hydro!ogic func t ion . Several types of quanti tat ive da;a were evaluated within the soil disturbance classes: static data (bulk density, saturated hydraulic conductivity, total, capillary, noncapillary pore space, and soil roughness) and dynamic data (mechanical resistance, volumetric soil moisture, subsurface water table depth). All data were collected from a long-term forest productivity study located in the Coastal Plain of South Carolina The study is a randomized complete block design with two harvest disturbance levels (wet-weather harvest vs. dry-weather harvest) and a maximum offive site soil disturbance (SD) classes. Disturbance classes included undisturbed (SDO), compressed but not rut ted (SDI), rut ted (SD2), deeply rutted (SD3). and churned (SD4). Analyses revealed that three s tat ic variables(soi1 bulkdensi ty , saturated hydraulicconductivi ty , mucropore pore space) and two dynamic variables (depth of the subsurface water tabk and mechanical resis tance) were signi f icant ly related to dis turbance. Al though undisturbed and compressed areas general ly were af fected less than the more severe disturbance classes, the three most severe disturbance classes, churned areas, deeply rut ted areas, and rut ted areas were not di f ferent from one another . Thus, i t appears v isual dis turbances do not necessarily equate to site damage. The overall implications are that visually determined soil disturbance classes have merit as indices of some soil and site changes, but they should not be equated to soil damage categories. South. J. Appl. For. 22(4):24S-250. . . Vehicular traffic associated with forest harvesting operations has the potential to compact and/or puddle forest soils (Hatchell et al. 1970, Aust et al. 1993, Scheerer et al. 1995). Although a wide variety of site and machinery factors influence soil disturbances, the likelihood of soil disturbance is enhanced on moist to saturated soils NOTE: W. Michael Aust is the corresponding author, and he can be reached at 228 Cheatham Hall, VPI&SU. Blacksburg, VA 24061-032&Phone: (540) 23 14523; Fax: (540) 23 l-3330; E-mail: [email protected]. Thii research w a s s u p p o r t e d b y W e s t v a c o C o r p o r a t i o n , S u m m e r v i l l e . S C , t h e N a t i o n a l C o u n c i l f o r A i r a n d S t r e a m !mprovement, I n c . (NCASI). Research Tr iang le . N C . a n d t h e U S D A F o r e s t S e r v i c e , S o u t h e r n R e s e a r c h S t a t i o n , A u b u r n , AL 36849 . Manuscr ip t rece ived Apr i l 14, 1997, accepted January 16, 1998. \ SJAF 22(4) 1998 245 (Mocha-ing alld Ra\vls 1 9 7 0 . (~reaicw’;lc,d S;mds IOSO. Ausl cl al. 1993.5). Wcl flats. rcfcrrcd lo regionally aS wCt flatwoods. pocosins (not true pocosins), or bays h2w fat topography and poor internal drainage. When \wt flats 2rc subjected lo fire or site preparation. rhcy arc dominated by pine species; wetter, less disturbed wet flats have a larger hardwood component (Harms et al. 199X). In the SOUI~Ieastern United States, fairly even seasonal distribution of rainfall in some years and very intense rainfall associated with hurricanes, tropical depressions, and even thunderstorms, combined with the relatively flat topography and poor drainage of the wet pine flats frequently result in site disturbance. Compared to undisturbed sites, compacted and puddled wet pine flats often have increased soil bulk density, decreased macroporosity and hydraulic conductivity, and elevated volumetric water content, resulting in impeded drainage conditions and inadequate soil oxygen for root respiration (Lockaby and Vidrine 1984, Aust et al. 1995). Growth declines of pine species on those disturbed sites may be attributed to the decreased soil aeration caused by the soil physical changes (Hatchell et at. 1970, Langdon 1976. Hatchell 1981. Karr et al. 1987). Several researchers have suggested that such disturbanccs may h2ve long-term consequences fOi lhc management of pine plantations (Foil and Ralston 1967, Scheercret al. 1995, Tiarks and Haywood 1996). Concern about potential site degradation issues associated with soil compaction and rutting are evidenced by the Sustainable Forestry Initiative by industrial forest companies (American Forest and Paper Association 1994, 1996); the development of forestry best management practices by states in the southeastern region (Aust 1994); andcontinued efforts to define and quantify forest soil health and develop indices that can be successfully used to predict forest productivity (Burger 1996). Unfortunately, the most thoroughly understood indices of soil disturbances and site productivity are labor-intensive, require large sampling time frames, and are unfamiliar to many foresters and logging supervisors. The purpose of this research is to compare and relate both static and dynamic variables that have been successfully used to quantify site disturbance with a fast, simple method of soil disturbance classification. Indices of Forest Harvest Disturbance Harvest-induced disturbances have been characterized by a variety of quantitative and qualitative measures, but most are used to quantify the eaSe with which roots can penetrate the soil and/or the movement of air and water in the soil. Intact soil cores are commonly collected to determine more than one-aspect of soil disturbance. Intact soil cores can be used to sample soil bulk density (Blake and Hartge 1986), which has been used toquantify soil compaction by forestand agricultural researchers for decades (Greaten and Sands 1980).Galeetal.(l99l)evaluatedlimitingsoiI bulkdensities for white spruce over a wide range of soil textures and concluded that spruce root growth was limited by bulk densities between I .46 and I .84 Mglm3. The inr;~ct soil cores used for soil hull, dcnsio samples can also bc used todc(cI-mint (o(al pore spa<<:. soil nlici-oporosity. and soil macroporosity, which are indices of soil aeration and drainage (Danielson and Sutherland 1986). Finally, the intact cores arc often used to measure saturated hydraulic conductivity, an index of soil water movement and potential soil drainage problems (Klute and Dirksen 1986). These techniques have been successfully used tocharacrerizc for-es{ soil disturbance for a variety of situarions in the southeastern Coastal Plain, including harvestin g machinery-soil interactions (Aust et al. 1993, McDonald et al. l995), effects of harvesting and site preparation on soil properties and tree growth (Gent et al. 1983, Tiarks and Haywood 1996), and effects of thinning during wet periods on subsequent stand growth (Karr et al. 1987, Reisinger et al. 1988; 1993). However, these intact soil core sampling techniques are almost invariably used to characterize research plots, as opposed to being used for forest management and planning applications. The techniques are relatively laborious and time consuming, and intact soil cores are very difficult to acquire during saturated soil conditions or after soils have been puddled. These difficulties as well as technology advances have persuaded forest managers and researchers to investigate the use of additional soil/site characterization pnramsters. Examples include soil strength measl;rcrrcnts as facilitated by recording penetromerers. mrasurenlents of near surface groundwater via newer types of stage recorders, and almost instantaneous measurements of volumetric soil water contents via Time Domain Refractometry (TDR) (Burger 1994)., as indices of soil disturbance, although these types of technology are relatively expensive as compared to soil cores. At present, no quantitative method of accessing site disturbance has been developed that can be used for common forestry applications. Therefore qualitative methods have been developed. Over the decade, numerous studies have attempted toquantify thespatial disturbances associated with wet-weather timber harvests and many of these studies have used modifications of the spatial soil disturbance classes originally developed by Miller and Sirois (1986).The various modifications of the soil disturbance classes generally include some recognition of soil litter layer disturbance, obvious soil compression caused by traffic, soil ruts caused by traffic, and mixing of mineral and litter layers.