Soil Compaction Affects Growth of Young Shortleaf Pine Following Litter Removal and Weed Control in the Missouri Ozarks

—More frequent use of heavy equipment in intensive forest practices can lead to soil compaction and reduced productivity. The growth of 8-year-old planted shortleaf pine (Pinus echinata Mill.) and seasonal soil moisture stress and soil temperature were measured on cherty silt loam soil from which surface organic materials (whole tree plus leaf litter) had been removed before applying compaction treatments. Three levels of compaction and two levels of understory control were compared. Trees were significantly taller and had more height and diameter at breast height (d.b.h.) growth in compacted treatments than in the no compacted treatment and when the understory was absent than when present. Mean seasonal soil moisture stress was much higher for compacted treatments than for the no compaction treatment during the measurement period, except for September. But differences were significant only for May (severe compaction>no compaction) and September (severe compaction<no compaction). Soil temperature, although higher for compacted treatments than for the no compaction treatment, was not significantly different between treatments. Both soil moisture stress and soil temperature were higher when the understory was present than when absent. Compaction affects nearly all properties and functions of the soil, physical as well as chemical and biological. Compaction causes a rearrangement of the soil particles resulting in the alteration of pore size distribution, decreased porosity, and changes in the movement and content of heat, air, water, and nutrients in the soil (Grable and Siemer 1968). These changes in bulk density affect infiltration, drainage, water availability, aeration, root exploration, and nutrient uptake, all of which can directly influence soil productivity. The compaction of soils during forestry operations reduces the rate of establishment of natural regeneration, has been shown to reduce tree growth for periods spanning at least a decade, and can have deleterious effects on tree form (Wronski and Murphy 1994). Forest harvesting, especially whole tree harvesting and litter raking, can remove considerable biomass, leaving the residual forest floor significantly disturbed. Severe compaction and organic matter removal can affect both the quality and productivity of the soil. Studies conducted in North America have shown that tree growth and forest productivity decrease due to compaction. For example, the reduction in ponderosa pine (Pinus ponderosa Dougl. ex Laws) height and volume growth attributed to soil compaction in southwest Oregon was still evident 17 years later (Froehlich 1979). On the Coastal Plains of South Carolina, oneyear-old loblolly pine height growth on compacted skid trails was 40 to 50 percent less than height growth on non-compacted soil (Hatchell and others 1970). While there is ample evidence supporting the reduction in growth caused by soil compaction, it has not always produced negative growth effects (Miller and others 1996, Cochran and Brock 1985). For example, on several California sites, the effect of compaction on 4-year-old ponderosa pine varied with soil texture and soil water (Gomez and others 2002a). These authors reported that compaction reduced the growth young ponderosa pine in California on fine textured soils due to increased soil strength, but soil compaction increased growth on a sandy textured soil due to increased water holding capacity. Coile (1948) reported that the growth of shortleaf pine increased as soil available water increased in the subsoil in the North Carolina Piedmont. Mohr (1896) concluded that sands were a poor growth medium for shortleaf and on the better sites subsoil textures were heavier than topsoil textures. Each soil has unique properties that interact to respond to compaction in different ways to affect tree growth. NC Research Station, 208 Foster Hall, Lincoln University, Jefferson City, MO 65102. Phone (573) 681-5575; fax: (573) 681-5578; e-mail: fponder@fs.fed. us