Surface Soil Root Distribution and Possible Interaction with Site Factors in a Young Longleaf Pine Stand

Interaction between soil bulk density and low soil water content may create root growth-limiting soil strengths. In a Louisiana longleaf pine (Pinus palustris Mill.) stand, soil strength at the zeroto 20.0-cm depth was assessed in response to no fire or biennial fires in May. At the 5.0to 20.0-cm depth, one-half of the measurements were characterized by root growth-limiting soil strengths regardless of fire history. Where soil strengths were root growth limiting, pine fine root biomass was about 24 percent lower than where soil strengths were not root growth limiting. Correlation between soil strength and pine fine root biomass was only observed where samples were collected distal to the longleaf pine trees, where soil strengths were high, and where biennial fire was applied. Further research is needed to determine whether repeated fire interacts with the relationship between soil strength and longleaf pine root growth on the west Gulf Coastal Plain. INTRODUCTION Soils in the western range of longleaf pine (Pinus palustris Mill.) are frequently characterized as poorly drained and fine textured (Peet 2006). High bulk densities are likely when soil texture is dominated by silt and clay (Fisher and Binkley 2000). Recently on the Kisatchie National Forest in central Louisiana, bulk densities of typical silt loam soils averaged 1.4, 1.5 to 1.6, and 1.6 to 1.7 g/cm3 for the A, B1, and B2 horizons, respectively (Patterson and others 2004, Sword Sayer 2007). Bulk densities >1.6 g/cm3 are known to restrict pine root elongation (Kelting and others 1999, Pritchett 1979). These root growth-limiting bulk densities are countered by root elongation along interped spaces and in macropores created by old roots and soil fauna (Van Lear and others 2000). Fortunately, these attributes also introduce spatial variation into bulk density measurements so that extreme values are not constant over large areas. During periods of sparse precipitation, low soil water content (SWC) interacts with bulk density to increase soil strength. Soil strength is the force required to advance through soil (Bennie 1996), and values >2000 kilopascals (kPa) are known to inhibit root elongation (da Silva and others 1994, Taylor and others 1966). When low precipitation evolves into drought, the negative effects of soil properties on pine root elongation are potentially far reaching on the west Gulf Coastal Plain. In effect, the land base with root-restricting soil characteristics is widened to include not only areas with high bulk densities but also areas that develop root growth-limiting soil strengths as the soil dries. Once again, in this situation, conduits produced by interped spaces, old root channels, and soil fauna allow root foraging for water and mineral nutrients. Efforts to restore longleaf pine ecosystems have been successful, in part, by the renewed use of fire as a management tool (Brockway and Lewis 1997). In some situations, repeated prescribed fire reduces understory woody vegetation but increases the growth of herbaceous plants and grasses (Haywood and others 2001). It is hypothesized that by manipulating understory vegetation, repeated fire also changes the amount and distribution of soil macropores that serve as conduits for pine root elongation. This, in turn, could affect soil strength, its spatial variability, and the relationship between soil strength and pine root elongation. As an initial step toward understanding the relationship between soil strength and longleaf pine root growth, the present study was conducted to survey soil strength, longleaf pine fine root biomass (FRB), and their relationship where competing vegetation was not controlled and where biennial prescribed fire was applied in May. MATERIALS AND MEHODS Study Site The study is located on the Kisatchie National Forest in central Louisiana at latitude 31°0'42.45" N, longitude 92°37'8.54" W. The soil is a Beauregard silt loam and Malbis fine sandy loam complex. A mixed pine-hardwood forest originally occupying the site was clearcut harvested in the mid-1980s, repeatedly burned, sheared and windrowed in 1991, and rotary-mowed in 1992 (Haywood 2002). In 1992, 15 treatment plots [22 by 22 m (0.048 ha)] were established and assigned 1 of 3 vegetation management treatments (no plant control, herbicide application, or mulching after planting) (Haywood 2002). In February 1993 and January 1994, one-half of each plot was planted at 1.8 by 1.8 m with container-grown longleaf pine seedlings from a Mississippi source. By age 3 to 4 years, seedlings were overtopped by competing vegetation in spite of the vegetation management treatments (Haywood 2002). Competing vegetation was manually and chemically eradicated in 1997 and 1998, respectively. In 1998, analyses of variance indicated that tree growth was significantly affected by vegetation management treatment but not by block, age, or their interaction with vegetation management treatment (Haywood 2002). The study was subsequently reconfigured with each of the original vegetation management treatments as one of three blocks, and random assignment of one of five treatments to each plot per block. 1 Plant Physiologist, U.S. Department of Agriculture Forest Service, Southern Research Station, Pineville, LA.