Thinning in a 28-year-old Water Oak Plantation in North Louisiana: Seven-year Results’

A Pl-acre. 28-year-old water oak (Quercus nigra L.) plantation on in old-field loessial site near Winnsboro, LA. was subjected to three thinning treatments during the winter of 1987-88: (1) no thinning, (2) light thinning to 180 dominant and codominant trees per acre, and (3) heavy thinning to 90 dominant and codominant trees per acre. Prior to thinning, the plantation averaged 356 trees and 86 p of basal area per acre, with a quadratic meati diameter of 6.7 in. Thinning reduced stand basal areas to 52 and 34 f? per acre for the light and heavy thinning treatments, respectively. After 7 years, thinning did not significantly increase stand-level basal area growth, but both thinning treatments produced shifts in stand structure. Diameter distributions of thinned stands were skewed to the larger diameter classes and crown class distributions of thinned stands were skewed to the upper crown classes. Both thinning treatments increased diameter growth of residual trees, but there were no significant differences between the two levels of thinning. Diameter growth of residual trees in the lightly thinned stand averaged 1.63 in. after 7 years, whereas those in the heavily thinned stand averaged 2.02 in. on this relatively poor site. Trees in the unthinned stand grew an average of only 1.04 in. INTRODUCTION Information on growth and development of southern oak plantations greater than 20 years of age is lacking. Most past research on oak plantations emphasized the development of suitable techniques for successful plantation estab l ishment (A l len and Kennedy 1989, Kennedy 1993) . but subsequent growth was generally followed for only a few yea rs . Through the Conserva t ion Reserve Program and the Wetland Reserve Program, many thousands of acres of marginal cropland in the South have been reforested, primarily to various oak species (Kennedy 1990). As these oak plantations develop and mature, the demand for information on how to manage them will increase. Land managers will need practical guidelines on thinning and other intermediate silvicultural operations. However, information on growth and yield, pattern of stand development, and the response of older oak plantations to silvicultural operations is sparse. Consequently, few guidelines currently exist for successful management of these older plantations. For these reasons, a thinning study was established in the winter of 1987-88 in two 28-year-old water oak plantations in north Louisiana. The study was designed to determine the diameter growth and stand structure responses of water oak to three levels of thinning. Stand parameters of the plantations prior to thinning were previously described by Krinard and Johnson (1988). Meadows and Goelz (1993) reported fourth-year responses to thinning. METHODS Study Area The plantations were established in 1960 on old agricultural fields of loessial soil on the Macon Ridge landform in Franklin Parish east of Winnsboro, LA. The fields were under continuous cultivation for several decades prior to the establishment of the oak plantations. The two plantations encompass 21 acres (a 14.5acre tract and a nearby 6.5 acre tract). Both tracts are privately owned. Both fields were planted at the rate of aporoximatelv 950 water oak seedlings per acre. Initial spading was variable, but appears to have been about 5 x 9 ft. No cultivation or other means of weed control was conducted after planting at either site. The plantations are located on the Calhoun-Calloway-Loring soil association (Krinard and Johnson 1988) that developed from wind-blown silt, or loess. Within these terrace sites, Calhoun soils occur on the flats and depressional areas, Calloway soils are found on low ridges, and Loring soils occur on slightly higher ridges. Well over half of the study area consists of Calhoun soils, with lesser proportions of Loring and Calloway soils. The soils are generally poorly to somewhat poorly drained, but some of the higher ridges are moderately well-drained. Permeability is slow across most of the area. These soils have low-to-medium natural fertility that has been reduced through many years of use for crop production. Texture in the upper soil horizon at both sites is silt loam. Soil pH is very strongly to medium acid and ranges from 4.5 to 6.0 across both sites. Calloway and Loring soils both contain fragipans at depths of 20 to 30 in. that limit effective rooting depth and hinder tree growth. The loessial soils of the Macon Ridge are somewhat unproductive for the growth of hardwood species, when compared to well-drained alluvial soils or the loessial hills on the east side of the Mississippi River. Broadfoot (1976) reported an average site index of 83 ft at 50 years for water oak on Calloway soils, but did not provide similar information for Calhoun or Loring soils. Based on the method described by Baker and Broadfoot (1979) we estimated that site index for water oak averaged 86 ft at 50 years across the entire study area, but ranged as high as 90 ft on the higher ridges. Treatments In the fall of 1986, the plantations were divided into 12 treatment plots, nine on the larger tract and three on the smaller tract. Each treatment plot was 150 ft by about 400450 ft (45 rows) and covered an area of about 1.4 to 1.5 acres. Three O.l-acre square measurement subplots were systematically established within each of the 12 treatment ’ Paper presented at the Tenth Biennial Southerf Siivicultural Research Conference, Shreveport, LA, February 16-18, 1966. * Principal Silviculturist and Principal Forest Biometrician, USDA Forest Service, Soutbem Research Station, Stoneville, MS 38776, respectively.