The Sprouting Potential of Loblolly and Shortleaf Pines: Implications for Seedling Recovery from Top Damage

Loblolly (Pinus taeda L.) and shortleaf (P. echinata Mill.) pine seedlings are frequently top damaged during their frst few years by browsing animals, insects, or forestry operations, but little quantitative information exists on the factors affecting recovery. Thus, we conducted two separate studies to evaluate potential recovery of seedlings from top damage under controlled conditions. The first study evaluated the effect of season of bum on recovery of I-year-old shortleaf pine seedlings. Although 99% of the seedlings were topkilled by the fires, survival of sprouting rootstocks exceeded 95% the year following the winter burn. However, no seedlings sprouted following the summer burn. Results indicate that winter prescribed fires have considerable potential in establishing natural shortleaf pine regeneration. The second study evaluated the effects of the extent and season of simulated browse damage on the recovery 1-year-old loblolly pine seedlings. Seedlings were clipped in both winter and spring at five positions: at the midpoint between the root collar and cotyledons, and so that 25, 50,75, and 100% of the height between the cotyledons and the tenninal remained after clipping. All seedlings clipped below the cotyledons died. Survival at 3 years for seedlings clipped above the cotyledons was 97% following winter clipping and 96% following spring clipping. Seedlings clipped in winter were larger at 3 years than those clipped during spring. Results of this study suggest that planting loblolly pine seedlings deep, with the cotyledons below ground level, may be an advantage in areas where browse damage is common. INTRODUCTION Loblolly and shortleaf pines (Pinus taeda L. and P. echinata Mill., respectively) are the most commercially important tree species in Arkansas, and stands may be successfully regenerated by either natural or artificial methods. In regenerated stands, pine seedlings are frequently top damaged by a wide variety of animals (rabbits (Sylvilagus spp.), white-tailed deer (Odocoileus virginianus Zimm.), and domestic livestock), insects (e.g., tip moths (Rhyacionia spp.)), forestry operations (harvesting, site preparation, release, and prescribed fire), and wild fire. Seedlings in natural stands may be at greater risk than in plantations, because they remain in size classes vulnerable to browsing for a longer period of time, and natural reproduction cutting methods frequently involve harvesting sawlogs in the vicinity of seedlings. Recovery ftom top damage depends on a species' sprouting ability. Although anecdotal observations indicate that the potential for recovery of both loblolly and shortleaf pine seedlings is good, there is little quantitative information on the factors affecting the recovery process. Thus, we conducted two separate studies over the last 5 years to evaluate potential recovery of seedlings from top damage under controlled conditions. The first study evaluated the effect of season of burn on the recovery of I -year-old shortleaf pine seedlings (Cain and Shelton 2000). The second study evaluated the effects of the extent and season of simulated browse damage on the recovery I-year-old loblolly pine seedlings (Shelton and Cain 2002). In this paper, we summarize results from the shortleaf pine study and update the loblolly pine study with an additional year of field measurements. METHODS Study Site-Both studies were located at the same study site on forest lands of the School of Forest Resources, University of Arkansas at Monticello. Elevation is 320 feet with a rolling topography. The soil is a Sacul loam (clayey, mixed, thermic, Aquic Hapludult), which is a moderately well-drained upland soil with a site index of 78 feet for lobIolly and shortleaf pines at 50 years (Larance et al. 1976). The growing season is about 240 days with seasonal extremes being wet winters and dry autumns. Annual precipitation averages 53 inches. The study site was a 0.2-acre cleared area at the edge of a 10year-old, naturally regenerated loblolly-shortleaf Research Forester and Research Forester (retired), respectively. USDA Forest Service, Southern Research Station, P.O. Box 35 16, Monticello, AR 7 1656-35 16. pine stand. The area occasionally received shadows from adjacent trees during the winter months but was in full sunlight during summer. The Shortleaf Pine Study-After stratification, seeds collected in 1996 were sown in containers with a 1: 1 peat-vermiculite mixture on December 3 1, 1996. Seedlings were grown indoors under 10 hours of full-spectrum fluorescent Iight from January to March 1997. In early April 1997, seedlings were transplanted to field beds at a density of 2 1 per bed. Each of the nine 6by 7foot beds was assigned one of three treatments: control, where no burning was done; summer bum on August 19, 1997; and winter burn on January 3 1, 1998. A pine forest floor was reconstructed within each bed using litter collected from a nearby stand. All burns were simulated headfires with wind provided by three electric box-fans. Seedling height was measured before burning. After burning, crown scorch was ocularly estimated. At the end of the 1998 growing season, groundline diameter and height were measured for the largest sprout from each seedling. Additionally, all sprouts were clipped at groundline, and their weight was determined on an oven-dry basis. Data were analyzed using analysis of variance for a randomized complete block design. This analysis differed from the overall study presented in Cain and Shelton (2000), which involved two oak species in addition to shortleaf pine. The blocking factor was position along the site's slope. Significance was accepted at P ~ 0 . 0 5 . The Loblolly Pine Study-Seedlings were grown in eight 6by 7-foot beds. After stratification, seeds collected in 1998 were lightly pressed into the soil at an 8by 8-inch spacing in late March 1999. At the end of the first growing season, seedlings were numbered using aluminum tags, and measured for groundline diameter, total height, and height to the cotyledons. Before applying treatments, the eight beds were assigned within four blocks based on seedling height, and then the blocked beds were randomly assigned to winter and spring treatments. The clipping that simulated browse damage was applied on February 15, 2000 for the winter treatment beds and April 4, 2000 for the spring treatment beds. Five clipping treatments were randomly imposed within each bed: at one-half the distance between the root collar and cotyledons, and to retain 25, 50,75, and 100% of the height from the cotyledons to the terminal's winter position. Living seedlings were remeasured for height and diameter in November 200 1 when they were 3-years old. In addition, seedlings were evaluated for multiple stems (when one or more stems were within 10% of the seedling's tallest stem) and damage to the terminal or associated buds by tip moths. Nonlinear regression (SAS Institute 1988) was used to predict the third-year height of individual seedlings from their first-year height; the percent of height above the cotyledons remaining after clipping; and the season of clipping, which was entered as an indicator variable. Second and third order interactions of independent variables were also tested in the full model. Variables were retained in equations if their regression coefficient significantly differed from zero at