Immature Loblolly Pine Growth and Biomass Accumulation: Correlations with Seedlings Initial First-Order Lateral Roots

Five to seven years after being graded by first-order lateral root (FOLR) numbers and outplanted, loblolly pine (Pinus taeda L.) seedlings were excavated using a commercial tree spade and root systems re-evaluated. Current competitive position of trees was related to initial FOLR numbers of I-O seedlings. Current FOLR numbers were comparable among tree size classes, but root diameters where the spade severed the root were different. The dominant and codominant individuals had much larger FOLR cross sectional area at the severe d point. The larger diameter laterals allow exploration of larger soil volume since they extended greater distances from the tree. Root biomass allometric equations were developed from excavating 175 individuals in 3 separate plantations. Root biomass was read ily predicted based on either stem diameter breast height squared (D2H), or total aboveground biomass. Approximately 75% of standing tree biomass was aboveground and 25% belowgroundfor all initial root grades, current crown classes, and sites. Subsoil compaction layers appeared to have a major impact on tree developmentatany specijic location within a plantation. Compaction layers affected heights and diameters but not root/top ratios or the relative competition position based on initial FOLR numbers. These compaction layers resulted in plate-like taproots that suggested furth er root penetration was unlikely. South. .I. Appl. For. 22(2):117-123. A series of nursery studies was conducted from 1984-1990 by the USDA Forest Service’s Institute of Tree/Root Biology (ITRB) in Athens, Georgia, to determine the heritability values for first-order lateral roots (FOLR) of half-sib loblolly pine (Pinus taeda L.) progeny. Seed from approximately 150 individual mother trees from the Georgia Forestry Commission’s seed orchard was used for this research. These nursery studies clearly demonstrated that development of FOLR was under considerable genetic control, with family means heritability (h2) values ranging from 0.60 to 0.77 (Kormaniket al. 1990, 1991). These studies also showed that seedling competitive status in the nursery was highly correlated with FOLR numbers, even when different nursery protocols were used to grow seedlings (Kormaniket al. 1990, 1991). When inferior stem characteristics repeatedly appeared among half-sib progeny having low FOLR numbers in No=: Manuscript received May 20, 1996, accepted May 21, 1997. Paul Kormanik is the corresponding author and can be reached at (706) 546-2435; Fax: (706) 546-2 143. This research was funded by U.S. DepartmentofEnergy Grant DE-A109-76SROO870 and by the Georgia Forestry Commission. our nursery heritability tests (Kormanik et al. 1990, 1991), the obvious question was whether the early nursery performance of individuals could be related to performance after outplanting. The importance of this issue was reinforced by Grigsby (1971) who reported that inferior type 1 seedlings, which were similar to our inferior seedlings, produced only about one-half the stem volume as the best two grades. In our long-term studies, the effect of initial FOLR numbers on subsequent growth has been substantial; however, some anomalies have occurred that have been difficult to reconcile based on initial root assessment and actual growth of individuals at specific locations within the plantations. Our current excavation research was designed to clarify variability in performance among and within seedling grades when they are outplanted under variable edaphic conditions. Traditionally, the primary seedling characteristics used for grading loblolly pine for outplanting has been root collar diameter (RCD) and height (HGT), with lesser emphasis on bud characteristics and needle fascicle maturity (Grigsby 1971, May 1984, Wakeley 1954, 1969). Characteristically, Reprinted from the Southern Journal of Applied Forestry, Vol. 22, No. 2, May 1998. Not for further reproduction. 117 seedlings from the higher grades outperformed those from lesser morphological grades. However, in spite of various refinements and modifications in grading standards (Dorman 1976) variation in volume production among individual trees exist both within and among grades and progeny. In general, this response has also emerged in these long-term tests, which raises questions regarding the value of grading nursery stock for artificial regeneration of forest stands. Root biomass has been studied in an attempt to explain variation in aboveground biomass. Root biomass estimates using allometric relationships and regression methodology to develop prediction models have been obtained from a few excavated trees in older stands (Pehl et al. 1984, Van Lear et al. 1984, Van Lear and Kapeluck 1995). In younger plantations, seedling excavation studies have shown significant correlations between aboveground and belowground biomass. Many such studies have been reported using seedlings of differing morphological stem grades, but few have included intensive long-term serial measurements of the same individual (Dorman 1976, Grigsby 197 1, Mexal and Burton 1978). There has been little research performed on using seedling initial root morphological characteristics as a major grading criterion. The purpose of this study was: (1) to determine the relationships between initial seedling FOLR number, subsequent root morphology several years after outplanting, and total standing biomass of immature sized loblolly pine and (2) to develop allometric equations for root biomass derived from stem measurements. Materials and Methods Plantation History Between 1987 and 1989, half-sib loblolly pine seedlings from 78 mother trees in the nursery trials for FOLR heritability were used to establish long-term studies at the Department of Energy’s (DOE) Savannah River Site located near Aiken, South Carolina. Over 15,000 seedlings were measured for RCD, HGT, and FOLR numbers, tagged, and machineplanted in three plantations at densities of 1500-1625 seedlings/ha. The number of graded seedlings planted were 4400, 4800, and 6600, respectively, for 1987, 1988, and 1989. In addition to number of FOLR, the individually tagged seedlings were placed in four vigor classes based primarily on FOLR numbers and their competitive status in the nursery beds (Kormaniket al. 1990, 1991). In the 1987 planting, low vigor seedlings were those with three or fewer FOLR and progressively higher vigor classes have FOLR numbers in the ranges of 4 to 5,6 to 7, and 8 or more. The vigor classes for seedlings outplanted in 1988 and 1989 were produced with a modified nursery protocol (Kormanik et al. 1992). Low vigor seedlings outplanted during these 2 yr had two or fewer FOLR numbers with progressively higher vigor classes having FOLR numbers in the ranges 3-5,6-7, and 8 or more. Site Description and Site Preparation All three plantations were established after naturally regenerated mixed pine/hardwood stands had been harvested from this upper Coastal Plain site. Pines were originally the predominant trees in these stands that had developed after land acquisition in the mid1940s. All stands were relatively flat with slopes of 2% or less. The soil in the youngest plantation 1 (age 5) was a Dothan series classified as fineloamy, siliceous, thermic Plinthic Paleudults. In plantation 2 (age 6) the soils were a gradation between the Orangeburg series and the Lucy series with Lucy sand being the primary soil at the sampling locations. The Orangeburg soils are classified as fine-loamy, siliceous, thermic Typic Paleudults while the Lucy soils are loamy, siliceous, thermic Arenic Paleudults. In plantation 3 (age 7), the soils were of Vauclose Series which are classified as fine-loamy, siliceous thermic Typic Hapludults. Soils in the plantations have been all classified as suitable for forestry with loblolly pine being the preferred species (USDA 1990). A distinct plow layer, characteristic of abandoned farmlands in the Piedmont and upper Coastal Plain soils, was present in plantations 1 and 3. No such plow layer was evident in the primarily Lucy soils of plantation 2 as this plantation had essentially unrestricted root penetration. Before the plantations were established, standing residuals were felled, the sites root raked, and the debris put into windrows approximately 100 m apart. A single post-planting treatment applied to these plantations was banded (30 cm) herbicidal application of glyphosate in July following outplanting. Tree Selection To determine how a tree’s competitive position related to both initial and current FOLR numbers, a group of three trees was selected for excavation from 14,18, and 15 locations in each of the three plantations, respectively. Tree selection was based upon the smallest, average, and largest tree dbh determined by measuring 25-30 adjacent trees at each excavation point. No gaps or missing trees w e re permitted adjacent to specific sampled trees. Preliminary soil profile examinations were undertaken to assure visually uniform soil characteristics at a specific sampling location. Few locations larger than 0.12 ha were found in any plantation that could be easily approached with the tree spade while maintaining the soil profile and tree constraints we wanted to maintain within any group of three trees. These excavation points were selected based on uniformity at site and stand conditions. Border trees that had to be removed to enter the stand were sampled to compare root relationships between border and inner row individuals. Tree Excavation A commercial tree spade that excavated a cone 1.22 m diameter x 1.37 m deep was used to extract 175 individual trees from the three plantations. A total of 50, 78, and 47 trees including border trees were excavated and evaluated from the three plantations. The different number excavated among the plantations is a reflection of the site variability and logistics of moving the large tree spade within specific plantations. Coarse root biomass (kg), ht (m), dbh (cm), aboveground biomass (kg), FOLR number, FOLR diameters (cm) at the point severed by the tree spade, depth (cm) of surface FOLR, and total root depth (cm) were obtained for each excavated tree. 118 SJAF 22(2) 19 9 8 Biomass Determinations Aboveground biomass was expressed as stem green weight obtained immediately after felling. For each tree, all fine and coarse roots were obtained from within the cone. On approximately one-third of these trees, all lateral roots to a 2-3 mm diameter in the soil outside the cone were excavated. Fine feeder roots less than 2 mm were sampled if they were attached to an excavated root. These root samples were washed of adhering soil and weighed to obtain root biomass. The allometric regression models for total root biomass were based on the trees where root data were obtained from within and outside the cone. Total root biomass was defined as the sum of these two root weights. We did not measure total root biomass outside the cone because feeder roots represent only a small percentage of the standing tree total biomass and a secondary consideration was that it was not practical to know exactly from which individual tree the sample originated. Statistical Analysis We computed descriptive statistics for aboveground and belowground components for trees within the plantations and the border trees. The aboveground components analyzed were dbh, ht, D2H, and total top biomass. The belowground components consisted ofFOLR number, surface FOLR depth, total root depth, cone root biomass, and total root biomass. FOLR cross-sectional area for a given tree was computed as the sum of the cross-section areas of each FOLR severed by the tree spade. The root/shoot ratio was defined as the ratio of belowground root biomass to total aboveground biomass, with the assumption that the small percentage of fine feeder roots not recovered was negligible. Differences in aboveground and belowground characteristics for border and inner plantation trees were compared using a two-sample t-test at the 0.05 significance level. This test indicated whether we could pool these data for further analysis. Correlation coefficients calculated between aboveground and belowground components were computed. Allometric relationships were developed between aboveground and belowground components. The typical loglog model was fitted to help linearize the relationship and ensure homogeneity of the variance. After parameter estimation, the log-log model was transformed back to arithmetic units using the bias correction of Baskerville (1972). Tree biomass distributions for top and root cone components were developed for each stand. Results and Discussion Characteristics and Relationships for Aboveground and Belowground Components No significant differences were found between inner and border tree characteristics for any of the stands except number of FOLR in the youngest plantation (Table 1). Thus, we pooled the inner and border trees to increase the sample size for further analyses. Basically, the aboveground and belowground components varied by soil conditions as much as by plantation age. FOLR depths (inner and border individuals combined) in the plantations were similar to each other. The FOLR depths observed in the Piedmont plateau and upper Coastal Plain soils are related to the depth of the plow layer. The old plow layer zone is characteristically the most friable, contains the most organic matter and has the greatest concentration of higher order roots. Lateral roots Table 1. Means of aboveground and belowground characteristics for the inner and border trees in each of the three loblolly pine plantations.’ Characteristic Plantation 1 (age) 5 Plantation 2 (age) 6 Plantation 3 (age) 7 Inner Border All Inner Border All Inner Border All (n = 45) (n = 5) (n = 50) (n = 54) (n = 24) (n = 78) (n = 40) (n = 7) (n = 47) Aboveground characteristics Dbh (cm) 7.8 Height (m) 4.80 D ’H (m ‘) 0.0338 Total top biomass (kg) 28. I Belowground characteristics Number of FOLR 13.6 FOLR cross section 49.6 area (cm ‘) FOLR depth (cm) 20.5 Total root depth (cm) 79.8 Root biomass (kg) 9.6’ Proportion of total tree 0.25’ biomass in roots Cone root biomass (kg) 7.3 Proportion of root o.792 biomass in cone Root/shoot ratio ’ No significant differences between inner and border trees within a stand were detected for any characteristic except number of FOLR (first-order lateral roots) in plantation 1. 6.2 7.7 7.8 7.2 7.6 4.40 4.76 5.29 4.83 5.15 0.0193 0.0324 0.0357 0.0301 0.0340 16.7 27.0 25.2 21.7 24. I