Pecan Tree Biomass Estimates

Allometric equations were developed for orchard-grown pecan [Carya illinoinensis (Wangenh.) C. Koch] trees. Trees, ranging in size from 22 to 33 cm in trunk diameter 1.4 m above the ground, were destructively harvested from two sites. The entire aboveground portion of each tree was harvested and then divided into leaves, current season’s shoots, and branches $1 year old plus trunk. Roots were sampled by digging a trench beginning beneath the trunk and extending to one-half the distance to an adjacent tree, then separating the roots from the soil. Roots were then divided into those less than 1 cm in diameter and those $1 cm in diameter. Equations in the form Y = eX were developed to estimate dry biomass of most tree components and the whole tree, where Y is the dry weight, e is the base of the natural logarithm, X is the trunk diameter at 1.4 m above the ground, and a and b are coefficients. A linear equation provided the best fit for estimating the weight of the current season’s growth. Power equations were also developed to estimate the weights of inner bark and wood for different size trunks or branches. Methods to estimate biomass of either individual trees or tree populations are useful for foresters, ecologists, and scientists. Biomass estimates of tree populations are valuable for such studies as ecosystem productivity, energy and nutrient flows, fire behavior, carbon utilization and sequestering, plus many other applications. Most models for biomass estimation have been developed and used by the forestry or ecology community. These models normally divide aboveground components into bole or main stem, bole bark, and crown (Parresol, 1999). Occasionally a fourth component, below-ground biomass, is estimated. Biomass equations for southern US hardwood and softwood species have been compiled by Clark (1987). More recently, a review of biomass estimators for tree species in the United States has been published (Jenkins et al., 2003), followed by a comprehensive database of biomass regressions for North America tree species (Jenkins et al., 2004). However, no equations have been developed to estimate pecan biomass. Jenkins and colleagues (2004) developed generic equations to estimate above-ground biomass of selected hardwood and softwood groups. They also developed equations to estimate the component ratios of hardwood and softwood species. Sampling protocols for individual tree and stand biomass have been reviewed recently (Snowdon et al., 2002). Techniques to estimate tree and stand biomass have also been reviewed (Catchpole and Wheeler, 1992). Below-ground biomass is typically sampled within a fixed distance from the trunk (Parresol, 2001). In many cases, only coarse roots are included in the analysis (Helmisaari et al., 2002). However, fine-root biomass can account for 18% to 45% of the total tree biomass, depending on species, age, and site (Fogel, 1983; Santantonio et al., 1977). In Scots pine (Pinus sylvestris L.), below-ground biomass accounted for 13% to 25% of the stand’s biomass depending on age, with 2% to 15% consisting of fine roots (Helmisaari et al., 2002). Both coarseand fine-root biomass increased with tree age. Fine roots accounted for 43% to 60% of the stand’s annual biomass production and used 45% to 63% of the tree’s nitrogen (N). Needle production accounted for another 27% to 34% of the N utilization, and the other tree components used 9% to 21%. A study using loblolly pine (Pinus taedaL.) reported that biomass was partitioned as 17% crown, 63% stem, and 20% roots (Van Lear and Kapeluck, 1995). Fine roots accounted for 11% of the root mass, but contained 24% to 30% of the nutrients in the root system. Fine roots and foliage made up only 4% of the stand’s biomass, but had one-fourth of the stand’s N and phosphorus (P). Biomass estimation of individual trees is also useful in horticultural studies, especially those involving mineral nutrition. Three recently published studies on N nutrition of pecan (Acuña–Maldonado et al., 2003; Kraimer et al., 2001, 2004) used equations developed from destructively harvested forest-grown black oak (Quercus velutina Lam.) trees (King and Schnell, 1972) to estimate pecan tree biomass components. Extrapolation of black oak models to pecan trees provides questionable information and their use entails major limitations and substantial error sources. First, and most obvious, is that such equations were not developed to estimate pecan tree biomass, a species exhibiting a different growth form than black oak. One striking difference is that Carya spp. produce about three times the leaf weight as black oak trees (Schnell, 1978). Acuña–Maldonado and associates (2003) developed equations from destructively harvested pecan trees to estimate leaf weight because of the discrepancy in leaf production between black oak and pecan. A second limitation is that equations for relatively shaded forest-grown trees do not accurately estimate the biomass components of relatively nonshaded urban-grown (Nowak, 1994) or open-grown trees. Typically in a commercial pecan orchard, grafted trees are initially trained to a modified leader and are subsequently managed with little or no pruning. These trees obviously exhibit morphological traits considerably different from those of forest-grown, shaded black oaks. The predictive ability of equations to estimate biomass of orchard trees is therefore likely to be most accurate if derived from orchard-grown trees. Another potential limitation of extrapolating existing equations Fig. 1. Typical growth forms of Maramec and Pawnee trees. Received for publication 21 Mar. 2006. Accepted for publication 18 Apr. 2006. Approved for publication by the Oklahoma Agricultural Experiment Station. Regents Professor. To whom reprint requests should be addressed; e-mail [email protected] 1286 HORTSCIENCE VOL. 41(5) AUGUST 2006 JOBNAME: horts 41#5 2006 PAGE: 1 OUTPUT: July 12 03:09:27 2006 tsp/horts/118440/01519