Pecan is an indigenous nut tree that has been growing over its present range in the United States for at least 8000 years (Hall, 2000). The native habitat of pecan is the mixed-stand riparian forests in the central United States extending into Mexico (Sparks, 2005). Archaeological and ethno-historic data indicated that it was an important food source for people inhabiting areas within its range during prehistoric and early historic times (Hall, 2000). Today, pecans remain a valued food source for a health-conscious public.
Pecans are produced in the United States from both native stands and planted orchards (Reid, 2002). Approximately 30% of pecans marketed in the United States are from native trees (Pollack, 2001), and 90% of those produced in Oklahoma are from native stands (Smith, 2006). Planted pecan orchards are similar to other tree fruit orchards in that they consist of limited genetic diversity (two or more cultivars) with trees planted in selected patterns at a fixed spacing. A unique characteristic of pecan orchards is they are planted with two planned tree removals to prevent excessive crowding during orchard life (Carroll et al., 2006).
Development of a native pecan grove consists of selecting an area with abundant pecan timber, removal of all woody species except pecan, and then thinning excess pecan trees to achieve the desired stand (Reid and Olcott-Reid, 1985). In contrast to a planted orchard, each tree in a native pecan grove is genetically unique. Tree removal to prevent crowding occurs simultaneously with tree replacement (naturally occurring seedlings in desirable locations are left); thus, trees within a native grove vary in size and age, and tree spacings are variable. Another distinctive characteristic of a native grove is that appropriate management allows them to remain productive indefinitely because younger trees seamlessly replace older trees that are damaged or when crowding becomes excessive.
Tree crowding causes stress from belowground competition for water and nutrients and aboveground for sunlight. Smith (1953) suggested that the more important limitation affecting production was shading. Excess shading of lower limbs reduces or eliminates production from affected areas, eventually resulting in limb death. Sparks (2005) pointed out that where pecans are native, soils are typically deep and fertile with excellent water-holding capacity, supporting Smith's belief that shading is frequently the most limiting stress affecting productivity.
Hinrichs (1961), working with native pecan tree stands in central Oklahoma, determined that maximum productivity occurred when tree density was 6.9 m2·ha−1 cross-sectional trunk area (30 ft2/acre cross-sectional trunk area). This result has been used as a guide for managing native tree density throughout Oklahoma (McCraw, 2006). Optimum tree density may be affected by climate, particularly rainfall, and soil characteristics. However, unless a particular input is excessively restrictive such as annual rainfall (Romberg et al., 1959), guidelines developed by Hinrichs (1961) should be suitable when sunlight is the primary input that limits production.
Allometric equations to estimate biomass have been developed for orchard-grown pecan trees (Smith and Wood, 2006). Equations relating trunk size with canopy size characteristics of native pecan trees would be useful to define tree density for maximizing productivity, aid in sprayer calibration for pesticide application, and for research applications. The objective of this study was to develop equations that relate trunk measurements to selected canopy size parameters and then use those equations to elucidate certain canopy characteristics at optimum tree density as defined by Hinrichs (1961).
Andersen, P.C. 1991 Photosynthetic characteristics of pecan and ten species of fruit crops with emphasis on sun tracking/non-sun tracking responses ARS–U.S. Dept. of Agr., Agr. Res. Serv. 96 168 174
Romberg, L.D., Smith, C.L. & Crane, H.L. 1959 Effects of irrigation and tree re-spacing (thinning) on pecan tree growth and nut production Texas Pecan Growers Assn. 38 60 75
Smith, C.L. 1953 Proper spacing of pecan trees for greater yields, in both planted orchards and native groves Proc. Texas Pecan Growers Assn. 32 35 37