Estimating leaf area of big sagebrush from measurement of sapwood.

Wyoming big sagebrush (Artemisia tridentata subsp. wyomingem&) plants were destructively sampled in May 1984 in southeast Oregon to examine the applicability of the pipe model theory to an arid land shrub. This theory proposes that total leaf area or leaf biomass of a plant is limited by the amount of conducting tissue available to service the leaves. Basal sapwood areas were measured for 20 Wyoming big sagebrush plants and correlated with the biomass and leaf area of the supported foliage. Additional samples of leaves were collected over a 1Zmonth period to evaluate fluctuations in leaf weight to leaf area relationships. Basal sapwood area of sagebrush stems was a reliable indicator of foliage biomass or leaf area (r2=0.95). Leaf area to weight relationships fluctuated signfficantly throughout the year with the most substantial changes occurring during the growing season. Although this species is an evergreen, the dynamic nature of foliage production and morphology demands intensive seasonal sampling to accurately predict leaf areas. Many aspects of range management and research require estimates of foliage biomass or leaf area. Estimates of biomass are critical to managers charged with allocating forage resources, and researchers investigating the rates of photosynthesis or transpiration must have indices of total leaf area to expand their data to landscape levels. Direct acquisition of such data requires timeconsuming sampling procedures and can be difticult to attain when plants support high densities of small leaves. Several workers have correlated canopy dimensions or expressions of canopy volume of arid land shrubs with the biomass of the supported leaves and twigs (Lyon 1968, Bentley et al. 1970, Rittenhouse and Sneva 1977). Rittenhouse and Sneva (1977) focused on Wyoming big sagebrush and correlated photosynthetic biomass with 2 measures of canopy width and a measure of maximum plant height. Best fit was obtained with log transformations of both dependent and independent variables and models employing combinations of all 3 independent variables. Coefficients of determination (rr) ranged between 0.73 and 0.97. Foresters have needed similar information (Gholz et al. 1979), and within recent years have detected strong linear relationships between stem sapwood area and leaf area or leaf biomass of several species of trees from the Abies, Acer, Castanopsis. Juniperus, Picea, Pinus, Platanus. Pseudotsuga, Sequoia, and Tsuga genera (Dixon 1971, Grier and Waring 1974, Waring et al. 1977, Whitehead 1978, Rogers and Hinckley 1979, Kaufmann and Troendle 1981, Waring et al. 1982, Marchand 1984). Similar relationships have been detected in shrubs of the Acer and Rhododendron genera (Waring et al. 1977). The premise behind this relationship is that total leaf area or leaf biomass is limited by the amount of conducting tissue available to service the plant. Shinozaki et al. (1964a and 1964b) refer to this as the pipe model theory. The objective of this research was to examine the applicability of this relationship to Wyoming big sagebrush (Artemisia tridentata subsp. wyomingensis), a medium stature, broadleaf arid land shrub dominating several habitat types within the Great Basin (Hironaka et al. 1983). Authors are range scientist, U.S. Department of Agriculture-Agricultural Research Service and associate professor, Eastern Oregon Agricultural Research Center, Squaw Butte Station, Star Rt. l-4.51 Hwy. 205, Burns, Oregon 97720. This article is Technical Paper No. 7620. Oregon Agricultural Experiment Station. Manuscript accepted 14 November 1985. Fig. 1. Stem outline and sapwood of a Wyoming big sagebrush plant. 333 Materials and Methods Sapwood to leaf area relationships were based on destructive sampling of 20 shrubs growing on a Wyoming big sagebrushThurber’s needlegrass (Stipa thurberiana) habitat type on the Squaw Butte Experiment Station in southeast Oregon. Phenology at the time of sampling was just prior to stem elongation and early leaf initiation. On 16 May 1984, crown dimensions of shrubs were recorded before harvest. Measurements to the nearest centimeter included the greatest diameter of the crown when viewed from above, a second maximum diameter perpendicular to the first, and the distance from the ground to the tallest actively growing tissue. Canopy openings were not subtracted from any dimension. A sample of leaves was collected from each plant, sealed in a plastic bag, and preserved by freezing for later evaluations of leaf area to leaf weight relationships. Samples ranged from 0.1 to 3.5 g (oven dry) in weight. No attempt was made to sample a constant number of leaves or relate sample size to the dimensions of the plants. After each shrub was cut near ground level, a 2 to 5 cm thick cross section was removed from the lowermost portion of the stem, the basal surface marked, and each cross section sealed in a plastic bag to prevent desiccation. Cut plants were placed in large, open plastic bags and allowed to air dry for approximately 1 month. Leaves were removed from woody portions with the aid of a soft-bristled paint brush, oven dried at 60° C for 48 hours, and weighed to the nearest gram. In each of the following 11 months 20 shrubs were randomly selected and additional samples of leaves gathered. These samples were used to evaluate seasonal fluctuations of the leaf area to leaf weight relationships. Frozen samples were thawed in the lab, the leaves stripped from the stems and sandwiched between 2 sheets of acetate, and leaf areas measured with a Li-Cor 3 100 leaf area meter to the nearest 0.1 mmr. Samples were oven dried for 48-hours and weighed to the nearest milligram on an analytical balance. The lighter colored sapwood of the stem cross sections remained clearly discernible as long as sections remained moist. If a section STEM OUTLINE