An evaluation of beta attenuation for estimating aboveground biomass in a tallgrass prairie.

The attenuation of beta particles by vegetation was evaluated as a nondestructive method for estimating aboveground biomass in tallgrass prairie in northtast Kansas. Regression equations using the sum of beta attenuation measurements for each of 5 height classes within the vegetation and mean midday leafwater potential as the independent variables were ustd to predict live and total biomass. Live and total biomass were better predicted on burned (r2z.91 and .88, respeetivtly) than unburned sites (P=.71 and .70, respectively). Greater variability in the relationship between beta attenuation and biomass in unburned prairie was a result of the lPrgt and variable amount of dead biomass on unburned sites. Dead biomass was poorly predleted by beta attenuation (+.24 .49). Beta attenuation predicted biomass in burned tallgrass prairie within 315% of harvest values until late Season vegetative stntscenct. In unburned prairie, prtdictions were poorer, but the technique could still be ustful if the required accuracy need be only f25%. Measurement of aboveground biomass by clipping and weighing the vegetation is labor intensive and destroys the plot for subsequent measurements for at least a year. These disadvantages prompted t&e development of several nondestructive techniques [see Tucker (1980) for review]. Due to the success of Teare et al. (1966), Mitchell (1972), and Johnson et al. (1976) with the beta attenuation technique in herbaceous vegetation, we evaluated beta attenuation as a nondestructive method for estimating aboveground biomass in a tallgrass prairie. This evaluation was accomplished by deriving regression equations from calibration plots and testing their predictive value. Material and Methods Research was conducted on the Konza Prairie Research Natural Area (KPRNA) near Manhattan, Kans. The vegetation is characteristic of bluestem prairie (Bailey 1980). Sites burned annually (late April) or left unburned were used in this study. None of the sites have been grazed recently, nor had the unburned sites been burned for 5 years. The soil at the study sites is a moderately deep Reading silt loam (Typic Argiudolls; Jantz et al. 1975). The apparatus constructed to measure beta attenuation consisted of 4 parts: a beta particle source, a Geiger-Mueller (GM) tube, a scaler, and a mounting frame. The beta source consisted of 10 capsules (l-cm diameter) containing 10 microcuries of strontium% each. A side-window GM tube was placed 50 cm from the source. The GM tube was connected to a Ludlum model 2000 scaler to record the number of beta particles received by the GM tube. The beta source, GM tube, and scaler were mounted on a rigid frame with a moving section that held the GM tube at a fixed horizontal distance from the beta source, but permitted the tube and source to be moved vertically. Five vertical positions, repreAuthors are research associates and professor of plant ecology, Division of Biology, Kansas State Universitv, Manhattan 66506. and Department of Botany. University of Wyoming, Laramie, dY 8207 1. Research was suoaorted bv NSF arant DEB 80-12 166. R. Sherwood. G. Radke, R. Rogers, C. McAfe&*J. Bog&h, S. Peterson, D. Loring, R. Peak, R. Ramundo, and T. Seastedt provided valuable field assistance. E. Finck, E. Evans, and M. Gurtz assisted in data analysis. G.R. Marzolf and C.E. Owensby reviewed an earlier version of this manuscript and comments by W.K. Lauenroth wereappreciated. Raw data and supporting documentation are stored in the Konza Prairie Research Natural Area data bank (data set code = PBAOI). 556 senting 5 height classes were used in this study (O-lO, lO-20,20-30, 30-40, and 40-50 cm). This height stratification allowed the integration of the density of vegetation over the height of the stand (Teare et al. 1966). Further details on the construction and operation of the frame are available from L.C. Hulbert (Director, Konza Prairie Research Natural Area). Twenty 20XSO-cm plots, systematically located along 50-m transects, were sampled in burned and unburned tallgrass prairie at 2-week intervals from late-May to mid-September (1983). Four beta attenuation measurements were taken at S-cm intervals at each of the 5 heights in each plot (total q 20/plot). Vegetation in each plot was clipped at ground level, sorted into live and dead biomass, oven-dried, and weighed. After the 20 plots were clipped, additional beta attenuation measurements were made at 40 points along an adjacent transect. These data were used to test the accuracy of the predictive equation developed from the clipped plot data. Also, during the 1982 and 1983 growing seasons, a comparison of predicted biomass values with clipped plot data was made in nearby sites which had either shallower or deeper soils. Two to four times during a daily sampling period, 10 replications of beta attenuation were measured at each height class in an area devoid of vegetation. These measurements were used to correct vegetation measurements for daily and seasonal differences in absolute humidity and electronic drift. Attenuation was calculated using the following equation: T:T Beta Attenuation = I ~ Ti;‘i where T? < is the number of beta particles transmitted through the vegetation (in 6 sec.) at a given height (htS and Ti? c is the number of beta particles transmitted through the air. Leaf water potential (Yld) was measured concurrently with beta attenuation hourly on a clear day late in the 1982 growing season to determine if diurnal variations in the water relations of the vegetation influenced beta attenuation. Measurements of Yluf were made with a Scholandertype pressure chamber on 5-7 fully expanded leaves of big bluestern (Andropogon gerardii Vitman), the dominant species in the study area. Additionally, midday Y1-r (1500 hr CDT) was measured for 5-7 big bluestem leaves each week throughout the 1983 growing season to examine the relationship between seasonal variation in Y\~l*at nd beta attenuation. Regression equations were developed with total, live and dead biomass as the dependent variables and the sum of the beta attenuation measurements for each of the 5 height classes (Xatt) and the mean midday Yl,r preceding the sampling date (X Y1.d) as independent variables. Data transformations and model fitting followed the techniques described by Neter and Wasserman (1974). Results and Discussion The relationship between aboveground biomass and Watt was exponential as reported by Teare et al. (1966) and Mitchell (1972). Because of heteroscedasticity of variance in data sets from burned and unburned sites, a log/log transformation was necessary to meet the assumptions of a linear model (ln(x+ 1); Zar 1974). Several combinations of the height class variables were used but Catt produced the highest coefficient of determination. JOURNAL OF RANGE MANAGEMENT 38(6), November 1985 Two aspects of the relationship between the moisture content of the vegetation and beta attenuation were evaluated: diurnal and seasonal changes in leaf water potential. When the diurnal course of Yi,tand beta attenuation were monitored concurrently (Fig. 1)