Estimates of Net Annual Aboveground Production: Sensitivity to Sampling Frequency

Abstract

We used frequently repeated measurements of growth characteristics in undisturbed study plots, and replicated harvests of shoots from nearby plots, to measure biomass accumulation of Typha latifolia in 1978 and 1979. The data were used to calculate net annual aboveground production (NAAP) using seven computational procedures: peak biomass; the Milner and Hughes; Smalley; Valiela et al.; a modified Wiegert and Evans; Allen curve; and the summed shoot maximum which we devised for this study. The procedures were evaluated with respect to their sensitivity to differences in sampling frequency by repetitively calculating NAAP using different sequential subsets of the study plot and harvesting data sets. Year—to—year differences in the relative accuracy of some methods (peak biomass; Milner and Hughes; Smalley; and Valiela et al.) were affected by cohort structure of the population. The Allen curve NAAP estimates were not affected by the differences in cohort structure. Our modified Wiegert and Evans method overestimated NAAP, and is inappropriate for use with Typha. As apparent sampling frequency declined, NAAP estimates for most methods declined and were more variable, especially for the more complicated methods. Relative to the standard used in this study (the summed shoot maximum), most methods underestimated NAAP by 18—38%, even with 1—3 wk sampling intervals. Underestimates were due to inadequate corrections for early shoot mortality, leaf turnover, and losses of portions of individual leaves. The procedure of estimating each cohort's production by plotting shoot number vs. mean biomass per shoot over time (Allen curve method) has inherent advantages relative to the other methods examined and was comparatively insensitive to changes in sampling frequency.