New applications for point intercept methods: Replacing manual sorting of current-year herbaceous biomass

Productivity is a primary indicator when assessing plant communities or their responses to various stimuli, but it can be difficult and costly to determine. Adding to this complication is the retention of standing litter from previous years’ growth intermingled with current-year biomass. The most accurate and commonly applied method for estimating productivity is to clip and manually sort vegetation to separate current-year and previous years’ growth. Because sorting is costly, it is often only done for subsamples or not done at all. Without accounting for previous years’ growth, productivity estimates will be biased upward for conditions that accumulate more standing dead material, which is problematic when comparing treatments such as fire or different levels of herbivory. We tested whether line-point intercept methods could be adapted to determine proportions of current-year and older vegetation and estimate current-year biomass directly or by calculating the product of percent current-year intercepts and total standing crop. Data were collected by seven observers across four years and five experiments in northern mixed prairie using a total of 306 plots. Point-intercept transects with 20-cm spacing between points were used to determine number of intercepts for current-year and older vegetation and compared with data derived by clipping, manually sorting and weighing herbaceous vegetation. Regression analyses were used to test relationships between 1) total standing crop and total point intercepts; 2) current-year biomass and current-year point intercepts; 3) percent current-year biomass based on manual sorting versus point intercept methods; and 4) sorted current-year biomass and point intercept-derived current-year biomass. Number of intercepts per 100 points was positively related to total standing crop and current-year biomass, but r2 values were only 0.49 and 0.53. The relationship between manually-sorted current-year biomass and that derived from the product of point-intercept percentages and total standing crop was positive, explained 96 % of the variation, and had a mean absolute error of 122 kg ha−1. Point-intercept sampling is more efficient than manual sorting and can provide more indicators, including cover and diversity. Data indicate point intercept methods are accurate and can replace manual sorting of current-year herbaceous biomass.