STIPA TENACISSIMA AERIAL BIOMASS ESTIMATED AT REGIONAL-SCALE IN AN ALGERIAN STEPPE, USING GEOSTATISTICAL TOOLS

Data from systematic sampling may be independent or autocorrelated. In the latter case geostatistical tools are used to identify the spatial patterns within the universe sampled. Special formulas have been deri ved by Russo & Bresler (1982) to estimate the variance of a value aver aged over several transect samples. We applied these formulas to the g reen biomass of the dominant perennial steppe grass, Stipa tenacissima or alfa, in a 400 km(2) region in North-West Algeria; thirty years ag o, this region was considered one of the best sites for alfa. A two-le vel sampling design was implemented with stratification of the region and systematic sampling within each stratum; globally the study includ ed fifteen transects, representing 713 1 m(2) quadrats. Autocorrelatio n up to 500 meters was detected in five semi-variograms or correlogram s, which were fitted to linear models with a sill. Biomass averaged on ly 165 (+/- 55) kg ha(-1). We discuss the processes that have lead to the rapid degradation of alfa steppes in northern Algeria and the vari ation in spatial patterns of alfa stands. Ignoring autocorrelation in systematic sampling leads to biased estimates of variances and standar d errors.