Non-destructive dry matter estimation of Alhagi sparsifolia vegetation in a desert oasis of Northwest China

Abstract

Question: Can above-ground biomass of naturally growing Alhagi sparsifolia shrubs be estimated non-destructively? Location: Qira oasis (37degrees01'N, 80degrees48'E, 1365 m a.s.l.) at the southern fringe of the Taklamakan desert, Xinjiang, NW China. Methods: Two methods were compared to estimate above-ground biomass (AGB) of Alhagi. At first shrub AGB was estimated by manual ground measurements (called 'allometric approach') of length, width and height of 50 individuals. Subsequently regression equations were established between calculated shrub canopy volume and shrub AGB (r(2) = 0.96). These equations were used to calculate AGB from manual ground measurements in 20 sample plots within the Alhagi field. Secondly, kite-based colour aerial photography coupled with the use of a Geographic Information System (called 'GIS approach') was tested. First and second order polynomial regressions between AGB data of the 50 individual shrubs and their respective canopy area allowed to automatically calculate the AGB of all remaining shrubs covered by the photograph (r(2) = 0.92 to 0.96). The use of non-linear AGB regression equations required an automatised separation of shrubs growing solitary or in clumps. Separation criteria were the size and shape of shrub canopies. Results: The allometric approach was more reliable but also more time-consuming than the GIS-based approach. The latter led to an overestimation of Alhagi dry matter in densely vegetated areas. However, this systematic error decreased with increasing size of the surveyed area. Future research in this field should focus on improvements of AGB estimates in areas of high shrub density.