Evaluation of the NDVI Index as a Source of Information on Aboveground Phytomass in Steppes

Abstract

The complexity of phytomass measurements on sample plots hinders the extrapolation of plot-related data to areas comparable to land use or landscape units. Vegetation indices calculated from satellite images are usually considered as indicators of green phytomass and are used for its areal estimates. The study solves the problem of establishing the information content of the normalized difference vegetation index NDVI depending on the fractional structure of living and dead aboveground phytomass, seasonal dynamics of the biological cycle, hydrothermal conditions and landscape position. We used the results of monthly measurements of aboveground phytomass fractions at 13 sites covered by Stipa zalesskii and Stipa lessingiana formations in the Burtinskaya steppe (Orenburgsky nature reserve) from May to September in 2015–2020. For each period, NDVI values were calculated from Landsat satellite images at all sites. Hypotheses about geobotanical, hydrothermal, phenological and landscape factors of NDVI informativity were tested by using the Spearman correlation coefficients, analysis of variance and multiple regression. The discrepancy between the seasonal peaks of NDVI and green phytomass is not consistent with the common opinion of a direct indicator value of NDVI. The total live biomass correlates more clearly with the index in June and July but weaker at the end of the season. NDVI turned out to be sensitive not so much to green phytomass as such, but to the mass and proportion of forbs and the ratio of live and dead phytomass. In late spring and early summer, NDVI is most closely associated with forbs while in July – with grasses. The hypothesis about the possibility of screening green mass with standing dead biomass was confirmed, which leads to a decrease in NDVI despite the absence of a decrease in green phytomass. NDVI may underestimate the real green phytomass if there is a sharp increase in the mass of dead biomass, usually in the second half of summer and early autumn. NDVI more adequately reflects the state of the aboveground phytomass of steppe communities that have not been exposed to fires for a long time, compared to burned communities and fallows.