ИСТИНА

Presently, the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) is a widely used index for the study of plant cover. It allows estimating the seasonal and long-term dynamics, since FAPAR-based data analysis is available for multiple satellite sensors. Volga-Akhtuba floodplain (VAF) wetland ecosystems are very sensitive to Volga river runoff, and particularly to spring inundation, which currently depends on Volzhskaya Hydropower plant (HPP) downstream water discharge regulation, as well as on the water natural inflow into the upstream reservoir. The plant communities in VAF are extremely heterogeneous, both spatially and temporally. We previously showed that different types of plant community have its unique features in seasonal vegetation index dynamics, which may have one, two or three local vegetation waves, clearly reflecting the actual in-situ dynamics. The present study of VAF plant ecosystem dynamics focuses on the assessment of vegetation indicators using SeaWiFS and MERIS FAPAR data. The specific FAPAR curves, model functions, and its parameters for 12 test sites represent the most common plant community types in VAF. We compare the period of relatively high Volga river runoff (1998–2005) with the period of relatively low runoff (2007–2011), with the extremely dry 2006 in between them. Our quantitative analysis of vegetation activity bases on a harmonic approximation of FAPAR time series. Instead of discrete data points, it yields a continuous function suitable for further analysis as a model yearly FAPAR curve. This methodology suggests a simple, clear and fairly universal approach to analyzing FAPAR time series. The analysis shows that in most test sites the model indicators changed considerably in 2007–2011, and 2006, as compared to the period of 1998–2005, due to the changes of the Volga river runoff, indicating significant changes in ecosystem structure and species composition. The in-situ observations also confirm these facts. In 2006–2011, the transformation of water meadows into secondary steppe or weed communities proved to be not a local processes, but occurring all over the VAF, being most critical in its upstream segment near the HPP. As follows from our study, the method provides weighted and unbiased quantitative analysis of plant ecosystem dynamics. It also gives a clear estimation of ecosystem health and allows detecting changes in its type for a given territory, e.g. arid wetland degradation under the unfavorable environmental conditions and human impact in our case.