Aboveground Carbon Stocks and Soil Carbon Pools in Sub-Mediterranean Ecosystems of the Black Sea Coast of Russia: A Descriptive Case Study of Two Contrasting Plots

Sub-Mediterranean ecosystems along the northeastern Black Sea coast remain insufficiently studied despite their importance for the regional carbon cycle and their sensitivity to human disturbance. This study presents an exploratory case study of aboveground biomass carbon stocks in two contrasting plant communities at the carbon-monitoring supersite of the Southern Branch of the Shirshov Institute of Oceanology RAS in Gelendzhik, Russia. The aboveground observations were interpreted together with soil carbon stocks and greenhouse gas fluxes previously reported for the same permanent plots. The investigated communities comprised a terraced pine–oak plantation (SP1) and a natural pubescent-oak shiblyak (SP2). Vegetation was surveyed using the Braun-Blanquet approach. Tree biomass was estimated using the generalized allometric equation of Chave et al., while aboveground biomass was converted to carbon using carbon concentrations determined by elemental analysis of sampled plant tissues. Herbaceous vegetation and forest-floor material were quantified by elemental analysis. Within-plot spatial heterogeneity was visualized using Python-based radial-basis-function (RBF) interpolation for exploratory mapping of local carbon distribution. Because each vegetation type was represented by a single 400 m2 plot, all comparisons are site-specific and exploratory and should not be extrapolated beyond the investigated communities. The aboveground carbon density was 64.1 t C ha−1 in the terraced pine–oak stand and 16.1 t C ha−1 in the natural shiblyak. Woody biomass dominated the plantation, whereas litter and herbaceous vegetation contributed proportionally more to the carbon pool in the shiblyak. Previously published soil organic carbon stocks at the terraced site exceeded the measured aboveground carbon pool, whereas the natural rendzinas were characterized by substantial carbonate-associated inorganic carbon stocks. C/N ratios of organic substrates ranged from 11 to 100, indicating a pronounced variation in substrate quality: higher C/N values in woody and coniferous fractions were consistent with slower decomposition, whereas lower C/N values in herbaceous material were consistent with faster mineralization. Exploratory spatial mapping revealed within-plot variation in aboveground carbon of up to two orders of magnitude. Together, these site-specific observations provide the first integrated description of aboveground biomass carbon together with previously reported soil carbon pools for contrasting sub-Mediterranean vegetation on the northeastern Black Sea coast. Despite its exploratory nature, this study establishes a baseline for future replicated, spatially explicit investigations of ecosystem carbon dynamics in sub-Mediterranean ecosystems along the northeastern Black Sea coast.