Carbon and Nitrogen Stocks in Deadwood and Soil after Mass and Isolated Windthrows in Broad-leaved Forests of the Kaluzhskie Zaseki Nature Reserve

Intact broad-leaved forests were studied as reference ecosystems for developing up-to-date assessments of the capacity of forests to sequester carbon. The main study object was an old-growth forest with isolated windthrows and oaks over 300 years old. We assessed (1) the volumes of deadwood on transects, C and N stocks in deadwood, and (2) C and N stocks in two soil types dominating in the area: Albic Retisols (RT‑ab) and Luvic Phaeozems (PH-lv). Soil sampling was carried out independently of the transects. The contents and stocks of elements, the density and thickness of the humus horizon were compared between the two soil types, as well as beneath and adjacent to fallen trunks of pedunculate oak. Estimates of stocks in the old-growth forest were compared with the same estimates obtained earlier at two plots of mass windthrows located on RT-ab soils and Albic Podzols. The results obtained show that (1) the mean deadwood volume on transects was 182 ± 28.4 m3/ha, more than half of which was represented by oak at the first three stages of decomposition. The C and N stocks in deadwood were 19.2 and 0.2 t/ha. The volume of deadwood in the old-growth forest was 40% lower than in the mass windthrow areas after 15 years. In terms of C stock in deadwood, the old-growth forest differed significantly from only one of the study areas; in terms of N stocks the areas did not differ significantly. (2) The stocks of C and N in the soils of the old-growth forest were significantly higher than in areas of the mass windthrow, primarily due to the greater thickness of the humus horizon. The PH-lv soils did not differ from the RT-ab soils in the term of density, but significantly exceeded them in terms of C and N content, by more than 2.5 times in term of horizon A thickness and by more than 4 times in terms of stocks. The content and stocks of C and N significantly decreased under fallen trunks in the old-growth forest in soils of all types, similar to what was previously observed in the area of mass windthrow on RT-ab soils. A conclusion was made about the importance of the humus horizon thickness parameter, which increases due to the activity of mesofauna and fallen tree uprootings. It is likely that the significant soil C buffer of intact forests is caused more by the presence of tree falls with uprooting than by the presence of deadwood itself.