250 years of disturbance dynamics in a pristine old-growth Picea abies forest in Arkhangelsk region, north-western Russia : a dendrochronological reconstruction
Abstract: To describe the historical patterns of natural disturbance regimes in European borealforests we conducted a dendroecological study in a primeval old-growth sprucedominated stands and reconstructed the long-term dynamics of canopy disturbances.We treated the radial growth releases of individual sub-canopy trees as the majorindicator of sudden openings in the forest canopy. Growth releases were detected byusing the formal approach only (strict criteria were applied). The reconstruction ofpast canopy disturbances was done by GIS-based analysis of spatial information onreleased trees over the study period.The study area was located in the transitional vegetation zone of the middle andnorthern taiga, on the watershed of Northern Dvina and Pinega rivers, North-WesternRussia.Spatial and temporal characteristics of canopy disturbances were studied within thearea of 1.8 ha along the two transects, 20x450 m2 each. All trees with DBH > 6 cm(dead and alive) and coarse woody debris (DBH > 18 cm) within transects weremapped and described (n = 2126) and all living and recently dead trees were sampledwith an increment corer (n = 1678) at the height 40 cm above the root collar.Stands were composed of Picea abies and Betula pendula, mean standing volume was211 m3/ha.Spruce was of multiple ages with pronounced regular peaks (cohorts) in trees agedistribution. At least four such cohorts were distinguished and represented peaks inspruce regeneration.No evidence of stand replacing events was found over the 250(300)-year period thatthe study covered. The dynamics was likely driven by small and middle-size canopydisturbances, occurring at varying frequencies. A detailed spatial disturbancesreconstruction reflecting the last 170 (160) years revealed a disturbance rate of about4% yearly mortality. Periodic increases in disturbance rate however played a majorrole in forest regeneration. Four such peaks were timed to decades 1850, 1890, 1930(1920 – for transect 1) and 1980. Disturbance rates at peaking decades were about60% and never exceeded 75% of the canopy area.Surprisingly little difference in disturbance rates was found among forest types withdifferent soil moisture. Though forests with higher soil moisture had slightly lowerdisturbance intensity and less pronounced regular peaks in disturbance rate overconsidered time span.
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