Ninth International Conference on Permafrost ... - IARC Research

Pyrogenic Dynamics of Cryosols and Carbon Pools in Open Forests ofNortheast EurasiaN.S. MergelovInstitute of Geography RAS, Moscow, RussiaThere is no sufficient data on fires and postpyrogenicfunctioning of ecosystems at the north tree boundary atthe Kolyma River Lowland (Russia). Our studies carriedout in larch open forests of the northeast part of this region(69°N, 161°E) showed that fires are among the major factorsinfluencing vegetation and soil successions. All loamycryohydromorphic soils (Turbic, Turbi-Saprihistic, Gleyi-Turbic & Endogleyi-Turbi-Histic Cryosols; Gelic Gleysols)formed at watersheds on loess-icy complex sedimentsrepresent various stages of postpyrogenic development.The most mature ecosystem of the study site is old (quasiclimax)larch open forest (~ 200 years old). Postpyrogenicrecovery of vegetation occurs through various nonstableecosystems: highly productive dense larch forest (30–60years old), larch open forest (60 years old), treeless areaswith grass and low shrubs (15–40 years old), and others (Fig.1).The strong fire in combination with absence of seeds fortree reproduction could lead to formation of treeless areaswith low shrub-grassy vegetation, stable to larch reproductionduring at least 30 years. In later stages, larch appears in suchareas forming open woodlands.The trend of postpyrogenic succession depends primarilyon the type (ground or crown fire, ground fires prevail)and intensity of fire. Among others important factors is theavailability of the sufficient amount of viable seeds. In thepresence of surplus amounts of seed, the plant successioncould develop through the stage of highly productive denselarch forest, which subsequently thins out.The trends in transformation of soil cover generally forinitial postpyrogenic stages (increase of the active-layerthickness, increase of soil moisture due to the melting ofice lenses and wedges, intensification of cryoturbations andgleyic processes) differ significantly in the later stages anddepend on direction of postpyrogenic plant succession (Fig.2).Figure 1. Sequence of postpyrogenic succession change.Development of dense larch forest with thick lichen-lowshrub–moss ground cover leads to a more stable state of thesoil cover since thermokarst processes are not so intensive.Active-layer thickness strongly decreases (from 120–140cm to 30–50 cm). Morphological gleyic and cryoturbationfeatures are not strongly expressed in the soil profile. Dueto autoregulation of the ecosystem, dense larch forest thinsout. This process is accompanied by raising of diversity ofthe ground vegetation: the share of mosses decreases whilethe share of lichens increases. The ground vegetation ofmature larch open forest comprises regular low shrub-mossand moss-lichen parcels-polygons. The difference in thermalcapacity of mosses and lichens—parcel-forming plants,leads to heterogeneity in thawing depth, which varies alsoon a regular basis. The average active-layer thickness underlow-shrub moss parcel is 25 cm; under moss-lichen parcelis 45 cm.The treeless areas with low shrub-grassy vegetation even30–40 years after the fire have high active layer thicknessof 90–120 cm, thermokarst processes are still active,and polygonal microtopography is formed. ThermokarstFigure 2. Soil properties alteration in dependence to fire age and/ortype of postpyrogenic succession.● = consecutive postpyrogenic development of larch open forest;ο = development through stage of dense larch forestLegend:Time passed after Postpyrogenic stages of vegetationthe last fire (years) recovery1 Burn without arboreal vegetation2 Burn without arboreal vegetation30 Burn without arboreal vegetation (lowshrub-grassy stage)60 Larch open forest60 Dense larch forest200 Old (quasi-climax) larch open forest211