Effects of Warming

There are two major scenarios of climate change in high latitudes (wet and dry) having, nevertheless, opposite effects on DOC export from terrestrial ecosystems.

Increased precipitation under "wet warming" exerts a significant control on the generation of runoff and DOC export from permafrost terrains. Streams draining permafrost-dominated watersheds have a more "flashy" hydrology than those draining permafrost-free watersheds (Woo and Winter 1993). A "flashy" hydro-logic regime is characterized by low baseflows but high stormflows, with a rapid onset following rainfalls (MacLean et al. 1999). Stormflows demonstrate an increase in DOC concentrations (Fig. 16.7b) in high latitude streams, which is indicative for near-surface pathways of runoff generation within catchments. The biochemical composition of stormflow DOC [e.g., aromaticity (specific ultraviolet absorption; SUVA), lignin breakdown products etc.] clearly reflects signatures of forest floor OC, though the magnitude of rainstorms affects the contribution from various soil horizons (Prokushkin et al. 2007). Correspondingly, in wetter climates, more of the runoff is generated from the soil organic layer, resulting in higher concentrations of DOC within streams (Fig. 16.7b,c) and increased overall export of DOC from watersheds.

Wildfires, assumed to be the main disturbance factor in the boreal biome, tend to increase in frequency and severity under drier climatic conditions. In general, fires exert significant control on biogeochemical cycling within watersheds in permafrost terrains. The examination of forested watersheds in Central Siberia has demonstrated that presumably all basins of the region were affected by wildfires in the past. Analysis of DOC fluxes in streams draining basins that were largely affected by fire (>90% of area) revealed a significant decrease of DOC concentrations in streams with recent fire-effects as compared to basins covered with more

O 1902


Fig. 16.8 Dynamics of DOC concentrations in small streams draining watersheds, which were totally burned in 1902, 1950 and 1993

aged forest ecosystems (Fig. 16.8). In terms of flux, DOC output from recently burned watershed in a dry year (2006) was only one fifth of that from watershed burned 100 years ago. Decreased discharge and respectively reduced DOC export may be caused by the larger water-holding capacity of the deepening active soil layer, which occurred after the fire event. Thus, under drier climatic conditions, fires imply two limitations of DOC release from watersheds: (1) decreasing mobile C-source (combustion of organic layer) and (2) free water (increased water-holding capacity of soil). Comparable concentrations of DOC in streams draining watersheds burned 50 and 100 years ago corroborate earlier estimates of a recovery time of 50 years for ecosystem structures (species composition, soil temperature etc.) (Abaimov 2005).

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