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Sap flow measurements, from July to August 2004, were coupled with micrometeorological, soil moisture, and soil temperature measurements to analyze forest water dynamics in irrigated and undisturbed (control) larch (Larix cajanderi) forest plots in eastern Siberia. Plots were irrigated with 120 mm (20 mm day -1 ) of water from 17 to 22 July. Sap flow measurements of ten trees at each plot were scaled up to daily stand canopy transpiration (E c ). Canopy transpiration at the irrigation and control plots was similar before irrigation. Forest evapotranspiration (E a ) was obtained from Ohta et al. (Agric For Meteorol 148:1941-1953 while E a in the irrigation plot was estimated based on the E c_irrig /E c_cont ratio. Rainfall during July-August was 63.4 mm but, after including water from thawing soil layers, the actual water input was 109.9 and 218.5 mm in the control and irrigation plots, respectively. Despite this large difference, a corresponding difference in E c (and E a ) was not observed [42.6 (61.5) mm and 46.4 (71.8) mm in control and irrigation plots, respectively]. Daily canopy conductance (g c ) increased as long as moisture was well supplied in the upper soil layers and evaporative demand was high. Soil moisture and rainfall contribution to E a was 36.9 and 24.6 mm in the control plot and 34.5 and 37.3 mm in the irrigation plot, respectively. Water supply from soil thawing layers in the control plot and high runoff (105.6 mm) rates in the irrigation plot accounted for the similarity in water dynamics. Under increased precipitation, the forest used less soil water stored from previous growing seasons.

A future increase in heavy precipitation events is predicted in boreal regions. An irrigation experiment was conducted in Taiga forest in eastern Siberia to evaluate the effect of heavy precipitation on greenhouse gas ([GHG] CO 2 , CH 4 , and N 2 O) fluxes in the soil. The GHG fluxes on the soil surface were measured using a closed-chamber method and GHG production rates in the mineral soil were estimated using the concentration-gradient method based on Fick's law. Irrigation water (20 mm day ) was applied continuously for 6 days (120 mm in total; the same amount as summer precipitation in this region). Greenhouse gas production rates in the organic layer (O-layer) were defined as the difference between the GHG fluxes and the GHG production rates in the mineral soil. Carbon dioxide flux was measured both in root-intact (R s ) and trenched plots (R mw ). The root respiration rate (R r ) was calculated as the difference between R s and R mw . Considering the root distribution in the soil, we regarded the CO 2 production rate in the mineral soil to be the microbial respiration rate in the mineral soil (R mm ) and microbial respiration rate in the O-layer (R mo ) as the difference between R mw and R mm . Irrigation increased both soil temperature and moisture in the irrigated plot. The R s , CH 4 flux and N 2 O flux during the irrigation period were higher in the irrigated plot than that in the non-irrigated plot (P < 0.05; mean R s ± standard deviation [SD] ) were 171 ± 20 and 109 ± 11, mean CH 4 flux ± SD (lg C m )2 h )1 ) were )5.4 ± 4.1 and )14.0 ± 6.5, and mean N 2 O flux ± SD (lg N m )2 h )1) were 1.6 ± 1.6 and 0.2 ± 1.1, respectively). Soil moisture had a positive effect on R mm and CH 4 production rate in the O-layer, a negative effect on R r , and did not affect R mo , the CH 4 production rate in the mineral soil or the N 2 O production rates in both the O-layer and the mineral soil. Soil temperature had a positive effect on R r and R mo . The increment of global warming potential of the soil mainly resulted from an increase in microbial respiration rates. Future changes in precipitation patterns in this region would accelerate decomposition of the soil organic matter.

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