Ecosystem CO_2 and CH_4 exchange in a mixed tundra and a fen within a hydrologically diverse Arctic landscape: 2. Modeled impacts of climate change

摘要

Climate change will have important effects on arctic productivity and greenhouse gas exchange. These changes were projected by the model ecosys under an Special Report on Emissions Scenarios (SRES) A2 scenario over the 21st century for a landscape including an upland tundra and a lowland fen at Daring Lake, NWT. Rising temperatures and precipitation caused increases in active layer depths (ALD) and eventual formation of taliks, particularly in the fen, which were attributed to heat advection from warmer and more intense precipitation and downslope flow. These changes raised net primary productivity from more rapid N mineralization and uptake, driven by more rapid heterotrophic respiration and increasing deciduous versus evergreen plant functional types. Consequently, gains in net ecosystem productivity (NEP) of 29 and 10 g Cm~(-2) yr~(-1) were modeled in the tundra and fen after 90 years. However, CH_4 emissions modeled from the fen rose sharply from direct effects of increasing soil temperatures and greater ALD on fermenter and methanogenic populations and from indirect effects of increasing sedge growth, which hastened transfer of CH_4 through porous roots to the atmosphere. After 90 years, landscape CH_4 emissions increased from 1.1 to 5.2 g Cm~(-2) yr~(-1) while landscape NEP increased from 34 to 46 g Cm~(-2) yr~(-1). Positive feedback to radiative forcing from increases in CH_4 emissions more than offset negative feedback from increases in NEP. This feedback was largely attributed to rises in CH_4 emission caused by heat advection from increasing precipitation, the impacts of which require greater attention in arctic climate change studies.