The role of driving factors in historical and projected carbon dynamics of upland ecosystems in Alaska

Abstract

Abstract. It is important to understand how upland ecosystems of Alaska, which are estimated to occupy 84% of the state (i.e., 1,237,774 km 2 ), are influencing and will influence statewide carbon (C) dynamics in the face of ongoing climate change. We coupled fire disturbance and biogeochemical models to assess the relative effects of changing atmospheric carbon dioxide (CO 2 ), climate, logging and fire regimes on the historical and future C balance of upland ecosystems for the four main Landscape Conservation Cooperatives (LCCs) of Alaska. At the end of the historical period ) of our analysis, we estimate that upland ecosystems of Alaska store~50 Pg C (with~90% of the C in soils), and gained 3.26 Tg C/yr. Three of the LCCs had gains in total ecosystem C storage, while the Northwest Boreal LCC lost C (À6.01 Tg C/yr) because of increases in fire activity. Carbon exports from logging affected only the North Pacific LCC and represented less than 1% of the state's net primary production (NPP). The analysis for the future time period (2010-2099) consisted of six simulations driven by climate outputs from two climate models for three emission scenarios. Across the climate scenarios, total ecosystem C storage increased between 19.5 and 66.3 Tg C/yr, which represents 3.4% to 11.7% increase in Alaska upland's storage. We conducted additional simulations to attribute these responses to environmental changes. This analysis showed that atmospheric CO 2 fertilization was the main driver of ecosystem C balance. By comparing future simulations with constant and with increasing atmospheric CO 2 , we estimated that the sensitivity of NPP was 4.8% per 100 ppmv, but NPP becomes less sensitive to CO 2 increase throughout the 21st century. Overall, our analyses suggest that the decreasing CO 2 sensitivity of NPP and the increasing sensitivity of heterotrophic respiration to air temperature, in addition to the increase in C loss from wildfires weakens the C sink from upland ecosystems of Alaska and will ultimately lead to a source of CO 2 to the atmosphere beyond 2100. Therefore, we conclude that the increasing regional C sink we estimate for the 21st century will most likely be transitional.

Assessing historical and projected carbon balance of Alaska: A synthesis of results and policy/management implications

“…The estimates of soil and vegetation carbon storage in this study were validated with data independent from those used in model development (see Genet et al. and Lyu et al. ).…”

“…In comparison, Genet et al. () and Lyu et al. () estimate that carbon storage in southeast and south‐central Alaska increased 2.67 Tg C/yr in uplands and 0.07 Tg C/yr in lowlands, respectively, from 1950 to 2009.…”

“…), and syntheses of carbon dynamics across upland ecosystems (Genet et al. ), wetland ecosystems (Lyu et al. ), and inland aquatic ecosystems (Stackpoole et al.…”

“…The upland and wetland analyses were further broken down by regions of the landscape conservation cooperatives (LCCs) in Alaska: (1) the Arctic LCC, (2) the Western Alaska LCC, (3) the Northwest Boreal LCC, and (4) the North Pacific LCC (see Genet et al. : Fig. 1).…”

Drivers of postfire soil organic carbon accumulation in the boreal forest

“…According to our results, increasing the water supply through greater amounts of precipitation could increase mineral soil C stocks through enhanced mineral weathering and leaching, releasing metal oxides that can bind to organic matter (Doetterl et al., ; Mikutta et al., ; Porras et al., ; Rumpel & Kögel‐Knabner, ). Otherwise, the projected increase in fire frequency suggested by models (Kloster & Lasslop, ; Wang et al., ; Wotton, Flannigan, & Marshall, ) could weaken the C capture function of boreal forests (Genet et al., ; Pan et al., ). Integrating direct and indirect effects of abiotic and biotic factors on C storage processes, as presented here through mechanistic models of C dynamics in boreal forest ecosystems, could improve our ability to account for C stocks and anticipate the response of boreal forests to global change.…”