Importance of Recent Shifts in Soil Thermal Dynamics on Growing Season Length, Productivity, and Carbon Sequestration in Terrestrial High-Latitude Ecosystems E.S. Euskirchen, A.D. McGuire, D.W. Kicklighter, Q. Zhuang, J.S. Clein, R.J. Dargaville, D.G. Dye, J.S. Kimball, K.C. McDonald, J.M. Melillo, V.E. Romanovsky, N.V. Smith ICDC7, Broomfield, CO, September 2005

(Serreze et al., Climatic Change, 2000) High Latitude Temperature Trends ( ) Annual data °C per decade

Spring: beginning of the growing season: Increasing temperature and light availability The snow melts Thawing of soil organic horizons Onset of photosynthesis Fall: end of growing season: Temperatures and light availability decrease Soils re-freeze Photosynthesis slows or ceases

Net ecosystem productivity could increase or decrease in response to changes in soil freeze-thaw regimes. Increases would be due to a longer growing season. However, enhanced productivity could be counter- balanced by increases in respiration from the soil heterotrophs.

The recent availability of remotely sensed spatially explicit data from high-latitudes provides an opportunity to evaluate if a large-scale process-based model captures changes in snow cover, soil freeze-thaw regimes, and growing season length. Satellite detection of recent changes in timing of pan-arctic spring thaw (K.C McDonald et al., Earth Interactions, 2004) Earlier thaw Later thaw Change in Day of Thaw (Days/Year) Pan-Arctic Growing Season Change

What are the implications of recent observed changes in snow cover, soil freeze-thaw regimes, and the timing and length of the growing season on terrestrial carbon dynamics, both retrospectively ( ) and prognostically (2001 –2100)?

Terrestrial Ecosystem Model couples biogeochemistry & soil thermal dynamics Soil Thermal Model (STM) Vegetation type; Snow pack; Soil moisture Soil temperature RARA RHRH LCLC LNLN Soil Temps. at Different Depths Upper Boundary Conditions Snow Cover Moss & litter Frozen Ground Thawed Ground Frozen Ground Lower Boundary Conditions Heat Conduction Moving phase plane Organic Soil Mineral Soil Prescribed Temperature Prescribed Temperature Snow Depth Moss Depth Organic Soil Depth Mineral Soil Depth Moving phase plane Heat balance surface Lower boundary Heat Conduction Terrestrial Ecosystem Model (TEM)

TEM Simulations & Model Validation -Conducted simulations focusing on terrestrial land areas above 30º N and retrospective decadal trends from the 1960s –2000 -Also conducted prognostic simulations focusing on using interpolated climate data obtained from a two dimensional climate model (Sokolov and Stone, 1998) -Performed simulations with transient CO 2 and climate data -Validated the TEM results with several remotely sensed datasets (Dye, 2002; McDonald et al., 2004; Smith et al., 2004)

8.0 –18.0 Weeks – Region – 28.0 Weeks – Region –37.0 Weeks – Region 3 Duration of Snow Free Period Based on simulation of the TEM for north of 30 o N Snow Free Duration Anomaly (weeks) Region 1 Region 2 Region 3 D. Dye = White lines TEM = Colored lines

Trends in the Duration of the Snow-Free Period Anomaly (Weeks) SlopeInterceptR2R2 Correlation Region 1 TEM Dye * Region 2 TEM Dye Region 3 TEM Dye *D. Dye, Hydrological Processes, –18.0 Weeks – Region – 28.0 Weeks – Region –37.0 Weeks – Region 3 Duration of Snow Free Period

Growing season length (GSL) change (days per year) Shorter GSL Longer GSL 3 Region (Years) Change in spring thaw (days earlier per year) TEMMcDonald et al. (1) Smith et al. (2) North America (1988 – 2000) Eurasia (1988 – 2000) Pan-Arctic (2001 – 2100) 0.36 (1) Earth Interactions, 2004 (2) Journal of Geophysical Research, 2004

Net primary productivity Heterotrophic respiration 9.1 g C m -2 yr -1 day g C m -2 yr -1 day g C m -2 yr -1 day g C m -2 yr -1 day Growing season length anomaly (days) Anomaly (g C m -2 yr -1 ) [R 2 ] = [p] <

9.5 g C m -2 yr -1 day -1 Anomaly (g C m -2 yr -1 ) Soil C Vegetation C 8.9 g C m yr g C m yr -1 Growing season length anomaly (days) [R 2 ] = [p] < Net ecosystem productivity -8.1 g C m yr g C m -2 yr -1 day g C m yr g C m yr

8.0 –18.0 Weeks – Region – 28.0 Weeks – Region –37.0 Weeks – Region 3 Trends in growing season length, productivity and respiration Greatest increases in GSL. Smallest increases in productivity and respiration Similar increases in GSL to Region 2. Greatest overall increases in productivity and respiration Similar increases in GSL to Region 3. Intermediate increases in productivity and respiration. Duration of snow-free period

(a) (b) Cumulative NEP (Pg C region -1 ) J F M A M J J A S O N D Month Boreal & tundra regions (60 – 90° N) Temperate regions (30 – 60° N) Source Sink

Conclusions Model simulations indicate strong connections between decreases in snow cover and changes in growing season length. These dynamics substantially influence carbon fluxes, including enhanced respiration and productivity in our analyses. Increases in productivity and respiration at high latitudes are not as large as those in lower latitudes. It is important to improve our understanding of the relative responses of photosynthesis and respiration to changes in atmospheric CO 2 and climate.

Acknowledgements Funds were provided by: The NSF for the Arctic Biota/Vegetation portion of the Climate of the Arctic: Modeling and Processes project within International Arctic Research Center at the University of Alaska Fairbanks The USGS ‘Fate of Carbon in Alaska Landscapes’ project