Growing season dynamics in high-latitude ecosystems: relations to soil thermal regimes, productivity, carbon sequestration, and atmospheric heating Bonanza Creek LTER Symposium February 2006

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

Outline of this talk: Part I: Examine changes in growing season length in high-latitude ecosystems (Based on Euskirchen et al., in press, Global Change Biology) Part II: Relate these changes to changes in energy

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 could 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

Date of leaf-out in Fairbanks (Chena Ridge) Data courtesy of J. Anderson

The increase in growing season length over the last 50 years averaged for 8 stations in Alaska having the longest and most consistent temperature records (Keyser et al., 2000). slope = 0.33 days/yr; p<

Part I: 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 Boreal region

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

Part I: 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.

Part II – What are the relative responses of changes in high-latitude carbon uptake due to growing season length increase versus changes in albedo on the climate system?

Air temperature anomaly (ºC, five year running mean) Year Air temperature anomaly (ºC) Five-Degree Latitudinal band (ºN) – ºC year – ºC year

The snow – albedo feedback loop Decreases in albedo Decreases in snow cover The greenhouse gas- ecosystem metabolism feedback loop Decreases in temperature Enhancements in productivity greater than enhancements in respiration Increases in temperature Increase in heat absorption Increases in greenhouse gases Enhancements in respiration greater than enhancements in productivity Increases in growing season length

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

Change in autumn snowfall Change in spring snowmelt Total change in snow cover duration Days per year earlier (or shorter for the ‘total’) Days per year later (or longer for the ‘total’) < >0.1 Anomaly -1.4 days decade days decade -1

SNOW: -Compiled seasonal data on surface energy balance (sensible-plus-latent heat flux to the atmosphere as a proportion of net radiation) by vegetation type. -Calculated monthly mean incoming shortwave radiation in TEM. -Estimated daily atmospheric heating depending on snow surface conditions. -Multiplied changes in snow cover (days per year) by changes in atmospheric heating (Chapin et al. 2005). ECOSYSTEM METABOLISM: A 4.4 W m -2 atmospheric heating change with a doubling of [CO 2 ]. 311 g C m 2 increase/decrease for each 1 W m -2 increase/decrease Translation of changes in snow cover and ecosystem metabolism to changes in radiative forcing Houghton et al., 2001

spring autumnspring & fall Changes in energy (W m -2 ) due to snow cover changes in: < ≥ 3W m -2 Cooling Heating

< ≥ 3W m -2 Cooling Heating Changes in energy (W m -2 ) due to changes in: snow cover ecosystem metabolism For boreal forests, changes in ecosystem metabolism (CO 2 + climate): : 0.00 W m : W m -2 changes in snow cover: : W m : W m -2

Change in energy (W m -2 ) due to changes in: Time periodsnow cover ecosystem metabolism ~ ~ (Negative sign represents negative feedback for the sink term, positive sign is positive feedback for a source term) Changes in snow cover had a much greater effect on energy than did changes in ecosystem metabolism Suggests the importance of considering other factors that may alter albedo. Pan-Arctic: north of 50° N

Foley et al., 2003 Reduced growing season albedo and increased spring energy absorption

Part II Conclusions: The effects of a longer snow-free season on atmospheric energy balances should considered in studies of climate change, particularly with respect to associated shifts in vegetation between forests, grasslands, and tundra. We should also consider other factors that play an important role in altering surface albedo, such as changes in fire regime, insect defoliation, timber harvest, and conversion to/from agriculture. And finally, how do these factors interact with changes in growing season length?

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