Project -- BOREAS
[ Go Back ]

Global Change Research in the Boreal Forest

Investigators: John S. Kimball, Steve W. Running

The Boreal Ecosystem-Atmosphere Study (BOREAS) and associated follow-on investigations constitute a multi-international effort to understand the influence of the boreal forest biome on global processes. A major goal of these investigations is to improve our understanding of the exchanges of radiative energy, heat, water, CO₂ and other trace gases between the boreal forest biome and the lower atmosphere in order to clarify their role in global change. Since 1993, field and remote sensing experiments at various scales have been organized to provide input and validation data needed to improve the accuracy of process models of these exchanges and to develop mechanisms for efficient, long-term monitoring of regional to global scale climate change effects within the boreal forest biome.

Research is being conducted at the University of Montana to identify and understand the important processes that determine the structure and function of boreal ecosystems. This work involves: 1) integrating remote sensing and field measurements across multiple spatial and temporal scales 2) calculating processes that are impossible or too expensive to measure and 3) developing more efficient and cost effective methods for long-term monitoring and assessment of ecosystem processes in Northern Hemisphere boreal and arctic regions using satellite remote sensing.

The BOREAS Study Region

The BOREAS study region consists of an area of approximately 1 million square kilometers covering portions of central Saskatchewan and Manitoba Canada. Within this region are two intensive study sites, each approximately 10,000 square kilometers in area. The terrain is relatively flat and vegetation cover is predominantly coniferous with low species diversity. Cold air temperatures and low solar radiation levels generally limit forest growth and restrict the growing season to only a few months between May and September. Four intensive study sites within the northern study area (NSA) represented fen (NFEN), young jack pine (NYJP), mature jack pine (NOJP) and mature black spruce (NOBS) forest types. Four sites were also examined within the southern study area (SSA) consisting of mature black spruce (SOBS), mature jack pine (SOJP), mature aspen (SOAS) and fen (SFEN) forest types. These stands were considered representative of the region and were subjected to intensive field studies involving virtually continuous eddy-flux tower measurements of water, energy and trace gas fluxes between the surface and lower atmosphere. These measurements were supplemented with detailed plot level measurements of biomass, soil and vegetation properties, as well as aircraft and space-borne satellite measurements of regional landcover characteristics. Scaling and integration of ecosystem properties between these various levels is being conducted using ecosystem process models.

Stand Level Results Summary

Understanding the cycling of water, carbon dioxide (CO₂) and other trace gases within the boreal forest and between the surface and lower atmosphere is critical for understanding the health and productivity of the region, as well as the importance of the boreal biome in enhancing or mitigating global change effects. The graphs above show BIOME-BGC daily simulations and tower eddy-flux measurements (Black et al. 1996, Goulden et al. 1998) of evapotranspiration (ET) and the net carbon flux (NEE) above mature black spruce (Fig. 1) and aspen (Fig. 2) stands within the BOREAS NSA and SSA. ET refers to the loss of water to the atmosphere through soil/plant evaporation and canopy transpiration. NEE refers to the residual difference between carbon (CO₂) uptake by the system through photosynthesis and carbon loss through respiration from maintenance, growth and decomposition processes. Negative NEE values indicate net carbon uptake by the system while positive NEE values indicate a net release of carbon to the atmosphere. Overall, both model simulations and field measurements indicate that mature boreal conifer stands range from near-zero to small carbon sources and sinks (i.e. ~ <1 Tonne C ha^-1 yr^-1) on an annual basis in response to seasonal and interannual weather fluctuations. Deciduous stands are generally more productive and have larger daily evaporation rates than conifer stands but are more sensitive to seasonal weather patterns, particularly in the timing of spring thaw. Both forest types are also generally less productive than similar stands in more temperate regions. Simulations of carbon exchange processes for these sites over longer time periods (i.e. >20 years) show interannual variations in NEE annual fluxes of +/-200 and +/-40 percent for deciduous and coniferous stands, respectively, indicating that productivity and carbon source/sink strength for the region varies spatially and temporally and is closely linked with short-term regional weather patterns (Fig. 3).

Snow is an important feature in the boreal forest environment, insulating the ground and regulating the regional energy balance during winter. The timing of snow melt and snow cover disappearance is also an important feature that generally initiates soil thaw and the onset of the growing season. The length of the snow free period also approximates the length of the growing season. Fig. 4 shows a comparison between snow depth observations and simulated snow cover water-equivalent within a mature jack pine stand in the BOREAS SSA. Daily snow cover was simulated using BIOME-BGC, while snow depth was measured using a sonic snow depth sensor (Shewchuck 1997). Long term simulations of snow cover characteristics show a large range of variability in seasonal snow depth, duration and timing of spring thaw (Fig. 5), which can have important ecological implications for the region.

Both ecosystem model results and field measurements indicate that the timing of spring thaw and the duration of the growing season are strongly linked to the carbon balance of boreal systems. Model simulations, eddy-flux CO₂ and biomass increment based net primary production (NPP) estimates have all shown strong linkages between spring thaw timing, ecosystem productivity and net surface-atmosphere trace gas exchange. NPP refers to the net difference between carbon uptake by the system through photosynthesis and carbon loss to the atmosphere through growth and maintenance respiration. Years with earlier spring thaws have generally been found to have significantly greater productivity (i.e. NPP) while years with delayed seasonal thaw periods are less productive (Fig. 6). Interannual variability in the timing of spring thaw over the circumpolar high latitudes may play a major role in regulating the amplitude and timing of the global seasonal atmospheric CO₂ cycle, as well as the annual carbon source-sink strength of boreal/arctic regions.

Regional Results Summary

The boreal forest represents a complex landcover mosaic where vegetation structure, condition and distribution are strongly regulated by environmental factors such as moisture availability, growing season length, disturbance (e.g. fire, logging) and soil nutrients. A major research effort has been directed towards improving our understanding of the linkages between climatological, physical, and biological (e.g. photosynthesis and respiration) processes within the region. Much of this effort has focused on detailed studies of individual stands (e.g., Black et al., 1996, Goulden et al., 1998). Extrapolation of stand level relationships to characterize large regions and longer time periods is difficult, however, because the boreal forest is spatially and temporally complex. As a result, the integrated effects of the various processes affecting productivity, evaporation and trace gas production are still poorly understood at regional scales. A fundamental strategy of BOREAS and other intensive large scale field experiments has been to extend stand level relationships to more regional scales using process models coupled with remote sensing and other integrative modeling techniques. To achieve these goals we use an ecosystem process model (BIOME-BGC) coupled with a daily regional weather generator (DAYMET) and remote sensing derived estimates of important surface features (e.g. landcover type, leaf-area) to simulate water, carbon and nitrogen cycling across the boreal landscape.

Weather conditions such as air temperature, humidity, precipitation and solar radiation are primary drivers of ecosystem processes and are required inputs for ecosystem process model simulations. Air temperature, humidity, solar radiation and precipitation were interpolated over the BOREAS study region using a daily meteorological interpolator (DAYMET), digital elevation information (elevation, slope, aspect) and weather data from a network of approximately 60 surface stations within and around the region (Fig. 1). 1994 mean annual air temperature (Fig. 2) and solar radiation (Fig. 3) are summarized above. Overall, the interpolated meteorological data show a general north-south gradient in solar radiation and a west-east gradient in air temperature and precipitation, which can have a substantial impact on regional ecosystem processes. Spatial interpolation methods were generally accurate to within 2.5 degrees celsius for air temperature and 30 percent for annual precipitation over the 3-year study period (1994 -1996). Interpolation accuracy was limited within the BOREAS region because of extremely low weather station densities (~ < 1 station per 16,667 km²), which are problematic for most high latitude boreal and arctic regions.

Landcover type and condition are major factors influencing ecosystem processes because different forest types (e.g. conifer vs deciduous) can behave quite differently under the same environmental conditions. A key BOREAS objective has been to quantify ecological processes at local scales (i.e. < 1 km²) and extrapolate results to regional scales consistent with Atmospheric General Circulation Model (GCM) and other global data sets which often define surface features at scales of 100 km² or more. Unfortunately, landcover features (e.g. wetlands) essential for characterizing regional fluxes may not be distinguished at coarse spatial resolutions. We used BIOME-BGC with remote sensing derived landcover (Fig. 4), and biomass maps to investigate the relationships between landcover spatial scale and estimated regional evapotranspiration (ET) and net primary productivity (NPP). These simulations were conducted across the Southern Modeling Sub-Area (SMSA) within the BOREAS SSA at length scales ranging from 30 m to 50 km. Fig. 5 shows remote sensing derived estimates of landcover features within the study area at 3 different spatial scales, illustrating the large reductions in surface complexity that occur at progressively coarser spatial scales. Spatial aggregation of landcover characteristics resulted in mean monthly NPP estimation bias from 25 - 48% and annual estimation errors from 2 - 14% (Fig. 6). These errors were primarily caused by a failure to adequately resolve deciduous and coniferous vegetation components within the landscape at coarser spatial scales. Error was reduced at longer time intervals because coarse scale overestimation errors during spring were partially offset by underestimation of fine scale results during summer and winter. ET was relatively insensitive to landcover spatial scale with an average bias of less than 5%. Factors responsible for differences in scaling behavior between ET and NPP included compensating errors for ET calculations and boreal forest spatial and temporal NPP complexity.

Net primary production (NPP) is the primary mechanism whereby carbon (i.e., CO₂) is removed from the atmosphere and stored in terrestrial vegetation. Seasonal and interannual variations in temperate and high latitude (30 - 60ºN) terrestrial NPP are also thought to be major factors regulating global atmospheric CO₂ concentrations. Recent studies present evidence of a trend toward increased amplitude of the seasonal atmospheric CO₂ cycle and enhanced photosynthetic activity of terrestrial vegetation linked to warmer spring-time temperatures, earlier seasonal snow melt and a lengthening of the active growing season for temperate and boreal regions. We used BIOME-BGC coupled with remote sensing parameter maps (e.g. landcover, biomass) and distributed daily weather information to assess the relative magnitude and spatial complexity of NPP across a boreal landscape and the sensitivity of the system to short-term, interannual weather variations. NPP simulations for the BOREAS SMSA region were found to be spatially and temporally complex (Fig. 7). NPP spatial variability was strongly controlled by the amount of above-ground biomass, particularly photosynthetic leaf-area, as well as landcover type (Fig. 8) . NPP simulations were strongly sensitive to year-to-year variations in seasonal weather patterns influencing the timing of spring thaw and deciduous bud-burst. Reductions in annual NPP of approximately 17 and 22 percent for 1995 and 1996 were attributed to 3 and 5 week delays in spring thaw relative to 1994 (Fig. 9). Boreal forest stands with greater proportions of deciduous vegetation were also found to be more sensitive to spring thaw timing than coniferous stands. These results highlight the importance of sub-grid scale landcover complexity in controlling boreal forest regional productivity, the dynamic response of the biome to short-term interannual weather variations and the potential implications of climate change and other large-scale disturbances.

References

Black, T.A. et al. 1996. Annual cycles of water vapour and carbon dioxide fluxes in and above a boreal aspen forest. Global Change Biology 2, 219-229.

Goulden, M.L. et. al. 1998. Sensitivity of boreal forest carbon balance to soil thaw. Science 279, 214-217.

Gower, S.T., J. Vogel, J. Norman, C.J. Kucharik, S. Steele, and T.K. Stow. 1997. Carbon distribution and above ground net primary production in aspen, jack pine and black spruce stands in Saskatchewan and Manitoba, Canada. Journal of Geophysical Research 102(D24), 29,029-29,041.

Keyser, A.R., J.S. Kimball, R.R. Nemani and S.W. Running. 1999. Simulating the effects of climate change on the carbon balance of North American high latitude forests. Global Change Biology (In press).

Kimball, J.S., A.R. Keyser, S.W. Running and S.S. Saatchi. 1999. Regional assessment of boreal forest productivity using an ecological process model and remote sensing parameter maps. Tree Physiology (In press).

Kimball, J.S., S.W. Running and S.S. Saatchi. 1999. Sensitivity of boreal forest regional water flux and net primary production simulations to sub-grid scale landcover complexity. Journal of Geophysical Research 104(D24) 27,789-27,802.

Kimball, J.S., P.E. Thornton, M.A. White and S.W. Running. 1997. Simulating forest productivity and surface-atmosphere carbon exchange in the BOREAS study region. Tree Physiology 17, 589-599.

Kimball, J.S., M.A. White and S.W. Running. 1997. BIOME-BGC simulations of BOREAS stand hydrologic processes. Journal of Geophysical Research 102(D24), 29,043-29,051.

Running, S.W., J.S. Kimball, A.R. Keyser, J.B. Way, K. McDonald, and S. Frolking. 1999. Recent advances in the use of satellite radar data to monitor freeze/thaw transitions in boreal regions. Eos 80(19), 213-221.

Shewchuck, S.R. 1997. Surface mesonet for BOREAS. Journal of Geophysical Research 102(D24), 29,077-29,082.

Developing a space-borne freeze/thaw monitor for cold regions...

[ Go Back ]