BOREAS in 1997: Experiment overview, scientific results, and future directionsSellers, P.J., et al. Journal of

Geophysical Research, Vol. 102, No. D24, December 26, 1997.

Boreal Forests

Encircles the Earth above 48°N Second in areal extent only to tropical forests Occupies 21% of the Earth’s forested land

surface Contains 13% of the carbon stored in

biomass and 43% of the carbon stored in the soil

Expected that warming will be greatest among these higher latitudes (43° – 65°N)

Importance of study

Large size of forests combined with magnitude of future climate changes could have significant effects Feedbacks, changes in carbon storage, changes

in ecological function of the ecosystem, etc. Our limited understanding of the exchange of

energy, water and carbon between the atmosphere and these land surfaces limits our ability to predict these future changes Also limits the reliability of climate models

Interactions between the boreal forest and the atmosphere

Physical climate system processes

Carbon and biogeochemistry Biophysics Ecology

Objectives

1. Improve the process models that describe the exchanges of radiative energy, water, heat, carbon, and trace constituents between the boreal forest and the atmosphere

Addressed at local scale (centimeters to a few kilometers)

2. Develop methods for applying the process models over large spatial scales using remote sensing and other integrative modeling techniques

Addressed at regional scale (10-1000km)

* Need to connect processes at these two scales

Multi-scale measurement strategy

Multi-scale nested design which permits knowledge at one scale to be translated and compared to that obtained or inferred at different scales

Experimental Design

Experiments took place over two years, 1994 and 1996

Project included 85 science teams and over 300 scientists

Individual projects divided into 6 groups Airborne fluxes and meteorology Tower fluxes Terrestrial ecology Trace gas biogeochemistry Hydrology Remote sensing science

Results Section Format

Carbon-Water-Energy Fluxes Small-scale fluxes and physiology with chambers and enclosures Stand and plot-level carbon-water-energy dynamics Landscape-scale carbon-water-energy dynamics and surface-atmosphere

boundary layer interactions Trace Gas Fluxes

Small-scale fluxes with chambers and enclosures Stand and plot-level trace gas dynamics with towers

Soil and Snow Moisture and Runoff Point measures and modeling of soil moisture dynamics Stand-level soil and snow moisture dynamics Landscape-scale precipitation and soil moisture dynamics

Remote Sensing Science Ground and aircraft measurements of biophysical and optical characteristics

and understory and canopy reflectance Radiative transfer models and algorithm development Landscape-scale land cover and biophysical characteristics algorithms Radiation and atmospheric effects

Carbon-Water-Energy Fluxes Small-scale fluxes and physiology with chambers and enclosures

Wintertime CO2 fluxes ranged from 0.5 to 1.0 g CO2 m-2 d-1

Lowest fluxes correlated with midwinter minimum temperatures Accumulated over the long winter, these fluxes contribute important

fraction of the annual carbon budget Maximum net photosynthesis occurred between 5°C and 8°C

Moss and surface net photosynthesis estimated to account for 10-40% of whole ecosystem uptake and 50-90% of whole ecosystem respiration

Shift from newer to older carbon (measured as CO2 release) as the winter wore on Suggests deeper soil source

Surface peats and mosses have sequestered an ave.of 40-60 g C m-2 y-1over the last 90 years Net decomposition in deeper soils released 20-50 g C m-2 y-1

Ave. net soil carbon exchange of +10 to -50 g C m-2 d-1 over the last century

Sites can vacillate between source and sink status based on climate variability

Carbon-Water-Energy Fluxes (2)

Small-scale fluxes and physiology with chambers and enclosures (2) Deep carbon storage seems to be a function of respiration,

decomposition, drainage and fire history Sites had slower decomposition rates due to high soil moisture

content and lower temperatures This results in more carbon accumulation in the soils

Deep soil respiration offset 15% of carbon uptake rates in wetlands and 45% in upland sites.

Photosynthetic capacity correlated well with stomatal conductance but decreased less steeply than did PAR through the canopy Good correlation between photosynthetic capacity and remote

sensing spectral vegetation indices Costs of respiration for the whole system estimated at 310-610 g

C m-2 y-1 Carbon use efficiency (ratio of net production to net photosynthesis)

averaged 0.44, 0.29 and 0.43 for aspen, black spruce, and old jack pine.

Schematic summarizing measurement of small-scale fluxes and physiology with chambers

Carbon-Water-Energy Fluxes (3) Stand and plot-level carbon-water-energy

dynamics Daily ave. summer albedos

0.083 – conifers 0.15 – aspens 0.20 – grass

Daily ave. winter albedos 0.13 – conifers 0.21 – aspens 0.75 – grass

Carbon-Water-Energy Fluxes (4) Stand and plot-level carbon-water-energy dynamics (2)

Conifers characterized by low evaporative fractions (ratio of latent heat to the sum of latent and sensible heat fluxes) and low CO2 uptake rates during the growing season

Before leaf emergence - most available energy converted to sensible heat flux, after leaf emergence – latent heat flux dominates

Forest canopy conductance directly proportional to forest leaf area index

Transpiration increases with air vapor pressure deficit until about 1 kPa and then remained almost constant with higher deficits

Factors restricting stomatal opening were low soil moisture, limiting atmospheric saturation deficit and low photosynthetic capacity of the needles. For jack pine, 20-40% of total energy exchange originated at the

forest floor underneath the sparse canopy

Carbon-Water-Energy Fluxes (5) Stand and plot-level carbon-water-energy dynamics (3)

Above a black spruce canopy, evaporation accounted for 43% of net radiation during the growing season

Soil surface respiration accounts for 48-71% of CO2 flux while foliage respiration accounts for 25-43%

Gross photosynthesis largely a function of PAR flux and air temperature with no apparent effects due to high evaporative demand or soil water content

Substantial productivity (carbon uptake) when high water table observed, loss of carbon at drier sites

Above ground net primary productivity for forest sites was 55-310 g C m-2 y-1

30-40% of the ANPP fell to the surface as detritus, 60-70% retained as biomass

LAI ranged from 1.25 for jack pine to 5.6 for black spruce Fine root net primary productivity ranged from 30 to 115 g C m-2

y-1

Cumulative carbon uptake and evapotranspiration rates calculated from measurements above the black spruce canpoy

Schematic summarizing measurement of stand and plot-level carbon-water-energy dynamics

Carbon-Water-Energy Fluxes (6) Landscape-scale carbon-water-energy

dynamics and surface-atmosphere boundary layer interactions Significant efforts were devoted to correlating

airborne fluxes with surface cover types Explores progressive decoupling of boundary

layer fluxes from surface features with increasing height.

Schematic summarizing measurements of trace gas fluxes, principally CH4 and CO

Soil and Snow Moisture and Runoff

Schematic summarizing measurement of soil and snow moisture runoff

Remote Sensing Science

Schematic summarizing methods of measurement for remote sensing science

Summary

Schematic summarizing gains in the physical climate system science due to BOREAS

Schematic summarizing gains in the carbon cycle science due to BOREAS

Schematic summarizing gains in ecology and remote sensing science due to BOREAS

Schematic showing how different elements of BOREAS science could combine to improve the performance and realism of global change models