TE-12 BOREAS Operations 1994

Objectives:
The goal of our coordinated research program is to gain an understanding of radiation and gas exchanges of canopy elements within a boreal forest and the coupling between radiation and gas exchange. The three objectives identified to achieve our goal are:

1. Characterization of boreal forest canopy optical properties during critical periods of the year. Leaf, needle, twig, and substrate optical properties were measured at the BOREAS Southern Study Area (SSA) as well as in the laboratory using various combinations of spectroradiometer and integrating sphere. Conifer shoot geometry was also characterized. Models will be used to enhance the understanding of key variables influencing canopy element reflectances and transmittances.
2. Characterization of gas exchange of boreal forest canopy elements during critical periods of the growing season. Responses of CO₂ exchange and stomatal conductance of canopy elements to environmental factors and diurnal courses of photosynthesis, respiration and stomatal conductance of canopy elements will be determined. Models will be used to describe the influence of relevant controlling variables on CO₂ and water vapor fluxes of boreal forest canopy elements.
3. Integration of foliage optical properties and gas exchange characteristics. Models will be used to couple radiation and gas exchange of canopy elements under diffuse and total radiation conditions via absorbed photosynthetically active radiation.

Places and Times of Measurements:

• Group I: FFC-T and FFC-W: SSA OBS and OJP. Jack pine and black spruce optical properties were measured on samples collected from the sites and sent to Lincoln, Nebraska for analysis. IFC-1: SSA OBS, YJP and OA; IFC-2 and IFC-3: SSA OBS, YJP and YA. Jack pine and black spruce optical properties and shoot geometry were characterized on samples collected in the field and taken to the SSA laboratory for analysis, two visits per IFC, sampling from the top and bottom parts of the canopy. Aspen samples were collected and analyzed in the field. Water potential was collected on surrounding samples in the field and in the laboratory at the time of analysis. Substrate element properties were measured once per IFC.
• Group II: May 29-Aug. 9 and Sept. 7-9: SSA YA and YJP with occasional measurements at OA, OBS and the Mixed site. Properties were measured at various times during this period. For canopy element gas exchange properties our primary emphasis was on aspen at SSA YA and jack pine at SSA YJP sites. Other species included were aspen at SSA OA, black spruce at SSA OBS and mixed sites and hazelnut and balsam poplar at SSA YA site.
• Group III: IFCs: SSA OJP and YJP. Measured once per IFC.

Problems and Caveats:
Calculations of needle transmittance in the visible portion of the spectrum occasionally yield negative numbers. The equation used in the calculation requires a measure of the non-intercepted fraction of the illumination beam (i.e., the fraction of light which passes between the mounted needle samples). The calculated transmittance is sensitive to any error in the fraction; our methods yield a fraction within 5% of the true value.
Accurate determination of conifer needle surface area by the volume displacement method required image analysis of needle cross sections to quantify the ratio of the perimeter to the square root of the cross sectional area.

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Last Updated: March 5, 1999