Long Term Carbon Flux Measurements in the Understory of an Old-growth Temperate Rainforest

Long term measurements of turbulence and scalar fluctuations are being conducted in the understory of an low elevation old-growth stand in the Pacific Northwest. Dominant tree species are Douglas-Fir (Pseudotsuga menziesii) and Western Hemlock (Tsuga heterophylla) with an age of 500 years. Dominant understory shrub species are vine maple (Acer circinatum), salal (Gaultheria shallon) and dwarf Oregon grape (Berberis nervosa). Continuous eddy covariance measurements commenced in July 1998 using a Gill HS sonic anemometer and a Li-Cor LI-6262 infrared gas analyzer at a height of 3 m. Gases are sampled with 5m long tubes and at a rate of 10 l/min.

The first year of data shows under-story and soil efflux to be between 4 µmol m^-2 s^-1 in the summer and 1 µmol m^-2 s^-1 during the winter months. The total efflux from the lowest layer of the forest accumulates to 7.5tC/ha. Interestingly the flux from the under-story and soil lags behind the total ecosystem uptake of CO₂. During the months of March, April and May the forest as a whole takes up carbon at the fastest rate throughout the year. At the 3 m level however fluxes are at the annual low. Low soil temperatures can be linked to this phenomenon. During the summer months of July and August 1998 soil efflux is at its highest. Ecosystem photosynthesis during this period is low because of summer drought (78 days consecutive days without rain) and subsequent water stress in the larger trees. The monthly averages show a rapid jump in the 3 m height exchange from May 1999 to June 1999 whereas the fall of 1998 shows a more gradual decline. Cold events contribute to the low soil efflux in May and can even shut the respiration completely down.

Understory photosynthesis is visible as a diurnal minimum in the carbon flux at 3 m during the middle of the day. The magnitude of this is not sufficient to reverse the soil respiration at any time. Preliminary comparisons show good agreement of the eddy-covariance data with chamber measurements taken on a monthly basis at the site.