Measuring Carbon Dioxide Exchange at the Old-growth Forest Floor
Overview
In addition to the above canopy EC system on the crane, we also have a flux tower located on the forest floor to measure below canopy gas exchange. The objectives of this tower are to: (1) measure fluxes of soil and understory carbon exchange, (2) investigate the seasonal and interannual dynamics of soil and understory exchange, (3) partition observed CO2 fluxes into contributions of forest floor (soil, coarse woody debris, bole) respiration and understory photosynthesis, and (4) investigate the influence of environmental drivers (soil temperature, soil water content on the dynamics of forest floor respiration.
Understory LAI (canopy height < 3 m) is estimated to be 0.86, roughly 10% of the stand total. Dominant understory shrub species are vine maple, salal and dwarf Oregon grape. A large amount of woody debris is present at the site. Carbon storage in the soil and detritus is 221 Mg C ha-1, of which 93 Mg C ha-1 are stored in the mineral soil (depth < 100cm), 95.5 Mg C ha-1 in woody detritus consisting of Coarse Woody Detritus and Fine Woody Detritus and 3.2 Mg C ha-1 in the fine non-woody litter (Harmon et al. 2004).
Instrumentation and Data Processing
The EC instrumentation are mounted at a height of 2.5 m above the forest floor. From 1999-2003, we used a closed path infrared gas analyzer (LI-6262, LiCor, Lincoln, Nebraska, USA) and a 3-D sonic anemometer/thermometer (Solent HS, Gill Instruments, Lymington, England, UK). In May 2007, the station was re-fitted and moved to the east side of the canopy crane. During this period, we used an open-path infrared gas analyzer (LI-7500, LiCor, Lincoln, Nebraska, USA) and a 3-D sonic anemometer/thermometer (CSAT3, Campbell Scientific, Utah, USA). Line-power is available at the understory flux tower.
Raw data are stored on a 23X datalogger (Campbell Scientific) and downloaded to the canopy crane computer via an Ethernet cable. Seven automated soil respiration chambers were installed on the forest floor in 2005. A comparison between the chamber and EC fluxes will be made in the near future.
Highlighted results
Maximum daily carbon exchange ranged from 5 to 7 g C m-2 day-1 in the summer months and was greatly reduced (but was still non-zero) in the wintertime due to lower soil temperatures, with daily values ranging from 0.5 to 1 g C m-2 day-1. Summer understory photosynthesis was shown to be up to 2 ?mol m-2 s-1 with some interannual variability. Annual estimates of soil and understory respiration average at 11.1 ± 1.5 Mg C ha-1 yr-1. The large observed annual soil efflux is consistent with the presence of large carbon stocks at the Wind River site.
We have used the understory flux tower data to derive an ecosystem respiration model for the Wind River old-growth forest. Nighttime soil and understory exchange is well described by an exponential function (Q10) of soil temperature during periods of moderate soil moisture, but moisture effects required a modification of the equation at low and very high soil moisture conditions in order to capture observed attenuated respiration fluxes.