Environmental Drivers of Carbon Exchange at Early Seral Stands: a Close-look at Summer Drought Effects
Although the Wind River area receives over 2300 mm of precipitation a year, only 10% of that falls during the months of July-October due to the presence of a strong Pacific High pressure system in the summer. In this project, I have measured eddy covariance fluxes during the spring, summer and autumn months to test the hypothesis that young forest stands are more vulnerable than mature forests to periodic water stress. These are overstory flux towers and measure forest ecosystem-atmospheric exchange of mass and energy above the forest canopy.
This is a two year long study. In late August of 2005, I set up a flux tower in Early Seral North, a 7-hectare stand of approximately 10-year old Douglas-fir. The area was clear-cut in 1994 and planted in 1996. A large proportion of the older forest remains in the form of snags, logs, and piles of harvested branches and other coarse woody debris. Ground-cover species include bracken fern, sword fern, salal, Oregon grape, native blackberry and grasses. Mass and energy fluxes were measured at the tower from August 2005 through October 2006, with missing data periods during the winter months.
In March 2007, I set up a second early seral flux tower in a Douglas-fir stand named Early Seral South. The trees range from 8 to 13 years old (determined through tree ring analysis) and were naturally established from the nearby forest. The site was clear-cut in 1988 and nearly all residual coarse woody debris was removed from the site. The soil was mechanically homogenized down to 1 meter deep. Nearly all of the ground-cover species are bracken fern and grasses. This site resembles an accelerated, old field succession – the conversion of abandoned cropland to forest.
The objectives of this study are to: (1) Continuously measure eddy covariance fluxes and air temperature, soil temperature, PAR, net radiation and soil moisture at the two early seral stands. (2) Identify the environmental controls on NEE at both young sites and compare these to the old-growth forest and (3) Identify and contrast time lags between environmental variables (e.g., soil moisture content, soil and air temperature, VPD, PAR) and NEE at the three overstory flux towers.
Instrumentation and Data Processing
Both early seral flux towers use the open-path infrared gas analyzer (LI-7500, LiCor, Lincoln, Nebraska, USA) and a 3-D sonic anemometer/thermometer (CSAT3, Campbell Scientific, Logan, Utah, USA). The EC instrumentation were mounted above the canopies using a 6 meter tall, triangular tower. The Li-7500 and CSAT3 were connected to a CR1000 datalogger (Campbell Scientific) equipped with a 2 GB memory card. In addition to the EC instrumentation, meteorological data were collected and included air temperature/relative humidity (HMP45C, Vaisala, Inc., Oy, Finland), net radiation (Q10, REBS), up- and down-welling PAR (Li-190SB, LiCor ), soil temperature at 5, 15 and 30 cm (CS107, Campbell Scientific), and soil water content at 3 levels ( 2 X 0-30 cm, 30-60 cm, 60-90 cm, TDR1000 + 6 CS610 probes, Campbell Scientific). Both stations were powered by a 110 W solar panel and a bank of 4, 12V batteries.