Ecosystem Studies and Management
Understanding the Forest Carbon Cycle in Harvard Forest: Throughfall Exclusion Experiment
Based on research findings from the long term study plots regarding the importance of summer drought on soil respiration rates and the confounding effects of temperature and soil moisture, WHRC researchers established a three-year throughfall exclusion experiment in 2001 to isolate the effects of experimentally imposed drought.
Methods and Measurements
These plots were located on a small plateau within the footprint of the eddy covariance tower. Three translucent roofs, each 5x5m2, were constructed 1.3m above the forest floor in order to simulate severe soil droughts during the growing season. The roofs were built of corrugated PVC panels with a rain gutter to exclude and remove all throughfall water from the exclusion plots, except perhaps small amounts that may have blown in horizontally around the perimeter. The exclusion plots were not trenched from the surrounding soil, because we did not wish to exclude roots from outside the plots. Throughfall water was excluded over 84 days from July 2 to September 24 in 2001 and over 127 days from May 7 to September 10 in 2002. Roof panels were removed in September to enable undisturbed leaf litter fall and snow fall to the forest floor in the exclusion plots. Measurement in the posttreatment period without manipulation continued until November 10, 2003. A control plot of same size was established 2m from each of the three exclusion plots.
Plastic roofing panels exclude rainfall from a plot. Soil respiration is measured within this exclusion area and in a control plot.
Soil respiration was measured using both manual and automated methods. Soil temperature and moisture were continually measured using type t thermocouple and Campbell Scientific soil moisture sensors (CS615). All data was stored on a Campbell Scientific CR10X datalogger.
Results
• Experimentally imposed summer drought immediately reduced CO2 efflux.
• This effect was reversed soon after natural throughfall was allowed back onto the treatment plots.
• Wetting events in the control plots produced pulses of CO2 efflux, even when small precipitation events wetted only the litter layer.
• In general, radiocarbon data indicated that heterotrophic respiration, primarily in the litter layer, was more affected by the throughfall treatment than was root respiration.
• However, notable periods of mushroom sprouting during natural and imposed drought episodically released CO2 with high radiocarbon content.
• The soil and litter layer may be a transient sink for C during drought years.
• Summer drought affects interannual variation of soil CO2 efflux and microbial activity within the litter layer.
Key Publications
Borken, W., K. Savage, E.A. Davidson, and S.E. Trumbore. 2006. Effects of experimental drought on soil respiration and radiocarbon efflux from a temperate forest soil. Global Change Biology 12: 177-193.
Borken, W., E.A. Davidson, K.E. Savage, E.T. Sundquist, and P. Steudler. 2006. Effect of summer throughfall exclusion, summer drought, and winter snow cover on methane fluxes in a temperate forest soil. Soil Biology and Biochemistry 38:1388-1395.
