Woods Hole Research Center

While down-and-dead wood (DDW) contributes to the carbon budget of forest, it is often neglected. This material is important to the ecology of forest ecosystems, and can be a major component in forest carbon budgets. It is particularly important in forests being actively managed. We examined the contribution of DDW to the forest carbon budget at Howland Forest by measuring the pool size and respiration rate of the DDW pool.

We used two approaches to quantify the decay rate of DDW. First, we estimated the rate of respiration from DDW by collecting pieces of dead wood at various stages of decay and measuring the rate of CO2 production using a portable infrared gas analyzer (see ‘Methods’). Second, we estimated decay rate using radiocarbon measurements to quantify the date of mortality, then combined that with measurements of wood density to quantify changes in density over time.

Segments of spruce and fir logs (between 100 and 1000g wet weight) were removed with a saw and placed in a 13 L sealable Rubbermaid container. Respiration rates were measured in the field at ambient temperatures by circulating air between the closed container and an infrared gas analyzer (same as that used for soil respiration). The CO2 concentrations were recorded every 30 seconds for 15 minutes and the rate of CO2 increase was calculated by linear regression. Each log was weighed field-moist and then dried at 60°C until the weight stabilized. The volume of each log was determined to calculate density. Rates of log respiration were calculated from the rate of CO2 increase, the chamber volume, the chamber air temperature, and the dry weight of the log. Regression analyses were used to develop temperature, moisture, wood density, and decay class functions. Analysis of variance was used to examine effects of species.

System for measuring deadwood respiration.

• Based on a strong positive relationship between temperature and respiration rate, annual temperature data, radiocarbon measurements, and estimates of the total amount of dead wood, we estimate that annual CO2 production from DDW is 0.10 Mg C ha^-1y^-1.
• Our results suggest that inputs to the DDW pool are roughly 0.3 Mg C ha^-1y ^-1.
• When combined with estimates of carbon losses from DDW, our results suggests that DDW may be accumulating at a rate of about 0.20 Mg C ha^-1y^-1, a number that is similar to that obtained for soils.