Date of Award

Fall 12-16-2022

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Forest Resources

Advisor

Shawn Fraver

Second Committee Member

Jodi Forrester

Third Committee Member

Ivan Fernandez

Abstract

As the climate changes, understanding the sources and sinks of greenhouse gases such as carbon dioxide (CO2) is increasingly important. However, several components of the carbon cycle within forests remain poorly understood. For example, knowledge gaps exist in our understanding of CO2 emissions from coarse woody material (CWM; logs and stumps), including how emissions change over time, how they are influenced by environmental variables, and how they compare to soil and ecosystem-level CO2 emissions.

To fill these knowledge gaps, we examined CO2 emissions from CWM at three sites. We sampled 18 red spruce (Picea rubens) stumps in a 32-year chronosequence of stumps (i.e., 0, 2, 4, 8, 23, and 32 years since harvest) at the Penobscot Experimental Forest (PEF) in central Maine; we sampled 12 sugar maple (Acer saccharum) logs in four harvesting treatments (small gap, large gap, thinned matrix, and unharvested control) at Dartmouth College’s Second College Grant (SCG) in northern New Hampshire; and we sampled 13 red spruce logs within the footprint of an eddy-flux tower measuring ecosystem-level fluxes at the Howland Research Forest in central Maine. At all of these sites, we sampled CO2 emissions using closed chambers fitted onto each piece of CWM, with repeated measurements taken with a Li-Cor LI-830 gas analyzer from May to November.

Our findings from stumps at the PEF suggest low initial CO2 flux rates (3.2 μmol CO2 m-2 s-1 at year 0) followed by a rapid increase, peaking 8 years post-harvest (24.3 μmol CO2 m-2 s-1) followed in turn by a decrease to very low rates by years 23 and 32 (1.4 and 1.7 μmol CO2 m-2 s-1, respectively). In the harvested treatments at the SCG, we found that CO2 emissions were highest for logs in small (31.8 ± 20.4 μmol CO2 m-3 s-1) and large gaps (29.6 ± 24.4 μmol CO2 m-3 s-1) compared to those in control (13.9 ± 8.3 μmol CO2 m-3 s-1) and thinned matrix (13.6 ± 8.0 μmol CO2 m-3 s-1) treatments. In this system, log CO2 flux rates increased with temperature; CO2 emissions increased throughout the day and generally peaked at about 13:00; and flux rates varied significantly within a log.

Finally, at the Howland Research Forest in central Maine, we found that when scaled up, logs emitted about 8.00 ± 1.09 g C m-2 yr-1, and stumps emitted about 0.115 ± 0.024 g C m-2 yr-1. In total, CO2 emissions from CWM contributed to about 0.72% of total ecosystem-level respiration and about 0.99% of previously measured soil emissions at the Howland Research Forest.

At two of our sites (SCG and Howland) we found that CO2 fluxes from CWM increased with increasing temperatures, and at Howland they increased with soil moisture as well. These results provide insight into the many factors influencing CO2 emissions from CWM. Our results may be important to consider for forest practitioners, who are increasingly interested in carbon management. In addition, our results can aid in the calibration of forest carbon models, and overall they improve our understanding of forest carbon dynamics.

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