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Fine woody debris dynamics after an ice storm


Tuesday, August 23, 2011, by Jakob Lindaas
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I used to walk through a forest, always looking up in wonder at the tall, sturdy trees and their vast canopies. But after this summer I have a newfound appreciation for what lies underneath these great sentries of the forest realm. Among the seasonal litterfall and the rotting remains of former protectors of peaceful succession, lay my study subjects. These are fallen soldiers of a war raged in December of 2008, between a mighty ice storm and the winter vigil kept by the mighty red oaks, their sidekick red maples, their hemlock allies, and their understory minions: beeches, yellow birches and a smattering of cherry, ash and other species. 

These branches and sticks that fell are called Fine Woody Debris (FWD) -- greater than 2cm in diameter but less than 7.5cm. They play an important role in the detritus part of the carbon cycle, which, along with above- and below-ground biomass and soils, form the three main reservoirs in the temperate forest C cycle. The Environmental Monitoring Site’s Eddy Flux Tower here at the Harvard Forest has been monitoring this cycle for over 20 years now, and biometry measurements in 33 plots surrounding the tower help separate and quantify the different C pools. 

As the ice storm’s impact on this carbon exchange began to be assessed, measuring and quantifying the amount of FWD that fell to the forest floor was very important. The summer after the ice storm, my mentor, Leland Werden, and two REU students measured the FWD in all 33 plots and tagged them with pink tagging. This summer we investigated how much the FWD has decayed in two years. 

Over a 10-week period, we re-found as many pieces as possible and re-measured their length, diameter, decay class, and other factors that influence decay rates. This process went much slower than it did 2 years ago, and we ended up confidently finding most of the pieces in only 8 of the 12 plots we re-measured.

After organizing the data to make sure we were comparing the exact same pieces, we found that overall volumes only experienced slight changes. So we measured the densities of the 6 main species and the 3 decay classes we found. We found significant changes in density, which translate to a C flux out of the FWD. However, due to the high spatial variability of the FWD among the plots, we could not confidently quantify the change in the Ice Storm FWD C pool. We were able to conclude that a disturbance event such as an ice storm to a temperate forest can result in a rapid initial increase in the C flux out of the FWD C pool.

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