The Complexity of Burning Wood

Here in New England, we’ve been burning wood for heat since the arrival of humans. And once Europeans started showing up in force four centuries ago, the forests have been a key source of warmth and power.

These days there are ongoing debates over the harvesting and use of “biomass.” As is the case with many debates, the issues tend to get oversimplified in an attempt to characterize biomass as “good” or “bad.” The science behind our understanding of those issues is more complex.

Let’s start with what we mean by biomass. The term refers to plant (or sometimes animal manure) material that can be converted to energy (or converted back to energy, as plant biomass is really energy derived from the sun via photosynthesis). Most often, around here anyway, when we talk about biomass we mean wood that is burned to generate heat, or burned to generate heat that’s converted to electricity.

At the Forest Society, we support the sustainable use of biomass. Not only do many of the timber harvests we conduct on our properties include harvesting low-grade wood as wood chips, we heat our Concord headquarters with a wood-chip gasification boiler.

As a practical matter, in the northern forest biomass most often refers to wood chips that come from what’s considered low-grade wood; that is, trees or parts of trees that aren’t of a quality to be harvested for timber (which is worth far more in dollars at the mill). Our northern New England forest has a lot of low grade wood, and there are various forest-management reasons that a landowner would want to remove some of that low-grade wood.

How that biomass harvesting is done, and the extent to which it’s done, generates a lot of the debate. Throw in additional debate about the economics of burning wood for electricity, and you’ve got a great example of energy conversion: sun grows plants, cow eats plants, man eats cow, man makes much hot air debating biomass.

But what does the science tell us about the impacts of harvesting biomass? As a forestry organization dedicated to the sustainability of forests, we work to understand what’s known and what’s still to be learned.

For example, forester Gabe Roxby, one of my colleagues at the Forest Society, wrote an interesting piece about a study he was involved in when he was still a forestry student that looked at whole-tree harvesting. Gabe writes:

“Whole-tree harvesting is the practice of cutting the entire above-ground portion of a tree and removing it from the forest. The different parts of the tree are used for different purposes – the trunk is often sawn into dimensional lumber, while the tree top and limbs can be chipped and sold to a biomass energy plant. Or in some cases, whole trees of low quality are chipped. This harvest method can be contrasted with “conventional harvesting”, which uses the tree trunk in the same way, but leaves tree tops on site to decompose. 

“However, concerns about whole-tree harvesting have existed since the 1970s, when machinery that first enabled its practice was developed. Since whole-tree harvesting removes nutrient-rich branches that would otherwise remain on site to decompose, wouldn’t this lead to soils that were less fertile? And over time, wouldn’t this lead to a decreased ability of the forest to support vigorous tree growth? This is a major concern to foresters, whose job is to sustainably manage forestland. 

“So what were the results? This study suggests that a single whole-tree harvest has no observable impact to these types of young regenerating stands, but does not rule out the possibility of long-term effects. Many other studies have examined this question from different angles and have found mixed results. Some find cause for concern from whole-tree harvesting and others find none.

“There are many caveats to every scientific study, and this one is no exception.  The first and foremost is that these are short-term results after a single whole-tree harvest. Second, this study only focused on a certain type of harvesting (small clear-cuts) in a specific forest type (northern hardwoods) within a limited geographic area (north-central NH and western Maine).”

And it focuses on only one of the numerous factors to consider as we seek to understand the impact of biomass harvesting and use.  Land managers seeking to make “wise use” of the forest will want to stay current with the latest research and understand that good science requires long-term observation, measurement, and consideration of a complex web of factors.

(If you’re interested in learning more about whole-tree harvesting, the full text of the article can be found here https://forestsociety.org/blog-post/forestry-research-whole-tree-harves…)

Gabe Roxby began working for the Forest Society in 2012 after graduating from UNH with an M.S. in Natural Resources and a focus in Forestry. He can be contacted at groxby@forestsociety.org. Jack Savage is the executive editor of Forest Notes magazine, published by the Society for the Protection of New Hampshire Forests. He can be reached at jsavage@forestsociety.org.