September 11, 2009

The carbon-wildfire paradox, part II

Article reviewed: Fuel treatment effects on stand-level carbon pools, treatment-related emissions, and fire risk in a Sierra Nevada mixed-conifer forest

By S.L. Stephens, J.J. Moghaddas, B.R. Hartsough, E.E.Y. Moghaddas, and N.E. Clinton, published in the journal Canadian Journal of Forest Research, available for download here

The plot line: They applied three typically-considered methods of treating a forest to reduce wildfire severity (prescribed burn, mechanical thinning, and burn + thinning) plus a control. Then they quantified the impact of each of these methods on carbon storage, release to the atmosphere, and potential loss from wildfire. As expected, the burn treatments released the most carbon into the atmosphere and had the least amount of total carbon stored on site. But the burn treatments were better at lowering the probability of massive loss in the future from a wildfire (although not “statistically” any better than the mechanical thinning). The authors argue that the losses of carbon from treatments that reduce wildfire severity are well worth it, considering that well-done treatments reduce the probability of massive carbon loss from future wildfires. The authors argue for better accounting when it comes to carbon credits, suggesting that carbon storage in wood products be considered.

Relevant quote: …should C [carbon] stored on site be maximized to ensure greatest short term benefit of C sequestration and potential C-related revenue, or should some of that C be removed using active treatments...thereby reducing total stored C in the short term but increasing fire resistance in the long term? Results from this study indicate that in fire-prone dry coniferous forests of the western United States that once burned frequently, the latter is the more prudent approach to storing C over the long term in these ecosystems.”

Relevance to landowners and stakeholders:

A forest landowner in California who is interested in maximizing standing forest carbon (to potentially sell carbon credits or to simply feel good about lowering their carbon footprint) might be tempted to implement a do-nothing approach. A do-nothing approach is indeed a good short-term method of sequestering carbon in a forest, especially if the forest has been disturbed recently (e.g. burned or logged). In the long-term, however, it increases the risk of total loss of carbon to wildfire. And, because of climate change and fire suppression, this risk of total loss has been steadily increasing over the past two decades. Thinking of the carbon you are sequestering as an investment, would you invest in a market where returns are unproven and risk has steadily been increasing (because of physical, biological reasons and not for socio-economic reasons)? I wouldn’t.  

I would, however, invest in a forest that was managed to balance objectives of high carbon sequestration, resistance to wildfire, and production of wood products that went into building materials or energy production. In this study, all three treatments (burning, mechanical thinning, and both together) were better (from a long-term carbon storage perspective) than doing nothing. Although the authors don’t state this, my interpretation of the results was that the mechanical thinning approach was the clear winner from a strictly carbon sequestration perspective.

Relevance to managers:

Here is my take on how burning and thinning can be implemented in order to decrease wildfire severity AND to minimize carbon release:

1.     Mechanical thinning- Try to avoid a net increase in surface fuel as a result of the treatment. This can be done by selling chipped material to a power plant (not always feasible depending on project location) or by doing whole tree removal, then burning slash at landings. Sell the merchantable logs to a mill. As this study showed, the carbon emitted from equipment is very small compared to the carbon that can be stored in wood products and the carbon that can be sequestered by the forest from long-term growth and protection from intense wildfire.

2.     Burning- Late summer/fall burns in the Sierra Nevada are more in sync with when fires occurred in the past. Consumption is also more likely to be “cleaner.” If intensity of the burn is high, then there is more carbon released but it also probably means that a follow-up treatment won’t be needed as soon. The authors of this study are reburning the burn-only treated stands after 7 years, while the mechanical + burn treatment (which burned much hotter) is projected to be ready for a burn after 17 years. While I am unconfident in the precision of this estimate, the point is that there is a tradeoff between fuel consumption intensity the persistence of the treatment effect.  

Critique (I always have one, no matter how good the article is) for the pedants:

This is a great study. I thought one of the most interesting parts of the study was the consideration of how much carbon was at risk to severe wildfire among the treatments. As far as I can tell, however, they are assuming that severe wildfire conditions AND an ignition will definitely occur in these stands. But what is the probability of it actually occurring? Should we consider the inverse of the weather percentile (2.5%) as the probability? Consideration of occurrence probability might change the interpretation of results considerably. Perhaps there aren’t good methods of calculating ignition/suppression probability... I heard from the authors on this issue: They are indeed working on this, but they need more ignition data and further analysis.

As far as I can tell from the graphs, the mechanical treatment was by far the “winner” in terms of carbon sequestration. Even if the probability of severe wildfire was 100%, the net loss of the thinned stands would be 20 tons of carbon per hectare. The net loss from the prescribed fire treatments were > 31 tons of carbon per hectare. Although mechanical thinning was the apparent “winner,” this was not pointed out. In fact, more emphasis was placed on the burning treatments as being the most effective. Am I missing something?