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If a Tree Falls in the Forest…Should We Use It to Generate Electricity?

Every summer vacation, we pack our tree-hugging family into the car and head for the Sierra Nevada mountains. In many respects, our trip this summer was just like any other year, complete with family bonding moments and awe-inspiring wilderness experiences:


I date myself with this reference                                                     Source

But our 2016 photo album is not all happiness and light.  This year, we saw an unprecedented number of stressed and dying trees. Forest roads were lined with piles of dead wood.


source                                                                       source

These pictures break a tree hugger’s heart. But they barely scratch the surface of what has been dubbed the worst epidemic of tree mortality in California’s modern history. According to CAL FIRE, over 66 million trees have died since 2010. And it’s not over yet.

The underlying cause is climate change working through drought and bark beetles. Warmer winters and drier summers mean this pesky bark beetle has been reproducing faster and attacking harder.  Drought-stressed trees are more vulnerable to fungi and insects. The big-picture impacts are devastating.

Acres of dying trees raise fundamental questions about how to preserve and protect our national parks and forests in the face of climate change. These existential issues were at the heart of President Obama’s speech in Lake Tahoe last week. But the epidemic also raises some more material questions. This week’s blog looks at the heated debate over what to do with millions of dead trees in the forest.

 66 million trees and counting

I’m an economist, not a woody plant biologist, so I have a hard time thinking in terms of millions of trees. With some expert assistance, I made the following ballpark conversion from trees to some more familiar metrics.

  • 66 million trees hold approximately 68 million tons CO2e.[1] To put that in perspective, California emits about 447 mmt CO2e annually.
  • If all 66 million trees were used to make electricity at existing biomass facilities (a very unlikely scenario), this would generate about 38,600 GWh.[2] To put this in perspective, California’s biomass facilities generated 7,228 GWh (gross) in 2015 .

Upshot is that 66 million dead trees is a big deal, no matter how you measure it.

There seems to be widespread – but not unanimous– agreement that leaving close to 40 million dry tons of wood (my rough estimate) in the forest will increase wildfire risk and intensity to unacceptable levels. So Governor Brown has declared a state of emergency and formed a tree-mortality task force to safely remove the dying trees, especially those that pose immediate danger. Having dragged these trees out of the forest, what to do with them?  Right now, many trees are being burned in open piles or “air curtain incinerators”.   tree3

Wood burning in an air curtain incinerator

CalFire plans to start running these incinerators 24 hours per day in the fall.  Yikes. The thought of incinerating wood in the forest 24/7 begs the question: are we better off using these trees to generate electricity? Researchers, including some esteemed Berkeley colleagues and forest service scientists, have been collecting some of the information we need to answer this question.

Forest-fueled electricity generation – at what cost?

Teams of researchers have been documenting the costs of biomass generation versus “non-utilization” burning  (i.e., burning trees in the woods to reduce fire risk). The punchline: Unless trees are located quite close to biomass generation facilities, the cost of extracting the trees, processing the wood, and transporting it to biomass generation facilities exceeds the market value of the wood fuel for electricity generation.  And this market value is falling as biomass generators struggle to compete with low natural gas prices and falling solar and wind electricity generation costs.

Some stakeholders argue that current market prices and policy incentives are failing to capture all the benefits of biomass generation has to offer. In particular, a growing body of research looks at  relative environmental impacts. The table below summarizes some recent estimates of the quantity of pollution emitted per kg of dry wood across different wood burning alternatives:

Biomass option Emissions (g/kg dry wood)
Air curtain incineration


Open pile burning 1834 0.7 0.6 10 5
Open pile burning 1894 7.5 5.0 62.5 3
Biomass to energy: gasification 1349 0.062 0.127 0.859 0.25
Biomass to energy: direct combustion 1349 0.111 0.028 0.768 0.45

Sources are here and Placer County Biomass Program. Biomass to energy conversion assume trees are 40 miles from the site of generation.

The first thing to note  is that the  estimates of CO2e emissions from electricity generation (1349 g/kg) are lower than emissions associated with burning wood in the woods, even though additional emissions are generated in the processing and transport of wood fuel. The reason is that these estimates are reported net of “avoided” CO2e emissions. In other words, researchers assume that if a kg of wood is used to fuel biomass generation, it will displace natural gas fired generation and  506 g of CO2e emissions associated with that gas generation. So 1349 g = 1856 g-506g.

It is standard to see avoided emissions from displaced electricity generation counted as an added benefit of biomass generation. Absent binding regulatory limits on GHG emissions, this can make sense. But in California, CO2e emissions are regulated under a suite of climate change policies, some of which are binding.  If the aggregate level of emissions is set by binding regulations, an increase in biomass generation will change the mix of fuels used to generate electricity, but not the level of CO2e emissions.

A quick walk in the policy weeds puts a finer point on this.  In California, an aggressive renewable portfolio standard (RPS) mandates the share of electricity generated by qualifying renewable resources (including forest-sourced biomass). So long as the RPS is binding, an increase in biomass generation will reduce demand for other qualifying renewable resources (such as wind or solar). But it should not reduce overall CO2e emissions from electricity if the biomass generation and the renewable resource it displaces are CO2e equivalent.

If avoided CO2e emissions are set to zero, the estimated CO2e emissions per kg of wood burned look fairly similar across non-utilization burning and biomass generation. In contrast, these alternatives differ significantly as far as harmful pollutants such as NOx and particulates are concerned. Aggregate emissions of these pollutants are not determined by mandated caps or binding standards.  And the quantity of pollution emitted per unit of wood burned differs by orders of magnitude across non-utilization versus electricity generation options.

It is not clear how differences in these (and other) emissions translate into differences in health and environmental damage costs. But accounting for these environmental costs would presumably reduce the net cost of  biomass generation relative to the more polluting alternative.

Dead trees fuel biomass policy developments…

No matter how you measure it, there’s a lot at stake in California’s dead and dying trees. Some of the wood can be harvested for timber. Some of the wood will be left in the woods to provide benefits to soil and wildlife. But given the current trajectory, lots of wood will be burned.

Many of the forest managers and researchers I talked to despair that biomass generation facilities are closing down just as air curtain incinerators fire up. They feel strongly that more of this dead wood should be used to fuel electricity generation. In response to these kinds of concerns, the California legislature recently passed legislation to support biomass power from facilities that generate energy from wood harvested from high fire hazard zones.  The bill is awaiting the Governor’s signature.

Increased support for biomass generation (over and above existing climate change policies) makes sense if the benefits justify the added costs. On the one hand, burning more wood at biomass facilities will incur additional processing, transport, and operating costs. On the other hand, it will generate less local air pollution as compared to non-utilization burning and other potential benefits (such as reduced ancillary service requirements vis a vis intermittent renewables). Getting a better handle on these costs and benefits will be critical if we are going to make the best of this bad situation.



[1] Assuming a mix of conifer species (pine, Douglas-fir, true fir, cedar), we estimate1800 green pounds per tree. X 66 million trees = 118.8 billion green pounds of wood available or 59.4 million green tons.  If we assume 35% moisture content (dead trees have less moisture) we have 38.6 million BDT (bone dry tons). Multiply the dry tons by 0.5 to obtain a comparable weight of entire tree’s sequestered carbon.  This gets us to 19.3 million tons of carbon. Multiply tons of carbon by 3.67 to get comparable weight in CO2e, and then convert to metric tons = 68.7 million tons. Thanks to Steve Eubanks, Tad Mason, and Bruce Springsteen for assisting with these calculations. All errors are mine.

[2] 1 bone dry ton generates approximately 1 MWh in existing biomass generation facilities.

25 thoughts on “If a Tree Falls in the Forest…Should We Use It to Generate Electricity? Leave a comment

  1. This was being tested, as of July 2015, are results applicable here? “The “Powertainer” — the unit is built in a 20-foot shipping container, hence the name — uses a gasification process to generate up to 150 kilowatts from biomass such as wood chips….a very small plant that can be moved as needed has two potential advantages that could reverse the usual economies of scale.

    “First, by moving close to the site of a biomass harvest, a portable plant can substantially reduce the cost of its biomass inputs. For large, stationary plants that must source feedstock from long distances, biomass trucking costs can easily account for more than half of fuel costs (see, e.g., Springsteen et al., this issue, page 142 ). In addition, because forestry operations tend to be seasonal and biomass can be stored only for a limited time before decomposition begins to create problems, it can be difficult to maintain a steady supply of fuel to a large plant throughout the year, which in turn can lead to shutdowns that increase the average cost of producing electricity.

    “Second, electricity generated and fed into the state’s power grid in remote locations can sell for a price well above the statewide average. Because of transmission constraints, it’s often difficult to keep the grid functioning properly in out-of-the-way spots — which makes additional generation capacity in those areas worth a premium.”

    • Interesting, but the assumptions are formulated in an office, not in real (and as the case may be, Climate Change influenced) conditions of California’s Sierras.

      California has year-round biomass resources, valley & foothills agriculture/orchards when the snow (see point above) socks in mountain locations. Decomposition? We’re talking logs 1 to 6 ft in diameter, not leaves & needles lying on the ground in wet areas (again a rarity in the arid Sierras)…. which begs the question, how many Sierra-sized trees would one of these units “burn-through” in a day?

      Grid connections? Where the trees are sourced, again unlikely to have power lines, as most of the transmission, sub-transmission & distribution lines come out of the river valley & bee-line it straight to the ridges where most Sierra homes & residents live. But more obviously, getting the logs to the “micro generators”, most likely means they have already been put on log trucks, and rolling miles to the mill becomes a smaller component of the supply chain cycle.

      That said, maybe mining is something where lessons/wisdom can be learned. Namely “draglines” and much more mobile high voltage cables for distant connections to substations.

  2. All the NOx in open burning will come from the fuel since natural convection biomass fires don’t burn hot enough to make it from Air. So why do the Biomass to Energy plants emit less NOx than open burning? Do they assume some kind of catalytic conversion or De-NOx technology?

    • They may inject air at the generator as well as the feedstock & combustion mixture is highly uniform. If the plants are modern enough, which they should be given most of California’s existing (dedicated to) waste to energy generators were built within the past 5-15 years, they may have supercritical [temp] boilers.

      Also, come combustion time, the ‘large’ wood has been crushed/grinded and moisture content is at or below ambient atmospheric moisture. They may also be striping the bark & removing the sap, entering it into the combustion feedstock as needed….. Ie, the saps can be added to increase burn temp, whereas too much bark in the burn mixture may cool the burn temp (think cedars, redwoods & Sequoia – all barks that nature designed not to burn).

      These are the general conditions at a paperplant/mill with onsite generation (very common) and the feedstock mix is highly regulated/automated. Burn temps are likely much higher than that of a ‘self-regulated’ burnpile, where complete, not partial, combustion is the name of the game.

  3. One of the key things that needs to be addressed is how to monetize the avoided emissions not just of forest waste for biomass – but also ag waste (where open burns are also making an environmentally deleterious comeback) and urban landfill. Offsetting these fuel costs (especially as the cost of forest fuel increases due to the tree mortality emergency) will capture the non-energy benefits of biomass and help keep energy prices for biomass in the market range. Losing a resource that is a far superior means of reducing carbon and shot-lived climate pollutant waste -conversion into electricity at facilities utilizing best available control technologies (BACT) to minimize carbon emissions in the generation process and take them much lower than natural gas – would be a major loss to California. Sustainable practices both for harvesting their organic fuel sources, and for how biomass energy uses fuels seasonally when they are harvestable and proximal (trees from late spring to early fall, then ag and urban waste when dead/dying trees aren’t available) make biomass in the 21st century a key resource. That biomass energy is also a good replacement for old, baseload fossil fuel plants and is also in many cases also a good ramping source of electricity to replace gas peakers and better balance solar and wind on the grid – is another attribute that needs to be acceptably valued. Biomass/bioenergy are key to managing our lower carbon future and to preserving forests/ag lands, and of addressing the growing problem of increased landfill waste.

    • Generally not dead standing…. aka already dried wood. Imposes higher maintenance costs all along the value chain cutting through dried (not green) wood. Sawmills tend to “water”, especially in arid California, their log piles to keep moisture content higher before they hit the blades. Felling dead trees (of unknown mortality date) also is inherently more dangerous.

      Colorado, post-beetle kill in the lodgepole pines, has found some success in harvesting for lumber…. mainly from the blue colorations found in the wood due to the fungus that grows after the beetle invasion, as well as the “rustic” look…. but the market is only so big for these products, i.e., how many “rustic” beds can the market purchase.

  4. Perhaps the greater gain in GHG reduction could be to burn the wood in CA biomass electric generators and export the power to states with less rigorous RPS and without Cap and Trade. Also the blossoming CCA portfolio managers could purchase the biomass electricity for their beyond RPS volume needs, thereby keeping it from glutting the RPS. But it would still suppress emissions in cap and trade (allowing other emitters to emit) Do the numbers change if you assume likely CH4 leakage from CH4 compressor seals etc to move natural gas to gas fired power plants?. After all, you counted the emissions to move wood to biomass plants.

  5. Thanks for this thought-provoking post. Not be familiar with the details of Calif climate policy (AB 32), this reaction is just a quick one. If Calif automatically assigns an “avoided CO2 emissions” credit for biomass combustion instead of fossil fuel combustion, well, that’s sad. It’s incorrect carbon accounting (see Searchinger+ Science 2009, Bioenergy of any form is only beneficial for the climate if its production *increases* the net rate at which CO2 is removed from the atmosphere (DeCicco Climatic Change 2013,, which can included avoiding CO2 emissions from biomass decay or combustion that would otherwise occur.

    This being said, what might make more sense is to treat trees that don’t get burned in the forest through air-curtain incineration, or are otherwise left to rot (slow “combustion” thru metabolism) or burn through a forest fire, as an offset. The offset in this case is the actual avoided CO2 emission that occurs in the location of the forest. All the rules of careful carbon offset accounting should apply. If an offset is generated by removing the wood from the forest, then that offset can be purchased along with the trees by a power plant operator and applied as a credit against the actual CO2 emitted when the wood is burned for power production. In principle, the situation is similar to an offset generated through avoided deforestation (such as a REDD program).

    So the next question is, does the price of carbon make this worthwhile economically? I’ll leave that one for Calif carbon market experts to evaluate.

    In any case, I suspect that AB 32 (incorrectly) treats bioenergy as carbon neutral at the point of combustion if it involves the sweeping assumption that any form of biomass burned for power represents an avoided emission. That error may require remedy by CARB before the best disposition of the tragically dying trees can be realized.

    • I suggest stepping back one step. The table in the blog shows all of the GHG emissions as though they are 100% incremental to a baseline with virtually zero emissions. That may be a fair assumption when the biomass has remaining lives of decades or centuries, but that’s not the case here. The dead biomass will now decay rapidly or go up in smoke. The existing alternative to burning or generating power needs to consider this new GHG emission rate (which will require some study.) And the stock nature of CO2 requires that we think about the cumulative emissions over different periods–not just per MWH or per year. If we look at it that way, we may say “We’re going to emit the equivalent of 68 MMT anyway. How do we get the best value out of this event?” That’s going to require more lifecycle and market analysis. But my intuition is that electricity generation is likely the preferred option, even over creating finished products for which there likely is no market unless we provide significant subsidies to that industry. Looks like we have to “pick our poison” regardless.

  6. Hello Meredith,

    I recently saw friends (former coworkers) from California who continue to work with utility forestry programs, and they recounted the same as you regarding the state of the tree mortality in the Sierras. They preface it with, it is far worse than what we saw in the ’90s by many orders of magnitude.

    Unfortunately, you missed the big story here…. so I’m a bit surprised. Basically, the rise of California’s energy darling, solar PV, is forcing the closure of one biomass generator after the other…. not just those “closer” to the forests where they are now increasingly needed, but in the Sacramento & San Joaquin Valleys where they usefully utilize agricultural biomass waste for energy production and help mitigate air quality concerns due to ‘under/un-utilized” biomass burning in situ.

    I recall in 1997 a slash burning operation in the Sierras roughly between Lake Tahoe & Yosemite that went horribly “wrong” when an atmospheric inversion came in, which is not uncommon to California. What was a smoke plume akin to that of a volcano, normally not an issue when winds are blowing towards the east, was caught in the weather inversion, the entirety of the valley (roughly Sacramento to Fresno) turned into a “smoke trap” and filled up, not unlike Winter/Spring valley fogs for many days in a row. The Valley was “dark” during that time.

    Here is an article from the LA Times from last summer worth your review & consideration….

    “Solar is in, biomass energy is out—and farmers are struggling to dispose of woody waste” – LA Times, December 31, 2015

  7. Re the “reduced ancillary service requirements [of biomass] vis a vis intermittent renewables,” these costs are being factored into the investor-owned utilities’ RPS procurement decisions. They are currently pegged at $3 and $4/MWh for solar and wind, respectively. While these costs are not California-specific and the numbers will probably increase (particularly for solar) after studies now underway are completed, they are likely to remain a relatively small fraction of total generation costs.

  8. No do not use it to generate electricity. Use it to make recycled wood products, keeping the carbon tied up and supplying more building materials. Wood burning plants are horribly inefficient electric power sources and huge carbon emitters. Bio fuels for electricity is an idea that should have ended with cave men.

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