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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. Certainly dead trees are useful for wildlife but where ponderosa pine dominates in eastern Fresno Co., virtually every tree I sas was dead, with some showing indications decay had alfready begun. The sher number of dead trees is far beyond what wildlife can utilize, and what happens after they fall to the cavity nesters and insect feeders? As for fungi and insects, there is plenty of dead material on the forest floor from just normal tree death. if all dead trees are left standing to fall when decay ends up with them falling in all directions, while it could ceate a banquet of overabundance for the species thriving on dead matter, while making it very difficult for the four footed residents of the landscape to move about among the fallen trees, much like people in tonado prone areas experience when they return to their damaged homes and businesses.

    More important in my mind is how will we (1) prevent major foresty alteration well beyond the areas of greatest tree loss from the inevitable wildfire that will igniite and been diriven by strong winds into areas where plenty live trees remain, and (2) whether we remove dead trees or leave them to fall in place, how many generations will it be before the forest we knew returns. If the Southern half of the Sierra continues to experience another year or two of below average precipitation and early snow melt, will the lower elevation conifer forests become an endangered plant community?

  2. Science, contradicting commons sense for more than 500 years.

    “In considering whether beetle-killed trees affect fire risk, researchers often consider two different dimensions. The first is the probability that a fire will occur. So, are we likely to see more wildfires in places with pulses of tree mortality from beetles? And second, when fires do occur in an area with beetle-killed trees, will it burn more severely?”

    The answer to the first appears probably not, and the 2nd, probably not, at least at higher elevations. And then I don’t understand this: “Just because the findings may not implicate beetle-killed trees in increased fire severity or probability in some places, there may still be other ways in which they affect fire behavior. And, if the trees are in places near homes and other infrastructure, they do still require mitigation, said Veblen, because they can be a danger.”

    And thanks re the answer on portable biopower generators.

    • Karen,

      Answers to #1 & #2 – “probably not”? That’s so far from reality, it almost borders on irresponsibility(???) The only probability is when.

      #1 – Take a stand of 100 trees, 90 of which are dead (for probability’s sake). Under live conditions – 5 trees out of 100, the probability of lightning striking a dead tree is 5%, to which the likelihood of it smoldering and causing a fire is higher, say 50% given a live forest holds moisture & that dead tree might be punky. So we are at 2.5% of a fire igniting from a single lightning strike in said “forest” of 100 trees.

      Now kill off 90 trees; moisture holding capacity of said forest drops to nil (5-15% in the Sierras), needles drop to the ground with moisture content again nil & directly proportional to atmospheric RH. The chance of a dead tree being struck in this “forest” is 90%, with chances of it igniting 50-100%, whether in the tree or at ground level with the tinderbox of dead needles. Chance of a fire igniting in said forest – 90% x (50-100%) = 45 – 90%

      #2 – See moisture content points above in high mortality kill zones, also these are not “park settings”, there is amble ladder fuels, whether naturally shaded dead limbs or various aged undergrowth is throughout the forest. 90% of a stand of trees dead with ambient atmospheric humidity (moisture content) of 5-15%, where moisture does not “re-enter” the tree after death, means flammable fuel load is at an all time high. Crown fires also generate their own local weather patterns.

      There may be higher moisture content in riparian or north facing slopes, but the idea that any percentage of these kill zones are water logged is marginal at best an assertion. A saving grace for the Sierras, it is one of the lowest lightning strike regions of the world. But it only takes one strike to burn a 100,000 acres. This is why California utilities spend several hundred million dollars a year keeping trees away from power lines and a single fire costing $100 million is not uncommon.

  3. I was drawn into reading your blog because of the great catchy title (good one!) and stayed because of the good analysis and learned about blue flames to boot. Thanks!

  4. What about the benefit to wildlife ecosystems from dead trees? Many species nest in dead trees and feed on the fungi and insects. Why not leave the dead trees to naturally decompose?

    • One should drive from Stockton, California to Bear Valley (Mnt Reba), stopping every 5-10 miles and noting the ecology in detail before offering this suggestion.

      The Sierras are not exactly the Namib desert, but they are much closer to it than a dense tropical or temperate rainforest.

      Except for some pockets of flat[ter] land that holds moisture and you’ll find the giants of the giants in trees, the Sierras are arid. Entire [dead] forests are baked in the sun, not a refuge for wildlife or a location where decomposition happens faster than a snail’s pace.

      The elevations just below the kill zones are made to burn & burn hotly; cresote bush, manzanita, digger (grey) pine. Evergreens (in general) and above this elevation kill off leafy trees (organic matter) with the acidity of their needles. There isn’t much decomposition occuring in the Sierra occuring without a live canopy to shade the thin soils.

      It’s not a matter of when a fire ignites, but how soon & how intense.

  5. Sorry, not meaning to “steal” this topic, but ultimately, the scale and topography of this problem is a conundrum, where doing the “right environmental thing[s]” is honestly out of the question and it’s little different than adding band-aids to bullet holes.

    Doing it “environmentally right” – selective harvesting & dragging out the slash (in order to get the highest CO2e abatement in contrast to natural decomposition) imposes significant cost adders (decreasing mechanized efficiency) while impairing the effective rate by which the process (wholesale clearing of all effected trees) needs to occur over a very short period of time.

    Every day, or every tree (slope, forest, etc), left “waiting” for prescribed treatment, increases the risk that Mother Nature takes over anyways with a lightning-strike initiated fire[s] or humans accidentally initiate a fire as well, or both.

    Then what, so much for “planned” CO2e abatement AND (more importantly), there is still standing dead (now many factors more difficult & dangerous to work in) and all the materials that would have become slash is burnt up (releasing its CO2) and cannot be used to stabilize the mountainsides, hence first rains – all hell breaks lose with remaining bio-nutrients washed into the reservoirs, not available for new plant life & decomposing anaerobically producing methane.

    So what is really needed is the fastest, most efficient (cost & energy) prescription on logging, trucking and bio-mass to energy (where this energy/electricity receives priority on the grid) and is generated 24/7/365. The only concern with trucking (moving the beetles) and spreading the outbreak, but as they are generally tree specific and the flora is different, this shouldn’t be a deciding factor.

    Looking at CAISO app currently, biomass gen is running at 200-250 MW in a state that imports significant GW and whose system is 15-25GW throughout the day/seasons.

    There is significantly more biomass generation in the state than 200-250 MW, but see post above regarding their continued closures due to the “preferential” treatment [priority] solar PV &/or CSP is recieving because it is the “right environmental thing” to do. And it isn’t just about displacing natural gas generators with biomass [so that solar & wind continue to get priority & everyone gets a warm feeling of doing the right thing], it’s about maximizing the benefits available when the alternatives are all “poison pills” as mentioned above.

    Clearcuts, skidding, trucking & biomass energy gen are about the only options left, but when one has gangrene in your arm, do you swear off amputation to save a hand that you are likely to lose anyways?

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