Post on 24-Jan-2016
Fuel treatment effects on forest carbon and wildfire
Malcolm North, Sierra Nevada Research Center, mpnorth@ucdavis.edu
Wildfires: A Large Source of Emissions
(Wiedinmyer and Neff 2007)
By one estimate annual forest growth can offset 6-10% of anthropogenic CO2
But these gains can be offset by emissions in fire-prone forests
Premises:
•In California’s fire-dependent forests, significant C can be released during wildfire
•The amount of that release increases with fire severity and size
•General objective: If possible increase C storage, reduce the risk of C loss due to wildfire and in the process minimize C emissions
•Forests need to be managed for more than just carbon sequestration
•Fortunately, forest restoration and C management share a common long-term objective: Redirect ecosystem C away from unstable (the growth of numerous, small trees) to stable pools ( the growth of fewer, large-size trees which are fire-resistance [i.e., pines])
•The question is how to get there and what are the tradeoffs between different means to that end
Removed Tree Size (dbh)
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C.StorageC.EmissionsFire.Sev..ReductionTree C Sequest RateEcosystem.Restoration
Conceptual Model of Tradeoffs in Fire-Dependent ForestsE
ffec
t %
*
* If Ladder AND Surface fuels are removed
What we know:The general shape of these curves
What we don’t know:The slope and inflection pointsHow these vary by forest type, productivity, etc.
Desired direction
‘Risk’High Low
‘Benefits’Present Future
Total Live Tree Carbon Stocks: 1865: 346 Mg C/ha Current Forest: 249 Mg C/ha
Potential for Increasing Forest C StorageLess carbon in modern fire-suppressed forests than active-fire (1865) forests due to loss of large trees
The plus is forests have potential to sequester a lot more carbon
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abcoabmacadepilapijeother/unk(1865)snagsCarbon1865 Carbon
>
Model estimate of wildfire emission is 38 Mg C/ha
Bonnicksen estimated 156 Mg C/ha average for CA wildfires (FCEM Report #2)
A forest structure (1865) of a low density of large pines has lowest wildfire and prescribed fire emissions
Untreated
Thinned and Prescribe Burned
Fire Direction
Treatment
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Su
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abcoabmacadepilapijeother/unk(1865)snagsCarbon1865 Carbon
a) control
b) burn only
c) understory thin
d) understory thin/burn
e) overstory thin
f) overstory thin/burn
> >
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1865
Control
Pre-treat
Post-treat
Pre-treat
Post-treat
Pre-treat
Post-treat
Pre-treat
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Live tree C and diameter distribution by species before and after 6 treatments
Desired: higher C stocks, greater density and a higher % of pine in the large dbh classes
Treatment
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Em
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TreesSnagsFuelsSoils/RootsLumberTotal Emissions
1865
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onl
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rsto
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Summary: What we may know and what we clearly don’t know
Fuels treatments: • Reducing ladder AND surface fuels reduces fire severity.
• Reducing surface fuels is key to restoring many ecosystem processes
• Thinning overstory trees (reducing crown bulk density) has a limited effect on reducing fire severity.
• Simulations (field data is absent) suggest treating 20-30% of a landscape can significantly reduce fire severity and extent
• Don’t know: How long fuel treatments remain effective
• How treatments affect residual tree growth (rate of C sequestration)
• When reducing crown fire risk, how resistant should the forest be made? (i.e. what percentile weather conditions should be targeted? Impacts of climate change?)
Carbon Dynamics:• Within the limits of current measurements: 60-70% of ecosystem C is above ground, with about 80% of that in live trees.
• In uncut forests, trees 5-25 cm dbh and 25-50 cm generally will contain about 5% and 15-20%, respectively of total live tree C.
• Fuels treatments reduce forest C, losses increase exponentially with tree size
• In most fuels treatments, fossil fuel use is a small % of C loss.
• C losses from milling waste and prescribed burn vary but are probably in the range of 5-20% of aboveground C • Estimates of prescribed fire emissions (15-25 Mg C/ha) are probably within the actual ‘ballpark’ but wildfire emissions (25-45 Mg C/ha) , while improving, are still very rough.
• Estimates of fire CO2 emissions are hampered by our lack of knowledge about C deposition, rates of atmospheric vs. soil incorporation of dead wood C, ‘real’ soil C loss, etc.
Summary: What we may know and what we clearly don’t know