Does it Require Wildfire to Kill Eastern Redcedar?
Eastern redcedar is a fire sensitive tree that is killed using prescribed fire. Fire intensities of 160 kilowatts per meter, equivalent to ~ 2-3 foot flame length, are required to kill cedar trees. Many prescribed fires are just below this intensity, but scientists and land managers have found that by targeting fuel load and or fuel moisture conditions, this threshold can be easily reached within the prescription of a prescribed burn plan. High wind speeds are not required to kill mature cedar trees.
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[00:00:05.230]You couldn't use fire, even a decade ago,
[00:00:08.690]to restore grassland if you had
[00:00:10.500]cedar trees over six feet tall.
[00:00:12.640]It was considered to be only appropriate
[00:00:15.300]if they were small trees, because it wouldn't work.
[00:00:18.660]Or because of perception that you couldn't control the fire.
[00:00:22.000]Both of those are outdated today, with what's known.
[00:00:25.050]There is a specific threshold to kill a cedar tree.
[00:00:27.730]That threshold has been quantified in the science,
[00:00:30.440]we have that publication, and it shows that
[00:00:32.670]it shows 160 kilowatts per meter.
[00:00:35.340]That is about the equivalent of
[00:00:38.070]a two to three foot tall flame lengths,
[00:00:40.210]which is associated with
[00:00:41.043]the amount of heat that's produced by a fire,
[00:00:43.090]the amount of energy it produces.
[00:00:45.220]So, relatively small flame lengths
[00:00:48.290]in order to actually get at that height.
[00:00:50.810]And that intensity depends on how tall cedar trees are,
[00:00:53.740]so it varies a little bit.
[00:00:54.700]It depends on certain conditions
[00:00:56.850]that actually were described by
[00:00:59.300]Van Wagner's equation in 1973.
[00:01:01.990]It just so happens that most prescribed fire conditions
[00:01:04.750]are right below that threshold.
[00:01:10.770]So the notion that it then takes wildfires
[00:01:15.770]is an interesting polarization of the idea.
[00:01:19.900]So on one hand, groups will say,
[00:01:23.350]low-intensity prescribed fires
[00:01:24.810]can't kill a six foot tall tree.
[00:01:26.370]On the other hand, they'll say,
[00:01:28.210]the only fires that can kill it are wildfires.
[00:01:30.750]So what you're seeing are people's experiences
[00:01:33.020]and observations coming into play.
[00:01:35.270]So the prescribed fires they set
[00:01:37.670]didn't do what they thought it would do.
[00:01:39.740]They're then on wildfires, fighting wildfires,
[00:01:43.530]and they're seeing these really intense,
[00:01:45.250]out of control extremes of fire
[00:01:48.780]that then are crowning out cedar trees, high spread rates,
[00:01:53.690]and they're using those to say what is required.
[00:01:56.890]And of course, they're two ends of the spectrum.
[00:01:59.350]There's, in reality, a threshold.
[00:02:01.350]Where does that threshold exist?
[00:02:02.980]Well, science has quantitatively and mathematically
[00:02:05.350]shown that it's really close to
[00:02:08.610]the low-intensity prescribed fires.
[00:02:15.250]The challenge with this ecologically is that
[00:02:18.290]if you wait until trees become established in grassland,
[00:02:22.310]it removes that fuel load underneath it.
[00:02:24.710]So even if you defer cattle,
[00:02:26.350]you're increasing fuel load at the edges of the trees,
[00:02:29.070]not right underneath them.
[00:02:30.730]So it's really hard to increase fuels
[00:02:33.360]after cedar trees have displaced the fuel.
[00:02:36.510]So that's an approach that has limited effectiveness.
[00:02:39.630]It can work, with the right conditions.
[00:02:41.660]But it's one way to increase intensity.
[00:02:43.710]What we were able to start to establish and do is
[00:02:45.790]show, well, where else in this biophysical mechanism of fire
[00:02:49.720]that has been really well understood
[00:02:52.010]could be targeted by groups to increase intensity slightly,
[00:02:56.640]reach the threshold at which you see mortality?
[00:03:00.010]And just fundamentally understanding how that process works
[00:03:02.950]between the fire and cedar trees.
[00:03:07.000]The common one that's been done, in addition to fuel load,
[00:03:09.970]is that other groups will target
[00:03:11.410]different moisture conditions.
[00:03:13.000]If you have drier fuels, again,
[00:03:14.590]it's a way to increase intensity.
[00:03:16.340]So those two are the most common that we see.
[00:03:19.750]A lot of groups will say that
[00:03:20.950]it also requires increasing wind speed.
[00:03:25.240]That is not done in practice and is not required.
[00:03:28.490]All you have to do is manipulate one of these parameters.
[00:03:31.810]If everything else is kept constant,
[00:03:33.730]we know with mathematical models that
[00:03:36.190]that can get your intensity high enough.
[00:03:38.600]So a lot of ecosystem managers will choose one of those two.
[00:03:41.640]Wind speed really changes how embers are transported,
[00:03:44.750]it changes the potential for a fire to
[00:03:47.780]start spotting and creating new fires.
[00:03:50.250]So usually, they do this in very low wind speeds,
[00:03:54.170]increasing either herbaceous fuel load
[00:03:57.300]or decreasing moisture conditions that they target.
[00:04:00.370]We did over 80 experimental fires to test those conditions.
[00:04:05.020]All of them were in wind speeds that were
[00:04:07.120]three miles per hour or less, somewhere in that range.
[00:04:11.200]It's all about fire intensity relative to
[00:04:14.670]a couple of other parameters.
[00:04:16.340]The easiest one to target and manipulate
[00:04:19.230]is the intensity of the fire.
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