PINE BEETLES AND FIRE HAZARD IN THE BLACK HILLS

The recent extreme infestation of pine trees in the Black Hills National Forest by the mountain pine beetle has raised concern that the large number of dead trees has increased the risk of wildfire, and that it is more likely that wildfires will escalate to enormous size. Some regard the enhanced risk as so great that it creates an emergency situation, requiring immediate and extreme intervention to ameliorate the danger to public safety and property.

Such concern is entirely natural, based on both intuition and considerable experience of forestry experts and firefighters. Dead trees rapidly lose their moisture content, which makes them more flammable. Dead pine needles fall to the forest floor, increasing the surface fuel load, and eventually the dead trees are more susceptible to falling, which increases the load of large fuels on the surface. The extreme number of dead trees due to pine beetle tree-kill has created a situation which many regard as “a tinderbox primed for wildfire.”

The extremity of public concern was well summarized by Richard Finn, a landowner in Sturgis, SD, who testified in a Congressional hearing in 2002:


Now, a massive swath of bug-killed trees marks the landscape. To a man, the fire experts with whom I’ve spoken assess this area as having tremendously magnified fire risk, so-much-so as to be regarded inevitable. They fear the fire intensity these conditions will create, coupled with poor access and steep terrain, will furthermore render any fire ignited in Beaver Park completely unstoppable. The infestation shows no sign of slowing, the fire danger shows no sign of lessening, and as such, I fear each day for the safety of my family.

What is lacking in these evaluations is hard evidence that pine beetle infestation greatly increases fire hazard. A growing body of scientific research suggests that this is simply not the case. As Roy Renkin, Vegetation Management Specialist for Yellowstone National Park, stated in a NASA press release about a detailed study of the relationship between pine beetle infestation and fire hazard in which he participated,


I’ve heard [the tinderbox analogy] ever since I started my professional career in the forestry and fire management business 32 years ago. But having the opportunity to observe such interaction over the years in regards to the Yellowstone natural fire program, I must admit that observations never quite met with the expectation.

Is it possible that pine beetle infestation has much less impact on fire hazard than is generally believed, even by many experts? What does the latest scientific research indicate? What can specific data from the Black Hills region tell us?

BLACK HILLS FIRE HISTORY

Historical data on wildfire occurrence in the Black Hills is tabulated by the Wildland Fire Suppression divison of the South Dakota Department of Agriculture. Fire history was reconstructed from historical reports dating back to 1910. Here are the reported fires, indicating the year in which they occurred (on the x-axis) and the acreage burned (on the y-axis):

Note that the y-axis is a logarithmic axis, so each doubling of fire size corresponds to a constant increase in the distance up the y-axis. Note also that there’s a thin dashed line at 200 acres burned (the dashed line may not be easily visible in many browsers, but if you right-click the graph and select “view image” you’ll probably get a much clearer view).

It’s obvious that prior to 1959, no fires were recorded which burned less than 200 acres. This is a selection effect, not because such fires didn’t happen, but due to the fact that smaller fires were less likely to be detected and recorded long ago than they are today. Therefore it’s a much better idea to restrict study to fires burning at least 200 acres, so that the data will be far less affected by under-reporting of small fires early in the record. We can call these large fires, and for most of the record, it’s very unlikely that fires at least as big as 200 acres burned (nearly a third of a square mile) would have escaped detection.

A simple, but effective, way to gain insight into the changing fire history of the Black Hills, is simply to count the number of large fires occurring each year:

There are visible signs of greater wildfire numbers in the last several decades. The increase seems to begin in 1985, which witnessed 5 large fires, the first year in the record showing more than 3. In fact, nine years since 1985 have had 3 or more large fires, an event which only happened twice before 1985, during the extreme drought of the 1930s.

Surely something has increased the risk of wildfire in the Black Hills. But what?

WILDFIRE IN THE WESTERN U.S.

A study published in 2006 (Westerling et al., Warming and Earlier Spring Increase Western U.S. Forest Wildfire Activity, Science, 313, no. 5789, 940-943) noted that wildfire activity has increased since the mid-1980s, not just in the Black Hills but througout the Western U.S. The authors studied the relationship of wildfire occurrence to both climate and land-use histories, concluding that climate rather than land-use factors dominated the increased fire occurrence:


… large wildfire activity increased suddenly and markedly in the mid-1980s, with higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons … The greatest increases occurred in mid-elevation, Northern Rockies forests, where land-use histories have relatively little effect on fire risks and are strongly associated with increased spring and summer temperatures and an earlier spring snowmelt.

Their study clearly indicates that warming spring and summer temperatures throughout the region have brought about earlier spring snowmelt, which causes drier conditions late in the year, particularly the latter part of fire season. They also cause warmer, drier conditions during much of fire season, which increases the risk of wildfire and exacerbates those fires that do occur. Their bottom line is that climate change, rather than land-use factors, dominates the recent increase in wildfire frequency. They particulary identified warmer Spring-Summer temperature as a major factor in fire risk in the Western U.S.

FACTORS INFLUENCING WILDFIRE

Another factor known to have a strong influence on wildfire is drought. Times of drought, especially severe drought, strongly enhance fire risk and severity. In fact, the earliest period for which the Black Hills show exaggerated wildfire occurrence was the 1930s, a time of severe drought throughout the region.

The situation can be clarified if the impact of known factors, in particular drought and increasing Spring-Summer temperature, can be accounted for in fire data, in order to isolate the remaining changes and look for their relationship to other factors. A mathematical model of fire occurrence based only on drought (as measured by the “Palmer Z-index“) and Spring-Summer temperature accounts for much of the change observed in wildfire in the Black Hills:

The residuals (what’s left over after the modeled factors are removed) still shows enhanced wildfire activity in recent decades:

Now we can compare these remaining, “residual” wildfire changes to other factors which may influence wildfire. One way to do so is to examine what’s called the cross-correlation function, which looks for relationships which may have a delayed effect over time. The cross-correlation of residual wildfire activity with pine beetle tree-kill shows no sign of any correlation at all:

There is, however, significant cross-correlation between residual wildfire activity and the amount of timber harvest over time:

This might suggest that greater timber harvest increases the risk of wildfire, with a time delay of about 12 years. But “it ain’t necessarily so.” Timber harvest has increased over the years, so to a large degree the timber harvest resembles a steady increase over time. Perhaps there has simply been an increase in wildfire activity over time due to some other factor, and the timber harvest simply mimics this effect, acting as a “proxy” for a simple time trend.

MATHEMATICAL MODELS INCLUDING OTHER FACTORS

A more complex model of wildfires, including drought conditions, Spring-Summer temperature, and timber harvest does a much better job of explaining the observed changes in wildfire activity. So too does a model using a simple linear time trend instead of timber harvest data:

Statistical tests comparing these two models find that they’re about equally good at explaining the observed changes — so it can’t really choose between them.

One can go further, including yet more factors in the model, but those attempts failed to improve the situation — neither more influence from timber harvest, nor the combination of timber harvest with a time trend, nor (more importantly) including pine beetle tree-kill in addition to other factors, gives a better model of Black Hills fire history. In fact, when pine beetle tree kill is included in the model it indicates that greater pine beetle infestation actually reduces fire risk — but this influence is not statistically significant so it’s likely to be merely a chance result brought about by the randomness in the data.

It is also possible to compare the likelihood of very large fires recently to previous decades. Since 1985 the Black Hills have witnessed more extreme wildfires than before, but comparing the distribution of fire size before and after the recent change in “fire regime” indicates that this is simply because there are more wildfires, period.

The bottom line is that data for the Black Hills region do not support the idea that pine beetle infestation has increased the risk of wildfire occurring, or of fires which do occur escalating to extreme size. This doesn’t prove that there’s no effect — absence of evidence is not evidence of absence — but it does show that even if an effect is present, it’s not nearly so severe as has been claimed in public and policy discourse.

RECENT RESEARCH

Several very recent scientific publications have specifically addressed the effect of pine beetle infestation on fire hazard. Some have indicated enhanced risk (Schoennagel et al. 2012, Effects of Mountain Pine Beetle on Fuels and Expected Fire Behavior in Lodgepole Pine Forests, Colorado, USA. PLoS ONE 7(1): e30002) but not as great as has often been claimed, some suggest little impact, or even reduced risk soon after infestation and possible enhanced risk much later (Simmard et al. 2011, Do mountain pine beetle outbreaks change the probability of active crown fire in lodgepole pine forests?, Ecological Monographs, 81(1), 2011, pp. 324, and Tinker et al. 2009, Reciprocal interactions between bark beetles and wildfire in subalpine forests: landscape patterns and the risk of high-severity fire, Joint Fire Science Program project no. 06-2-1-20). This is an area of active research and a consensus has not yet emerged, but current research definitely belies the notion that pine beetle infestation creates a “tinderbox primed for wildfire.”

Clearly, the Black Hills region, like most Western U.S. forests, is at greater risk of wildfire than during most of the 20th century. But there is little, if any, evidence to implicate pine beetle infestation as the culprit in that increased risk. Other factors, including drought, changing climate conditions, and perhaps changes in forest management (including fire suppression) are at work, and their impact seems to overwhelm the results of pine beetle infestation.

Wildfire hazard is a crucial issue which must be addressed with as clear as possible a perception of the actual risk factors. A focus on pine beetle infestation seems misplaced, threatening to draw attention away from factors which have strong and demonstrable impact on fire hazard and to divert limited resources to less productive strategies. Surely, excessive rhetoric about the urgent fire danger posed by pine beetle infestation, sometimes to the point of hysteria, does not serve the public interest.

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68 responses to “PINE BEETLES AND FIRE HAZARD IN THE BLACK HILLS

  1. FWIW, there is a lot of research, some going back several decades, which does indicate that increased logging, for various reasons, does lead to increased to incidence of wildfire.

    [Response: I found a little of it. Among other things, clearing can alter the microclimate of the forest floor, allowing more sunlight to warm the surface and increasing the likelihood of ignition. There are other possible factors too — but I don’t recall them.]

    • Debris from logging and increased access by humans are others…

      • Gavin's Pussycat

        Many lumberjacks smoke…

        Another suggestion FWIW is that cutting makes the forest younger. Younger trees may die easier from drought, increasing fire hazard.

    • On the Black Hills, with near-perfect weather pattern for ponderosa pine (reliable spring rains), once the canopy is opened the forest floor erupts with thousands of stems per acre of “doghair” seedlings.

      In 10-15 years, these become pole-sized “jackpine” that can carry a ground fire easily into the canopy.

  2. Interesting. Another area of study where the exclusion (or rather, the correct estimation) of the smaller events from the available records is essential in my opinion is the volcanic record of the holocene. I tried in here estimate the frequencies of +4 VEI events, which may inject sulfates to stratospheric levels, during history, but I there are probably many uncertain events that weren’t included in the dataset I used.

  3. This photographer has a lot of images of the historic Black Hills with modern photographs taken from the same vantage point for comparison.

    • There appears to be a lot of areas afforested since the earlier photographs. Are there any estimates of acreage of the area under forest that could be used in the analysis?

    • Thanks, JCH. It’s a beautiful area, and one often overlooked (no pun intended) despite the attractions of Mount Rushmore (and the ongoing Crazy Horse memorial project).

      Interesting to observe that, for the most part, there appears to be more tree cover in most of the modern images than in the corresponding historic ones. I know that in the East forest cover has been increasing for quite a while, but I didn’t expect that to apply to South Dakota (if indeed it does, when examined more rigorously.)

    • The increased tree cover is most likely due to the aggressive fire suppression policies followed by the forest service since the early 20th century. This leads to increased fuel loads and consequently more severe fires. In environments where P. Ponderosa tends to dominate historic (prior to suppression policies) fires were fairly small, low intensity and frequent which kept the understory fuel load low and the forest itself fairly open.

      • When I was in college I worked summers as a cave guide in the Black Hills, so I have a fondness for the area. Got to map and survey with Herb and Jan Conn in Jewel Cave, which was a neat experience. For those who are old enough, magazines used to have photographs of climbers patching the cracks on Mt. Rushmore. For a very long time, Herb and Jan were the only two allowed to do that task.

  4. I find it a bit odd to count number of fires when their sizes vary so wildly. Is there a reason to use that measure rather than acres burned?

    It probably makes little difference in the end, but I don’t see why one fire burning 10k acres would be so different from five fires doing the same.

  5. The full Black Hills Fire Risk analysis paper (2.7mb pdf) can be found here.

  6. This photographer has a lot of images of the historic Black Hills

    Is it me, or is there a case for suggesting that there appear to be more trees in almost every one of the contemporary images compared to the historical?

  7. Is there any estimate of the change in the number of people going into the hills for recreational purposes, and whether that has had any influence?

  8. Another factor increasing the risk of severe fire over time is the fire control. In normal condition, small fire arise all the time and bur the dead biomass. If those fires are suppressed dead biomass accumulated and makes the whole setting more vulnerable to bigger fires. I would not be surprise is more logging is correlated with a more effective fire suppression.

  9. Perhaps the changing climate in the Black Hills is favorable to pine beetle infestation. The infestations and the increased fire risk are both consequences and not cause and effect.

  10. I think the leap from area burned in Figure 1 to the number of large fires (> 200 acres) may be masking some insight. For example in 2000 there were 5 large fires; however, the largest was about 500 times larger than than the smallest, but they are given equal weight in your model.
    Could that large fire been a mereger of several fires?
    It may be that there are fewer fires after a large burn, since there there are less burnable acres.

    [Response: It’s important to examine both fire severity and the likelihood of fire occurrence (which is tied to the number of fires rather than their size). This study did both, by analyzing time series of number of fires, and comparing size distribution between different time spans.

    The extraordinarily large fire of 2000 (which was about four times as large as the next-largest fire on record) was the Jasper fire, which was caused by arson.]

  11. JCH, be careful, only the earliest views there show what the Black Hills were like before mining and settlement. Photos of most any mining region in the West show that the forests were quickly stripped of timber large enough to be used as mine tunnel supports and cribbing.

    But even with the earliest views, keep in mind that the Black Hills were a managed landscape before Custer’s expedition, so the expansion of tree cover is a relatively recent development in the wake of the demise of the mining industry and the advent of concerted fire suppression.

    • Jim, whites were in the area before the Custer Expedition, but in very small numbers. Afterwards, there was a mining boom.

      I think most mining operations started after the Custer Expedition. Homestake was the only large, long lasting mine.

      To me the issue would be the burning activities of Native Americans. My impression has always been they tended to live outside the hills.

    • JCH, I’m aware that the mining boom occurred after the Custer expedition, which is why I used it as a demarcation point. The Homestake was the largest operation, but far from the only one. If you want to see photos of what the Hills looked like during the boom and how extensive the mining development was you might want to take a look at the book “Railroads of the Black Hills.”

      True, the various plains tribes that made use of the Black Hills did not reside in the Hills, but that does not mean that they did not manage what was a major resource for them.

  12. After areas near Melbourne in Australia had fatal bushfires (Australian for wildfires) a couple of years ago, on a day when the maximum temperature reached 46.5 C, and there were very strong, dry northerly winds, there was much debate about how to reduce the damage next time.

    Much of the focus was on controlled burns, where the undergrowth is reduced through controlled fires(“prescribed burns”) lit in spring. Typical angry, middle-aged, self styled experts blamed the green movement for a lack of prescribed burns.

    A recent study which looked at the likelihood of houses surviving bushfires found that it was really only the amount of vegetation close to the house that mattered. And that if prescribed burns were used, they were more effective when close to houses (as close to 500m) rather than kilometres away.

    But of course that won’t stop another round of environmentalist bashing the next time an opportunity to blame them for something arises.

    • Another feature of many types of Australian bush is that regular firing, of the sort that proponents of “controlled burning” like to advocate for bushfire suppression, tends to select for species that respond well to firing.

      The thing is, these species also tend to be more flammable than other species, and so present a greater risk for long-term fire control. The response is to fire them more frequently in order to (temporarily) reduce that risk, with the consequence that they grow back with even more enthusiasm. And on it goes…

      I see this on the scale of hectares on my own property, where it is taking a lot of effort to restructure communities of currently-flammable understorey species that have resulted from other people’s “control burning” in past decades.

      Another aspect that tends to be lost in the hysteria to pre-emptively burn everything in sight is that (in certain eucalypt associations, at least) ground litter flammability increases with time only for a relatively short period (6-8 years seems to be a period accepted by some fire ecologists I spoke with a number of years ago), beyond which the bottom layers retain moisture effectively and compost away. In such composting accumulations it appears that the fire risk reaches a plateau (and may even then diminish), and over time possibly permits assemblage composition to change to less flammable associations.

      I saw a lot of evidence for this in the transects I established for my (faunal) PhD fieldwork, wherevery flammable schlerophyll forest had rapidly outgrown (relatively less-flammable) rainforest in valleys after recent bushfires, and where rainforest had slowly pushed into schlerophyll forest where there had been a period of many decades without fire.

      Of course, such instances do not translate into blanket generalisations, but they go to show that the simple strategy of ‘fuel-reduction’ burns might not always be so fuel-reducing over time. They certainly can have many unintended consequences, the alteration of ecosystem structure not the least amongst them.

  13. Is there any evidence in the data you have of a change in frequency of the wildfires in later vs earlier months of the season? Sounds like “longer wildfire seasons” suggests that.

    [Response: The data included only the year of fire occurrence, not the time of year. It is a very interesting question.]

    • Another aspect is elevation. The fires are overwhelmingly (>80%) occurring in the foothills around the perimeter of the Black Hills, not in the higher elevation interior, which would have cooler temperatures and higher rainfall.

      The drought index might capture some of this, but the spatial pattern is a very interesting aspect of fire occurrence.

  14. There has been aforestation throughout most of the western U.S. ponderosa pine forests. This forest is best described as a savannah and its grassy understory component used to suppress recruitment of ponderosa pine through direct competition for water with tree seedlings and by providing fine fuels which promoted frequent, light fires that killed pine seedlings. Thus was the savannah sustained and made up of large pines, relatively few young trees and frequent fires that didn’t kill the canopy.

    In my opinion it was grazing and the removal of grasses starting in the 1900s that did the savannah in through dense aforestation. I say this because the affects of grazing were first reported in Ponderosa Pine savannahs in the 1920′s by USFS rangers, where they cited an increase in thickets of young trees. Others cite fire suppression, though I can’t see how you can argue that point out west in the early 1900′s. Others cite lack of ignitions from native Americans and this is borne out by studies of fire histories. Probably it is a combination of grazing’s removal of the grasses and a lack of ignitions which promoted dense recruitment of seedlings into the fire accessible mid-story where they either act as ladder fuels or where there densely packed crowns can carry a crown fire.

    Removal of the canopy through logging also destroys the savannah, but it should be able to recover if regular prescribed fires are implemented afterwards. However, this doesn’t occur because the native grasses are long gone. Thus you get a dog-hair thicket of regrowth after logging.

    • “In my opinion it was grazing and the removal of grasses starting in the 1900s that did the savannah in through dense aforestation.”

      At least out there in California and the far west (I don’t know about the Black Hills, but I suspect a similar story) grazing also caused replacement of native bunch grasses with a predominantly-european annual grass cover. The annual grasses ‘brown up’ substantially earlier in the season, and create a uniform (not patchy) flammable grass cover for more of the fire season.

  15. Horatio Algeranon

    Global Warming’s Unlikely Harbingers: The West is heating up — and barkbeetles are moving in for the kill talks about the effect of warming temperatures (particularly at high latitudes and altitudes) on beetle infestation.

    The article relates that ten years ago, 2 scientists and a mathematician produced a model which predicted that if temperatures increased, the beetles in some areas would shift from a two year life cycle to a one year cycle, thereby enhancing infestations.

    But as we all know (and are reminded ad nauseam by the Wills of the World), “All Models are worthless — and those that deal with climate (an average, by definition) are the most worthless of all”, even (particularly?) when the model predictions turn out correct (at least qualitatively).

    They started watching beetles march through whitebark pine on Galena Summit in the Sawtooth Mountains. Last year, even their highest-elevation study site got hammered: The whitebark pine on 10,000-foot-high Railroad Ridge, an area that Logan and his coworkers have monitored for more than a decade, was hit hard by the mountain pine beetle. Sure enough, as temperatures warmed, beetles at these sites shifted from a two-year to a one-year life cycle — just as the model predicted.

    Horatio has very fond memories of the Stanley, Idaho area (rafting the Middle Fork and Main Salmon River and backpacking and climbing in the Sawtooths), so this hits home.

    …but, of course, Horatio must keep repeating to himself, “It has nothing whatsoever to do with climate change” — and must constantly remind himself of the “Word of the Wills” about models.

    The Models are worthless
    As worthless can be
    ‘Specially when they deal
    With the life of a tree

    On the issue of beetles and fire, one researcher — Bill Romme (note that he does not go by “Will”) even says that in some cases, beetle infestation may actually decrease fire hazard:

    Romme and other scientists sent a letter to Interior Secretary Gale Norton last December, arguing that beetle outbreaks in the piñon pine forests of the Southwest may reduce, not increase, the danger of large, intense fires in the tree canopy. When piñon needles drop to the ground, Romme explained, the tops of the trees are less likely to burn.

    • “When piñon needles drop to the ground, Romme explained, the tops of the trees are less likely to burn.”

      Sounds like the arboreal version of “die young and leave a beautiful corpse.” Also like cold comfort.

  16. Jim E, thanks for answering my [almost] rhetorical question. A compelling explanation.

  17. Daniel J. Andrews

    Interesting. Now I’m wondering how that applies to the spruce kills in the Yukon as the spruce bark beetle kills vast areas. Flying by plane you see reddish-brown swathes covering the landscape and when working up there I heard time and again the tinderbox analogy. I’ll try and start digging up some data now that you’ve shown the way.

    [Response: Be advised that each case can be different. For instance, pine needles contain a highly flammable oil which breaks down rapidly when the tree dies — this reduces flammability and offsets the increased flammability due to drying. Also, dead pines lose a lot of their fuel load in the canopy (up to 40%), which again might not be the same for spruce. The fact that pine beetle tree kill doesn’t have much influence on fire risk, doesn’t necessarily translate to other species. Each case should be evaluated independently.]

  18. Well I did a wiki and found this;

    http://en.wikipedia.org/wiki/Mountain_pine_beetle#Fire_Hazards

    http://www.nasa.gov/topics/earth/features/beetles-fire.html

    I’d also have questions of the quality of the dataset prior to the satellite era. Not too sure that Smokey the Bear had ranger stations everywhere that had total coverage of the forests.

    Changes in the forest cover/type versus time and changes in human populations (specifically smaller settlements and single homes versus time and human campsites and forest accessability) might also bias the dataset.

    Changes in FS land use practices (fire suppression versus letting nature take its course) would appear to be the main reason for increased forest fires (all other things being equal)?

    A few personal anecdotes, In 1996 the north side of the Grand Canyon forests were mostly off limits due to potential fire hazards. In the late summer of 2007, while flying over eastern Oregon via commercial jet, I could see literally hundreds (if not thousands) of forest fires, in the late summer of 2005 while visiting the Smokey Mountains it was interesting to find out about the deforestation/reforestation logging history and the current problem areas of various biological infestations.

    Regardless of the frequency/duration/intensity/size of forest fires, does anyone think that the pine beetle problem is a good thing for our forests?

    On a side note, I’ll be relocating to Colorado sometime in the spring/summer/fall, I’ll be on a very long camping trip “out west” ranging from Mexico to Canada in the meantime.

    • The earlier data mainly comprises aerial surveys from helicopter and fixed-wing planes, hand-drawn on maps, and later digitized in some kind of GIS program. The degree of error is quite a bit larger than current systems. I have done some of both the aerial surveys and map digitization myself here in western Canada, very early on in my career. The quality of the data is very dependent on the abilities of the individual, and coverage tended to be biased toward commercially important species and forest stands.

  19. Sturtevant, Brian R., Brian R. Miranda, Douglas J. Shinneman, Eric J. Gustafson, and Peter Thomas Wolter.

    In press.

    Comparing modern and presettlement forest dynamics of a subboreal wilderness – Does spruce budworm enhance fire risk?

    Ecological Applications. [doi:http://dx.doi.org/10.1890/11-0590.1%5D

  20. Pete Dunkelberg

    Thanks Tamino for sorting this out. It’s not surprising that killing a number of pines in a forest renders the canopy less flammable – once you think about it ;)
    ….
    Terrestrial ecosystems burn, excepting permanent lakes and rivers. Each ecosystem or community has its characteristic fire frequency. A change in fire frequency may well change the plant community into a different one, with a very different number of trees or different dominant species in many cases. When drought comes, swamps may burn. The accumulated organic material may burn as a muck fire. The swamp may be set back to a marsh, possibly to become a swamp again in fifty years.

    A forest of mostly large mature trees and a quite walkable understory is quite normal. Tropical rainforest is like this. Forest or savanna? Various combinations of soil, hydrology, fire frequency and perhaps grazing seem to make the difference. There is much rainforest in the Amazon region, prime rainforest in Guyana, and a large expanse of wet prairie (savanna, grassland) in between. The wonderful longleaf pine forest once made much of the US Southeast park-like, with a very diverse understory. This ecosystem functions as a forest for a few species, but as a grassland for a large number of species. Another ecosystem, dry prairie, is maintained by frequent fire and seasonal inundation and seems treeless at first glance. On closer examination there are clumps of oaks with many hundreds of stems. These tiny trees are Quercus minima, and less than knee high in this environment.

    • A forest of mostly large mature trees and a quite walkable understory is quite normal.

      Pete, come visit us Down Under sometime, and enjoy the wonders of our sclerophyll forests – unless one has the advantage of a previously-forged track or of the passage of a recent bushfire, “quite walkable” usually tends to be a pipe-dream, even in mature forests!

      Our rainforests can sometimes have clearish understoreys, but there are plenty that don’t – bushwalkers in the warm temperate regions will know about lawyer vine (Smilax spp.) that creeps around all sorts of other understorey species, and in Tasmania the infamous ‘horizontal’ (Anodopetalum biglandulosum) has a fearsome reputation. There’s also a range of Poaceae and Cyperaceae (amongst other monocots) that can nicely fill the space between trees – the finely (though dangerously) serrated Gahnia grandis, for instance, can make it impossible to climb 2 metres away from a stream bed if one has no cutting tools.

      Our Nothofagus associations sometimes have the parkland-like northern hemisphere look, but even they can often be filled with thick undergrowth.

      There is one forest type that I’ve worked in though that is almost always clear in the understory – the Pinus radiata timber plantations!

      • Aye, I’ve also been lucky enough to hike through some of New Zealand’s national parks (northern south island), which I imagine are somewhat like the Aussie forests you describe. We kept to the trails there, but, while I have bushwacked through US pine forests, I would not have wanted to make the attempt through what I could see off the trail. My uncle used to lead tramps through the bush, and would describe the necessity of carrying clippers with you in order to be able clear a way you could just squeeze through.

  21. Pete Dunkelberg: “Terrestrial ecosystems burn, excepting permanent lakes and rivers. ”

    Never been to Cleaveland, huh?

    • I censored myself on that one. . . Pete’s comment was so nicely written.

      Besides, the Cuyahoga hasn’t caught fire since 1969, and likely won’t ever again.

      The Oracle of Wiki says:

      The most polluted portions of the river now generally meet established aquatic life water quality standards except near dam impoundments. The reasons for not meeting standards near the dam pools are habitat and fish passage issues rather than water quality.

  22. Also, it was apparently a more regional problem than the mythology recognizes:

    http://www.environmentalcouncil.org/priorities/article.php?x=264

    (Three other rivers that burned.)

  23. Pete Dunkelberg

    Whoops! I guess I don’t know much about Cleaveland and its pollution problems. I do vaguely recall an old joke about a really lousy contest where the prize was a weekend in Cleaveland. ;)

  24. Cleveland, guys! No ;’a’ !

  25. Robert,

    Nice find! Thanks for posting.

  26. Sorry to be OT, Tamino, but please eviscerate this when you get a chance:

    http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2012/02/14/BA8N1N7HNQ.DTL

    And great news about the Heartland documents! With what’s at stake the only word to describe these folks is ‘Evil’ (with a capital E).

    • Christy has been pimping this result for years. Snowfall in the Sierra is really, really noisy varying wildly from year to year. It would be very difficult to sort out a comparatively small signal from the vast amount of noise.

      Also it looks as though the author has done a pretty good job of debunking Christy’s work w/o any help from us…

      • “uses data from a ragtag collection of people, many of them amateurs.”

        Didn’t stop Pielke Sr from pimping it uncritically on his blog. Which is kind of ironic given how concerned he claims to be about data quality issues.

  27. Additionally, t seems like kind of a strawman argument, doesn’t it? AFAIK, regional-scale modeling isn’t generally all that robust yet, so his claim that his result contradicts the models is likely unjustified. (Unless there is a robust model result for the Western Sierras, that is.)

  28. A useful and relevant paper (both for this thread and the drought thread) is:

    J. R. Marlon et al (2012) “Long-term perspective on wildfires in the western USA” PNAS in press Open Access

    http://www.pnas.org/content/early/2012/02/09/1112839109.abstract

  29. FWIW, I spent a week backpacking through the Black Hills a couple of years ago, and witnessing the brown mountainsides of dead trees immediately called up the question of fire hazard in my mind. (As in, I am definitely getting out of ‘here’ before making camp.) I think there should be little question that fire spreads more readily when the wood has less moisture content, and dead wood generally has less moisture content than live trees.

    At the same time, once the tree is fully dead for a little while, the needles fall to the ground and rot, and that makes it harder to build up enough heat for fire to spread up the next tree.

    I also observed that in the areas reforested after being logged, there was a higher than natural density of smaller trees. Being packed too tightly together forced them to grow thin as they struggled to out-compete their neighbors for light. This was so common a problem that the local practice is to try to thin them out in order to prevent the too skinny trees from being snapped or blown over en masse during a storm. (And let me tell you, being up in elevation when a storm blows through is…maybe “thrill” is not quite the right word.) Also, they are only marketable after having grown to a specific diameter, and too many too close together actually lowers yields. I would hazard a guess that tightly packed, small trees in poor health also increases the fire risk.

    Anecdote: You call tell an area that has been logged from an area that has suffered some other damage by the absence or presence of random large trunks that remain. A recent fire leaves obvious char marks, and a recent beetle infestation leaves obviously intact, but dead, trees. After some years, when the aspen/birch have started to take over, it can be hard to tell fire from beetles, at least from a distance. I was looking at an area and having a hard time deciding which of the latter had happened; so, I asked a local, “Both”, was the reply.

  30. There are many things that contribute to the number and severity of wildfires. In the past there was a concerted effort to not allow any wildfires. Only in the last 10-20 years has there been the acceptance that fires are necessary. You hear all the time about prescribed burns. But the areas burned usually remain small. Mostly due to fact that starting almost any fire in the mountain is extremely dangerous. Their ability to control or extinguish a fire is very limited. These issues cause many knock down effects. Most of the area is semi-arid. Here in the Denver area we average only about 15″ of moisture a year. Because it’s so dry dead plants and trees don’t decay quickly. I can show you trees that died more that 100 years ago that are still standing. Fallen leaves, branches and pine needles take many decades to decompose. All of this adds up to tremendous quantities of fuel over extremely large areas.

    Wildfires are a necessary part of the ecosystem. They remove the fuel that allow fires to reach the very large sizes. They also kill and destroy many of the trees infected with pine beetles. Wildfires are also an important part in the life cycle of many trees. Lodgepole pines are an example. Their pine cones need the heat from something like a wildfire to open and disperse their seeds

    Pine beetles are not the cause but a symptom of an unhealthy forest ecosystem. An ecosystem that’s been strangled by the US Forest Services’ past policies on wildfires.

  31. Highly recommended:

    A Conspiracy of Optimism: Management of the National Forests Since World War Two
    Paul W. Hirt
    ISBN 10: 0803223757 / 0-8032-2375-7
    ISBN 13: 9780803223752
    Publisher: Univ of Nebraska Pr
    Publication Date: 1994

  32. Have you considered the effect of salvage logging of infested trees?? and after fires? There is not necessarily an independent variable:)

  33. I don’t come here much any more so I missed this post.

    Two things:

    o This post treats all forests as the same. This is incorrect. The Black Hills forest is used to frequent small fire and now no longer gets such small fires. Lodgepole forests generally go up in stand-clearing fires in most places where they exist.

    o AFAICT most professional forest ecologists state that the combination of increased drought and MPB are going to be bad for lodgepole stands. Not just one factor, but a combination of factors. That is what we are dealing with in the west. Not just one factor, in our forests, but a combination.

    IMHO this is a case where simple statistics doesn’t tell the story. You need some ecology in there as well to direct the statistics.

    • This post treats all forests as the same. This is incorrect.

      Dale, several others and myself indirectly made this point above. It’s worth emphasising though specifically in the context of the OP.

      Never-ending complexity is certainly one of the joys of ecology.

  34. Definitely need some ecology with the stats as noted by several commenters above. Big issue is stage of infestation and associated forest structure. Researchers out of UMontana & USDA have been tackling this issue with combined ecosystem/wildfire models – some interesting results thus far. They covered a lot of this in a USDA-sponsored webinar series on forest disturbance http://www.fs.fed.us/rmrs/presentations/future-forests/fire-risk/