The Genesis of Old-Growth Forests
Note: due to the crush of work I have accepted recently, I do not have time to prepare new posts for the next week or so. The following is a repost from May, 2008 [here, here, here].
by Mike Dubrasich
Iain Murray, the author of The Really Inconvenient Truths: Seven Environmental Catastrophes Liberals Won’t Tell You About - Because They Helped Cause Them, [here], wrote in Chapter 4 of that book:
With wildfires burning, it is useful to turn to the wisdom of the ancients. When the pioneers first entered the great forests of America, they found that the Native Americans had managed the forests for centuries. Their woodlands contained very few big trees—maybe fifty such trees per acre.
Apparently the Indians had set regular, low intensity fires which burned away accumulations of undergrowth, deadwood, dying trees and particularly small trees growing between the big trees. The larger trees were unharmed, because of their thick fire-resistant bark.
That in a nut shell is the way our old-growth forests developed. Frequent anthropogenic fire gave rise to open, park-like forests, largely uneven-aged at large-area scales. Forest scientists refer to such trees as “older cohort” because they are quite different than the even-aged thickets of trees (younger cohort) that arose following elimination of anthropogenic fire (aka “Indian burning”).
True old-growth forests contain older cohort trees. Those trees are remnants of the the former open, park-like forests that covered much of forested North America, and they may also be viewed as relics of our ancient culturally-modified landscapes.
In this 3-part series, I discuss in greater detail how our old-growth forests came to be here. The issue is important, because we must understand how old-growth forests arose in order to protect, maintain, and perpetuate them. If we value old-growth, and that seems to be a widely-shared value, then it is vital to understand their development.
Part 1 — Historical Forest Development Pathways
Fifteen thousand years ago the Wisconsin Glaciation reached its maximum extent. Continental ice sheets and tundra covered much of North America. The forests of today did not exist at that time, except in refugia in southern regions, coastal strips, and isolated mountain tops. Most of the acres covered by forest today were not forests 12,500 years ago.
Then, in accord with variations in the Earth’s orbit known as Milankovich Cycles, the planet warmed and the ice retreated. Over time, forests developed where they did not exist before (at least not for the previous 90,000 years or so). The progressions of vegetation change (from ice/tundra to forest) that actually occurred are called the historical forest development pathways.
Historical forest development pathways are not theoretical; they are the actual changes in vegetation types and species that actually occurred in actual landscapes. Since we have no time machines, we must reconstruct and deduce historical forest development pathways from empirical evidence of the past.
The evidence for the Ice Ages includes fossil pollen and spores from lake and bog cores, ancient middens, ice cores, ocean bottom cores, volcanic deposits, and the use various radioisotope measures such as C14 and O18. That evidence indicates that 18 to 20 glacial stadials, the last one being the Wisconsin Glaciation, have occurred over the last 1.6 million years. The stadials last roughly 90,000 years each and are separated by 10,000-year-long interglacials, such as our Holocene.
During the stadials, continental ice sheets formed over 1/3 of North America. Tundra and steppe extended far south of the ice sheet edge. Forest species were extirpated across much of North America and hung on only in coastal refugia to the south. Then during interglacials, plants and animals moved back north to recolonize the newly warmed regions, only to be extirpated again 10,000 years later.
The climatic turbulence during the Pleistocene (the last 1.6 million years) has led to “temporal” plant and animal assemblages. The plants associations of today are unlike past associations. Designating a modern vegetation type as characteristic of a region is to raise artifact to principle. During the Pleistocene, vegetation has exhibited a dynamism that approaches chaos.
The collection of empirical evidence of past forest development must be locational as well as temporal. Not all forests are alike. Many different species mixes and types of forests exist across North America today. Obviously, there have been many different forest development pathways. No one pathway happened everywhere, or else all forests would look the same.
Forests across North America have certain similarities, however. It appears that all or almost all tracts of forests of every type have experienced quite a bit of fire during the Holocene (during the Wisconsin Glaciation the ice and tundra did not burn, and the forests of today did not exist).
We also know from common experience that fire can kill trees and sometimes all the trees in a forest, regardless of forest type. Putting two and two together, it is widely concluded that a particular kind of fire was an important element in the actual historical forest development pathways.
That is, fire has been an important disturbance agent, and forest development is disturbance-related. Forests change when disturbed, especially when fire is the agent. The manner in which forests have changed tells us something about the type of fires that occurred.
One theory of historical forest development is that today’s forests consist of even-aged stands where the trees seeded in at roughly the same time following a “stand replacement” fire. That theory was at first widely assumed to be true for almost all forests. It was certainly true for younger tracts of forest which had arisen in even-age fashion following well-documented stand replacement fires.
As more and more forests were investigated for actual age distribution, though, anomalies in the early theory began to be discovered. The general anomaly observed is that many forests, particularly older, untouched forests, are not even-aged. Instead, many (if not most) older forests are distinctly multi-cohort. That is, forests often have two or more widely divergent age classes of trees.
The empirical evidence tends to disprove the “stand replacement fire” theory, at least in regards to older forests. Their development pathways must have been different than that.
The evidence suggests that many (if not most) North American forests were at one time (120 to 500 years ago) open and park-like with widely spaced, large, uneven-aged trees, and those forests were conditioned to be that way by frequent, non-stand-replacing fires. Historically, frequent fires were light and low-burning, and that those fires individually did not kill many of the bigger trees.
That is, the actual historical development pathways for many (if not most) of our forests involved frequent light fires, not stand-replacing fire.
Nowhere is this more apparent than in the Biscuit Burn and in other severe burns of the last few decades in southwest Oregon. Typically the forests that have burned severely were strongly multi-cohort with older cohort trees of 150 to 600 years of age. The younger cohorts were dense thickets that arose subsequent to the frequent fire era. Also typically, most or all older cohort, old-growth trees were killed in the catastrophic, stand-replacing fires. The vegetation that arises after such fires is sclerophyllous brush with a few conifer germinants.
It is clear that the new “forests” will be nothing like the old forests. In fact, it is probable that the new “forests” (brushfields) will burn again after 15 to 50 years of new fuel development. We know from reburned areas such as the Silver Burn (1987) within the Biscuit Burn (2002) that the “replacement stands” are highly flammable. After reburns no conifer seed sources are left, and the new “forest” becomes a permanent shrubfield.
The conversion of forest to brush by fire has been recognized in Arizona pine forests as well. Barbara A. Strom and Peter Z. Fulé of Northern Arizona University did a study of post-fire forests, published in the International Journal of Wildland Fire in Feb., 2007:
Strom, Barbara A.and Peter Z. Fulé. 2007. Pre-wildfire fuel treatments affect long-term ponderosa pine forest dynamics. International Journal of Wildland Fire 16(1) 128–138.
Abstract: The 2002 Rodeo–Chediski fire, the largest wildfire in south-western USA history, burned over treated stands and adjacent untreated stands in the Apache–Sitgreaves National Forest, setting the stage for a natural experiment testing the effectiveness of fuel reduction treatments under conditions of extraordinary fire severity. In seven pairs of treated–untreated study sites measured 2 years after the fire, thinning was strongly associated with reduced burn severity. Treated areas had more live trees, greater survival, and reduced fire intensity as indicated by crown base height and bole char. Ponderosa pine regeneration was patchy but more dense in treated areas. We assessed decade- to century-long effects of the pre-wildfire fuel treatments using the Forest Vegetation Simulator (FVS). Differences between treated and untreated areas were projected to persist for several decades after the fire in terms of stand structure characteristics and for at least 100 years in terms of species composition, with ponderosa pine making up ~60% of basal area in treated areas but only 35% in untreated areas. Future ecosystem development may take the trajectory of recovery to a ponderosa pine/Gambel oak forest or of a shift to an alternative stable state such as an oak-dominated shrubfield, with untreated areas more apt to undergo a shift to a shrubfield state. Current management decisions about fuel treatments have multi-century legacies.
To summarize, the authors found that after a modern severe fire, untreated pine forests are apt to shift to a stable shrubfield state. That is, modern stand replacement fires do not give rise to forests; they give rise to permanent fire-type brush.
Thus the historical forest development pathways of the past must have been different than those we see today. They must have been different because they gave rise to open, park-like forests with old trees, not permanent brushfields. The big difference: historical pathways had frequent light fires, not infrequent stand replacement fires.
Frequent, regular fire is an artifact of human presence and human ignition. Fire has been a non-natural, human-controlled disturbance in the forests of North America for 10,000 years or more. No doubt lightning fires occurred, but they occurred in landscapes “pre-programmed” by people. Lightning fires in open, grassy forests quickly dropped to the ground and acted just like people-set fires.
A problem we face today is a lack of understanding about the actual, historical forest development pathways. Understanding is crucial because the historical pathways must be replicated if we are to save our heritage forests from extinction. If we allow Mother Nature to have Her way, our forests are condemned, and our legacy to our grandchildren will be brushfields, not forests.
Part 2 — Open, Park-like Forests and the Anthropogenic Mosaic
Throughout the Holocene human beings have significantly altered Western Hemisphere landscapes in purposeful ways; i.e., people burned the land to make it human-friendly, generally-speaking. Frequent, regular, seasonal, fire aided hunting, gathering, agriculture, and other human endeavors.
Evidence of frequent fire can be seen in fire scars and the structure and composition of old-growth forests of today. Most are dense thickets of mostly young trees with a a few emergent old-growth trees, but 150 years ago they were open and park-like forests. Forests stands can be backdated by counting rings and compiling age distributions. Extensive evidence indicates that 150 to 200 years ago in forests across the western US there were only 5 to 20 ancient and giant trees per acre, of different ages, widely-spaced, crowns not touching, except in occasional groupings, trees otherwise often singular and separated by 40 or 50 feet or more from their nearest neighbors. The understory was grassy, nearly devoid of shrubs, although small pockets of low-growing bushes and younger trees appeared here and there.
The old forests were not a continuous blanket across the landscape. Indeed, ancients forests occurred in widely dispersed pockets, more like islands of trees in an ocean of praire. Even in the cool, moist Pacific Northwest, forests were fragmented. Treed areas were found along streams in dendritic (finger-like) patterns. In between were grassy slopes, vast berry fields, and veldts with wild food crops, such as tarweed (Madia) tracts, bracken fern brakes, and camas meadows.
Human beings have resided in North America for at least 12,500 years (some evidence suggests 30,000 years or more). Human beings have been Masters of Fire from the beginning (some evidence suggests H. sapiens arose 200,000 years ago in Africa), and when humanity arrived in the New World they brought extensive fire skills with them. As capable as Clovis people were at knapping fluted spear points, they were even better at setting fire to the landscape.
After 12,500+ years of deliberate, skillful, landscape burning, human beings had induced a cultural landscape, an anthropogenic mosaic, across the continent. Human beings favor prairies and savannas — such vegetation types provide more subsistance than dense forests.
Early Euro-American explorers reported this same open, park-like condition in forests from Mexico to Canada, from the Rocky Mountains to the Pacific Coast. Western pioneers spoke of forests so clear of brush and thickets that they could drive wagons for miles through avenues of towering trees.
The structure of open, park-like forests (OPLF’s) was similar across much of western North America, but the species composition varied. That is, regardless of tree species, forests were open with tall, widely-spaced trees and grasses or prairie type-plants underneath. Pre-Columbian forests were more savanna-like, except in fire refugia.
In the mountainous Southwest open forests were made up almost exclusively of ponderosa pine. In the northern Rockies open forests contained ponderosa pine, western larch, and Douglas-fir.
The Sierra Nevada mixed-conifer open forests had sugar pine, ponderosa pine, Douglas-fir, and incense cedar, with the occasional true fir. Sequoia forests with trees as old as 1,500 years were also found in the Sierra’s. Much of the Sierra’s below 5,000 feet were oak savannas, principally black oak (Quercus kelloggii).
The Southern Oregon Cascades open forests had the same or similar species, though proportionally less pines. The eastside Oregon Cascade open forests were also mixed-conifer, dominated by ponderosa pine, but with scattered western larch, Douglas-fir, and grand fir. Eastside Coast Range and interior valley open forests were dominated by oaks. (Oak forests with only one or two trees per acre are probably better termed savannas, but the crossover point from savanna to forest is somewhat subjective.)
All these open, park-like forests had grassy understories, but in wetter and more northwesterly regions bracken fern replaced the grasses, and Douglas-fir replaced the pines. The westside Cascades and Coast Ranges, Puget Sound, and San Juan Islands all had Douglas-fir/bracken fern open forests pre-Contact.
Across the continent open, widely-spaced, old trees maintained by frequent fire were the most common structural form of forest in the West over the last 10,000 years.
Frequent fire was necessary to maintain the open, park-like conditions. No other factor logically could have been responsible, and the fire scars and charcoal are present and evident. Frequent fire must have removed most small trees, leaving only the most fire-tolerant grasses, shrubs, or ferns, as well as the fire-resilient, older trees.
The fire regime must be considered annual if we look at the extent of pre-Columbian prairies. They covered most of the continent. Every year fires burned seemingly everywhere. At the fine grain, every exposed and burnable acre got burned at least once every 3 to 5 years. Another way to say that is one-fifth to one third of the continent burned every year.
Without regular fires at very short intervals grasslands become brushfields or dense forests. Those other vegetation types did exist, or came to exist in the Holocene, but only in fire refugia: places sheltered from fire. In the coastal fog belt, coastal river bottoms, in deep canyons in the mountains, and at high elevations, fire was restrained by coolness, wetness, and topographic shields. When the opportunities arose, pioneer tree species ventured out of fire refugia into the fire-washed prairies.
The prairie and savanna fires were anthropogenic. Ignited by humans in the lowlands (and in particular places in the uplands), the fires spread downwind and uphill across vast tracts of land. The fires were not local or patchy; it was not a lightning-fire regime landscape.
It took human intelligence and coordination to ignite vast tracts every year. Lightning is too infrequent and chaotic in timing and location, except (perhaps) in the more arid regions of the West. In the wettest parts of the region, lightning and lightning fires are rare, yet the fire history extensive.
From the prairies arose the savannas. First the ground was completely burned over again and again, and then gradually a few trees got established and grew large enough to withstand the frequent fires.
People created the conditions that led to old-growth forests, or induced them, or nudged nature into creating them. Without thousands of years of human manipulation of western N.A. ecosystems, specifically with anthropogenic fire, there would have been few old-growth forests here today.
Part 3 — Restoring Old-Growth Forests
Most North American forests arose during millennia of frequent, human-set fires, i.e. anthropogenic fire. For millennia, anthropogenic fires were so frequent that they engendered open, park-like forests wherein individual trees grew to great ages.
During the last 150 years (more than that in eastern regions) anthropogenic fire has been eliminated (along with most of the humans who set them). The absence of anthropogenic fire has allowed our forests to accumulate an abundance of fuel in the form of young trees, duff, debris, litter, dead trees, etc.
When fires enter modern forests, they combust hugely and decimate the forests. Vast tracts formerly covered with trees are converted to brushfields because the intense heat, fueled by extreme fuel accumulations, kills everything-heritage old growth trees included.
Stand-replacement fire is not the historical development pathway that led to our old-growth forests. Old-growth forests of today are strongly multi-cohort, and the older trees arose under a different ecological pathway, one of regular, frequent, seasonal anthropogenic fire.
Formerly forests were open and park-like with widely spaced trees. Because frequent, light-burning fires in open, park-like forests did not kill all the trees, individual trees lived for incredibly long lifespans. Those conditions gave rise to ancient trees. Modern forest conditions preclude the opportunity for trees to attain great ages.
To save our old-growth, and to restore the development pathways that lead to old trees, we must also restore the appropriate forest conditions and disturbances.
Landscape restoration is a complex undertaking because many vegetation types were present long ago. These include prairies, grassy slopes, vast berry fields, and savannas with wild food crops, bracken fern brakes, and camas meadows. A system of foot roads webbed the landscape. Traditional human use areas were everywhere.
Only a portion of the landscape was forest, but those portions are the concern of this essay. Restoration of non-forest areas is also important, and very worthy of consideration, study, and action. I concentrate on the forested areas in this essay, however.
Restoring forests to historic conditions, and restoring the development pathways that lead to old-growth (long-lived) trees, requires the re-creation of open, park-like conditions and regular, frequent, seasonal, anthropogenic fire.
Modern overly-dense forests, with 150 years of fuel build up, must be thinning mechanically, the leave trees widely spaced, the excess fuel loadings removed, and then they must be carefully but repeatedly burned with low-running fires during the fall months.
Fire itself has been suggested as a tool to reduce fuel loadings. Sadly, prescribed fire, wildland fire use, or other common intentional fire types often do more damage than good. They add to the fuel loading by killing all the green trees. They do not reduce fuel loading; indeed such fires often increase fuels. Modern forest fires encourage beetles and fungi that kill any trees that made it through the flames. Forests are less healthy and more prone to catastrophic fire after the “treatment” than before.
In contrast, preparing a forest to receive a beneficial fire requires the removal of most of the 150 years of fuel buildup prior to ignition. At the very least, excess fuels must be piled and burned in winter before any attempt is made to broadcast burn the forest floor.
Fire is indiscriminate, but people are not. People can and must make the choices about which trees to leave and which to remove. Large and healthy trees with ample crowns, thick bark, and low height-to-diameter ratios should be left at wide spacings, with crowns separated. Leave trees should also be pruned of low limbs which can ladder fire up into the crowns. Accumulated duff and other fuels should be raked away from the leave tree bases.
If a forest is prepared to receive fire, it doesn’t matter if people set the fires or lightning ignites them. In either case, the prepared forest will not be devastated. Trees will live through repeated fires and thus attain great ages. The historical development pathways of old-growth will be restored.
The outcomes of restoration forestry also include production of wood products from the thinnings, protection of watersheds and landscapes from catastrophic fires, and protection, maintenance, and perpetuation of wildlife habitat.
Stewardship of forests is an ancient art and practice. Our forests are not “wilderness, untrammeled by man” but instead are homelands and tended landscapes of deep antiquity. Restoration forestry also restores the traditional human connections with nature and the land.
Foresters have done it, and we know that preparing forests to receive fires works.
We need new institutions and a new cadre of foresters and fire tenders who have been trained in restoring old-growth development pathways and application of the appropriate treatments. We need a School of Restoration Forestry where the subject can be studied. Forests are diverse, and each patch on each forest requires careful, informed guidelines for restoration. A School of Restoration Forestry could educate students about how to do that in different forest types and conditions.
We need a movement, a national discussion about the role of restoration in our forests, and how restoration can reduce firefighting costs, prevent catastrophes, and protect, maintain, and perpetuate our priceless heritage forests.
Abandonment is not stewardship. Wildfire is not silviculture. Only active restoration forestry can save our old-growth forests, and create the conditions whereby trees can attain great ages. Old-growth forests have not and do not arise by accident. We must assume the responsibility and recreate the ancient practices if we wish to protect, maintain, and perpetuate old-growth forests.
