23 May 2008, 3:20pm
Forestry education Saving Forests
by admin

The Genesis of Old-Growth Forests, Part 1

Iain Murray, the author of the preceding post, is not a forester or forest scientist. Yet his understanding of the development of our American forests is remarkably cogent and perceptive:

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 15,000 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 100,000 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.

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.

The collection of empirical evidence of past forest development must be locational as well as temporal. The evidence is specific to place and time.

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 in 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, 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 fire 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.

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. The new theory holds that historical 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 burns of the last few decades in southwest Oregon. Typically the forests that have burned were strongly multi-cohort with older cohort trees of 150 to 600 years of age. 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.

The historical forest development pathways of the past were 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 devastating stand replacement fires.

Furthermore, 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.



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