SOS Forests

The Genesis of Old-Growth Forests, Part 2

Part 2 — Open, Park-like Forests

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.

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.

Open, park-like forests were widespread for millennia, but not anymore. Now, they are few and far between. If historical OPLF’s haven’t been logged, as most were, or burned up, as many remnants have been lately, then they have become overgrown. The remnant OPLF older cohort trees are dying from insect and fungus attacks and other mortality factors related to moisture stress because the old-growth trees are unable to compete for soil moisture with the thicket of young trees around them.

The demise of OPLF’s is common, through fire or stress-related mortality, but nowhere are OPLF’s arising spontaneously in nature. They must have become established somehow because they were definitely here, but the ecological processes that lead to OPLF’s do not seem to be happening anymore.

It is a puzzle, then, to deduce the natural processes by which forests become open, and park-like, with widely spaced trees, over grass or fern understories. We have no modern examples of this forest genesis process to go by. The phenomenon doesn’t seem to be occurring anywhere today.

When modern western forests catch fire and burn, they do not leave behind an OPLF. Modern forests are thick with fuels, and forest fires tend to crown into canopy fires. The result is total tree mortality. Then the brush sprouts, and tree seedlings move in over time. A thicket of trees and brush arises, not a savanna/woodland with prairie grasses underneath. The thickets are fire-prone, burning again in a decade or two, and permanent fire-type brushfields result.

In subalpine forests where lightning is the natural ignition source, forests are patchy. Pockets of single species occur, 5 to 40 acre areas often with stands of pure lodgepole pine, or pure noble fir, or pure western larch (whatever species had the next good cone crop). There are also mixed-conifer patches. On the ridgetops, where lightning strikes most often, small brush patches occur, from 1/10 acre to 50 acres or so. The fire regime is a spotty affair, with ignition coming from the sky, here and there, now and then (a-periodically), into a forest with a very short warm/dry season, cool nights year-round, and deeply divided terrain. This regime leads to patchy fire, not landscape fire, and hence a patchy forest. But subalpine forests are rarely OPLF’s. On any given acre, fire returns too infrequently (every 15 to 50 years, not annually).

It’s a real conundrum, then. How did open, park-like forests arise in the first place? What was the forest genesis process? What fire regime leads to OPLF’s? How did hundreds of millions of acres of open, park-like forests arise? For many millennia, most forests in western North America had large, scattered trees with grasses or ferns below. What ecological processes caused or induced this widespread vegetation type?

The answer: the grasses did not invade the forest, the trees invaded the prairie. The prairie came first; the scattered trees came second.

More precisely, fire came first. Fire, frequent fire, annual fire, was the ecological agent that created or induced the grass and fern prairies. Open, park-like forests arose when trees invaded prairies subject to annual fires.

The fire regime must be considered annual if we look at the extent of the grasslands. They covered most of the western 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 western continent burned every year.

Without regular fires at very short intervals the grasslands would have 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.

Many seeds were cast into the turf, fewer germinated, and fewer yet survived the first fire, or the second, or the third. As the years piled up, however, a few lucky trees survived it all and grew tall and thick-barked. From then on, the low-running, lightly-fueled prairie fires would sweep by those lucky few trees with less than fatal consequences. Working outward from their refugia for millennia, scattered trees gradually occupied, in a patchy fashion, much of the western grasslands.

OPLF’s are really savannas with an abundance of trees, or with trees so large and tall that they seem to be abundant. The trees have invaded the grasslands, not the other way around.

The prairie 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. Vastness is an important part of the picture. Western OPLF’s occupied hundreds of millions of acres. Pure prairies, without any trees, also occupied hundreds of millions of acres. (The vegetation type most common today, dense thicket forests, has been a rare and patchy type until relatively recently). The sorts of fires that produce prairies are vast fires, landscape fires, not spotty patch burns.

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.

From the prairies arose the OPLF’s. 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 caused OPLF’s, or induced them, or nudged nature into creating them. Without thousands of years of human manipulation of western N.A. ecosystems, specifically anthropogenic fire, there would have been no open, park-like forests here.