Journeys in ancient and old growth forests

What is old-growth forest?

The diverse structure of an old-growth forest

You might think, five years after publishing a book about old-growth forests, that I could easily tell you what an old-growth forest is. But even though I’ve answered the question countless times during interviews, on a speaking tour, even at parties, I still find I can get bogged down in definitions involving minimum ages, stand history, human disturbance, coarse woody debris, etc. Over time I’ve gotten closer to a simple answer to a complex question, but bear with me, I’m still going to wade through technical terms for a while before I get there.

Robert Leverett summarizes the multitude of ways that scientists and forestry professionals have struggled to define old-growth forest, in the opening chapter of Eastern Old-Growth Forests: Prospects for Rediscovery and Recovery. He groups old-growth forest definitions into four categories:

  1. Definitions that emphasize lack of disturbance by humans (at least post-colonization); there are abundant old trees some of which are approaching the maximum old-age for the species.
  2. Definitions that use a minimum age (typically around 150 years) combined with presence of old-growth characteristics such as logs, snags (standing dead trees), canopy gaps etc.; some human disturbance may be permitted.
  3. Definitions that emphasize stand development, in particular climax forest – that is, the forest is in a stable state where trees are dying of old age and being replaced, and may continue to be stable for centuries.
  4. Definitions that use an economic threshold. Old-growth stands are past the economic optimum for harvesting – usually between 80-150 years, depending on the species.
forest evolution in layers 5_Layer 0
A new forest begins

Categories one and three describe very old, undisturbed forest. Two and four describe middle-aged, forest that may have had some human disturbance. I would argue that there isn’t one type of definition that’s better than the others; the variation in definition reflects the variety of forested landscapes. For example a category one definition, with its prohibition on human disturbance, would exclude just about everything we consider old-growth forest in Ontario south of Algonquin Park, except perhaps cliff-growing ancient cedars. Most forests have at least had a little firewood cutting or selective tree removal during the long history of settlement. However forest-grown trees in southern Ontario have been aged to over 500 years old – to say there’s no old growth there would clearly be false. All the other categories would likely allow some of southern Ontario’s old forest through the filter, but widely varying amounts. On the other hand category three, with it’s emphasis on stand development (trees dying of old age and being replaced by new trees), would exclude majestic 250-year-old red and white pine stands in Temagami – since pines can live for at least 500 years, these are middle-aged stands.

forest evolution in layers 5_Layer 3
Trees grow rapidly, competing for light

The solution to the apparent contradiction is that old-growth forest is not an end point, but a continuum. On our managed forest landscape, forests that have passed the optimum age for commercial harvesting are relatively rare – so it makes sense to begin definitions at or beyond this threshold. It’s no coincidence that tree growth begins to slow around this age (which is variable by species), so when trees are occasionally felled by wind, insects, disease, etc., gaps will remain in the canopy for a while. The number of large logs and snags (dead standing trees) will increase over time, often for centuries. Eventually, in theory, the forest will enter a steady state where trees are continuously dying and growing, and it is a very uneven-aged forest. This is the ultimate old-growth forest implied in category three definitions. However in some northern forest types, forests are commonly (though inconsistently) reset by natural disturbance before they enter a true steady state, and may rarely enter the climax stage.

forest evolution in layers 5_Layer 5
Growth begins to slow

Some jurisdictions have adopted official definitions in order to implement old-growth forest policies. In 2003 the Ontario Ministry of Natural Resources published Old-Growth Forest Definitions for Ontario, which are tailored to individual forest types, and would fall into Leverett’s category two definitions (specifying a minimum age, and limited or no prohibition on human disturbance). The definitions include age of onset and, somewhat problematically, stand duration. It’s intellectually honest to acknowledge that an old growth forest may not last forever, but putting a fixed number on it (sometimes as little as 30 years in the Ontario definitions) is harder to justify and may have more to do with forest management than ecology.

Though tree species may transition as a forest continues to age, and average tree age will level out or even decline, many other old growth characteristics will continue to increase. The characteristics of old-growth forest may include:

forest evolution in layers 5_Layer 13
As trees die, gaps form in the canopy
forest evolution in layers 5_Layer 21
The forest is uneven-aged with abundant dead wood. This may take 300-500 years.

Most of these characteristics are only just getting started when an old-growth forest reaches a minimum age, they will continue to increase usually for centuries. So all of the definitions are correct, they just aren’t defining the same stage of old growth. Keeping this perspective when working with the minimum-age definitions is essential.

How to make sense of all this? Look around in your own local area and ask, what is the most intact forest – where natural processes still operate, often despite human interference; and where trees are allowed to grow old? If you’re in Toronto, maybe it’s High Park, and if you’re in Temagami, it is probably the Obabika forest. On some level this is how many old-growth forest definitions were arrived at – they were reverse-engineered to match the scattered, damaged remnants of the forests that once occurred on the landscape. I think that’s fine, at least until we let more of the landscape grow truly old – maybe, with some planning and some luck, our great-great grandchildren will find they want to tighten up the definitions.

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5 Responses to What is old-growth forest?

  1. Foresters are always looking for new terms, or redefining old terms, to more clearly define “best practices” in order to harvest that which should not be harvested. Old-growth is a an-made classification. That is why the term old-growth is impossible to define. Actually, there is no limit to the age of a forest and no reason to conclude that a forest is “old.” Let’s stop pretending that loggers care for the forest. If they did they would not use triage words and log. The species would define “their” forest as the SPECIES’ FOREST and it is ageless. Old-growth is a term used by the logging industry or in some cases it is desperately used by true environmentalist to save a species’ forest. The word WOODLAND is another example. In Massachusetts the State has re-classified tree plantations or would be tree plantations as woodlands and they are eager to use the term old-growth in their triage map of “woodlands.”

    • Hi Richard, the idea of a species forest has merit, however old-growth forest would be a sort of ultimate species forest (for some species) because it is unique habitat, rare on the landscape, and there is a subset of species use it preferentially or even require it. By not attempting to define and preserve it, it will likely disappear at least from managed landscapes. The battle to leave large tracts of landscape un-managed has been going on much longer than we have been alive, and I hope will continue long after we are gone with greater success. Nevertheless, it doesn’t mean efforts to preserve smaller very important areas, or the entirely separate effort to improve forest management, are without value. From a more subjective standpoint, I know old-growth forests exist and are different from the much younger managed landscape around them, because I’ve been in many of them.

  2. I was always taught the the terminology had four divisions:

    1. Ancient forest = that which has never been disturbed (at least pre-colonization in the U.S.)
    2. Old growth forest = that which has been cut at least once (100 – 200 years old)
    3. Managed forest = that which has on-going logging every 50 years or so
    4. New forest = monoculture or tri-culture “forests” that have been re-planted as a timber “crop”.

    Ancient forests are extremely rare in my experience since just about every easily-reachable square mile of the U.S. has been logged or disturbed at one time or another (especially in the eastern U.S.). Old growth are less rare, but have the earmarks which are typical (i.e. stands of 1 – 3 species of trees, very little diversity of the understory, etc.). I love the enthusiasm which people exhibit over protecting old growth, but I have no doubt that a majority mistakenly think they’re protecting “virgin” forest, which is almost never the case.

    • Our definitions are slightly different than yours, we consider old-growth forest to overlap with ancient forest. We have ancient forest landscapes that were never logged but were disturbed by wind or fire within the last 150 years, and may not be old growth. We also have old-growth forest that has had some human disturbance, often highgrading of select tree species, which may not be ancient forest. And we have many areas that qualify for both. Ancient forest is relatively rare, but it is untrue that old-growth forest is almost never virgin forest. An estimated 0.5% of the landscape escaped logging in the eastern United States, which seems very small but means that ancient (undisturbed) old-growth forest is found covering many thousands of hectares in parts of the Appalachians, Adirondacks, Catskills, etc. In Canada as one moves north there is a significant amount of ancient old-growth forest in Algonquin Park, Temagami, Algoma, and the boreal forest (and yes, there is old-growth forest in the boreal, where fire cycle are sometimes, and in some places, as long as 1100 years). It is true that the ancient forest that remains is almost never where growing conditions are best, as I described in my most recent post.

  3. Old cedar trees have 30 to 40 growth rings per inch. This wood was good for making cedar shakes and good for wood carving because of the tight rings.
    Second growth cedar had growth rings of 15 to 20 rings per inch. Cannot be used for cedar shakes or carving. Fir trees also had really tight growth rings.

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