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:
- 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.
- 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.
- 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.
- 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.
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.
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.
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:
- large diameter logs and snags
- old trees
- openings in the forest canopy
- undisturbed soil layers
- uneven forest floor, characterized by pits and mounds left by windthrown trees
- multiple vegetation layers, from understorey and shrub to canopy and supercanopy
- high diversity in the herbaceous layer
- lichen and fungus abundance and diversity
- late-successional (shade tolerant) tree species
- absence of human disturbance
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.