Our readers often have questions about old-growth forests: what they are, how to identify them, their relevance to climate change mitigation, and (absent a time machine) how to replicate their characteristics in younger forest stands. For help answering these questions, we turned to forest ecologist Bill Keeton, and asked him to answer our questions as comprehensively as he could in fewer than 2,000 words.
A professor at the University of Vermont’s Rubenstein School of Environment and Natural Resources, Keeton is the co-editor, with Andrew Barton, of a recently published compilation of technical reviews, Ecology and Recovery of Eastern Old Growth Forests. He chairs the International Union of Forest Research Organizations Working Group on Old-Growth Forests and studies old-growth temperate systems on four continents.
What are Old-Growth Forests?
Here’s what they aren’t: they’re not static and don’t always conform to the archetypical “cathedral old-growth” we might imagine. Rather, they are constantly changing – shaped by natural disturbances, climate, and succession. Nor are they necessarily “untouched” by people, although this point is contentious and best thought of as a continuum rather than a strict standard. For example, they may be missing American chestnuts, American elms, and large beech trees, all killed off by human-introduced pests and pathogens. And of course, Native Americans profoundly shaped forests in many parts of the eastern United States for millennia.
But what the East’s remaining old-growth forests do share in common is a degree of continuity with the pre-colonial past. They are remnants of the primary forests that once blanketed much of the East, that were themselves a mix of successional stages and varied habitat conditions as influenced both by natural disturbances and aboriginal activity. One of the most important aspects of continuity is the carryover of biological legacies – organic matter, key structures such as dead and living large trees, and organisms above and below ground. This contrasts dramatically with younger, secondary forests regenerated on old fields or after clearing of primary forests; those typically lack biological legacies.
Old-growth continuity can redevelop as the legacy of land-use history fades, for example, in areas we might designate as old-growth reserves. These days, scientists are thinking a lot, for example, about the community of biota and microorganisms in the soil that can redevelop over time, such as super intriguing mycorrhizal fungi that form symbiotic networks connecting trees and other plants belowground.
Perhaps the single most defining characteristic of old-growth is complexity. There is complexity in the forest structure, in the community of organisms both above and below ground, as well as in functional processes, for example, how carbon is taken up through photosynthesis, and how the forest canopy intercepts and slows the infiltration of rainwater into the soil.
What do Old-Growth Forests Look Like?
They don’t all look the same. Depending on the site, you may have to look closely to recognize old-growth characteristics. Sometimes you find the full set of classic old-growth indicators, other times only some of these. However, as a rule, these forests appear to be “messier” and more architecturally complex than younger forests. Perhaps the best way to describe them is to describe what it’s like to walk through a typical northeastern old-growth stand.
You see trees of all ages and sizes, including many unusually large trees. If you look up, you’ll notice the ragged, intricate architecture in the canopy overhead, and the diversity of light environments it creates. There are areas of disturbance and regrowth; in some areas, you may have to crawl over a mess of large downed logs or through a jungle of gap-regenerated saplings. You trip through the hobblebush, and as you pass by a large tip-up mound, you find holes in the mud where winter wren are nesting and an abundance of yellow birch and elderberry rooting on top.
The aesthetics of old-growth forests may not be immediately appealing to all. But with a retrained eye, you can appreciate the messy consequences of disturbances and tree death as niches that foster biodiversity, creating a palpable richness of habitats. Once recalibrated, the aesthetics of eastern old-growth are every bit as awe inspiring as one might have hoped for.
There are not many opportunities left for such an experience though. East of the Mississippi, less than half a percent of the old-growth that once existed remains, but those residual stands offer an extraordinary resource for forest science. And in some places – including, in the Northeast, parts of the Adirondacks – we are not just talking about small, isolated fragments, but entire landscapes composed of “primary,” never-cleared forests.
These places give scientists the opportunity to learn how old-growth forests change dynamically over broad scales. And we have learned that these dynamics are directly related to the functions or services that old-growth provides, such as habitat for late-successional biodiversity, high levels of carbon storage, and exceptionally high-quality forested streams. And there is much we can learn from natural forest dynamics – including the importance of biological legacies – that can guide how we practice sustainable forestry in managed forests.
What are Climate Change Considerations Related to Old-Growth?
If there is one core lesson from more than a half-century of research on eastern old-growth forests, it is that a dynamic system is a healthy one. Instead of the classic view of forest succession, which assumed a linear march through predictable stages, research on old-growth forests has revealed a much more complicated model. There are multiple pathways that forest succession and development can take. Disturbances – insect outbreaks, storms, and other events – periodically redirect forest development and create a diversity of stand structures, each with somewhat different habitat conditions and functional processes, including carbon uptake.
This dynamic “multiple pathways” model is relevant to predicting how forests may respond to climate change. It offers important insights, as we work to understand how forests may change, and how the mix of biodiversity and ecosystem services may shift.
For decades, many have thought that old-growth forests are greenhouse gas sources, emitting more carbon dioxide to the atmosphere through respiration than they absorb through photosynthesis. But more recent evidence suggests the opposite. In many (although not all) cases, old-growth forests remain net carbon sinks for centuries. In other words, they continue to sequester more carbon than they lose, adding to the bank already deposited in living and dead biomass, both above and below ground. Conserving that bank of already-sequestered (that is, stored) carbon in high biomass forests such as old-growth can help fight climate change.
Here’s another important point about climate and old-growth forests – a benefit that colleagues and I described in the July 2019 issue of Global Change Biology. Our paper used data from 18,500 forest inventory plots spread across the temperate-boreal ecotone of eastern North America – from Minnesota to Maine and from Nova Scotia to New Jersey. Feeding the data through the super-computing cluster at the University of Vermont, we found something unexpected. Not only did the old-growth plots offer more biodiversity and ecosystem services than other forest plots, but their communities of organisms and ecosystem services seem especially resistant to climate change.
This is not to say that climate change does not pose grave risks to old-growth forests. Of course it does. But we now know that increasing the proportion of complex forest structures on the landscape is a good strategy to promote climate resilience. Our argument is that this would complement (not replace) other adaptive forest management approaches.
How Can We Manage for Old-Growth Characteristics?
There is clear value in protecting remaining old-growth forests, here at home and abroad. But can we actually restore more old-growth? In fact, the proposition that we might one day restore eastern old-growth is no longer theoretical, as was the case when the idea was originally proposed several decades ago. At least a half dozen experimental studies since then have proven that it is possible to actively restore old-growth characteristics in redeveloping secondary forests; some unmanaged forests will recover on their own through natural processes.
The lion’s share of this endeavor will need to focus more narrowly on adding greater complexity to managed forests, and foresters in our region are already leading the way using a variety of innovative silvicultural techniques. For example, gap-based silviculture, the irregular shelterwood method, and variable retention harvesting (in simple terms: leaving varying numbers of live trees in a harvested area as well as downed wood) are increasingly used to enhance structural complexity and age-class diversity in secondary stands, while resulting in favorable regeneration, growth, and timber yield. Such “natural disturbance-based” approaches add complexity to managed stands, but are not designed for “full-on” old-growth restoration. For that objective we may need something different.
In my own work in Vermont, I have tested a system called Structural Complexity Enhancement or SCE. Initiated almost 20 years ago, the experiment emulates natural tree mortality and disturbance processes, to push stand development along faster. It employs a variety of silvicultural techniques in tandem, many of which will be familiar to forest managers in the Northern Forest region. Each technique targets a different process of stand development or structural feature. My colleagues and I created small, irregularly shaped gaps to free up growing space for saplings and to promote seedling growth. We placed the gaps strategically to “crown release” many of the large, dominant canopy trees; previous work had shown that this method can improve growth in larger trees. Instead of uniformly marking harvest trees (in other words, marking for a harvest that results in evenly spaced thinning or selection) we used variable density marking to create more complexity in the forest canopy. We felled some trees and left them as downed woody debris. We toppled others to create tip-up mounds. And we girdled trees to increase the number of snags, which are vital habitat for many wildlife species.
It worked! The experiment enhanced habitat for a range of late-successional organisms, resulting in increases in the populations or species richness of herbaceous plants, salamanders, and fungi. The tree regeneration results were interesting, showing ups and downs over time in seedling recruitment, survival, and establishment. But after 13 years of monitoring, the experiment resulted in diverse and abundant regeneration, although competition with beech sprouts was a problem in certain patches. As with other studies, we are looking at beech control methods that might improve establishment and growth in the sapling class.
On the economic side, SCE is never going to maximize revenue from wood products, producing only about 60 percent of the merchantable volume in the more conventional single-tree and group-selection harvests we compared against. But viewed as a restoration treatment or as a component of multifunctional forest management, the study showed that SCE could pay for itself and, when site and market conditions are favorable, generate enough profit to make it attractive for some landowners.
Perhaps most exciting, however, was the effect on carbon sequestration and storage. SCE resulted in much higher carbon storage than the conventional selection harvests, an effect attributed to both the higher structural retention after harvest and unexpectedly high sequestration (that is, carbon uptake) rates. Prospects look good for SCE and other types of old-growth silviculture as forest carbon markets emerge and incentivize exactly these kinds of practices.
What’s the Future of Old-Growth?
Old-growth in some form will persist, even if forest composition changes, species ranges shift, and natural disturbances are altered. Adapting to change will be just as relevant for old-growth management as it will be for other types of forestry.
Conservation of old-growth forests, and management of some younger stands to redevelop elements of the complexity found in old-growth, have clear carbon storage and climate benefits. They will also enhance the resiliency of forest ecosystems to stresses in the environment. For example, ecologists have long known that complex canopies can shield, to some degree, forest microclimates. Complex forest structure can also mitigate the impact of extreme rain events, for instance, by slowing runoff, trapping sediment, and absorbing flood energy in streams.
Although the future is uncertain, this much is clear: eastern old-growth forests, and forests managed for old-growth complexity, will and must be an element of a sustainable, resilient landscape. With care and attention, future generations will have the same experience of walking through an old-growth forest that ours has enjoyed.
Web Extras
We’ve noticed an uptick in questions from readers about old-growth forests: what they are, how they relate to carbon sequestration, and how to simulate some of their characteristics in younger, managed stands. For the spring issue, we recruited forest ecologist and old-growth expert Bill Keeton to share some his insights on the topic. However (like old-growth forests themselves) this is an incredibly diverse, rich topic to cover.
Readers interested in exploring more on the topic may want to start by reading two articles from past issues, Big Reed Forest Reserve: A Place Out of Time, by Joe Rankin, and Reconstructing the Past: Maine Forests Then and Now, by Andrew Barton, Alan White and Charles Cogbill.
Andrew Barton, one of the authors of this second article, is also the co-editor along with Bill Keeton of a recently published compilation of technical reviews, Ecology and Recovery of Eastern Old Growth Forests.