Research review: Tradeoffs between mitigation and adaptation approaches

Forests are complex systems, both because of their many parts and processes and because of the diverse human values governing their use.  Sound forest management approaches recognize this complexity and balance multiple objectives including clean water, wildlife habitat, recreational uses, timber production,  and scenic beauty, based on landowner objectives. In recent years, two new objectives have emerged as priorities:  mitigation of changing climatic conditions and adaptation to them. Recent research in Minnesota and the Lakes States has shed light on the tradeoffs between these objectives.

Mitigation:  Forests and carbon

Red pine logs
Red pine logs

Trees are about 50% carbon by dry weight. This massive amount of carbon is derived from atmospheric sources; indeed, forests store over 80% of aboveground carbon globally. Atmospheric carbon is widely understood to be a major driver of climate change, and forests play a substantial role in reducing atmospheric carbon levels. Forest management research and practice are increasingly recognizing the importance of carbon sequestration and storage in forests. In general, thinning and final harvests that retain some large standing trees are preferable in terms of carbon storage because they retain the carbon on site. (Alternatively, processing harvested wood into long-lived wood products such as structural lumber or furniture can lead to long-term storage of forest-derived carbon.)

Adaptation: A focus on resilience

While instrumental records indicate changes over the past several decades in Minnesota’s climate, it is difficult to predict the nature of future changes. Trees live a long time, particularly in cold climates like Minnesota’s.  Uncertainty about future conditions is a problem for landowners and foresters. As such, landowners and forest managers increasingly recognize the need to focus on developing forest ecosystems that are resilient to a variety of disturbance regimes and conditions. Different tree species have different resource requirements to sustain healthy and vigorous growth.  For instance, while jack pine can thrive on dry, sandy relatively nutrient-poor sites, basswood and sugar maple require richer, moister soils. And even within a species, resource needs can change dramatically throughout their lives. Large, older trees need more resources to sustain themselves and can be more vulnerable and less resilient to windstorms, drought, or other disturbances than younger trees of the same species.

Young mixed spruce and hardwood stand near Grand Rapids, MN
Young mixed spruce and hardwood stand near Grand Rapids, MN

Forest management to promote resilience thus focuses on maintaining or increasing complexity in terms of stand composition (e.g. the number of species present), structure, and age. Stands that are more complex in these ways tend to have higher adaptive capacity and resilience to changing climate or disturbance regimes. Stands that lack high “response diversity” may be more vulnerable to changes in climate and disturbance.

Tradeoffs

The problem?  While some management objectives are highly compatible, others are less so. Research on red pine-dominated systems and northern hardwoods in Minnesota and the Lakes States by Tony D’Amato, John Bradford, Shawn Fraver, and Brian Palik suggests that forest management for both climate mitigation and adaptation (in other words, to store more carbon and to be more resilient to changing conditions) may fall into the second category. “For example, results illustrate that maintaining higher stocking levels (i.e. enhancing mitigation by increasing carbon stores) decreases stand-level structural and compositional complexity (i.e., reduces possible adaptation potential).”

This tradeoff was particularly clear in northern hardwoods systems. Single-tree and small-gap selection treatments led to dominance by a relatively small number of shade-tolerant species. This in turn reduced the system’s response diversity and vulnerability to changing conditions. Similarly, maintaining high stocking and long rotations (e.g. large, old trees) in fire-prone regions or in stand types vulnerable to insect outbreaks may reduce resilience and  increase ecosystem vulnerability to disturbance.

A white pine shelterwood harvest, Saint Louis County MN. For more on this method, see Chapter 4 of Extension's Woodland Stewardship book.
A white pine shelterwood harvest, Saint Louis County MN.

How can these two important objectives be reconciled? Silvicultural approaches that retain some large trees during harvest treatments, yet also create conditions that lead to increased stand response diversity may be part of the solution. Strategies like shelterwood harvests, in which mature trees are left standing to create mixed light conditions to favor regeneration and growth of seedlings of desired species may be seen as more beneficial. (For more on shelterwood and other silvicultural systems, see Chapter 4 of Extension’s Woodland Stewardship book.)

Relevance to family forest owners

Family forests can deliver tremendous value to landowners, value that goes beyond revenue or resources obtained from timber sale or harvest. Forest health is likely to become a more prominent issue in Minnesota as higher storm frequencies, drought, and other climatic events continue to increase in both frequency and severity.  Active management to increase resilience to these events will be the key to maintaining healthy forest lands. For landowners, that may mean an increased focus on maintaining stands containing a variety of tree species, ages, and sizes. Landowners may choose to maintain both rapid growth in the form of young trees well suited to their sites and a high level of carbon storage in the form of large, mature trees. A professional forester can help design forest management plan to do this that is appropriate to the site and specific landowner objectives.

Caring for and managing forest land has never been simple. But research like this gives landowners new tools to leave a lasting and positive legacy on the land.

Sources:

A.W. D’Amato, A.W., J.B. Bradford, S. Fraver, and B.J. Palik. 2011.  Forest management for mitigation and adaptation to climate change:  Insights from long-term silviculture experiments.  Forest Ecology and Management 262 (2011): 803-816.

Bradford, J.B., and A.W. D’Amato. 2012. Recognizing trade-offs in multi-objective land management. Frontiers in Ecology and the Environment 10(4):210-216.

(PDF downloads of both articles are available from the UMN Forest Resources Silviculture Lab site.)

Eli ‘s work addresses Minnesota forest ecology & management. He’s based in St Paul.

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