Old Growth

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Old growth cedar on McRae Creek, H.J. Andrews Experimental Forest
Al Levno

Scientific, social, and economic perspectives on old-growth forests and our treatment of them have changed dramatically over the last 150 years in the Pacific Northwest. Research at the H.J.A. Andrews LTER has been central to many those changes in the last 30 years, when perceptions about old-growth shifted from an unproductive forest to be eliminated through exploitation to a valued ecosystem to be protected and restored (Spies and Duncan 2009).

Research by Andrews scientists on ecosystem structure, composition and function in the late 1970s and early 1980s laid down a fundamental ecological model of old-growth conifer forests that has stood the test of decades of empirical and experimental work and has been revised to incorporate new knowledge from long-term studies of forest ecosystems. This work sparked a global interest in locating, studying and managing older forests.

The initial simple model of stages of Douglas-fir development: regeneration (0-20 years), young forest (20-80 years), mature forest (80 to 200 years) and old growth (> 200 years) has been replaced with a more nuanced set of structural stages and multiple pathways in which forest development over many centuries can move through a variety of stages of structure, composition and spatial heterogeneity (Franklin et al. 2002) following very diverse pathways. Historical studies have revealed that the fire regimes that shaped these forests are more complex and diverse than originally thought. For example, while some old-growth forest have developed without significant fire for more than 400 years, others have experienced low to moderate severity fires that created diversity in live and dead structures and species composition.

Study of old-growth and the processes that disturb these forests indicates the critical importance of biological legacies: the fires that killed and regenerated these old forests left many live and dead legacies that help to maintain biodiversity and ecosystem function such as carbon sequestration through time. This finding has led to development of new silvicultural systems throughout the PNW region and the world that promote retention of live and dead trees to create ecological complexity even where timber management is a significant goal.

The role of PNW old-growth in carbon cycling indicated these forests have some of the highest amounts of carbon per area in the globe. Simulation models based on long-term research at the Andrews Forest indicated proposals to liquidate these forests to increase carbon sequestration would have the opposite effect, releasing large amounts of carbon to the atmosphere.

More recently, long-term studies of growth and mortality in old-growth forests have indicated that mortality rates of trees in these have increased rapidly in the last 50 years suggesting that regional warming and increased water deficits may be contributors to widespread change in older forests (van Mantgem et al. 2009). This finding has accelerated interest in developing management strategies to increase the adaptive capacity of forests within the region to climate change. It also is spurring further research to help us better understand the demographic processes associated with populations of old trees.

For further reading: 
Spies, T.A. and S.L. Duncan, eds. 2009. Old growth in a new world: A Pacific Northwest icon reexamined. Island Press. 344 p.
Franklin, J.F., T.A. Spies, R. Van Pelt, A. Carey, D. Thornburgh, D.R. Berg, D. Lindenmayer, M. Harmon, W. Keeton, and D.C. Shaw. 2002. Disturbances and structural development of natural forest ecosystems with silvicultural implications, using Douglas-fir forests as an example. Forest Ecology and Management 155:309-423.
van Mantgem, P.J., N. L. Stephenson, J. C. Byrne L. D. Daniels, J F. Franklin, P. Z. Fulé, M. E. Harmon, A. J. Larson, J.M. Smith, A.H. Taylor and T.T. Veblen. 2009. Widespread Increase of Tree Mortality Rates in the Western United States. Science 323, 521-524.
For further information: 
Thomas A. Spies
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