Science You Can Use: Multi-age, multi-use: Tips for transforming Rocky Mountain ponderosa pine forests from even-aged to multi-aged structures

Have you ever looked at a towering ponderosa pine forest and wondered how old the trees are? Were all the trees of a similar age, known as an “evenaged structure,” such as often results from a reforestation effort where all the seedlings were planted at the same time. Or were there a mixture of young and old trees? That sort of “multi-aged structure” in ponderosa pine forests is desirable as it can meet many of the goals of multi-use land management. A forest stand with different-aged trees can support timber production, livestock grazing, recreation opportunities, wildlife habitat, and native plant diversity. 

Until recently, the steps that a silviculturist could take to move a forest in that direction were vague, and multi-aged practices were often incorrectly perceived as inherently complex, rigid, or unsuited to a shade-intolerant species. 

Photo Credit

Forest Service photo by Neil Williams.

A multi-aged ponderosa pine stand at Primm Meadow, MT, managed by The Nature Conservancy. Many cohorts of trees of widely varying age are present in the overstory, stand density is low, and the spatial structure is variable.

Additionally in some regions it has been tough for silviculturists to learn the skills needed to create a multi-aged structure because there simply aren’t many good examples to learn from. Now, a new practical guidebook fills this gap. 

To help silviculturists transform even-aged ponderosa pine forests to multi-aged structures, Neil Williams (a postdoctoral fellow with the Oak Ridge Institute for Science and Education) and Justin Crotteau (a research forester with Forest Service Research and Development) worked with Forest Service colleagues to create a new guidebook. Their ‘how-to’ enables managers to plan and visualize forest transformations from multiple angles while responding and adapting to the unexpected. The authors’ goal is to help guide land managers through the forest transformation process and show that multiaged silviculture in ponderosa pine stands doesn’t need to be cumbersome and resource intensive. A diagram showing the start and end points for a hypothetical transformation to multi-aged structure accomplished primarily using the variable retention harvest method. 

Transformation is a process that begins with an “existing condition” and is complete when the desired multi-aged structure, the “desired future condition”, has been reached. But these starting and ending points may look very different from one forest stand to the next. The guidebook addresses this variability by exemplifying a range of common “existing condition types” and some of the diverse potential “future conditions” that can be maintained in ponderosa pine forests. 

Later chapters describe the wide range of silvicultural practices that can be used to build a transformation sequence that links the existing condition to the desired future condition. This includes methods for regenerating new ponderosa pine, tending existing trees in the stand, and controlling stand density. Methods range from “light touch” treatments that simplify managing multi-aged stands, to more complex approaches. The authors also include hypothetical examples and real-world case studies showing ponderosa stands 10 to 30 years into the multiaged structure transformation process. 

The printer-friendly guidebook contains reference pages that can be used in the field. Although it is centered on the Northern Rocky Mountains, the content is adaptable throughout ponderosa pine’s range across inland western North America. “This is a guidebook we hope will reduce complexity and perceived red tape to encourage foresters that they do have the skills and knowledge to make transformational changes in forest structures already,” said Crotteau.