We all believe we know what is meant by multi-aged silviculture. It summons images of a forest where there are trees of many sizes, ages and species all growing together in a harmonious, integrated self-sustaining environment. The image is clear, but the underlying mechanisms and constraints are not.
The first portion of the talk was spent dispelling myths about uneven-aged forests, what they are and how they work. The myths are:
- Uneven-aged stands must have a reverse-J diameter distribution with all size or age classes represented. This assumes that there are a small number of fully mature trees with an understory of many smaller, younger trees. Real forests are a lot more complex than this.
- Uneven-aged stands exhibit a balance between size classes. Each age class occupies an equal amount of growing area. This actually imposes a human-created model on the forest where people take what the forest grows and prunes it back to match the model, rather than adjusting the model to what they see develop.
- Uneven-aged stands maximize biodiversity. Biodiversity is actually better served when there is a distribution of different age-classes within an area. Some organisms thrive best in young stands, others thrive in old-growth, and they’re not the same organisms. Mingling them together in an uneven-aged stand serves neither.
- Uneven-aged = all-aged Age structure is not the same as size structure. Stand structure and species composition are critical elements of the stand. A forest can have trees that are all the same age, yet still exhibit an array of sizes and have both a canopy and an understory, due to the species composition and relative competitiveness.
- Uneven-aged stands are more “natural” Truly natural stands have an array of naturally occurring structures, including even-aged and multi-aged. Natural stands containing all ages classes together are actually rare.
- Uneven-aged systems assure sustainability Measurements on very old European test plots are showing that growth rates are increasing in what should be a steady-state system under the harvest theory used. It demonstrates a need to keep adjusting management parameters to account for changing environmental conditions.
- Uneven-aged stands are only suitable for shade tolerant species Uneven-aged stands can be developed for shade intolerant species as long as the tree spacing is sufficient to allow the young trees sufficient sunlight. As soon as the canopy closes, however, the shade tolerant understory trees will have the advantage.
- The Central European model for selection silviculture is appropriate for other systems The base species for central European forests are European beech, Norway spruce and silver fir. All of these species are very shade-tolerant, and they recover from selection harvesting well because they don’t have the sun requirements of pines. This harvest model should not be used for forests with high sun requirements.
- Uneven-aged stands are less productive. Test plots on ponderosa pines showed that the statistical differences between even-aged and multi-aged stands are insignificant. Water stresses may be a factor for forests with hot dry summers.
- Multi-aged stand productivity is unaffected by density and structure. A multi-aged stand can perform much like an even-aged stand, exhibiting an array of dominant, co-dominant, intermediate and suppressed trees, where the dominant trees are growing vigorously and the suppressed trees scarcely at all. The big trees typically grow better, so a stand with loads of little trees, like the reverse-J may be hampering overall productivity.
So, now that the myths are dispelled, what does Dr. O’Hara believe are good management parameters?
- A wide array of age classes is not necessary when the size variation caused by dominance and suppression are sufficient to create the desired variety.
- Generate models that mimic normal disturbance regimes for the forests being managed. That which is appropriate for redwoods is not appropriate for pines and vice versa.
- Models are approximations – let the forest drive the model, not vice versa.
- In general, longer cutting cycles are desirable due to the reduced disturbance in the forest. Every 20 to 25 years is good in pine forests. Cutting cycles to maintain a multi-aged stand in Redwood/Douglas firs needs to be about every 10 to 15 years since the trees grow so quickly they lose their multi-age structure quickly.
The University of California Forest Research and Outreach program presented this webinar on February 25, 2013. Hear it for yourself at http://ucanr.edu/sites/forestry/Webinars/Current_Research_at_UC_Berkeley_webinar_series/February_25,_2013_Dr_Kevin_OHara/
Below are references to papers that cover the content that Dr. OHara discussed on the webinar. Unfortunately we are not able to post these papers or distribute them because of copyright issues. Authors are, however able to send out copies to those who inquire, so please contact Kevin directly (email@example.com) if you would like a copy of one of these papers:
- O’Hara, K. L. 1996. Dynamics and stocking-level relationships of multi-aged ponderosa pine stands. Forest Science 42(4): 1-34, Monograph 33.
- O’Hara, K.L. 1998. Silviculture for structural diversity: A new look at multiaged systems. Journal of Forestry 96(7): 4-10.
- O’Hara, K.L. 2001. The silviculture of transformation: a commentary. Forest Ecology and Management 151(1-3)81-86.
- O’Hara, K.L. 2002. The historical development of uneven-aged silviculture in North America. Forestry 75(4): 339-346.
- O’Hara, K.L., and R.F. Gersonde. 2004. Stocking control concepts in uneven-aged silviculture. Forestry 77(2): 131-143.
- O’Hara, K.L. 2006. Multiaged forest stands for protection forests: concepts and applications. Forest Snow and Landscape Research 80: 45-55.
- O’Hara, K.L., and L.M. Nagel. 2006. A functional comparison of productivity in even-aged and multiaged stands: A synthesis for Pinus ponderosa. Forest Science 52: 290-303.
- O’Hara, K.L., H. Hasenauer, and G. Kindermann. 2007. Sustainability in multiaged stands: An analysis of long-term plenter systems. Forestry 80(2): 163-181.
- Berrill, J.-P., and K.L. O’Hara. 2009. Simulating multiaged coast redwood stand development: interactions between regeneration, structure, and productivity. Western Journal of Applied Forestry 24(1): 24-32.