Silviculture
Today's
powerpoint file
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- We've talked about a wide range of tools used to manage vegetation.
Today, we'll discuss harvest-regeneration methods used in forestry. Our
objectives for the next 3 days are to (1) create a vocabulary and (2)
introduce concepts regarding stand replacement of forest stands
- Until the 1700s, common practice was to go into a stand of trees every
few years and remove trees
- --> development of silviculture, specifically clearcutting as a system
of harvest and subsequent regeneration
- Clearcutting was the first silvicultural system; it was
developed to replace degraded stands
- Produces even-aged stands
- Clearcut: removal of all merchantable material in a stand; see image
here
- Some states have laws or regulations governing maximum size of
clearcut units--Arizona has no such regulations (defers to NFMA, which
has no arbitrary size constraint)
- Advantages:
- Can easily handle stands in any condition
- Can artificially introduce appropriate trees in a single step
- Can realize large genetic gains with artificial regeneration
- OR
- Can maintain high-quality trees via natural regeneration from adjacent
stands
- Relatively low skill level; easy to administer
- Low costs for harvesting, management, administration
- No damage to residual stand
- High quantity and availability of herbs, browse (at least
initially)
- High water yield
- Disadvantages:
- Minimal site protection:
- High erosion potential
- Regeneration uncertain
- Leaves large amount of slash, which must be removed
- Non-continuous cash flow
- Windthrow in adjacent areas
- Aesthetically unappealing
- Generally decreased biological diversity
- Application
- Best-adapted for intolerant early-seral trees, especially those with
easily disseminated seeds
- In arid or semi-arid regions, northeastern aspects preferred (cooler,
moister microclimate)
- Patch
- Alternate-strip
- Natural regeneration into harvested strips from "leave" strips
- Harvest "leave" strips after trees in harvested strips are
reproductively mature, or artificially regenerate them
- Provides "landscape corridors"
- Progressive-strip
- Prevailing winds blow across residual stand --> waves of
regeneration
- Less windthrow than with alternate-strip method
- Coppice (synonymous with natural asexual reproduction)
- Simplest harvest-regeneration system
- Usually clearcut --> even-aged stand (aboveground)
- Reproduction accomplished by sprouting from stumps and roots (within
1-2 years)
- Advantages and Disadvantages similar to clearcut
except:
- (+) Excellent re-occupation of site
- (+) Rapid volume growth --> short rotations
- (-) Must have market for small material (i.e., fuelwood or fiber
production)
- (-) Limited to tree species that sprout profusely (few conifers, most
angiosperms)
- (-) Reduced vigor late in rotation, or after multiple rotations
(roots are old, so each coppice --> decline in vigor earlier in
rotation)
- (-) No potential for genetic improvement (no change in genetic
composition)
- Application:
- Use relatively small diameter trees (for evenly distributed site
occupancy); this is consistent with fiber production or fuelwood
production
- Cut when dormant --> more vigorous sprouting
- Cut stumps at an angle, to reduce water collection --> decay
- Good species: fast-growing species with minimal rot
- If there is uneven site occupancy (i.e., widely spaced trees in some
areas) due to large trees in previous stand--or if you want to introduce
new genetic material
- plant seedlings ("standards") in these gaps (this is called "coppice
with standards")
- Partial harvests (i.e., do not remove all merchantable trees in
one visit)
- Seed-Tree Method
- Produces even-aged stands
- Harvest all trees except 10-25 trees/ha for natural regeneration
("seed" trees); see image
here
- Residual stand should have "best" characteristics and should have
potential for good seed production (e.g., full crowns)
- Seed trees usually (but not always) are harvested after next stand is
established (usually 5-15 yr after initial harvest)
- Best adapted for intolerant trees with easily disseminated seeds; best
adapted for northerly aspects, in SW US (~ clearcut)
- Beware: there is usually considerable pressure to cut good trees and
leave poor-quality trees --> successively poorer stands
- Advantages and Disadvantages similar to clearcut
except:
- Uniform distribution of seed (contrast with naturally regenerated
clearcuts)
- Windthrow to seed trees (or in adjacent areas, sensu clearcut)
can be high
- Harvesting seed trees may damage regeneration (consider harvesting on
snow), and is more expensive than single entry per rotation
- Site prep is challenging, especially with fire
- Shelterwood Method
- Goal: regenerate site under shade and protection of existing trees
- Produces even-aged stands
- Application: series of cuts are made toward the end of the
rotation
- Preparatory cut (1-several, removing 25-40% of stand); see image
here
- "Prepares" trees, with respect to seed production capability or
windfirmness
- Seed cut (1, removing 25-50% of stand)
- Open canopy to favor regeneration of desired species
- Removal cut (1-few, all within 20% of rotation age, removing remainder
of stand)
- Remove what foresters call "crop" trees, in a manner which will
minimize environmental stress to regeneration
- All within 20% of rotation age
- Advantages and Disadvantages are "intermediate" between
harvest-regeneration strategies with higher or lower harvest levels
- Selection Methods
- Produces uneven-aged stands
- Harvest individual trees or groups of trees (preferably of similar age
and size within a group) and rely on remaining trees for protection and
regeneration
- Single-tree selection is best adapted for tolerant species (usually no
site prep); common in intensively managed German forests
- Group selection is suitable for tolerant or intermediate species
(usually mechanical site prep)
- Advantages
- Continuous cash flow --> popular for small woodlot owners
- Continuous tree cover --> high aesthetic, conservation values
- Also --> excellent site protection from invasion of off-site species,
erosion
- High biological diversity (generally)
- Minimal slash disposal (especially if decomposition is rapid)
- Well-adapted for all aspects
- Disadvantages
- Expensive
- High degree of skill required, time-consuming to administer
- Difficult to monitor inventory, growth (most mensurational techniques
were developed for even-aged stands)
- Difficult to protect young age classes
- Site prep limited to mechanical only (herbicide drift, fire may damage
adj. trees)
- Although our focus has been on conventional harvest-regeneration
systems, you should recognize that these systems are merely points on a
continuum of removing all overstory to removing none
- Intermediate treatments
Definition: | any manipulation in a stand that occurs between two
regeneration periods |
Note: | definition does not consider
uneven-aged mgmt. (in which regeneration is ± constant), but the
concept does |
Objectives: | control species composition, spacing, growth
form, and growth rate |
- Release Cuttings
- Applied early in stand development {saplings (< 15 cm DBH) or
smaller}
- Designed to control species composition and size structure
- Rarely yield merchantable material (treated as a "cost" in economic
analysis)
- Primary types of release cuttings: cleaning, weeding, liberation
- Improvement Cuts
- Release trees are pole size (15-30 cm) or larger
- Therefore, harvested material is often merchantable
- Purpose: release trees that will improve composition, form, and/or
growth of residual stand
- Often initial cuts in previously unmanaged (or poorly managed) stands
--> better condition for subsequent management
- In practice, locate and favor desirable trees rather than locating and
removing
undesirable trees
- Removing trees is expensive
- If removing a tree doesn't favor a desirable tree, it's not worth
removing
- Thinnings
- Objectives:density control (not composition control)
- capture volume that would otherwise be lost to mortality
- Total volume in thinned stands < unthinned stands @ end of rotation
- Thinning may increase yield of merchantable volume (i.e.,
increase value); may also provide early return on investment
- Thinnings are classified based on economic criteria (pre-commercial
vs. commercial) or biological criteria (based on relative position of
crowns: low, high, selection, mechanical, free)
- When released, young trees respond much more quickly and vigorously
than old trees
- Pruning
- Artificial removal of branches from selected portions of the stem, to
increase quality and value of trees and/or to alter vertical structure
- 17' pruning height is common
- Sanitation and Salvage Cutting
Sanitation: | removal of trees that have been attacked or
appear likely to be attacked by insects and/or disease to prevent spread
to other trees (often not economic) |
Salvage: | removal of dead or dying trees for economic
gain (death is usually by means other than "normal" competition-induced
mortality, e.g., fire, insects, disease, lightning) |
- Of course, it is not always desirable to remove damaged trees, with
respect to habitat requirements for some animal species
Additional Information (also see assigned readings):
Burns, R.M. and Honkala, B.H. (technical coordinators). 1990. Silvics
of North America (Volume 1, Conifers; Volume 2, Hardwoods). USDA
Agriculture Handbook 654, Washington, D.C.
Daniel, T.W., Helms, J.A., and Baker, F.S. 1979. Principles of
Silviculture, 2nd edition. McGraw-Hill, New York.
Ffolliott, P.F. and Gottfried, G.J. 1991. Mixed conifer and aspen
regeneration in small clearcuts within a partially harvested Arizona mixed
conifer forest. USDA Forest Service Rocky Mountain Experiment Station
Research Paper RM-294, Fort Collins, Colorado.
ForestyImages, online catalog of
forest management images.
Gottfried, G.J. 1992. Growth and development in an old-growth Arizona
mixed conifer stand following initial harvesting. Forest Ecology and
Management 54:1-26.
Jones, J.R. 1974. Silviculture of southwestern mixed conifers and aspen:
the status of our knowledge. USDA Forest Service Rocky Mountain
Experiment Station Research Paper RM-122, Fort Collins, Colorado.
Smith, D.W. 1986. The Practice of Silviculture, 8th edition. John Wiley
& Sons, New York.
Young, J.A. and Young, C.G. 1992. Seeds of Woody Plants in North
America. Dioscorides Press, Portland, Oregon.