Thinning Practices in Southern Pines - With Pest Management Recommendations
T. Evan Nebeker – Respectively, professor, Department of Entomology,
John D. Hodges – Professor, Department of Forestry, Mississippi State University,
Mississippi State, MS,
Bob K. Karr – Assistant professor, Department of Forestry, Mississippi State University,
Mississippi State, MS, and
David M. Moehring – Professor (deceased), Department of Forestry, Mississippi State University,
Mississippi State, MS.
United States Department of Agriculture, Forest Service, Technical Bulletin 1703, December 1985.
Growth of Trees and Stands
The principles of forest management are not greatly different from those
affecting other agricultural crops. Trees, like other crops, require light,
water, nutrients, space, and protection from insects and diseases. The
fundamental growth processes are quite similar. The major difference is the
length of time required to reach maturity. Given this difference, the
economics of intensive management of a system as extensive as a forest has not
always seemed favorable.
The growth potential of a tree is determined genetically, but actual growth
is determined largely by the environment. Numerous environmental factors
affect growth; of these, water, nutrients, and light intensity are most easily
manipulated.
Height and Diameter Growth
Height growth in the four major pines of the Southeast is indeterminate.
Additional flushes (multinodal growth), particularly during midseason, reflect
current soil moisture conditions. However, the initial terminal bud is
formed in the year before extension, and the height growth from the extension of
that bud is closely related to availability of soil water during the late summer
of bud formation. If height increment is plotted against age, growth
begins slowly at first, climbs more steeply, then flattens out (Prodan 1968).
It is in these steeply climbing intermediate years, the grand period of growth
between ages 10 and 30, that foresters try to regulate growth through thinning.
Diameter growth is also closely related to availability of soil water.
Cambial cells begin dividing in early spring when soil water is not limited and
stop in late summer when conditions are reversed. Across this ring of
annual growth, early wood cells are abruptly followed by late wood cells whose
greater density increases the specific gravity. Although the transition
from early to late wood is not a well-understood process, , a decrease in soil water availability usually precedes
the formation of late wood cells and a continued moisture deficit stops cell
division. However, cell division may begin again in midsummer to late
summer with increases in soil moisture, as evidenced by false annual rings.
In addition, late wood cells continue to form until late summer or early fall if
soil water is available (Moehring and Ralston 1967). It is partly through
these biological principles that growth increments of individual trees can be
regulated through thinning practices or stand density control.
Stand Development
Growth of stands is influenced by site quality, age, species, stocking level, and forestry practices.
A tree's environmental standing can be expressed through the concept of site
quality. The site index is an integration of several environmental
factors, but emphasizes the quality and quantity of soil nutrients and water.
The rate of stand development increases with increased site index. Thus,
the carrying capacity of a given unit of land for tree production increases with
increasing site quality.
 Growth response as a result of thinning (from Burton 1982). |
In an even-aged pure stand, the stages of development are similar throughout
the stand at a given age, although more advanced stages are reached earlier on
better quality sites than on low-quality sites. For similar stockings, a
stand on a high-quality site will require thinning earlier than one on a
low-quality site.
Part of the popularity of even-aged silvicultural management is the simplicity of stand structure. By definition, most trees are of similar
age, reaching sapling status – and to a lesser extent, pole and sawlog status – at
roughly the same time or stage in stand development. However, taller trees
with larger diameters and crowns suppress the growth of neighboring trees, which
may become overtopped and eventually die. (Four standard crown classes are
recognized in forestry: dominant, codominant, intermediate, and overtopped or
suppressed.) As a stand matures, the natural process of competition
concentrates the growth potential of the stand in the dominant and codominant
trees. |
 Uneven-aged southern pine stand, showing size classes. |
Stocking
What are the implications of stocking and stand development? Proper
stocking is a term commonly used but as difficult to apply as to define.
First of all, proper stocking is the number of trees per acre that fully
utilizes the site's potential to grow trees. It follows that a
high-quality site has a higher carrying capacity and, if properly stocked, would
carry more trees per acre than a low-quality site. Second, a given site
may be properly stocked, once the carrying capacity is reached, with an initial
spacing as low as 450 trees/acre or as high as 1,000 or more trees/acre.
Third, the rate of diameter growth on individual stems and of stand development
differs considerably with spacing. Both the rate of diameter growth and the age at which carrying capacity, in basal area, is reached are greater at wide spacings than at narrow spacings.
It is these concepts of stocking, carrying capacity, and stand dynamics that
form the biological basis for spacing and thinning to achieve management
objectives.
The preceding discussion establishes the following premises:
- Stand differentiation, or stages of development, occurs at earlier ages on
high-quality than on low-quality sites.
- Competition directs growth potential of the stand toward dominant and
codominant trees.
- Competition promotes crown differentiation in the stand.
- Realized growth potential is a function of site, stand, and environmental
conditions.
- Stand development is a predictable process.
- Site quality, as an integration of environmental factors, is a major
determinant of the rate of stand development.
- Once the carrying capacity of a site is reached, total volume is similar
over a wide range of stocking.
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