Good Forestry in the Granite State:
Recommended Voluntary Forest Management Practices for New Hampshire
Table of Contents >> 2.1 New Hampshire Forest Types << 2.2 Forest Structure >> 2.3 Regeneration Methods

2.2 FOREST STRUCTURE

BACKGROUND

Managing forest structure can meet landowner objectives including a sustainable flow of forest products, wildlife habitat, aesthetics, clean water, and other benefits.

Forest structure is the horizontal and vertical distribution of layers in a forest including the trees, shrubs, and ground cover (which includes vegetation and dead and down woody material). Structure looks at the proportion of small, medium, and large trees and is usually reported as trees per acre by diameter class. These age- or size-class groupings are further defined as seedling, sapling, pole, and sawlog.

Size Class Groupings

                                      Diameter in Breast Height
                                           (DBH) in inches
Seedling...............................up to 1
Saplings...............................1-4
Pole......................................5-11
Sawlog.................................12 and larger

Forests can have a simple structure or they can be very complex. Based on the range of ages among the different levels of structure, forest stands are defined as even-aged, two-aged or uneven-aged.

Even-aged structure means a stand has one distinct age and size class. (An age class is comprised of trees within 20 years of age). They are often less diverse and composed of fewer species than other structures. Most of the tree diameters come close to the average stand diameter. A plantation provides an extreme example of an even-aged structure.

Two-aged stands are often, but not always, a result of human intervention and may be a temporary condition as management works towards developing an even-aged or uneven-aged stand. Structure within these stands will often have patchy or partial overstory canopies with a well-defined second story, or layer, of either poletimber or seedlings and saplings.

Uneven-aged structure means a stand has three or more age classes. This type of structure is a result of increasing species, age- and size-class diversity within a stand. Different species grow at different rates, and a distinct overstory canopy may no longer be recognizable. Each species or age class exhibits an average stand diameter of its own, and smaller diameter classes may contain more trees per acre than the next larger one. Uneven-aged stands are considered balanced when they have three or more age classes occupying approximately equal areas. When this is achieved, the stand can be considered self-sustaining.

Wildlife biologists and foresters are often interested in structure because of its relationship to timber flow, biological diversity and wildlife habitat. Other chapters in this publication address habitat issues. The focus of this chapter is on the role of structure in maintaining a flow of timber products over time.

OBJECTIVE

Maintain a sustainable flow of quality timber through control of stand and forest structure.

CONSIDERATIONS

RECOMMENDED PRACTICES

Even-aged Management

Tree Size

Percent Of Acres

Seedling/Sapling

20-30

Pole

25-35

Sawlog

35-55

These targets are based on rotation ages of about 80 to 120 years (shorter if there is a predominance of short-lived species such as aspen, white birch or balsam fir). They are most applicable at the landscape scale or on large properties (several thousand acres or larger).

Uneven-aged Management

Tree Diameter

Percent Basal Area
(of Sq. Ft./ Acre)

Percent Nos.
(of Trees/ Acre)

6-10

30-50

60-80

12-14

20-30

15-20

16-22+

25-50

5-20

Examples (using the mid-range in above categories):

(1) If a stand contained a basal area of 100 square feet per acre, 40 square feet per acre may represent trees 6-10 inches in diameter at breast height (DBH), 25 square feet may represent trees 12 to 14 inches DBH and 35 square feet may represent trees 16 inches DBH or greater.

(2) If the stand contained 100 trees per acre, those same classes may contain 70, 17.5 and 12.5 trees per acre respectively.

CROSS REFERENCES

2.1 New Hampshire Forest Types; 2.3 Regeneration Methods; 2.4 Managing for High-Value Trees; 6.2 Cavity Trees, Dens and Snags; 6.3 Dead and Down Woody Material; 6.4 Overstory Inclusions; 6.6 Temporary Openings Created by Forest Management; 6.7 Aspen Management; 7.5 Old-Growth Forests.

ADDITIONAL INFORMATION

DeGraaf, R.M., M. Yamasaki, W.B. Leak, and J.W. Lanier. 1992. New England Wildlife: Management of Forested Habitats. USDA For. Serv. Gen. Tech. Rep. NE-144. 271 p.

Frank, R.M., and J.C. Bjorkbom. 1973. A Silvicultural Guide for Spruce-Fir in the Northeast. USDA For. Serv. Gen. Tech. Rep. NE-6. 29 p.

Lancaster, K.F., and W.B. Leak. 1978. A Silvicultural Guide for White Pine in the Northeast. USDA For. Serv. Gen. Tech. Rep. NE-41.13 p.

Leak, W.B., D.S. Solomon, and P.S. DeBald. 1987. Silvicultural Guide for Northern Hardwood Types in the Northeast (revised). USDA For. Serv. Res. Pap. NE- 603. 36 p.

2.1 New Hampshire Forest Types << 2.2 Forest Structure >> 2.3 Regeneration Methods

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