Section 15
Stand, Site, Growth, Yield

Tree Growth and Form

To gain insight into growth of stands and trees, and how silvicultural practices can influence them, requires familiarity with growth patterns and the resulting shape of individual trees. Trees make their annual growth by extension of shoots and roots and by thickening of stems and roots. We characterize these two increment processes by two partial parameters: height and diameter increment.

A. Height growth

The rhythm of annual height growth is characterized by the occurrence of maximum increment--the "grand period of increment". After a slow start, the rate of growth increases to a maximum and then falls off again. Thus, the seasonal pattern of growth closely parallels the pattern of growth as a function of age.

If we plot cumulative growth, we obtain the typical sigmoid curve of growth showing total height attained by the tree at any particular age (Figure 2a).

If instead the annual height increments are plotted against each year of the corresponding ages, a height increment curve is obtained. In the height increment curve there are two points of inflexion and the one point of culmination. The two points of inflexion mark the division between three characteristic phases of increment curves: juvenile phase, phase of full vigor, phase of decline (Figure 2b).

Factors Influencing Height Growth

  1. Species characteristics. Light-demanding species reach maximum of current height increment earlier than shade-tolerant species. The curve both climbs and falls more steeply.
  2. Site. Effect of site such that on better sites culmination of increment occurs earlier than on poor sites.
  3. Competition during early years suppresses height increment and postpones age of maximum growth rate sometimes by 50 to 100 years. A striking acceleration of height growth can be induced by heavy thinning before the culmination of increment.

FIGURE 2 Height growth and height increment curve of a spruce tree. (From Assmann 1970).

B. Diameter growth

Every year a tree forms a new layer of wood; we see it in the form of an annual ring. We usually measure this increment at DBH. If we compare diameter growth curves and diameter increment curves for different tree species on the same site, we find that they are analogous to those of height growth. That is, culimination of diameter growth of light-demanding species occurs early in comparison with shade-tolerant species. On the other hand, performance of light-demanding trees drops rapidly and sharply during later years.

We use both DBH and basal area (BA) to characterize growth but we must remember that the area increment of the cross section of a tree does not depend on the annual ring width alone (i.e. the diameter increment) but also very much on the diameter of the cross section of stem where it accrues. The larger the initial diameter, the larger the sectional area increment for a given annual ring width (or for a given diameter increment). As a result, the basal area increment at breast height, or any chosen stem height for that matter, culminates considerably later than the corresponding diameter increment. This is demonstrated by Figure 3 which shows that in the spruce tree chosen as an example basal area increment does not culminate until age 38. That is 13 years later than diameter increment. Even after diameter increment has started to decrease, BA increment may continue to increase for awhile, because the narrowing annual ring is added to an increasing cross area.

FIGURE 3 Diameter increment (id) curve (top) and BA increment (ig) curve (bottom) of a spruce. (From Assmann 1970)

The available growing space has an important influence on diameter and BA increment of individual trees. With increasing growing space, BA increment of individual trees increases until the maximum utilizable growing space is reached. When this limit has been reached, a yet more liberal provision of light cannot produce further increment.

Diameter and BA Increment at Different Stem Heights

Annual ring width decreases from base of the stem toward the middle and increases again toward base of the crown and in the crown. Consequently, BA decreases with height. This is most pronounced in older trees because younger ones have wide rings, and more pronounced on poor than on good sites.

C. Volume Increment

Volume increment is an expression of a tree’s vigor. We have already seen that height and radial increments, its two components, do not coincide in time. Height increment culminates earlier, BA increment later. The BA increment is added to a continually lengthening and thickening tree body so that culmination of volume increment is reached even later than that of BA increment. In the example given in Figure 4 at age 62, there are 14 years after culmination of BA increment at age 48, and 29 years after culmination of height increment at age 33.

FIGURE 4 Curves of current annual volume increment (c.a.i.) and mean annual increment (m.a.i.) of a spruce (tree is same as in Figures 2 and 3). (From Assmann 1970).

The shape of the volume increment curve depends on genetic and environmental factors. For instance, volume increment is more rapid in shade-tolerant than intolerant species.

Open position in the stand speeds up volume increment; overhead or dense shade from neighboring trees suppresses it. Thus, in multi-storied stands with wide age differences, growth often makes irregular progress (Figure 5).

FIGURE 5 Current and mean volume increment curves of a spruce in a multi-storied stand. Increase in c.a.i. probably effect of release. (From Assmann 1970).

Volume culmination of the m.a.i. begins considerably later for individual trees than for entire stands. This is a consequence of the progressive reduction of the number of stems in a stand, whereby the growing space is increased and the per acre efficiencies of the trees are progressively reduced, i.e. we have lower increments per unit area of ground occupied. The decrease of mean increments and mean yields per acre, which sets in when the crop has reached certain average dimensions, applies not only for pure even-aged stands but for all types of stands.

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