Tree Growth

Generally, radial growth of hardwoods begins each year after leaves are fully grown. Growth is completed by late August. The length of this "grand period," shorter than for conifers, seems to be influenced by late winter temperatures. As height growth depends on the amount of carbohydrate produced the previous year, rather than on the current-year's photosynthesis, terminal elongation takes place in early spring before leaf buds develop into foliage. Trees cease height growth in

Figure 4.4 Old-growth hardwoods in the Georgia Piedmont. These oaks doubtless seeded-in under pioneer species like the southern pines that, in turn, arose following fire. Eventually, these broadleaf trees captured the site.

summer, their leaves continuing to produce carbohydrate through photosynthesis for storage. Some species with long seasonal growth periods, flowering dogwood for instance, probably use the current year's carbohydrate production for additional increment. When conditions are favorable, hardwoods produce multiple shoot growth flushes, each flush followed by a rest period during which root elongation takes place.1

Rainfall distribution during the year may influence radial growth. Species within a genus, like oaks among the Quercus, vary in their responses to such patterns. Some tissues for some species withstand considerable desiccation, yet regain turgidity when adequate moisture is restored. Drought resistance of post and blackjack oaks, for example, is primary active. Such plants withstand dehydration, rather than being able to prevent dehydration (secondary active, like cacti), or to enter a resting stage (passive, like ferns, mosses, and lichens). Shoot to root ratios, as they affect transpiration rates and the area of moisture-absorbing root surfaces, could influence drought resistance, but the ratio of the weight of above-ground to below-ground parts of a tree is more closely tied to taxonomic than to ecological relationships.2

In stands of pines mixed with a variety of broadleaf species, the effect of drought is first noticed among the upland oaks. Foliage of these hardwoods may be shed by midsummer, while pines are retarded in exhibiting the effect—seedlings may not appear to be dying until autumn. Radial growth measurements that depend on ring-width tallies may mislead, because false rings also develop. There may be no way to differentiate between false and true rings. For example, frost that follows spring cambial activity causes such false rings. They are exhibited especially in cross-sections of southern magnolia growing in moist soils near streams. Provenance seems not to play a significant role in the rate of growth of broadleaf trees, as studies with sugar maple indicate.3

Lesser plants play a role in nurturing deciduous forest trees. Lichens contribute ammonia nitrogen, especially in beech forests. In addition to adding nitrogen to the soil in forms available to plants, legumes improve tree growth in young stands because these bean-producing stems grow during the cool season without smothering seedlings. Inoculating the soils with mycorrhizae in which maple and black cherry grow increases seedling growth.4

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