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species

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Abundant

Abundant or

Scarce

Scarce

scarce scarce reclamation because they will more quickly form a canopy that protects the soil from erosion.

From a forestry perspective, the functional characteristics of early succes-sional species or pioneer species make them relatively easy to manage (see Chap. 5). Clear-cutting may be necessary to obtain the necessary light for germination and growth of desired species. The pulse of nutrients released by burning or other forest disturbance may also stimulate the growth of early suc-cessional species, such as species of the genera Cecropia, Musanga, Macaranga, Ochroma, and Trema. These species produce large quantities of small seeds that are capable of dormancy and are called "orthodox" (Whitmore 1998). Orthodox seeds usually remain viable for relatively long periods of time and form the seed bank in many forests. Dormant seeds are often already present in the cleared area and more are quickly blown or carried in after an area is cleared.

In contrast to management for pioneer species, management for species of the mature forest is often difficult due to seed characteristics. Seeds of many primary forest species are large and heavy and fall straight down. Other species, such as those of the commercially important Dipterocarpaceae family, have winged seeds, but dispersal is still not far from the parent tree. Other characteristics relate to the viability of seeds. Almost all the seeds of some species are viable, while for other species only a few germinate. Length of time that seeds remain viable also varies. Seeds of some mature forest tree species often have little or no dormancy. They germinate as soon as they reach the forest floor, or within a few days. Unless they encounter favorable conditions within days after falling, they never germinate. These types of seeds are called "recalcitrant" (Whitmore 1998), somewhat of a misnomer according to Janzen and Vazquez-Yanes (1991) who said, "Such a terminology is akin to labeling an animal 'recalcitrant' if it cannot be maintained without food."

The characteristics of many mature forest species mean that most will spread only very slowly unless the seedlings are transported and planted by humans. It also means that the seeds must be collected very soon after falling if they are to be useful. Specific procedures to preserve, store, and germinate the seeds when desired must be developed for each individual species. Because of such special requirements, the physiological ecology of each species must be well understood in order to achieve successful management. The high specific gravity of the wood of many mature forest species is also a consideration in forest management because it means that the volume accumulation of these species will be relatively slow. However, such high-density wood often brings a good price. It is also often resistant to decay and can be used for pilings and other submerged support beams.

The secondary successional progression outlined in Table 3.2 is often called relay floristics (Whitmore 1991) and frequently occurs on large areas of abandoned agricultural land when there is no further disturbance. Early pioneers can modify the environment in such a way as to make establishment of later species more feasible. Such modifications include nitrogen enrichment of the soil by nitrogen-fixing bacteria associated with leguminous tree species such as those of the genera Inga, Acacia, and Albizia. Other influences of pioneer species on the microenvironment may include provision of shade which increases soil moisture and lowers soil temperature (necessary for many mature forest species). Pioneer species can also provide habitat for seed-dispersing animals. However, if the soil in the successional area has not been heavily disturbed, microhabitat may be more favorable for all species, and the successional progression may resemble more simultaneous colonization in which the species that become established depend simply upon which seeds arrive first in the disturbed area.

When secondary succession progresses according to the pattern of relay floristics (Table 3.2), we can expect to observe the following ecological trends which are potentially important for management:

• Species diversity and mutualisms will increase.

• Gross photosynthesis increases at first, as the quantity of leaves in the successional forest increases. Respiration also increases, but not as quickly. Net primary production (the difference between gross photosynthesis and respiration) increases in early stages of succession and as a result accumulated biomass increases. Later, however, more and more of the gross photosynthesis is used for maintenance and less for biomass accumulation. Eventually, biomass of the forest reaches an approximate steady state (Fig. 2.7).

• The trend of nutrient cycling during the course of succession may depend on the nature of the soil on which the succession is occurring. If the soil is low in nutrients, the cycle may become tighter as nutrient-conserving mechanisms play an increasingly important role (Odum 1969). If the soil is high in nutrients, the cycle may become looser, because early in succession, available nutrients go towards building biomass. However, as maturity approaches and biomass no longer increases, available nutrients are leached from the soil (Vitousek and Reiners 1975).

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