Before considering the silvicultural options for promoting natural regeneration, the sources of regeneration will first be reviewed.  This review will include consideration of species that typically regenerate from seed and those that spread vegetatively (e.g., with rhizomes or root sprouts) or recover vegetatively after natural or human-induced damage (e.g., coppicing or pollarding).

Seed production

To assure sufficient natural regeneration after harvesting and to coordinate seed collection for planting programs, basic biological information about the timing of reproduction (phenology) and modes of pollination are needed.  Where the target species lack seed dormancy and are only sparsely represented amongst understory seedlings and larger advanced regeneration, timing of harvesting operations is critical to assure sufficient regeneration.  Collecting phenological data is not technically difficult, but a fairly large number trees growing under the range of environmental conditions characterizing the sites to be managed should be monitored.  If the pollen vector (e.g., wind, insect, bat or bird) is not known, once the season of flowering has been determined, studies on pollination should be carried out.

Ultimately, all plant species rely on seeds (or spores in the case of ferns, mosses, and other non-vascular plants) for regeneration.  Except in a few cases of non-sexual formation of seeds through apomixis, seed production is preceded by miosis to form gametes (pollen and ovules), pollen transfer, and fertilization.  Without the gene mixing associated with these processes, the effectiveness of natural selection is reduced and rates of evolutionary adaptation are consequently slowed.  Seed dispersal also promotes colonization of new sites, and escape from sites that have deteriorated in their appropriateness for regeneration.  Nevertheless, many species persist by vegetative means; these mechanisms are discussed later in this chapter.

Germinating seeds can be newly produced or long dormant in the soil or attached to mother plants.  Although many tropical plants produce seeds that are viable for only a few weeks or months after production, others have the capacity to remain dormant for years, germinating when environmental conditions are appropriate.  Densities of viable buried seeds in surface soils can be extraordinarily high (Table 8.x—Densities and life forms of buried seeds in tropical forest soils).  Harvesting activities that result in disturbance of surface soils (e.g., log skidding), promote germination of buried dormant seeds and may result in locally high densities of regeneration. Unfortunately, seed dormancy in soil is apparently more common among weedy species than it is among commercially important species.  It is therefore important to remember that silvicultural treatments that promote seed production and germination, and seedling establishment, may foster regeneration of desirable and well as undesirable species.

Long-term storage of viable seeds in fruits or cones that remain attached to mother trees is characteristic of some species of pine and some trees in the Myrtaceae (e.g., some Eucalyptus and Melaleuca), as well as some shrubs in the Proteaceae (e.g., Banksia spp.).  Seeds of many of these species are released only after the passage of a fire; such taxa are referred to as being serotinous. Regeneration of serotinous species is favored by the seed-bed preparing effects of fires.

Advanced regeneration and advanced residuals

The understories of many forests and woodlands contain substantial populations of seedlings, saplings, poles, and subcanopy trees of commercial species.  Trees larger than 5-10 cm dbh, but smaller than the minimum commercial diameter are often referred to as “advanced residuals”, whereas the others are called “advanced regeneration.”  In both cases, age variation and silvicultural responsiveness of individuals of the same size can be substantial.  The physiological mechanisms are not yet clear, but it appears that during long periods of suppression, plants of many species lose the capacity to grow rapidly when environmental conditions improve, such as when the canopy is opened by timber harvesting.  In contrast, many species of moderately to extremely shade-tolerant species rely on post-disturbance regeneration from seedling “banks.”

Among the seedlings in the seedling bank may be individuals representing several age cohorts, i.e., the products of different fruiting episodes. Soon after fruiting, seedling densities are high but then diminish with time.  Seedling mortality rates can be very high and spatially patchy due to the impacts of hervivores and the damping-off fungi that rapidly infect seedlings that are stressed from being crowded and shaded.