Module 10: Promoting Natural Regeneration of Commercial Species

Securing adequate natural regeneration for future harvests is a central but often hard-won goal for forest managers.  Despite the challenges and the popular perception of forest management as necessarily involving tree planting, natural regeneration has a number of advantages over artificial regeneration (e.g., hand or machine planting of seeds or seedlings).  One of these advantages is that because the seed sources for natural regeneration are individuals that successfully reproduced in the stand, it is reasonable to expect that they are genetically well adapted to local biotic and abiotic conditions. For plantation managers, in contrast, mis-matches of species, provenances, and genotypes to local site conditions is commonplace. Furthermore, transplanted seedlings often suffer high mortality rates and, if planted poorly, may grow slowly or develop deformed stems even if they do survive.  Natural regeneration is also generally less expensive than artificial regeneration, but it is not always “free.”  In any event, where natural regeneration is relied upon, management interventions are generally less drastic than where seeds or seedlings are planted.  Lessening the impacts of stand regeneration operations, in addition to saving money, has the advantage of reducing the effects of forest management on biodiversity and ecosystem functioning (e.g., stream sedimentation and nutrient cycling).  This is not to say, however, that methods for securing natural regeneration are always gentle. On the contrary, where natural, stand-regenerating disturbances include fires, hurricanes, or other major perturbations, the appropriate natural regeneration treatments are also likely to be severe.

For various reasons the debates over the environmental soundness of natural forest management have disregarded the simple observation that management, by definition, involves favoring some species or structures over others.  Perhaps the feeling that managed forests are not important for conservation is at least partially inspired by managers who focus solely on timber production, to the detriment of non-timber forest products, ecosystem services, biodiversity, and the whole range of other forest values.  Even where silvicultural treatments reflect a broader perception of forests than just a source of wood volumes, management necessarily involves some degree of forest “domestication.”  In particular, treatments designed to increase the yield of valuable forest products often generally involve conscious manipulation of species composition and stand structure.

Successful natural regeneration of harvested species without causing unnecessary harm to other species or forest processes requires substantial ecological and more specific silvicultural knowledge.  For example, the reactions to harvesting and other stand manipulations of commercial species, weeds, and other taxa need to be known.  Forest managers thus need to be aware of the intervals between seed crops (e.g., mast year frequencies), the distances to which seeds are dispersed, the probability of seedling establishment and survival, and the relative growth rates of commercial species and the species with which they compete.  Due to improperly designed or implemented harvesting operations, and application of inappropriate forest management systems, as well as due to uncontrollable factors (e.g., droughts), attempts at natural regeneration often result in low stocking or uneven spacing of potential crop plants.

Another problem with natural regeneration that is worth mentioning is more political and social than biophysical: stands that are gently managed using natural regeneration may not look like they are being managed at all.  This problem is likely to crop up where apparently unmanaged forests are subjected to encroachment and conversion, or where governmental policies require more obvious evidence of forest management to secure or maintain land titles.  Unfortunately, too many decision-makers equate forest management with tree planting thus jeopardizing stands regenerated naturally.

A major challenge for forest managers is developing sufficient understanding of the regeneration mechanisms of the species for which the area is being managed as well as of the other species that influence forest development.  Plants regenerate in a variety of ways, both sexually (i.e., by seed), and vegetatively (e.g., from rhizomes or coppicing from cut stumps).  Among sexually reproducing species are those that produce seeds that lack dormancy (i.e., they either germinate or die soon after maturing), and others that produce seeds that may remain dormant in the soil for many years.  Species that regenerate vegetatively may simply sprout back after being damaged or spread extensively by root sprouts or stem layering.  Vegetative expansion is fairly rare among tropical trees, at least those that grow to be large, but is common among other growth forms (e.g., vines, grasses, and ferns).  Sprouting of naturally broken or felled trees, in contrast, is commonplace.  Due to a variety of factors including intense seed predation, herbivory, and the effects of pathogens, natural regeneration may not result in fully stocked stands.

Silvicultural methods for encouraging natural regeneration is the subject of this chapter. Reducing harvesting damage to advanced regeneration (e.g., seedlings and saplings) is stressed and discussed first because uncontrolled logging is a major cause of regeneration failures in many forests.  How harvesting operations can be designed to promote natural regeneration is also discussed at length.  Finally, site preparation methods for enhancing seedling establishment are reviewed.

Topic 10.1: Protecting Future Crop Trees (FCTs)

Topic 10.2: Sources of Regeneration

Topic 10.3: Vegetative Expansion and Resprouting of Damaged Plants

Topic 10.4: Regeneration Requirements

Topic 10.5: Natural Regeneration-Promoting Silvicultural Treatments