Natural regeneration can be promoted by a variety of silvicultural treatments, the most severe and potentially efficacious of which are intimately associated with harvesting activities.  In fact, harvesting activities should be designed, scheduled, and implemented to assure sufficient natural regeneration for the next crops.  Other silvicultural interventions can be prescribed where necessary, but often can be made unnecessary through appropriate harvesting.

It seems reasonable to attempt mimicking the natural disturbances to which a stand has been subjected when designing harvesting regimes. After all, following this logic, the composition and structure of the stand to be managed would not be as they are were not the component species at least resilient to these disturbances.   Furthermore, regeneration of some species, including the focal species for management, might have actually been promoted by these disturbances.  Mimicking the natural disturbance regime also makes sense if the primary environmental objective for management is to maintain, as much as possible, pre-intervention structure and composition.  Unfortunately, the disturbances responsible for the present structure and composition of a stand might have been catastrophic or otherwise difficult, expensive, and environmentally worrying to mimic.

Determining what happened in a forest hundreds of years or even just decades ago is extremely challenging.  Stand or population-regenerating disturbances often occur infrequently, with long periods of relatively minor disturbances in between.  For example, regeneration of a number of commercially important timber trees is apparently favored by hurricanes that are followed by fires, or by the human-induced mimic of such a catastrophe—slash-and-burn agriculture.  Environmental concerns will certainly not be assuaged by silviculturally mimicking such disturbances, nor can we be confident that the silvicultural objectives will thereby be attained.

Despite these misgivings about mimicking natural disturbance regimes with silvicultural treatments, in many forests there is no better way to start.  In any event, it is important to proceed as gently as possible, and to modify the recommended silvicultural treatments as experience and information accrue.

The wide range of approaches to harvesting fall along a continuum stretching from the lightest of interventions (e.g., harvesting of dead branches) to the very severe treatments such as clearcutting followed by mechanical scarification of surface soils in large areas.  For convenience the range of harvesting treatments are generally divided up into discrete categories; it is important to recognize that these categories are artificial given that the treatments actually fall on a continuum.  It is also important to recognize that components of different systems can be combined in ways appropriate to the particular species and stands under management.  In natural forest management, it is of the utmost importance to tailor silvicultural treatments for each stand, and to not even hope for an “off the shelf” solution to your particular silvicultural problems. This need for insight and creativity makes silviculture of natural forests both challenging and fun.  Given the wide range of products and services for which forests have recently begun to be managed, the challenges and the fun are only likely to increase.

Retention of seed-producing individuals

Natural regeneration can be promoted in a variety of ways, but regardless of the silvicultural approach, a prerequisite for population-level sustainability is maintaining a sufficient density of viable seeds and advanced regeneration and/or of seed producing or vegetatively regenerating individuals.  For species that are poorly represented by advanced regeneration or in the buried seed bank, retention of seed-producing individuals is generally of the utmost importance.  The minimum density of retained seed-bearers needs to be empirically determined given that it is likely a function of a large number of factors including both propagule and site characteristics.  For example, the required density of retained individuals of a dioecious species that produces large seeds is likely greater than for a species with perfect flowers and small, wind-dispersed seeds.  The location of seed trees relative to skid trails, felling gaps, and other canopy openings may also be critical. For example, on the Yucatan Peninsula of Mexico, regeneration of Swietenia macrophylla (mahogany) is promoted by retention of seed trees upwind from such openings.

To avoid genetic deterioration of future generations, individuals retained as seed sources should be well formed, large, and healthy.  In the case of seed trees, they should have straight boles with no evidence of spiral grain, full crowns, and no evident defects.  Recommending retention of large seed trees is based on the observation that what appears to be an uneven-aged stand may actually be a single age cohort of trees. Given that variability in growth rates are at least partially based on genetic differences, it makes sense to retain the faster-growing genotypes.  Although they too are greatly influenced by environmental factors such as windstorms, stem and crown characteristics are also influenced by genotypic characteristics. Futhermore, large healthy trees are very likely to survive the shock of crown isolation and to produce larger and more frequent seed crops.  Unfortunately, recommendations for seed tree retention are completely opposite of what would be preferred by harvesters who are only interested in maximizing short-term profits and thus want to harvest all of the largest and best formed individuals.

Soil scarification

Seedling establishment of some tree species is promoted by mechanical scarification of the surface soil. Most species that benefit from this treatment are small-seeded and light demanding.

Promoting coppicing

For species that can be managed for stump sprouts, it is important to know how to most effectively promote coppicing.   Evaluating coppicing is made a bit complicated by the variety of uses for coppiced shoots. For example, large numbers of small diameter shoots may be preferred when poles for bean trellises are desired, but one or two stems per stool would be preferred where growing large dimension timber is the goal.  There is also variation among species in whether they coppice better from high or low stumps.  In many species, coppicing ability increases with stump diameter to some intermediate size, after which it declines.  It has also been reported for a number of species that stumps made by angled saw cuts are less prone to rotting than flat stumps created with an axe.  Basal rotting can be a severe problem in coppiced stands, especially where the coppice rotations are long.  Because it is difficult to grow large rot-free stems from coppice, trees of preferred timber species are grown from seed are interspersed among coppiced trees in an ancient silvicultural system referred to as “coppice with standards.”

When harvesting stems from species that are naturally multiple stemmed, such as many palms harvested for their edible terminal buds (e.g., Euterpe oleraceae and Bactris gasipaes), harvesting guidelines need to be developed to avoid severe damage to the retained ramets or the entire genet. Similarly, thinning of stems sprouting from a stump can enhance growth rates or improve the forms of retained ramets.  Often the decision of whether or not to thin coppice stands hinges more on financial considerations (e.g., are the thinnings marketable?) than on strictly silvicultural concerns.