Topic 11.3: Enrichment Planting

Enriching standing but depleted forests by under-planting of valuable species is an oft-prescribed silvicultural treatment. Whether nursery-grown seedlings or transplants from the wild (=wildlings) are planted in felling gaps, log landings, or along cleared lines through degraded forest, enrichment planting is apparently a very attractive approach to solving a fundamental silvicultural problem.  Unfortunately, the desired results have seldom been obtained despite huge investments in planting.

As much as 50 years ago, tropical foresters had outlined the prerequisites for successful enrichment planting. These guidelines called for planting quality seedlings under conditions that are appropriate especially in regards to light availability, and then tending the outplanted seedlings until they are large enough to grow unaided. This process  often requires annual removals of lianas and cutting back of encroaching vegetation for several years. Despite the availability of these guidelines the considerable requirements for species-site matching, appropriate site preparation, careful planting, and substantial and prolonged post-planting treatments are frequently ignored at great expense.  Nevertheless, when properly planned and implemented in appropriate sites, enrichment planting sometimes has a role to play in natural forest management.

Seedlings transplanted into the forest with the planting medium in which they were grown in a nursery represent the most common planting material used for forest enrichment.  Seedlings from seeds or small wildlings are generally grown in black polyethyene bags filled with various mixes of sand, forest soil, and perlite. The presence of forest soil is intended enhance the probability of the seedlings getting inoculated with the appropriate mycorrhizal fungi and other symbionts.

Growing tree seedlings in bags or pots is challenging in many ways, and how the seedlings fare in the nursery determines the likelihood of successful outplanting.  For example, species that characteristically develop deep tap roots are notoriously difficult to propagate in the nursery in such a way that they can be easily transplanted. Root pruning can reduce handling problems, but taproots of some species recover only very slowly after being damaged.  Similarly, seedlings of some species never seem t recover from being “pot-bound” (i.e., the effect of growing in a container too small for their root system). This effect is particularly noticeable after the roots have started to grow in a spiral around the container. Various modifications on the basic circular container (e.g., square or ribbed) can help reduce this problem, but timely out-planting is the best way to avoid root system constriction.

Transplant shock, an ill-defined series of problems that develop soon after out-planting, often takes a great toll among young seedlings.  For plants coming from a nursery where they were frequently watered and fertilized, life in the forest must seem very harsh, at least initially. To reduce transplant shock, nursery stock is often “hardened-up” by withholding fertilizer and cutting back on irrigation for a few weeks or months prior to out-planting.  This rough treatment causes the seedlings to change in many ways but the main objective is to reduce shoot growth and stimulate root growth, thereby increasing the root:shoot ratio.  Seedlings with a higher root:shoot ratio are better able to withstand herbivory and field conditions where water is frequently in short supply. Root:shoot ratios can also be increased by removing foliage or even cutting off the entire shoot. The latter method is used when transplanting large growing stock into particularly drought prone conditions; it has been successfully used, for example, for regenerating Tectona grandis in dry forests in South Asia.

Bareroot seedlings transplanted from the forest or extracted from nursery beds are also sometimes used for enrichment planting.  Considerable root damage occurs during seedling extraction and transportation; this damage changes root:shoot ratios in the wrong direction and increases the likelihood of transplant shock.  Post-planting mortality rates of bareroot stock can be very high, but they are lighter and easier to transport and the considerable costs of growing seedlings for several months or more in a nursery are avoided.

Before out-planting seedlings, most enrichment planting guidelines call for opening 2-5 m wide lines or 10-40 m diameter circular gaps from the forest floor up to the sky.  Although enrichment planting under pre-existing canopy gaps initially entails less labor, scattered gap plantings are difficult to locate during tending operations and costs are thereby increased considerably.

Enrichment planted seedlings that survive transplant shock still have to compete with pre-existing vegetation. Grasses and vines are particularly troublesome because they often flourish under the open canopy conditions provided for the planted seedlings.  Larger trees bordering the clearings also respond by rapidly extending branches into the openings and casting shade on the planted seedlings.  Even many of the plants cut during line or gap clearing resprout and often grow more vigorously than the planted tree seedlings.

Because the species most commonly used for enrichment planting are almost always fairly light-demanding, successful out-planting requires opening the canopy and keeping it open until the planted trees are tall enough to reach the light without further assistance.  Depending on the height of the pre-existing canopy and the height growth rates of the planted seedlings, commitment to tending operations could be for as much as 10 years.  Enrichment planting combined with tended natural regeneration in Sabah, Malaysia.).

Depending on the planting density and the severity of treatment of the residual forest between planting lines and gaps, enriched stands can become almost indistinguishable from even-aged plantations.  Where only one species is selected for planting and silvicultural treatments of the residual forest are severe, the resulting stands may be susceptible to many of the problems with pests and pathogens suffered by monospecific plantations.  Too few enrichment planting efforts have been successful to evaluate the severity of this problem, but it is nonetheless worthwhile to keep in mind.