Topic 11.1: Planting Materials

Plantations can be established using a wide range of planting materials.  Seed collected from “plus” trees (=trees of good form and rapid growth) in mature stands or from seed orchards of selected plus trees can be directly sown on prepared sites or planted in nurseries for later transplanting.  Vegetative propagation (e.g., from cuttings or tissue culture) is used when seed is not readily available or for the purposes of rapidly propagating promising genotypes.  Depending on tree species, the products desired, and other management issues, stands harvested by clearcutting are replanted or managed by coppicing.

Selecting the appropriate species for planting is a complicated process that needs to reflect the types of products desired, site capability and other ecological constraints, availability of planting material, and availability of the required silvicultural knowledge. Appropriate matching of tree species, provenance (=place of origin), and genotype (for cloned or otherwise genetically selected planting material) to the site conditions determines whether a plantation will be a silvicultural success or failure (financial success and failure are determined by productivity plus a complex set of other factors).  One of the reasons that a few taxa dominate the plantation forests of the world (e.g., Pinus spp. Eucalyptus spp., Acacia spp., Gmelina arborea, and Tectona grandis) is that so much is known about their silvics.  As knowledge accumulates about other promising species and planting materials for them become more available, the diversity of plantation forests should increase.

The debate over the virtues of native and exotic tree species for use in plantations has raged on for many decades and is unlikely to be resolved anytime soon.  Opponents of the use of exotic species point to the instances of planted introduced species becoming becoming invasive in natural ecosystems (e.g., Pinus spp. in montane grasslands in Africa).  They also denounce plantation forests for their generally low species diversity.  Proponents of the use of exotics tend to emphasize the great volume increments that are possible in well-managed plantations and the profits that can thereby be realized. They also often point out that plantation monocultures influence ecosystems processes (e.g., carbon sequestration and water retention) in ways similar to natural forests and certainly in ways preferable to the degraded vegetation that the plantations have replaced.

Another debate about plantations, although a less fervent one, is over the issue of monospecific vs. mixed-species stands.  Mixing tree species either within or between stands has the potential benefit of reducing the risks of losing entire plantations to species-specific pests or pathogens.  Monoculture agriculture and forestry do indeed seem particularly susceptible to outbreaks, but with a few prominent exceptions (e.g., Hevea brasiliensis plantations in Brazil where the species and its most threatening pathogen are both native), they have been successfully controlled by proper sanitation methods (e.g., slash burning) and pesticide application.  Defenders of monospecific plantations generally stress the ease with which they are established and maintained. Stands composed of trees of different species growing at different rates and hence being harvested at different times are more difficult to manage than uniform, monospecific stands.

Better matching of species and genotypes with sites and genetic improvements in plantation forestry crops have in some cases allowed substantial increases in yields from one rotation to the next.  Nevertheless, if site conditions are allowed to deteriorate due to soil compaction, erosion, or a variety of other management-induced problems, geneticists will not be able to compensate for the mistakes of silviculturalists.  Because most site damage occurs during the inter-rotation phase, careful management of the processes of harvesting, site preparation, and replanting are critical.