FOREST PRACTICES THAT AFFECT GENETIC DYNAMICS

Forest level events caused by forest management and utilization practices, or due to inadvertent changes which result from climate change or environmental disasters, can be expected to induce certain kinds of changes to the genetic processes that in turn affect the evolution and sustainability of forests. If forest management has only a slight impact, or if environmental changes are small, these events may not have significant genetic consequences. However, the forest manager needs to consider the impact of forest practices on genetic aspects of sustainability. Some of these forest level events are described below, and displayed below as a matrix in Table 1.

TABLE 1. Genetic consequences of different types of utilization

HARVESTING OF WOOD

The term “logging” is often used for widely divergent activities related to the harvesting of wood. In many countries, certain areas of forest are scheduled for conversion to other lands uses, usually agriculture, in national land use planning. Clearing of these “conversion forests” is by means of clear-cutting. Although the term logging is often applied to this form of harvesting, it is not considered relevant for the purposes of these criteria and indicators, as this is obviously a non-sustainable activity from a forestry point of view, and is designed as such.

Harvesting of wood, or logging, directly impacts the genetic resources of the commercial species through changes in their population age and density distributions. Non-commercial species, both plant and animal, will also be affected, either due to intentional or accidental damage during the logging process, or due to the resulting changes in environmental conditions. Forest-dependent animal species will also be affected. For any animal species with small initial populations, the effects of drift would be enhanced. Directional selection would also be likely to affect at least the logged species, but the changed growing conditions would also affect the conditions required for survival and reproduction of other species. For isolated populations of any type of species, the response to selection can be strong, and for plant species with specialized pollen or seed dispersal mechanisms, logging would also be expected to change the mating system.

GRAZING

Grazing of livestock in forests is a very common practice, especially in tropical seasonal forests. Grazing will have a thinning effect on regenerating vegetation and changes in the population density of grazed species could affect levels of genetic drift. Since grazing may also compact soils and alter stand structure, it may induce selectively significant environmental changes. It could also affect populations of forest-dwelling herbivores.

FIRE

Intense fire that inflicts heavy mortality and reduces population sizes, would increase drift. Migration may increase if the migration vectors are abiotic, but may decrease if fire adversely affects biotic dispersal agents. Direct selection may be felt on traits that affect fire resistance. Very strong fires would however indiscriminately remove all genotypes.

HARVESTING NON-TIMBER FOREST PRODUCTS (NTFP)

Harvesting parts of trees or shrubs may involve removing reproductive structures such as fruit or seed, and directly reduce the effective size of the pool of reproducing parents. Fruit and seed are often harvested very intensively, resulting in severe genetic impacts (see e.g. Ganeshaiah et al. 1998). Direct selection will affect reproductive traits, but the strongest genetic effects would be felt on the mating system and gene migration dependent on the harvested parts. If non-reproductive parts are harvested, such as leaves, then only indirect selection will affect the viability or fecundity of affected individuals, and this could be genetically significant. When whole plants are harvested the effects of reduced population size will be genetically significant. Harvesting of animals (such as hunting and fishing), may have as severe impacts as harvesting of whole plants.