As a result of forest management practices or as the side effects of climate change, environmental disasters, or other unpredictable events, resultant changes in genetic variation are attributable to changes in the basic evolutionary processes. These processes are:
1. Random genetic drift, the non-directional changes in genotypic frequencies among generations due to random chance in small populations
2. (Natural) Selection, relative differences among genotypes in viability or reproductive success
3. Migration, the exchange of genes between populations that differ in genotypic frequencies
4. Mating, the process mediating the recombination and assortment of genes between generations.
Mutation, a fifth evolutionary force, is excluded from consideration here, since we are primarily concerned with effects of human interventions and relatively short term processes, of less than ten generations; mutation rates are not likely to be significant in such cases.
All the above evolutionary processes affect the levels and distribution of genetic variation and the present state of the genetic resources is a result of their joint effects over time. Variation within populations may be increased by migration if populations were originally distinct, but at the expense of variation between populations and possibly with decreased fitness. The size of the receiving population may be increased, but the adaptability of the resultant genetic material is dependent on the forms of selection and migration that also occur.
It is also clear that the present-day condition is not necessarily an optimum state for adaptation to future conditions. However, regardless of any difficulties inherent in defining an optimum condition (and noting that so-called “undisturbed forests” may not any longer exist -), we assume that if the processes that maintain the levels and distribution of genetic variation remain intact, then the biological capabilities for adaptability are expected to be in at least as good a condition in the immediate future as they are now. The exact levels and distribution of genetic variation would be expected to have changed during evolutionary history, but our central concern is for the processes and the structures necessary to maintain those processes, and not for the present distribution of variation as an end in itself.
In the absence of any other information, it is possible to use some general rules of thumb of population genetics that suggest that population sizes of several hundred or a few thousand are sufficient to indefinitely maintain quantitative genetic variation and to save even rare alleles (Franklin 1980). For species with random mating and without inter-population divergence, simple counts of reproductive adults may be sufficient for assessing genetic viability, but such rules are imprecise, and are of limited value for the complex structure of many forest tree populations. Local populations may normally be much smaller and transient, and genetic divergence among populations may be an important adaptive feature of the species to conserve. Especially in the species-rich tropics, many species are widely dispersed and contiguous populations of large size are an aberration. Therefore, it is necessary not only to measure static population size parameters, but also to monitor the effects of forest practices on adaptive genetic processes.
The effects of various types of biotic and abiotic changes that are evident at the forest stand level usually affect, or are the result of, more than one of the above genetic processes. For example, changes in the abiotic environment can change relative genotypic viabilities, but it can also affect pollinator behaviour and other inter-species interactions. Because biotic interactions, such as in pollinator-plant relationships, are generally more complex in the tropics than in temperate ecosystems, the indirect effects of several, simultaneous environmental changes are likely to also be more complex in the former, and to operate on a variety of time scales.
Populations may be established by only a few individuals, which results in a limited sample of the species’ genetic variation in each population. Also, populations may be periodically reduced in size and consequently lose genes at random. Thus, levels of genetic variation are directly affected by processes of genetic drift, and forest practices affect drift. Forest operations or fire may effectively reduce population sizes, leaving few individuals to contribute seed or pollen. In addition, sources of migration may be cut off, and the effectiveness of pollinators may be reduced, all leading to a reduction in the effective population size (Ne) and to random losses of variation by drift. Frequently, the number of mating individuals and their reproductive output may be much smaller than the total number of adults or number of flowers. Furthermore, inequities in mating frequencies and non-random mating generally further reduce Ne. If the reproductive population remains small, the random drift of alleles will severely reduce genetic variation that may be significant for future adaptability for several generations.
Directed selection, whether resulting from natural or human-induced pressures, favors certain genotypes over others and therefore acts to maintain population viability and reproductive success through adaptive responses. However, some genes would be lost at higher rates than expected by drift alone, and very strong selection may reduce population sizes below recuperation levels. Thus, selection can simultaneously reduce genetic variation, and change population averages. Both of these shifts may influence future adaptability. Events that lead to genetically significant selection may be unintentional, for example, a single event such as harvesting of wood, could result in changes in several traits, not only those considered desirable for harvest. Thus, traits such as juvenile growth rates, reproduction age, and pollinator behaviour, can be affected by logging as well as by the timing and design of harvests. Furthermore, selection on one trait may induce genetically correlated changes in other traits and in genetic processes. Hence, even simple natural and man-induced selection may have multiple effects on the distribution and dynamics of genetic variation, and may thus affect present and future viability and reproduction.
Changes in the degree of seed and pollen exchange between populations directly affect the divergence between populations and indirectly affects drift within populations. Conversely, high migration rates may also increase local levels of genetic variation. The mating system is also affected by both seed and pollen migration as the pool of potential mating partners is altered. Isolation between populations can be affected by the increase or decrease of barriers to exchange between populations, distance between them, or changes in pollen or seed vectors. The form and rates of migration mediate the genetic divergence between partially independent populations of a species (i.e. meta-populations) and affects the distribution of genetic variation. The creation of forest islands by fires or creation of patch openings within continuous forests, can change migration rates and the relative sizes and patterns of openings vs. forest cover can strongly influence genetic divergence. For species that are dependent on local extinctions and subsequent colonization, an additional factor is the proximity of an adequate recruitment pool of immigrants.
The mating system that mediates the inter-generational process of evolution is also subject to change caused by forest level events. The mating system determines the composition of the male and female mating pools, the extent to which germplasm is exchanged between individuals (outcrossing rates), and the rates of immigration and emigration. Thus, the levels of heterozygosity, the relative importance of vegetative reproduction, and the exposure of mutants to selection, are all affected by changes in the mating system. Forest practices such as harvesting for wood, thinning, or conversion of some areas of forests to other uses, can cause changes in change population density, pollen and seed dispersal rates and dispersal mechanisms, and in timing of flowering; these, in turn, will cause changes in the mating system. Populations that are usually large but suddenly undergo severe reductions may also be exposed to inbreeding depression and loss of seed viability, reducing their immediate capacity for survival.
