Genetic variations exist in virtually all forest organisms; they are particularly important in tree species and in the pests and pathogens that attack them. The genetic basis of forest pathology and its influence right up to the landscape level was the basis of the 1999 Montreal symposium of the American Phytopathological Society. Much of what was discussed then has been updated (Lundquist and Hamelin, 2005) in a volume that emphasizes how rapidly long-lived trees and forests can be devastated by pathogens and microscopic organisms such as fungi, bacteria, phytoplasmas and viruses. Pathogens have important effects on biodiversity, greatly influencing plant populations of both natural forests and plantations. The numbers of important indigenous species may be greatly reduced; some may be eliminated altogether. On the other hand pathogens themselves contribute to the biological diversity of the ecosystems they inhabit.
Severe devastation of the cypress Cupressus lusitanica by the cypress aphid Cinaria cupressi in East Africa appears to have been facilitated by the narrow genetic base of the planting stock. Similarly, Dutch elm disease in the UK (Section 5.4.5) was particularly bad because many of the 25 million trees killed were English elm Ulmus procera that all derived from a single clone brought to the UK from Italy by the Romans 2000 years ago for use in supporting and training grape vines (Gil et al., 2004). Considerable care is taken to provide an adequate genetic base in forests of radiata pine Pinus radiata in New Zealand where particular clones are often employed. Trees of the same clone are genetically identical, those of different clones may vary quite widely in features such as growth and immunity to pests and diseases. Though these forests are monocultural they are not usually monoclonal; it has been argued that such forests should contain at least 7-25 genetically unique clones or else seedlings from seed lots with more than 16 parents.
The fact that individuals of a self-incompatible species such as the primrose Primula vulgaris are unable to breed with other genetically identical individuals has the advantage of ensuring that reasonably large populations remain genetically diverse. This perennial herb of moist, shaded habitats has a wide North Atlantic and Mediterranean distribution; most English individuals grow in woodlands but primroses can be quite abundant in hedges and old grasslands protected from drought. The distylous nature of this species, whose flowers have either short or long styles, is controlled by a super-gene. Plants with pin-eyed flowers are double recessive (ss) and those with thrum-eyed flowers are heterozygous (Ss). In thrum-eyed flowers the stigma is half-way up the centre of the perianth tube and the stamens are at the top in the eye of the flower, while in pin-eyed flowers the reverse is true.
Fewer alleles (alternative forms of a gene) can be physically contained in smaller populations, and some even of these can be progressively lost by small random changes in gene frequency (called genetic drift) leading to even lower genetic variation in the population. The effects of this on small populations can be severe. Habitat fragmentation and the resulting decline in population size have adversely influenced the reproductive success of primrose in Belgium, where it is a rare and declining species. Brys et al. (2004) examined 16 populations near Bruges and found that population size strongly affected reproductive success; plants from small populations produced significantly fewer fruits per plant and seeds per fruit.
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