Longevity And Persistence In Marginal Habitats

Examples of long-lived trees can often be seen at treelines. The two species of bristlecone pines (Pinus longaeva and P. aristata) are probably the most famous of ancient trees that have survived for millennia on high ridges in hostile montane environments. Pinus longaeva is the best known from the Methuselah tree in California's White Mountains which has an age of over 4700 years (Fig. 4.39). The oldest known living Great Basin bristlecone pine had 4862 countable annual rings when it was cut down in 1974 (Currey, 1965).

Only 260 km away in the southern Rocky Mountains in Wyoming and extending through Colorado into New Mexico can be found the Rocky Mountain bristlecone pines (P. aristata). Some individuals of this species growing under arid environmental conditions achieve ages in excess of 2000 years, and one living specimen has been found which is at least 2435 years old (Brunstein & Yamaguchi, 1992). A study designed to determine if bristlecone pines show any signs of ageing examined live trees of P. longaeva ranging in age from 23 to 4713 years. No differences were found in annual shoot growth increments. The hypothesis that ageing results from an accumulation of deleterious mutations was examined by comparing pollen viability, seed weight, seed germinability, seedling biomass accumulation, and frequency of putative mutations, in trees of varying ages. None of these parameters had a statistically significant relationship to tree age. It would appear that the great longevity attained by some Great Basin bristlecone pines is unaccompanied by deterioration of meristem function in embryos, seedlings, or mature trees, and it might be concluded that the concept of senescence did not apply to these trees (Lanner & Connor, 2001). Despite the continued virility of the younger tree growth, older parts of the trees will inevitably exhibit deteriorating growth forms and heart rot. The bristlecone pines are not alone in their capacity to survive as trees to a great age in marginal sites. The monkey puzzle (Araucaria araucana) is an ancient species now restricted in the wild to a small area in central Chile and to the drought zone that lies between northern Argentina and Brazil at 37° to 40° S (Hueck, 1966). Many trees reach an age of about 1300 years and an age of 2000 years has been recorded (Lewington & Parker, 1999). Due to its thick fire-protecting bark, adult Araucaria araucana is better adapted to disturbances by volcanic activities and fire than other potentially competing species. Also in the southern hemisphere the Antarctic beech (Nothofagus moorei) can attain ages between 1000 and 3000 years old. Again this is a tree of marginal areas inhabiting mountain tops and ridges in eastern Australia.

The longevity of trees in marginal areas can give rise to what can be described as a perched treeline, where the upper limit of trees exceeds the capacity to reproduce without shelter from mature trees. An example of this is seen in another southern hemisphere beech, the lenga (Nothofagus pumilio). During warm

Fig. 4.39 Pinns longaeva in the Schulman Grove, White Mountains, California. Only part of this very ancient tree is still alive. (Photo Dr A. Gerlach.)

periods the upper limit of forest advances up the mountains in Chile and Argentina and then maintains its position during a subsequent period of climatic decline due to the longevity of the established trees. Old growth stands dominated by deciduous Nothofagus pumilio occupy more stable substrates, and probably represent the last stage of post-glacial succession. This long-lived tree species has recorded ages of over 200 years with a canopy which appears to be a mosaic of even-aged, old growth patches (Armesto et al., 1992). Nothofagus pumilio therefore forms an abrupt alpine timberline (Fig. 4.4) and seedling emergence and density decreases with altitude within the forest so that the overall probability of adult establishment decreases with increasing altitude and becomes very low once the protection by the tree canopy is no longer available.

Persistence is therefore an advantageous property in marginal areas as it permits peripheral populations to wait for a suitable climatic window when they can again reproduce. In Orkney (Scottish Northern Isles) aspens

(Populus tremula), which are currently climatically limited from setting seed, appear to have persisted for centuries, possibly throughout the Little Ice Age due entirely to the vegetative renewal of their clones (Fig. 4.40).

The value of clonal proliferation in arctic and subarctic habitats has been closely studied in arctic species (Callaghan et al., 1992). The polar willow (Salix polaris) has been found on extensive patches on small islands in the Spitsbergen archipelago with only one sex present, which suggests the presence of extensive clonal colonization (Crawford & Balfour, 1983).

Although clonal plants in general possess less genetic variation than those that reproduce sexually this does not mean that they are devoid of variation. In aquatic habitats large stands of the clonal common reed (Phragmites australis) have been shown to contain more than one genetic unit (Koppitz, 1999). Amphibious plants, although morphologically and physiologically well-adapted to surviving in habitats with fluctuating

Marginal Ecosystem
Fig. 4.40 General view and detail of aspens (Populus tremula) growing in Britain's most northerly natural woodland at Berriedale on Hoy (Orkney). These trees are currently unable to set seed and may have persisted for centuries through vegetative renewal of their clones.

water tables, frequently require a respite from inundation in order that seeds can germinate successfully and establish young plants able to withstand flooding. Thus many amphibious species, e.g. Glyceria maxima, Acorus calamus (in Europe), survive almost entirely by vegetative reproduction. Apomictic species are also highly successful in marginal areas. Here again recent research is beginning to show that this highly specialized form of asexual reproduction can also contain within certain species groupings a significant degree of sexual variation (Chapman & Bicknell, 2000).

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