Relict Species And Climate Change

Climatic change lies at the base of the restricted range of most relict species. Species that were once widespread and now exist in isolated 'islands' provide examples of a marginal existence that in many cases can be related to specific causes (Milne & Abbott, 2002). Not all relict species are in danger of extermination as some of the colonies that still exist, although only covering a fraction of their former distribution, frequently contain populations that are both numerous and viable in terms of regeneration. Endangered relict species are those that exist only in small populations throughout their remaining distribution and which could be eliminated easily by exceptional climatic events or human disturbance. Relict species represent a challenge to the view that for species to survive in a changing world they either have to migrate or adapt. Their continued survival excites curiosity and notice as they are frequently remarkable examples of ancient and now isolated biota that have been fortunate enough to find a habitat where survival has been possible despite a changing world. To encompass the disparate biogeo-graphical histories of relict species it is convenient to consider them under two headings, namely evolutionary relicts and climatic relicts.

1.6.1 Evolutionary relicts

Evolutionary relicts include groups such as the cycads (Cycadaceae) comprising about 100 species in nine genera, all with limited distribution and confined to tropical and subtropical regions. In a sense, these species, which are sometimes referred to as living fossils, could also be considered as climatic relicts, but from a time that has long since passed. These slow-growing cycad species were once dominant features of the vegetation in Mesozoic times when the temperature gradient between the Equator and the poles lay between only 10 °C to 20 °C and much of the world had a tropical climate. Many evolutionary relicts are also found in geographically isolated areas where their secluded existence protects them from excessive competition and predation. Two extreme examples are the Chinese maidenhair tree (Ginkgo biloba) and the dawn redwood (Metasequoia glyptostroboides).

Ginkgo biloba (Figs. 1.15-1.16) is truly a relict species that is apparently identical to 200 million-year-old fossil specimens. It had long been thought that the species was extinct until the German botanist and physician Engelbert Kaempfer (1651-1716) discovered the tree in Japan on a visit to Buddhist monks in Nagasaki in February 1691. Apparently, ginkgo trees had survived in China and were mainly found in mountain monasteries where they were cultivated by Buddhist monks. From China they were introduced to Japan and Korea. Ginkgo seeds were brought to Europe from Japan by Kaempfer. Whether ginkgo still exists in the wild in China is not certain. Some trees can be seen growing naturally in two small mountainous areas on the border between Zhejiang and Anhwei provinces (Tian Mu Shan Reserve) (Fig. 1.15), in central China in Hubei province and in western China in Guizhou and Sichuan provinces. They may, however, have been seeded there from cultivated trees in temple gardens (Kwant, 2006).

Other notable discoveries ofrelict species include a deciduous member of the Taxodiaceae, the dawn redwood (Metasequoia glyptostroboides, discovered in 1941 and first collected in 1947 (Fig. 1.17). More recently, the Wollemi pine (Wollemia nobolis, Araucariaceae) was discovered in 1994 by intrepid abseilers into a ravine in the Australian Blue Mountains in a remote area of the Wollemi National Park (New South Wales, Australia). With only around 40 adult trees known to be growing in the wild, it is one of the world's rarest plant species, and this relict population shows no genetic variation between individual trees.

The genus Araucaria itself, formerly of worldwide distribution in Triassic, Jurassic and Cretaceous times, now has a relict distribution with the natural distribution of the classic xerophytic tree, the monkey puzzle (A. araucana), now confined to between 37° S and 40° S in the arid zone of Argentina and Chile. Similarly, the Norfolk pine (A. heterophylla) is confined to Norfolk Island, and A. bidwillii is the tallest tropical tree to occur naturally in Queensland, Australia. A combination of plate tectonics and climatic change has reduced these species to isolated areas, usually in warmer climates which resemble the more widespread conditions of uniform warmth that existed on Earth before the Cenozoic climatic decline.

1.6.2 Climatic relicts

Climatic relict is a term usually applied to plants that have had their distribution curtailed more recently and in which a direct link with more recent climatic change is readily observed. Coastal environments now provide the only refuge for some of the coniferous trees that once had a worldwide distribution. The Californian redwood (Sequoia sempervirens), in common

Fig. 1.15 Mature tree of Ginkgo biloba growing in Lichuan county, Hubei province (China). In this province are many ginkgo trees, possibly growing in the wild. (Photo Wei Gong, reproduced with permission from the web pages of Cor Kwant, 2006.)

with several species of Chamaecyparis and Taxodium, had a transcontinental distribution during the Tertiary period but are now found only in very restricted coastal habitats. Apparently the oceanic niche, with its diminution of climatic extremes, together with a reduction in competition from more recently evolved tree species, provides many relict tree species with an environment in which they are still viable. As with many of the other dominant species of these coastally restricted forests, survival appears to be due in part to the longevity of these species, which can exceed 1200 years for S. sempervirens and 3000 years for the Alaskan cedar (Chamaecyparis nootkatensis) (Laderman, 1998a).

Many of the tree species in these maritime forests share characteristics that have similarities with the ericoid species of the oceanic heathlands of north-west

Fig. 1.16 Foliage of the maidenhair tree (Ginkgo biloba). A relict species in China and apparently identical with fossil specimens 200 million years old.

Europe. In form they are usually evergreen, and possess sclerophyllous foliage. Ecologically they inhabit nutrient-poor soils, and have a requirement for high levels of atmospheric moisture. They also illustrate the limiting effects of such oceanic environments, which have been described as 'success through failure' in that these coastally restricted forests survive in regions where other species have found it impossible to survive (Laderman, 1988b).

The genus Rhododendron subsection Pontica is remarkable among Tertiary relict groups for its disjunct distribution (Section 4.5.2). Pontica lineages have survived the Quaternary in south-west Eurasia, south-east North America and north-east Asia with little or no subsequent speciation (Milne, 2004).

It is not necessary, however, to have to search the shores of the Pacific Ocean or the mountains of Asia for examples of arborescent refugia species with an ericoid growth form. The strawberry tree (Arbutus unedo) is a member of the Ericaceae that has long been a member of the Irish flora. There has been debate as to whether the strawberry tree is truly indigenous to Ireland or whether monks introduced it during their pilgrimages in the early Christian period to the Iberian Peninsula to shrines such as Santiago de Compostela. However, investigations of the pollen record have now shown it to have been present for over 3000 years in County Kerry and 2000 years in County Sligo and this tree can therefore be added to the worldwide list of ericoid trees that have survived as relict forests on islands (Mitchell, 1993). Whether or not this tree survived in Ireland or in some neighbouring area at the time of the Late Glacial Maximum (18 000 BP) has not yet been determined.

Many species that once had a wide distribution at middle latitudes during post-glacial times retain a scattered and fragmented occurrence on mountains. These climatic relicts are particularly noticeable in Europe due to the predominantly east-west orientation of the mountains as compared with the more favourable corridors for adjustment to climatic change that are found in the north-south orientated mountains such as

Fig. 1.17 Botanic garden specimen of the dawn redwood (Metasequoia glyptostroboides). This deciduous conifer was only discovered in 1945 and seeds collected in 1947; it has subsequently been introduced widely to western gardens.

the Andes and the Rockies in the New World, and the Altai mountains of Asia, and the Urals at Europe's eastern fringe. Some montane species are currently in retreat due to climatic warming at a rate that can be observed over decades (see Chapter 10).

+1 0

Post a comment