THE DISTRIBUTION AND ECOLOGY OF CONIFERS

The 614 species of conifers cover a large proportion while the Greenland icecap prevents any form of of the land surface of the earth. The map on p. xxx vegetation. The steppes of North America, Central shows that the greatest covering of land by conifers is Asia, Tibet and Mongolia, parts of Africa, and clearly to be seen in the northern hemisphere. This is Patagonia in South America can have some conifers due to the extensive conifer forest of the boreal zone here and there, but are also largely without them. 23 in Eurasia and North America. There are only few Although a few conifers occur in deserts, most of the species occurring there and the diversity increases world’s deserts, i.e. the deserts of central and western dramatically further south, while the areas being Asia, Arabia, North Africa (Sahara), and Australia, covered decrease. The area indicated to be covered are devoid of conifers. Another vegetation type by conifers on the map is, of course, not uniformly mostly devoid of conifers is lowland tropical rain- covered with conifers, they usually occur together forest, such as the Amazon Basin in South America with angiosperms or grow in patches too small to and the Congo Basin in Africa but also in smaller separate on this scale. There are already big gaps at areas like the Yucatán Peninsula in Mexico and the around 30º North, which continue across the equator southern lowlands of New Guinea. Other large areas into the southern hemisphere. In the far south there without conifers have been stripped of their natural is much less land, but except Antarctica – which has vegetation and are now occupied by agriculture and been omitted from the map for obvious reasons – it urbanization, especially in Western Europe, India, is quite well covered with conifers, with few big gaps. Bangladesh and northern China. However, some Conifers are present in nearly all the major veg- of the remaining areas devoid of conifers cannot be etation types of the world. They are absent from only resolved with an ecological explanation. Large parts a few and are very rare in some others. In the far of South America and larger parts of Africa and the north, a few conifers occur in tundra vegetation, Indian subcontinent are the major gaps in conifer mostly in the transition zone from boreal forest to distribution that have no explanation in terms of tundra. In the High Arctic of northern Canada and unsuitable climate or soils. Did these parts ever have northern Siberia (cold deserts) they are left behind, conifers, and if so, why did they loose them?

The global distribution of conifers shown on a world map with approximate equal area projection. The black areas on the map are not uniformly covered with conifers; they occur together with angiosperms or in patches too small to separate on this scale. Discounting the Mediterranean coastal areas, which origin. Such widely distant occurrences of taxa in biogeographically belong to Eurasia, Africa is the the Cupressaceae seem to throw the separation of poorest continent for conifers today. In Sub-Saharan extant conifers into northern and southern origins Africa we only have Widdringtonia (Cupressaceae) into doubt. Araucaria may have originated before with four species, Afrocarpus (Podocarpaceae) with the separation of Pangea into two super-continents, five species andPodocarpus with four species. Vast but as far as we know, the Cupressaceae evolved after areas of Sub-Saharan Africa have no conifers at all. that event. The Indian subcontinent south of the Himalaya and adjacent hills has only a single indigenous conifer: One final, intriguing observation about present-day Nageia wallichiana (Podocarpaceae). There are plenty conifer distribution is that more than half (ca. 330) 24 of suitable areas for conifers in both, as the successful of all species occur on the Pacific Rim. All families plantation of conifers for forestry in Africa and India and 83% of all 70 genera are represented. The Pacific demonstrates. The causes are most likely of a histori- Rim is the system of mountain ranges and islands cal kind, having had effect over time spans on a geo- around the Pacific Ocean in both hemispheres, logical scale. It appears that isolation, caused by the mostly forming part of so-called subduction zones break-up of the southern supercontinent Gondwana where oceanic crust plates slide beneath the conti- and the increasing separation of its constituent land nents, causing volcanism and thrusting up of moun- masses, is a major factor behind the gaps in conifer tains and islands in the process. Going clock-wise an distribution of land masses of Gondwanan origin. starting at 10 o’clock the diversity centres for conifers Connections between land masses remained much are Japan, the Pacific North-west of the United States longer in place with the break-up of Laurasia, the and Vancouver Island, California, southern Mexico northern super-continent, which also became less and Guatemala, southern Chile, New Zealand, fragmented. Both dispersal and vicariance played a Tasmania, New Caledonia, Fiji, the coast of New part in the history of conifer distribution leading to South Wales and Queensland, New Guinea, Borneo, the present situation. However, due to the size, prox- the Philippines, and Taiwan. All of these areas have imity, and orientation of land masses of Laurasian many species in several genera and families. In con- origin, all situated in the northern hemisphere, dis- trast the coastal areas of the Atlantic and Indian persal accounts for much of the distribution of gen- Oceans do not have such diversity of conifers, with era and species. Losses in the past could often still be the exception of Morocco, which has a moderate made up for by new arrivals. diversity of 12 species, and Florida, with 10. The The long history of the assemblage and subse- explanation is probably again historical: none of the quent fragmentation of the two super-continents, other oceans are nearly as old as the Pacific Ocean, Laurasia in the north and Gondwana in the south, is which has therefore provided coastal mountains and apparently still reflected in the distribution patterns islands suitable for well moisturized montane forests of conifers across the globe. If we look at the dif- from well before the origin of the angiosperms. ferent taxa this is even more clearly demonstrated. At the family level, Cephalotaxaceae, Pinaceae, In the north, around the Arctic Circle, the conifer Sciadopityaceae and Taxaceae are Laurasian, whereas forest or taiga begins where the tundra ends. The Araucariaceae, Phyllocladaceae and Podocarpaceae taiga is not entirely homogeneous. It is interrupted are Gondwanan, although Araucaria once extended by lakes and swamps and dissected by large riv- into Laurasia. Cupressaceae as a family are cosmopol- ers. There are few conifer species and most of them itan, but the 30 genera that make up this family today belong to the family Pinaceae. In some areas two or are divided in northern and southern hemisphere three of these conifer species may occur together, but groups, with a few ‘trespassers’, some only known very often the forest is formed by only a single spe- from the fossil record. Some of these exceptions cies. The environment of these conifers is dynamic. are intriguing enough, though. The South African Disturbance by fire and flooding is an integral part of genus Widdringtonia has been found in the Upper the taiga ecosystem, and natural disturbance, mostly Cretaceous of North America and Austrosequoia in the form of fires, often covers large areas. Storms, wintonensis from the Cretaceous of Australia is very however, are rare. The air burst of a large meteoroid similar to Sequoia or Sequoiadendron of Laurasian or comet fragment in June 1908 in Siberia, known as