Global and trans-continental distributions

In this chapter we present a world map with the distribution of all species and maps of the families and genera with a wide, trans-continental but not highly fragmented distribution. Small maps of all families are given together to compare their global ranges, but those with a more limited distribution are presented and discussed in the following chapters relevant to where they occur.

Global Distribution of

Map GTC-1. Global distribution of all conifer species. The boreal conifer forests of Alaska, Canada, Scandinavia and Russia are shaded in this map to show their extent. Available localities from herbarium collections are inadequate to show the extent of occurrence (EOO) and area of occupancy (AOO) of the widespread boreal conifer species. [Source of data on extent of boreal forest zone: www.fao.org/geonetwork/srv/en/metadata .show?id=1255]

The global distribution of all 615 conifer species shows distinct patterns. In the N a belt of boreal coniferous forest dominated by a small number of species encircles the globe; it is shaded on the map and would be dense with dots if the herbarium collection data had been available. To the S of this boreal forest zone the nearly 37,000 herbarium specimen records in the Conifer Database give an accurate general picture of conifer distribution at this scale. One pattern that stands out is that conifer species are most abundant in major mountain systems, e.g. in W North America, Mexico and Central America, in the Andes of S Chile, in Europe from Spain to Greece, in the Himalayas, and Japan, and in New Guinea and New Zealand. Other areas of abundance are perhaps less expected, such as the Atlantic side of the USA and SE China, where forests tend to be dominated by angiosperms. Less densely populated but equally surprising may seem parts of Australia and parts of Malesia other than New Guinea, where angiosperms are once again the dominant trees of the forests. Some small islands are rich in conifers, most notably New Caledonia, Taiwan and Tasmania. Where are conifers scarce or absent? Apart from regions such as Antarctica, the High Arctic and major tundra zones, steppes and deserts, there are some very large areas devoid of conifers where this absence is perhaps less obvious. In South America these are the Amazon Basin and Mato Grosso, in Africa the Congo Basin and West Africa, in Asia the Indian subcontinent and most of the Tibetan Plateau. For the latter, adverse climate is probably the cause, with a growing 12 global and trans-continental distributions season too short for shrubs and trees due to its very high altitude. The Amazon and Congo Basins are excellent habitats for tree growth; they harbour the world’s largest tropical rainforests. Coni- fers occur in tropical rainforests, mostly in uplands but in some regions also in lowlands. In the latter, they tend to occupy sites with very poor soils, where tree growth is much less vigorous and consequently competition less intense, leaving space for slower growing conifers. Are such sites absent from the Amazon and the Congo basins? The rivers that come into the Amazon from the N, especially the Rio Negro, carry water very low in minerals, indicating that they spring from and run through regions with poor soils. Ecology seems not a sufficient explanation and possibly the absence of conifers in these large tropical river basins also has historical (contingent) causes. The absence of conifers in West Africa is even more of a conundrum, perhaps until it is realized that the entire sub-Saharan continent is poor in conifer species (see Africa chapter for more details). Much of Africa has been subject to major and rapid climatic fluctuations during the later Neogene which will have caused the extinction of many forest species (website: natureNEWS (4/1/2011): The Drying of East Africa). This must have made it less likely for the remaining species to be able to re- colonize; indeed only one species, Podocarpus milanjianus is now extremely widespread in Africa (map AF-18 on p. 469) and it is inferred to have been the species that spread during periods of global cooling in the Pleistocene (Morley in Turner & Cernusak, eds., 2011). This leaves the great- est enigma of all: the near absence of conifers in the Indian subcontinent. All but one species of conifer in are confined to the Himalaya or to the hills in the far NE of the country and these nearly all belong to Pinaceae, the subfamily Cupressoideae in Cupressaceae, or Taxaceae and are of Laurasian origin (see below). The exception is wallichiana (map MA-55 on p. 323) with a disjunct population in the Western Ghats of S India. During an extended period in the Meso- zoic and early Cenozoic, India was a continental craton moving on the Indo-Australian Plate as a large island northwards, from temperate S latitudes through the tropics to tropical/temperate N latitudes and finally colliding with the Asian continent, pushing up the Himalaya and adjacent mountain systems. India has a good Mesozoic record of fossil conifers, all of Gondwanan affinities (Sitholey, 1963). Conifers went extinct during that long journey, but what caused this? Perhaps climate change or the disaster of the Latest Cretaceous Deccan Traps, an event that itself caused climate change, or a combination of these factors. In a subcontinent with elevations generally below 1000 m and situated for the most part in the tropics, the natural vegetation is tropical to subtropical forest dominated by angiosperms. India could only have been repopulated by conifers from Asia, and few apparently succeeded to gain a foothold. Of these, only Nageia wallichiana, a broad-leaved tree species in the , survived into the present, occupying a few loca- tions in the Western Ghats mountains which rise to >2500 m.

Global Generic and Specific Hotspots

Not only are conifers unevenly distributed across the world (map GTC-1 on p. 11), there are also disparities in the taxonomic diversity among the regions in which they occur. Some areas are diverse and others are poor in genera and/or species. To show where high diversity is concen- trated we have chosen to calculate the diversity of conifers at the spatial level of 1 degree cells and plot the highest diversity levels only on global maps. All eight families, 83% of 70 genera, and slightly over half of all conifer species occur in 14 centres of diversity around the Pacific Ocean (Farjon, 2008). First noted by Engler (1926), Li (1953) sought to explain this concentration around the Pacific at a time before continental drift and plate tec- tonics were better understood and accepted. The centres of conifer diversity around the Pacific Rim are all mountainous, often on islands, and have oceanic climates with moderate tempera- tures and high precipitation. Separation and isolation, especially on islands, promotes specia- tion. The great age of the Pacific as an ocean basin, going back to the origin of the oldest extant families, also played a role. Continuous movement of its plates towards shifting land masses, creating subduction zones where they contacted and thereby causing volcanism, island-forming and mountain-building, provided suitable habitats during the entire period of the evolution of