The two previous GeoCurrents posts examined the biological significance of continents by looking at the distribution of animals. It is time now to turn out attention to plants.
One of the most influential divisions of the world into “floristic kingdoms” is that of botanist Ronald Good, found in his 1947 book The Geography of Flowering Plants. As can be seen in the map below, Good’s floristic regionalization scheme has important similarities with, as well as differences from, Alfred Russel Wallace’s pioneering zoogeographical map. The Americas, for example, are similarly divided not at the isthmus of Panama but rather across Mexico. In the most striking divergence, northern North America does not stand alone but is instead connected with northern Eurasia and North Africa in the massive Holarctic kingdom. Most of Africa is likewise linked to southern Asia, along with Australia’s Cape York Peninsula, to form the “Paleotropical” kingdom. The rest of Australia combines with most of New Zealand to form another kingdom. Southwestern Africa, in contrast, forms its own diminutive Cape Kingdom, which is distinguished from the rest of Sub-Saharan Africa by its Mediterranean climate. Good also differentiated an Antarctic floristic kingdom, which has no counterpart in any zoogeographical system that I have seen. Unfortunately, this region is not easily discerned on the Wikipedia map that I have posted, despite its prominent label. It can be seen, however, in the pale color used for southern New Zealand and the far southern tip of South America. All told, Good’s map of flora departs more sharply from the architecture of continents scheme than do similar maps of fauna.
In the 1970s, Good’s scheme was modified and expanded by the Armenia botanist Armen Leonovich Takhtajan. Unfortunately, the maps that I have found of Takhtajan’s floristic regions are difficult to parse, as can be seen in the one reproduced below. Suffice it to say that he identified six floristic kingdoms, which he subdivided into 35 “regions” and many more “provinces.” Takhtajan’s kingdoms are essentially the same as those of Good, although some of their boundaries differ. Takhtajan, for example, maps an expansive Antarctic Kingdom encompassing most of Chile, Argentina, and New Zealand, as well as a host of islands scattered over a vast swath of the southern hemisphere. Despite its name, the region barely touches Antarctica itself, reaching only the northern extension of the Antarctic Peninsula. This restriction is hardly surprising, as the northern Antarctic Peninsula is the only part of the continent that supports any vascular plants, and it has only two (Antarctic hair grass, Deschampsia antarctic, and Antarctic pearlwort, Colobanthus quitensis).
The existence of an Antarctic floristic kingdom that excludes Antarctica might seem odd, but it makes sense in terms of geological history. When this biogeographical region began to form, Antarctica itself was both vegetated and closely linked to other landmasses, including South America and Zealandia (the submerged continent that is now mostly limited to New Zealand). The map posted below shows the arrangements of the major landmasses in the early Cenozoic Era some 60 million years ago, almost 30 million years before the formation of the Antarctic ice cap.
The Antarctic floristic region therefore does partially reflect continental divisions, but not those of the present day. Despite the passage of many millions of years, the similarities of the plant life found across this far-flung “kingdom” are striking. As noted in a Wikipedia article:
According to Ronald Good, about 50 genera of vascular plants are common in the Antarctic floristic kingdom, including Nothofagus [the southern beeches, dominant in many forests] and Dicksonia. Takhtajan also made note of hundreds of other vascular plant genera scattered and isolated on islands of the Southern Ocean, including Calandrinia feltonii of the Falkland Islands, Pringlea antiscorbutica of the Kerguelen Islands, and the megaherb genera of the New Zealand Subantarctic Islands.
Other botanists have developed their own floristic regionalization schemes, not all which include an Antarctic region. In 2001, for example, C. B. Cox deleted both the Antarctic and Cape kingdoms while dividing the vast Paleotropical region on partly continental lines, distinguishing an African from an Indo-Pacific kingdom. His map thus has closer affinities to Wallace’s zoogeographical map than it does with the floristic schemes of Good and Takhtajan. But Cox still deviated from the architecture of continents by placing northern North America and northern Eurasia in the same floristic zone.
The most useful global map of floristic regions that I have found is the one produced by Encyclopedia Britannica (posted below). This map largely follows the work of Good and Takhtajan. It divides plant “kingdoms” into “subkingdoms,” “regions,” and “divisions,” and clearly depicts the floristic affinities of widely scattered islands.
The “Macaronesian division” of the Boreal (Holarctic) Kingdom, is of particular interest. Macaronesia covers several islands and archipelagos in the northeastern Atlantic: the Azores, the Canary Islands, the Cape Verde, and Madeira. Many of these islands are – or were – noted for their laurisilva, or laurel forests, which require high humidity as well as stable and mild temperatures. Such conditions were once far more widespread than they are today. As noted in a Wikipedia article:
Laurel-leaved forests, called laurisilva, once covered most of the Azores, Madeira, and parts of the Canaries at an altitude of between 400 and 1,200 metres (1,300 and 3,900 ft), the eastern Canaries and Cape Verde being too dry. These forests resemble the ancient forests that covered the Mediterranean Basin and northwestern Africa before the cooling and drying of the ice ages. Trees of the genera Apollonias, Clethra, Dracaena, Ocotea, Persea, and Picconia, which are found in the Macaronesian laurel forests, are also known, from fossil evidence, to have flourished around the Mediterranean before the ice ages.
The Britannica map also depicts New Caledonia as forming its own division, part of the Indo-African (or Paleotropical) Kingdom. Although New Caledonia is part of the mostly submerged continent of Zealandia, it does not have New Zealand’s Antarctic floristic affinities. Evidently, the piece of continental crust on which New Caledonia sits was formerly submerged, with the modern island group emerging roughly 30 million years ago due to complex tectonic process. What make the flora of New Caledonia unique is not its ancient Antarctic origin, but rather its adaptation to soils infused with metal, especially nickel and chromium. As explained in a Wikipedia article:
The native flora [of New Caledonia] evolved many extremophile species that thrive in environments sufficiently toxic to put invasive plants at a disadvantage. Many areas, mainly on Grand Terre have some very high concentrations of metalliferous rocks. Their mineral content is poorly suited to most foreign species of plants. Ultramafic rocks also contain elevated amounts of chromium and nickel which may be toxic to plants. As a result, a distinctive type of vegetation develops on these soils.
Not surprisingly, the economy of New Caledonia, a dependency of France, is heavily dependent of nickel mining and smelting. Roughly 10 percent of the world’s nickel reserves are found in New Caledonia.
