Phytochorion

A phytochorion, in phytogeography, is a geographic area with a relatively uniform composition of plant species. Adjacent phytochoria do not usually have a sharp boundary, but rather a soft one, a transitional area in which many species from both regions overlap. The region of overlap is called a vegetation tension zone.

In traditional schemes, areas in phytogeography are classified hierarchically, according to the presence of endemic families, genera or species, e.g., in floral (or floristic, phytogeographic) zones and regions, or also in kingdoms, regions and provinces, sometimes including the categories empire and domain. However, some authors prefer not to rank areas, referring to them simply as "areas", "regions" (in a non hierarchical sense) or "phytochoria".

Systems used to classify vegetation can be divided in two major groups: those that use physiognomic-environmental parameters and characteristics and those that are based on floristic (i.e. shared genera and species) relationships. Phytochoria are defined by their plant taxonomic composition, while other schemes of regionalization (e.g., vegetation type, physiognomy, plant formations, biomes) may variably take in account, depending on the author, the apparent characteristics of a community (the dominant life-form), environment characteristics, the fauna associated, anthropic factors or political-conservationist issues.

Explanation
Several systems of classifying geographic areas where plants grow have been devised. Most systems are organized hierarchically, with the largest units subdivided into smaller geographic areas, which are made up of smaller floristic communities, and so on. Phytochoria are defined as areas possessing a large number of endemic taxa. Floristic kingdoms are characterized by a high degree of family endemism, floristic regions by a high degree of generic endemism, and floristic provinces by a high degree of species endemism. Systems of phytochoria have both significant similarities and differences with zoogeographic provinces, which follow the composition of mammal families, and with biogeographical provinces or terrestrial ecoregions, which take into account both plant and animal species.

The term "phytochorion" (Werger & van Gils, 1976) is especially associated with the classifications according to the methodology of Josias Braun-Blanquet, which is tied to the presence or absence of particular species, mainly in Africa.

Taxonomic databases tend to be organized in ways which approximate floristic provinces, but which are more closely aligned to political boundaries, for example according to the World Geographical Scheme for Recording Plant Distributions.

Early schemes
In the late 19th century, Adolf Engler (1844-1930) was the first to make a world map with the limits of distribution of floras, with four major floral regions (realms). His Syllabus der Pflanzenfamilien, from the third edition (1903) onwards, also included a sketch of the division of the earth into floral regions.

Other important early works on floristics includes Augustin de Candolle (1820), Schouw (1823), Alphonse de Candolle (1855), Drude (1890), Diels (1908), and Rikli (1913).

Good (1947) regionalization
Botanist Ronald Good (1947) identified six floristic kingdoms (Boreal or Holarctic, Neotropical, Paleotropical, South African, Australian, and Antarctic), the largest natural units he determined for flowering plants. Good's six kingdoms are subdivided into smaller units, called regions and provinces. The Paleotropical kingdom is divided into three subkingdoms, which are each subdivided into floristic regions. Each of the other five kingdoms are subdivided directly into regions. There are a total of 37 floristic regions. Almost all regions are further subdivided into floristic provinces.

Takhtajan (1978, 1986) regionalization
Armen Takhtajan (1978, 1986), in a widely used scheme that builds on Good's work, identified thirty-five floristic regions, each of which is subdivided into floristic provinces, of which there are 152 in all.

I. Circumboreal region

 * 1 Arctic province
 * 2 Atlantic Europe province
 * 3 Central Europe province
 * 4 Illyria or Balkan province
 * 5 Pontus Euxinus province
 * 6 Caucasus province
 * 7 Eastern Europe province
 * 8 Northern Europe province
 * 9 Western Siberia province
 * 10 Altai-Sayan province
 * 11 Central Siberia province
 * 12 Transbaikalia province
 * 13 Northeastern Siberia province
 * 14 Okhotsk-Kamchatka province
 * 15 Canada incl. Great Lakes province

II. Eastern Asiatic region

 * 16 Manchuria province
 * 17 Sakhalin-Hokkaidō province
 * 18 Japan-Korea province
 * 19 Volcano-Bonin province
 * 20 Ryūkyū or Tokara-Okinawa province
 * 21 Taiwan province
 * 22 Northern China province
 * 23 Central China province
 * 24 Southeastern China province
 * 25 Sikang-Yuennan province
 * 26 Northern Burma province
 * 27 Eastern Himalaya province
 * 28 Khasi-Manipur province

III. North American Atlantic region

 * 29 Appalachian province (forested areas extending east to include the piedmont and west to the start of the prairies)
 * 30 Atlantic and Gulf Coastal Plain province
 * 31 North American Prairies province

IV. Rocky Mountain region

 * 32 Vancouverian province
 * 33 Rocky Mountains province

V. Macaronesian region

 * 34 Azores province
 * 35 Madeira province
 * 36 Canaries province
 * 37 Cape Verde province

VI. Mediterranean region

 * 38 Southern Morocco province
 * 39 Southwestern Mediterranean province
 * 40 South Mediterranean province
 * 41 Iberia province
 * 42 Baleares province
 * 43 Liguria-Tyrrhenia province
 * 44 Adriatic province
 * 45 East Mediterranean province
 * 46 Crimea-Novorossijsk province

VII. Saharo-Arabian region

 * 47 Sahara province
 * 48 Egypt-Arabia province

8A. Western Asiatic subregion

 * 49 Mesopotamia province
 * 50 Central Anatolia province
 * 51 Armenia-Iran province
 * 52 Hyrcania province
 * 53 Turania or Aralo-Caspia province
 * 54 Turkestan province
 * 55 Northern Baluchistan province
 * 56 Western Himalaya province

8B. Central Asiatic subregion

 * 57 Central Tien Shan province
 * 58 Dzungaria-Tien Shan province
 * 59 Mongolia province
 * 60 Tibet province

IX. Madrean Region

 * 61 Great Basin province
 * 62 Californian province
 * 63 Sonoran province
 * 64 Mexican Highlands province

X. Guineo-Congolian region

 * 65 Upper Guinean forests province
 * 66 Nigeria-Cameroon province
 * 67 Congo province

XI. Usambara-Zululand region

 * 68 Zanzibar-Inhambane province
 * 69 Tongoland-Pondoland province

12A. Zambezian subregion

 * 70 Zambezi province

12B. Sahelo–Sudanian subregion

 * 71 Sahel province
 * 72 Sudan province

12C. Eritreo–Arabian subregion

 * 73 Somalia-Ethiopia province
 * 74 South Arabia province
 * 75 Socotra province

12C. Omano-Sindian subregion

 * 76 Oman province
 * 77 South Iran province
 * 78 Sindia province

XIII. Karoo-Namib region

 * 79 Namibia province
 * 80 Namaland province
 * 81 Western Cape province
 * 82 Karoo province

XIV. St. Helena and Ascension region

 * 83 St. Helena and Ascension province

XV. Madagascan region

 * 84 Eastern Madagascar province
 * 85 Western Madagascar province
 * 86 Southern and Southwestern Madagascar province
 * 87 Comoro province
 * 88 Mascarenes province
 * 89 Seychelles province

XVI. Indian region

 * 90 Ceylon (Sri Lanka) province
 * 91 Malabar province
 * 92 Deccan province
 * 93 Upper Gangetic Plain province
 * 94 Bengal province

XVII. Indochinese region

 * 95 South Burma province
 * 96 Andamans province
 * 97 South China province
 * 98 Thailand province
 * 99 North Indochina province
 * 100 Annam province
 * 101 South Indochina province

18A. Malesian subregion

 * 102 Malaya province
 * 103 Borneo province
 * 104 Philippines province
 * 105 Sumatra province
 * 106 Java province

18B. Papuan subregion

 * 107 Celebes province
 * 108 Moluccas and West New Guinea province
 * 109 Papua province
 * 110 Bismarck Archipelago province

XIX. Fijian region

 * 111 New Hebrides province
 * 112 Fiji province

XX. Polynesian region

 * 113 Micronesia province
 * 114 Polynesia province

XXI. Hawaiian region

 * 115 Hawaii province

XXII. Neocaledonian region

 * 116 New Caledonia province

XXIII. Caribbean region

 * 117 Central America province
 * 118 West Indies province
 * 119 Galápagos Islands province

XXIV. region of the Guayana Highlands

 * 120 The Guianas province

XXV. Amazon region

 * 121 Amazonia province
 * 122 Llanos province

XXVI. Brazilian region

 * 123 Caatinga province
 * 124 Central Brazilian Uplands province
 * 125 Chaco province
 * 126 Atlantic province
 * 127 Paraná province

XXVII. Andean region

 * 128 Northern Andes province
 * 129 Central Andes province

XXVIII. Cape region

 * 130 Cape province

XXIX. Northeast Australian region

 * 131 North Australia province
 * 132 Queensland province
 * 133 Southeast Australia province
 * 134 Tasmania province

XXX. Southwest Australian region

 * 135 Southwest Australia province

XXXI. Central Australian or Eremaean region

 * 136 Eremaea province

XXXII. Fernandezian region

 * 137 Juan Fernández province

XXXIII. Chile-Patagonian region

 * 138 Northern Chile province
 * 139 Central Chile province
 * 140 Pampas province
 * 141 Patagonia province
 * 142 Tierra del Fuego province

XXXIV. region of the South Subantarctic Islands

 * 143 Tristan-Gough province
 * 144 Kerguelen province

XXXV. Neozeylandic region

 * 145 Lord Howe province
 * 146 Norfolk province
 * 147 Kermadec province
 * 148 Northern New Zealand province
 * 149 Central New Zealand province
 * 150 Southern New Zealand province
 * 151 Chatham province
 * 152 New Zealand Subantarctic Islands province

Regionalization according to Wolfgang Frey and Rainer Lösch (2004, 2010)

 * Notes:(with focus on Europe, matching the image on the right)
 * The central European region and the central Russian region are sister regions.
 * The border between them is similar to the Fagus sylvatica limit (January, day-time temperature average: above -2 °C).
 * The border between the central Russian region and the boreal region is similar to the Quercus spp. limit (Day-time temperature average: above 10 °C, 4 months per year).
 * The border between the boreal region and the arctic region is similar to the tree line, taiga/arctic tundra limit (July, day-time temperature average: above 10 °C).
 * The border of the Atlantic region is the limit of no frost (average), Gulf Stream influence.
 * The warm islands in the Atlantic Ocean are in the Macaronesia region: isolated populations in a more humid environment.
 * The Mediterranean region is similar to the occurrence of wild Olea europea and wild Cistus salviifolius (Olea europea is grown very North in Italy).
 * The border between the submediterranean region and the central European region is similar to the alpine arc (upper Rhone, upper Rhine, lower Danube), a weather barrier.
 * The Pontic region border is similar to the tree line/ steppe limit (less than 450 mm precipitation per year).
 * The Turanian region has a semi-arid climate.

Liu et al. (2023, 2024) Regionalization
Critiquing previous attempts for their lack of phylogenetic relationships in the construction of their regions, Liu et al. incorporated distribution data alongside phylogenetic relationships to configure their realms. This led to the classification of eight realms organized into two super-realms and each composed of a number of sub-realms.
 * Gondwanan Super-Realm
 * 1 African
 * 2 Indo-Malesian
 * 3 Australian
 * 4 Novozealandic
 * 5 Neotropical
 * 6 Chile-Patagonian


 * Laurasian Super-Realm
 * 7 Holarctic
 * 8 Saharo-Arabian

Differences from Takhtajan's floristic kingdoms mainly focus on emphasizing the uniqueness of certain realms that he had as subdivisions within kingdoms. Two examples are separating some kingdoms into two separate realms, as happened to the Paleotropical and Antarctic kingdoms, reasoning that they have been separated form each other for long enough time to constitute a different phylogenetic trajectory. The merging of the Cape floristic kingdom with the African realm was based by the low endemism of higher taxonomic ranks, which could be found outside the cape region in the rest of Africa. The final major change is the separation of the Saharo-Arabian realm from the Holarctic kingdom, though they admit the northern boundary is not clear, with flora from the Holarctic being found within this area.

After publishing their regions, Dr. Hong Qian criticized Liu et al. for the inclusion of nonnative distributions in their analyses. In response to this, the group cleaned their data to remove nonnative ranges and reassessed their regions. They suggest that the previous inclusion of exotic species did not significantly affect their mapping and found that the cleaned data revealed the same floristic realms.