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,^[1] or also in kingdoms, regions and provinces,^[2] 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".^[3]

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.^[4] 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.^[5]

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)^[6] 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,^[7] mainly in Africa.^[8]

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.

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).^[9][10] His Syllabus der Pflanzenfamilien, from the third edition (1903) onwards, also included a sketch of the division of the earth into floral regions.^[11]

Other important early works on floristics includes Augustin de Candolle (1820),^[12] Schouw (1823),^[13] Alphonse de Candolle (1855),^[14] Drude (1890),^[1] Diels (1908),^[15] and Rikli (1913).^[16]

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.^[17]

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.^{[18][19][20][21]}

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

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

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

32 Vancouverian province

33 Rocky Mountains province

34 Azores province

35 Madeira province

36 Canaries province

37 Cape Verde province

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

47 Sahara province

48 Egypt-Arabia province

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

57 Central Tien Shan province

58 Dzungaria-Tien Shan province

59 Mongolia province

60 Tibet province

61 Great Basin province

62 Californian province

63 Sonoran province

64 Mexican Highlands province

65 Upper Guinean forests province

66 Nigeria-Cameroon province

67 Congo province

68 Zanzibar-Inhambane province

69 Tongoland-Pondoland province

70 Zambezi province

71 Sahel province

72 Sudan province

73 Somalia-Ethiopia province

74 South Arabia province

75 Socotra province

76 Oman province

77 South Iran province

78 Sindia province

79 Namibia province

80 Namaland province

81 Western Cape province

82 Karoo province

83 St. Helena and Ascension province

84 Eastern Madagascar province

85 Western Madagascar province

86 Southern and Southwestern Madagascar province

87 Comoro province

88 Mascarenes province

89 Seychelles province

90 Ceylon (Sri Lanka) province

91 Malabar province

92 Deccan province

93 Upper Gangetic Plain province

94 Bengal province

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

102 Malaya province

103 Borneo province

104 Philippines province

105 Sumatra province

106 Java province

107 Celebes province

108 Moluccas and West New Guinea province

109 Papua province

110 Bismarck Archipelago province

111 New Hebrides province

112 Fiji province

113 Micronesia province

114 Polynesia province

115 Hawaii province

116 New Caledonia province

117 Central America province

118 West Indies province

119 Galápagos Islands province

120 The Guianas province

121 Amazonia province

122 Llanos province

123 Caatinga province

124 Central Brazilian Uplands province

125 Chaco province

126 Atlantic province

127 Paraná province

128 Northern Andes province

129 Central Andes province

130 Cape province

131 North Australia province

132 Queensland province

133 Southeast Australia province

134 Tasmania province

135 Southwest Australia province

136 Eremaea province

137 Juan Fernández province

138 Northern Chile province

139 Central Chile province

140 Pampas province

141 Patagonia province

142 Tierra del Fuego province

143 Tristan-Gough province

144 Kerguelen province

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

Notes

(with focus on Europe, matching the image on the right)^[22][23]

• 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.

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.^[24]

• 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.^[25] 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.^[26]

• Frodin, D.G. (2001). Guide to Standard Floras of the World. An annotated, geographically arranged systematic bibliography of the principal floras, enumerations, checklists and chorological atlases of different areas. 2nd ed. (1st edn 1984), pp. xxiv, 1100, .Cambridge University Press, Cambridge, [17].