Review of definitions (Dec. 2002)

Salvador Rivas-Martínez


Phytosociologic, bioclimatic and biogeographic terms, notions and units

Terms, index, notions and more detailed information can be found in Rivas-Martínez (1996, 1997) Rivas-Martínez, Sánchez-Mata & Costa (1999) and "Global Bioclimatics" (in progress, multicop.).

1. Phytosociological terms and notions

Phytosociology. Science dealing with biocoenosis from a botanical perspective (phytocoenosis or phytosyntaxa). In other words, it is concerned with plant communities, their relationships with the environment and the temporary processes modifying them. With all this information, by means of inductive and statistic methods, based on the reality of the phytosociological releve, it attempts to create an universal hierarchic typology with the association as the basic unit of the syntaxonomical system. Nowadays we distinguish, besides classic or Braun-Blanquet"s Phytosociology (association level), Dynamic-Catenal or Integrated Landscape Phytosociology, whose units are the series or sigmetum (Dynamic or Successional Phytosociology) and the geoseries or geosigmetum (Catenal Phytosociology).

Association. Fundamental and basic unit of the Phytosociology. It is a plant community-type with particular floristic, biogeographic, ecological, successional, historic and anthropogenic features. It owns a particular ecological and geographic territory, some characteristic and differential species or a precise characteristic statistically reliable combination of species and diagnostic bioindicators. It may be determined from the comparative study of releves, in which the floristic composition is annotated and quantified, like the rest of the ecological and geographic information of a particular homogeneous plant community. Making association releves is the most important part of the phytosociological research. Associations with similar floristic composition, dynamic stage, structure, habitat or vicariant distribution may be taken together in types or units of higher rank (alliances, orders, classes).

Climax. Final equilibrium and self-perpetuating stage in geobotanic succesion. Plant communities that territorially represent the final stage or potential natural vegetation of any vegetation series. Adj.: climactic, climactical.

Climactic dominion. Area in which a climatophilous plant association has really or virtually the meso-climax function. Taking into account its usual pluri-teselar diversity, more homogeneous lower ranked units could be recognized inside.

Potential natural vegetation. Steady plant community which should be present in an area, as a consequence of the progressive succession, if there were not human influences. In practise, potential vegetation is considered synonymous to climax and to primitive vegetation (not altered yet by humans). We must distinguish between the climatophilous series of the natural potential vegetation (the mesic one or edaphically in the middle of every territory) and the edaphophilous series or permanent plant communities (the xerics and hydrics possible series in the same territory).

Succession. Natural process by which certain plant communities or vegetation stages replace each other in a particular order, within the same place unit or Tesela. We may speak of progressive succession, which tends upwards to the stable optimum or climax (progression, preseral stages), and of regressive succession, the opposite (regression, subseral stages). The progressive and regressive processes do not follow necessarily the same stages.

Sigmetum. Typological unit in Dynamic Phytosociology (Symphytosociology), it is also called Synassociation or Vegetation Series. It is a geobotanic notion that tries to express all the plant communities, or collection of stages, that can be found in similar teselar places as a result of the succession processes: and so, a Geosigmetum includes not only the representative vegetation type of the mature stage, or head series, but also the initial or subserial communities replacing it.

Catena. Ensemble of contiguous plant communities ordered by any changing ecological factor (temperature, moisture, topography, etc.). It is the landscape representation of the zonation phenomena.

Geosigmetum. Also denominated geosigmassociation or geoseries, it is the basic unit of the Integrated Landscape Phytosociology (Geosymphytosociology). It seeks to be the catenal and successional expression of landscape science. It is built up with the contiguous sigmeta or vegetation series. It is the most important notion and unit in the Phytosocological Landscape Science, the one which has been used in our bioclimatic and biogeographic new approaches. If we integrate to a series their contiguous ones, that is, if we take into account, besides the succession, the catenal phenomenon, -for instance, the climatophilous and edaphophilous series that can be in contact- we have this more complex and integrated unit known as geoseries or geosigmetum.

Microgeosigmetum. Vegetation transects or microcatenas constituted by microsigmassociations, usually with reduced surface. They are delimited by exceptional microtopographic and edaphic features which, in a small piece of ground, cause numerous ecological niches and teselas, covered by non-stratified permanent plant communities whose dynamic balance seems to have been reached. Under these conditions, the reference to a mature plant community of the theoric regional sigmeta is not possible or should be ambiguous. In general, the microgeosigmeta are monostratum vegetation-types ordered in microcatenas according to their determinant mesologic features. The most auspicious biotopes for the microsigmetum development are the cliffs and rock crevices, bogs, snowdrifts, mobile dunes, shores of lakes and ponds, springs, etc. The study of these neighbouring plant communities complexes must be made within their geomorphologic and ecological limits, following the increasing degree of their causing ecological factor.

For their hierarchic ordination (-etum, -ion, -etalia, -etea) it must be noted their biogeographic location, their bioclimatic peculiarities, as well as their statistical fidelity to the rest of communities also present in the microcatena, not to mention the possible vicariances with similar microcatenas in other territories.

Types of forests. In the following table we summarize both forest types and the correspondent tree life-forms, according to the heigth at their canopy top. We consider as tree a plant having a permanently woody main stem or trunk and usually developing branches at some distance from the ground; an extense or small group of trees interlocking branches or canopies form woodlands or forests. Shrub is a woody plant, like a tree, having multiple permanent stem branching from o near the ground; a group of shrub, form shrubberies, scrubs, or scrublands.

Microforest 3-12m Microphanerophyte (small tree)
Mesoforest 12-24m Mesophanerophyte (medium tree)
Macroforest 24-50m Macrophanerophyte (tall tree)
Megaforest > 50m Megaphanerophyte (gigant tree)

2. Bioclimatic terms and notions

Bioclimatology. Ecological science dealing with the relations between the climate and the distribution of the living species on the Earth. The aim of this subject is to determine the relation between certain numerical values of temperature and precipitation and the geographic distribution areas of single plant species as well as of plant communities. Latterly, information from biogeocenosis has also been added. Recently, useful knowledge from the Dynamic-Catenal Phytosociology and from landscape science, that it is to say, from the vegetation series and geoseries, has been incorporated.

Until now, there have been few bioclimatic classifications and systems proposed for global use. Among the best known, those of Köppen (1918), Gaussen (1954, 1955), Troll & Paffen (1964), and Walter (1970, 1976, 1985) could be mentioned. Although most of them are good, and even widely accepted, we do not consider that they have provided adequate solutions and correlations to some important aspects of occurrences in the geobiosphere, particularly in vegetation approaches.

Bioclimatic Units. In the new "Worldwide Bioclimatic Classification System" proposed (Rivas-Martínez, in progress), five macrobioclimates, twenty-seven bioclimates and five bioclimatic variants are recognized. The macrobioclimate is the highest typological unit of our bioclimatic classification. It is an eclectic biophysical model, delimited by means of climatic and vegetation values, with a wide territorial jurisdiction. The five macrobioclimates are: Tropical, Mediterranean, Temperate, Boreal and Polar. Each of them, and every one of their subordinate units or bioclimates, is represented by a characteristic group of plant formations, biocoenosis and plant communities. Within almost every bioclimate, a number of variations in the seasonal rainfall patterns of rain allow us to recognize the bioclimatic variants. Additionally, within every bioclimate, variations in the thermic and ombrothermic values make it possible to distinguish the bioclimatic belts: thermotypes and ombrotypes. If macrobioclimates, bioclimates, bioclimatic variants as well as bioclimatic belts (thermo- and ombrotypes) are taken into account, about three hundred isobioclimates find territorial representation on the Earth.

Bioclimatic Variants. Typological units which can be recognized within macrobioclimates. With the use of these units we clarify several climatic peculiarities regarding rainfall patterns. We distinguish the following bioclimatic variants: Steppic, Submediterranean, Bixeric, Antitropical and Pluviserotinal.

Steppic: Bioclimatic variant (Stp), which can be recognized within the Mediterranean, Temperate, Boreal and Polar macrobioclimates. Its characteristic features are: the Continentality Index must be higher than 18 (Ic > 18), the summer quarter rainfall must be more than 1.2 times that of the winter quarter [Ps > 1.2 Pw], the Ombrothermic Index must fall within 0.1 and 4.6 [0.1 < Io < 4.6], and, at least during one summer month, the rainfall in mm (Psi) must be less than two and a half times the temperature in centigrade degrees [Psi < 2.5 Tsi]. The steppic character can be recognized in many continental vegetation-types by the xerophytic appearance of their communities, adapted to the hydric limitation during both solstices [Ps > 1.2 Pw].

The most characteristic steppic vegetation-types on the Earth, according to these isobioclimates, are the Temperate areas, known as steppes and steppic forests in Eurasia, or the extensive prairies or wooded prairies in North America. The Steppic Mediterranean vegetation-types of a xeric and desertic character are also common. The steppic "tundra" and "taiga" formations which belong to the Boreal and Polar macrobioclimates, are restricted to territories with low summer rainfall.

In general, we can assume that the steppic character is mainly a type of relatively high continentality together with an attenuated summer drought or mediterraneity as well as with drought during the winter solstice.

Submediterranean: Bioclimatic variant (Sbm), which can be recognized only within the Temperate macrobioclimate. Its characteristic feature is that at least during one summer month the rainfall is less than twice the temperature [Iosi = Psi/Tsi < 2, Psi < 2Ti] or during the two consecutive dryest summer months, the rainfall Ps2 is less than two and a half times the temperature [Ios2 = Ps2/Ts2 < 2.5, Ps2 < 2.5 Ts2].

The most characteristic temperate submediterranean vegetation-types are the plant communities growing along the ecotones between the Temperate bioclimates without summer drought and the typical Mediterranean bioclimates with a summer drought period of more than two months.

Thermotypes. The threshold thermotype horizon values based on Thermicity Index (It), Compensated Thermicity Index (Itc), and Positive Temperature (Tp) for the Mediterranean, Temperate and Boreal macroclimates are listed below. Tp is used only if Continentality Index Ic >= 21 or It, Itc < 120.

Mediterranean It, Itc Tp
Lower inframediterranean 515-580 > 2650
Upper inframediterranean. 450-515 2450-2650
Lower thermomediterranean 400-450 2300-2450
Upper thermomediterranean. 350-400 2150-2300
Lower mesomediterranean. 280-350 1825-2150
Upper mesomediterranean.. 210-280 1500-1825
Lower supramediterranean. 145-210 1200-1500
Upper supramediterranean. 80-145 900-1200
Lower oromediterranean - 675-900
Upper oromediterranean. - 450-675
Lower cryoromediterranean. - 150-450
Upper cryoromediterranean - 1-150
Gelid mediterranean. - 0
Temperate It, Itc Tp
Lower infratemperate 445-480 > 2450
Upper infratemperate. 410-445 2350-2450
Lower thermotemperate 355-410 2175-2350
Upper thermotemperate. 300-355 2000-2175
Lower mesotemperate. 240-300 1700-2000
Upper mesotemperate 180-240 1400-1700
Lower supratemperate. 100-180 1100-1400
Upper supratemperate - 800-1100
Lower orotemperate - 590-800
Upper orotemperate. - 380-590
Lower cryorotemperate. - 80-380
Upper cryorotemperate - 1-80
Gelid temperate. - 0
Boreal   Tp
Lower thermoboreal. - 750-800
Upper thermoboreal - 700-750
Lower mesoboreal - 600-700
Upper mesoboreal. - 500-600
Lower supraboreal - 440-500
Upper supraboreal - 380-440
Lower oroboreal. - 230-380
Upper oroboreal. - 80-230
Lower cryoroboreal - 40-80
Upper cryoroboreal. - 1-40
Gelid boreal - 0

Ombrotypes. The threshold of ombrotype horizon values, based on the Ombrothermic Index (Io), that we recognize in the world are listed below. Arid, hyperarid and ultrahyperarid types only exist in Tropical and Mediterranean macrobioclimates.

Types Io
Ultrahyperarid. < 0.1
Lower hyperarid. 0.1-0.2
Upper hyperarid 0.2-0.3
Lower arid. 0.3-0.6
Upper arid. 0.6-1.0
Lower semiarid. 1.0-1.5
Upper semiarid. 1.5-2.0
Lower dry. 2.0-2.8
Upper dry 2.8-3.6
Lower subhumid. 3.6-4.8
Upper subhumid. 4.8-6.0
Lower humid. 6.0-9.0
Upper humid. 9.0-12.0
Lower hyperhumid. 12.0-18.0
Upper hyperhumid 18.0-24.0
Ultrahyperhumid. > 24.0

Continentality Index. It is the figure in Celsius degrees that represent the yearly thermic average interval expressing the range between the average temperature of the warmest and coldest month of the year (Ic = Tmax-Tmin). (*) It could be divided in semihyperoceanic (Ic 11-13) and euoceanic (Ic 13-17).

Types Subtypes Ic
Hyperoceanic (Ic 0-11) Extremely hyperoceanic 0-4
Euhyperoceanic 4-8
Barely hyperoceanic 8-11
Oceanic (Ic 11-21) Euoceanic (*) 11-17
Semicontinental 17-21
Continental (Ic 21-65) Subcontinental 21-28
Eucontinental 28-46
Hypercontinental 46-65