North American boreal and western temperate forest vegetation

Salvador Rivas-Martínez, Daniel Sánchez-Mata & Manuel Costa

Itinera Geobotanica 12:5-316 (1999)

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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 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, have been incorporated.

Until now, there have been few bioclimatic classifications and systems proposed for global use. Among the best known, those of Köppen (1918), Thornthwaite (1931, 1933, 1948), Gaussen (1954, 1955), Troll & Paffen (1964), Holdridge (1967), Walter (1970, 1976, 1985) and Tuhkanen (1984) could be mentioned. Although most of them are good, and even widely accepted, we do not consider that they have provided adequate solutions to some important aspects of occurrences in the geobiosphere.

For several years, we have been trying to achieve a relevant worldwide bioclimatic classification. Our purpose is to accomplish a bioclimatic typology, always quantifiable, which should show a close relation between the plant formations, vegetation series and climatic numerical values, expressed by parameters and easily calculated bioclimatic indexes. The increasingly detailed knowledge of the vegetation distribution pattern on the Earth, as well as the modifications in the appearance and composition of the potential natural vegetation and its substitution stages, caused by climatic, edaphic, geographic and anthropic factors, are making it possible to recognize bioclimatic and vegetation boundaries with great accuracy and objectivity.

Once the limits or boundaries of the vegetation series and macroseries have been determined and charted, the threshold of the climate values, which discriminate them, can be statistically calculated. Thus, the areas of the bioclimatic units (bioclimates, thermotypes and ombrotypes) have been progressively delimited and adjusted. The established biophysical models have shown a high reciprocity in the climate-vegetation binomial, which allows us to make bioclimatic and biogeographic maps which correspond very closely with reality all over the Earth. As a practical consequence we have achieved a reciprocal predictive value for any place on the Earth if we know only one of the two variables, either the climatic data or the vegetation types.

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, biocenosis and plant communities. Within almost every bioclimate, a number of variations in the seasonal rainfall patterns of rain allows 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.

Premises: From the conceptual point of view, the new bioclimatic classification (Rivas-Martínez, 1996a, 1997) is based on the following fundaments, premises and theses:

1. In Bioclimatology there must be a close and reciprocal relation between climate, vegetation and geographic territories; that is to say, between bioclimates, vegetation series and their respective biogeographic units.

2. Between the parallels 23ºN and 23ºS, solar radiation is practically zenithal all year round and, in consequence the vegetation as well as the macrobioclimate at any given altitude, is considered Tropical. The Tropical, Temperate and Mediterranean macrobioclimates can be found within the subtropical latitudinal belts (23º to 35ºN and S), depending on the temperature and pattern of rainfall. The seasonal photoperiods delimited by the parallels 35º and 52ºN and S represent a rigid boundary for many species and plant communities, and all the macrobioclimates can be found within this latitude with the exception of the Tropical and the Polar macrobioclimates. Beyond the parallels 66ºN and 66ºS, due to the extreme difference in length between day and night during the solstices, the vegetation at any given altitude as well as the macrobioclimate are considered either Boreal or Polar.

3. The average monthly temperature range between the most extreme months of the year (which, when expressed in degrees centigrade, is the simple Continentality Index) has a great influence on vegetation distribution and, as a result, on the boundaries of many bioclimates. The most significant limit values and units for the Continentality Index are: 0 to 3 (extremely hyperoceanic), 3 to 7 (euhyperoceanic), 7 to 11 (barely hyperoceanic), 11 to 18 (euoceanic), 18 to 21 (semicontinental), 21 to 28 (subcontinental), 28 to 46 (eucontinental), and 46 to 65 (hypercontinental).

4. The pattern of rainfall throughout the year is as important as, or even more important than, the amount of rain in relation to the composition and distribution of plant communities. These variations or rainfall patterns determine the macrobioclimates (Tropical, Mediterranean and Temperate) as well as their subordinate units (bioclimates, and bioclimatic variants).

5. Almost all the Anglo-Saxon classification systems define the Mediterranean climate as a subtropical warm temperate type, with abundant winter rain but with summer drought, related with sclerophyllous forests and preforests. We consider that there is a broad Mediterranean macrobioclimate, extratropical in latitude (that is, not found within the equatorial and eutropical belts), whose pattern of that in rainfall is the opposite of the Tropical and Temperate macrobioclimates. The Mediterranean macrobioclimate is characterized by a summer drought of at least two consecutive months in which P < 2T. The drought can persist for up to twelve months a year in the Desertic and Hyperdesertic Mediterranean bioclimates. Depending on the rainfall, the Mediterranean vegetation structure conforms to several evergreen or deciduous forest types (Pluviseasonal Mediterranean), microforests or dense shrubby terrain (Xeric Mediterranean), halfdeserts or low density scrublands (Desertic Mediterranean), as well as hyperdeserts without woody climatophilous vegetation (Hyperdesertic Mediterranean). It must be remembered that the plant communities growing in territories with Mediterranean bioclimates differ in structure and floristic composition from those in the Temperate and Tropical bioclimates with a similar rainfall; that the Mediterranean macrobioclimate can be found within the subtropical and the upper temperate belt (23º to 52ºN and S); that the Xeric and Desertic Mediterranean bioclimates are present in broad territories inside every continent; and finally, that the Pluviseasonal Mediterranean bioclimate has its territorial optimum in the countries situated in the western regions of the continents, beside the oceans or seas.

6. Up to now, all the classification systems have placed every high mountain on the earth, whether in a boreal or equatorial zone, in a single bioclimatic belt type, known as the "Mountain Climate". We consider the mountains only as colder, and generally wetter altitudinal variations of the macrobioclimates at the foothill. Every mountain range has a specific vertical zonation of flora and vegetation. Thus particular bioclimatic altitudinal belts must be recognized for mountains in every macrobioclimate.

It is evident that the mountains situated in the tropics have an almost equinoctial yearly sun pattern, whilst in the upper temperate and boreal zones the day span varies widely throughout the year. As a consequence, the daily temperature cycle in tropical mountains shows, all the year round, an almost continuous pattern of severe night frosts and high day temperatures, which implies there is a daily thaw. In the mountains located at higher latitudes, there is a long winter with no thaw and, up to some altitudes, a short cool summer without frosts. Therefore tropical and extratropical mountain flora and vegetation, independently of possible migrations in glaciation periods in mountains with a North-South orientation (such as the American ranges), are formed by floristic and vegetation elements whose origin is mostly in the flora of the foothills (Tropical, Mediterranean, Temperate, etc.). Although there are morphological convergences between tropical and extratropical high mountain vegetation, there are also great divergences. In the rainy tropical high mountain areas, such as the Andes, the high altitude formations have their own character and an almost continuous activity all year round. Examples of these formations are: puna grassland (Calamagrostietea vicunari), paramos with caulirosulated plants (Espeletion), compact cushion bogs (Plantagini-Distichietea muscoidis) and the extensive cold deserts determined by cryoturbation and solifluxion phenomena (Anthochloo-Dielsiochloetea floribundae). These ecological environments and habitats are completely different from those in the temperate mountains where, as occurs in the Alps or in the Canadian Rocky Mountains, an upper montane and subalpine coniferous belt covered with snow for six months of the year (Vaccinio-Piceetea, Linnaeo-Piceetea glaucae) is followed by a grassy, nano-shrub, peat bog and snowfield alpine belt with vegetative activity for three or four months a year at the most (Carici-Kobresietea bellardii, Loiseleurio-Vaccinietea, Scheuchzerio-Caricetea fuscae, Salicetea herbaceae).

In summary, we consider that mountains represent only altitudinal variations in temperature and rainfall which, in most cases, can be expressed by zonation of bioclimatic altitudinal belts (combinations of thermo- and ombrotypes) of the macrobioclimates from the neighbouring valleys and plains.

7. In the Eurasian continent, the alpine orogeny mainly constitutes a continuous mountain system group with an East-West orientation. This obstacle has to a great extent limited plant migratory movements during subsequent major climatic changes. Thus, after the extinctions of the glaciation periods, the recolonization of the large central Asian transverse mountain ranges (Himalaya, Karakorum, Hindu Kush, etc.) by flora and vegetation from the neighbouring tropical belt during the interglacial, and subsequent Holocene periods was a very slow process. As a consequence, we have established an altitudinal limit of 2000 m as a frontier between the Tropical and the Temperate and Mediterranean macrobioclimates in Asia (70º to 120ºE), and this 2000 m altitudinal limit should also be recognized in North Africa between the parallels 26º and 35ºN.

8. Until now, every bioclimatic classification system has recognized a single type of desertic climate for every desert in the world. We recognize, apart from the polar and high mountain pergelid and athermic cryodeserts, two Desertic Tropical and two Desertic Mediterranean bioclimates, depending on the annual pattern of rainfall. The Desertic Tropical and Hyperdesertic Tropical bioclimates have the maximum of their scarce rain within the four months after the summer solstice (tropical rainfall pattern), while in the Desertic and Hyperdesertic Mediterranean bioclimates most of the precipitation takes place between the autumn and spring equinoxes, and is somewhat greater than the scarce rainfall of the four months following the summer solstice (Mediterranean rainfall pattern). The flora and vegetation in both Tropical and Mediterranean deserts are very different from each other, as they are adapted to such antithetic rainfall patterns. A clear example of this is the Sahara Desert, whose central-northern territory has a Mediterranean rainfall pattern and a Saharan-Arabic vegetation with evident Holarctic roots (Gymnocarpo-Atractyletalia, Rhoo-Periplocion angustifoliae, etc.), while the western and southern Sahelian zones, with tropical rainfall pattern (summer rains), have a Sudano-Zambezian desertic flora and vegetation (Panico-Acacion).

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 degrees centigrade [Psi < 2.5 Tsi]. The steppic character can be recognized in many continental vegetation-types by the xerophytic appearance of its communities, adapted to the hydric limitation in 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. In the Holarctic, the vegetation-types whose mature phases are either marcescent deciduous forests or xerophytic coniferous forests constitute the most characteristic vegetation of this bioclimatic variant (Quercetalia pubescentis, Pino-Juniperion sabinae, Pino scopulorum-Pseudotsugion glaucae).

Bixeric: Tropical bioclimatic variant (Bix), characterized by two annual periods of aridity (P = 2T) of at least one month in each of the solstice quarters (Tr1, Tr3), separated by another two rainy periods during both equinoctial quarters. This variant does not affect either the Pluvial Tropical bioclimate or the Hyperdesertic Tropical bioclimate.

The most characteristic vegetation types related with this bioclimatic variant are distributed throughout the Central-Eastern African mountains and made up of perennial plants with sclerophyllous small leaves. The phyletic relationships between these vegetation types and the sclerophyllous Mediterranean European and North African forests are shown by the fact that both share some characteristic genera such as Olea, Rhamnus, Pistacia, Jasminum, Smilax, Juniperus, etc.

Antitropical: Tropical bioclimatic variant (Ant), restricted mainly to the equatorial and adjacent territories in which rainfall of the winter solstice quarter (Tr1 y Tr3 within the northern and southern hemispheres, respectively) is wetter than the summer solstice quarter (Tr3 y Tr1 within the northern and southern hemispheres, respectively). This bioclimatic variant does not affect either the Pluvial Tropical bioclimate or the Hyperdesertic Tropical bioclimate.

The antitropical vegetation-types are very similar in structure to the pluviserotinal or typical tropical-types with equivalent ombrotype, usually with monzonic rainfall pattern. However, from the floristic point of view, the antitropical flora is rich in endemic species due to the isolation and speciation caused by the antithetical phenological periods.

Pluviserotinal: Tropical bioclimatic variant (Pse) in which the rainfall of the two first summer solstice months (early summer, Pes, June and July within the Northern Hemisphere, and December and January within the Southern Hemisphere) is at least 1.3 times lower than that of the two following months (late summer = serotinal, Pls) [Pls > 1.3 Pes]. This bioclimatic variant does not affect either the Pluvial Tropical bioclimate or the Hyperdesertic Tropical bioclimate.

In the tropics, the pluviserotinal bioclimatic variant is most common in the western regions of the continents, such as western Africa, western India, western tropical America, etc. In all these territories, the period of heaviest monsoon rains usually occurs between August and October.

Thermotypes and ombrotypes: The threshold thermotype and ombrotype horizon values: Thermicity Index (It), Compensated Thermicity Index (Itc), Positive Temperature (Tp) and Ombrothermic Index (Io), for the Tropical, Mediterranean, Temperate, Boreal and Polar macroclimates are given in figures 2 and 3.

Thermotype horizons Acro. It, Itc Tp
Lower infratropical Litr 810-890 > 3350
Upper infratropical Uitr 730-810 3100-3350
Lower thermotropical Lttr 610-730 2900-3100
Upper thermotropical Uttr 490-610 2700-2900
Lower mesotropical Lmtr 395-490 2400-2700
Upper mesotropical Umtr 320-395 2100-2400
Lower supratropical Lstr 240-320 1575-2100
Upper supratropical Ustr 160-240 1050-1575
Lower orotropical Lotr 105-160 750-1050
Upper orotropical Uotr (50-105) 450-750
Lower cryorotropical Lctr - 150-450
Upper cryorotropical Uctr - 1-150
Athermic tropical Tra - 0
Thermotype horizons Acro. It, Itc Tp
Lower inframediterranean Lime 515-580 > 2650
Upper inframediterranean Uime 450-515 2450-2650
Lower thermomediterranean Ltme 400-450 2300-2450
Upper thermomediterranean Utme 350-400 2150-2300
Lower mesomediterranean Lmme 280-350 1825-2150
Upper mesomediterranean Umme 210-280 1500-1825
Lower supramediterranean Lsme 145-210 1200-1500
Upper supramediterranean Usme 80-145 900-1200
Thermotype horizons Acro. It, Itc Tp
Lower oromediterranean Lome - 675-900
Upper oromediterranean Uome - 450-675
Lower cryoromediterranean Lcme - 150-450
Upper cryoromediterranean Ucme - 1-150
Athermic mediterranean Mea - 0
Thermotype horizons Acro. It, Itc Tp
Lower infratemperate Lite 445-480 > 2450
Upper infratemperate Uite 410-445 2350-2450
Lower thermotemperate Ltte 355-410 2175-2350
Upper thermotemperate Utte 300-355 2000-2175
Lower mesotemperate Lmte 240-300 1700-2000
Upper mesotemperate Umte 180-240 1400-1700
Lower supratemperate Lste 100-180 1100-1400
Upper supratemperate Uste (20-100) 800-1100
Lower orotemperate Lote - 590-800
Upper orotemperate Uote - 380-590
Lower cryorotemperate Lcte - 80-380
Upper cryorotemperate Ucte - 1-80
Athermic temperate Tea - 0
Thermotype horizons Acro. It, Itc Tp
Lower thermoboreal Ltbo - 750-800
Upper thermoboreal Utbo - 700-750
Lower mesoboreal Lmbo - 600-700
Upper mesoboreal Umbo - 500-600
Lower supraboreal Lsbo - 440-500
Upper supraboreal Usbo - 380-440
Lower oroboreal Lobo - 230-380
Upper oroboreal Uobo - 80-230
Lower cryoroboreal Lcbo - 40-80
Upper cryoroboreal Ucbo - 1-40
Athermic boreal boa - 0
Thermotype horizons Acro. It, Itc Tp
Lower mesopolar Lmpo - 230-380
Upper mesopolar Umpo - 80-230
Lower suprapolar Lspo - 40-80
Upper suprapolar Uspo - 1-40
Athermic polar Poa - 0

Figure 2. Thermotype horizon values and acronyms of Tropical, Mediterranean, Temperate, Boreal and Polar macrobioclimates, expressed in Thermicity Index (It), Compensated Thermicity Index (Itc) and Positive Temperature (Tp). (1) In the territories north of 45ºN and south of 49ºS with Tp 600-900, hemiboreal thermotype name is used at < 1000 m altitude and Ic > 21 as well as at < 400 m altitude and Ic <= 21.

Tropical and Mediterranean
Ombrotype horizons Acro. Io
Ultrahyperarid Uha < 0.1
Lower hyperarid Lhar 0.1-0.2
Upper hyperarid Uhar 0.2-0.3
Lower arid Lari 0.3-0.6
Upper arid Uari 0.6-1.0
Lower semiarid Lsar 1.0-1.5
Upper semiarid Usar 1.5-2.0
Lower dry Ldry 2.0-2.8
Upper dry Udry 2.8-3.6
Lower subhumid Lshu 3.6-4.8
Upper subhumid Ushu 4.8-7.0
Lower humid Lhum 7.0-10.5
Upper humid Uhum 10.5-14.0
Lower hyperhumid Lhhu 14.0-21.0
Upper hyperhumid Uhhu 21.0-28.0
Ultrahyperhumid Uhh > 28.0
Temperate, Boreal and Polar
Ombrotype horizons Acro. Io
Semiarid Sar < 2.2
Lower dry Ldry 2.2-2.9
Upper dry Udry 2.9-3.6
Lower subhumid Lshu 3.6-4.8
Upper subhumid Ushu 4.8-6.0
Lower humid Lhum 6.0-9.0
Upper humid Uhum 9.0-12.0
Lower hyperhumid Lhhu 12.0-18.0
Upper hyperhumid Uhhu 18.0-24.0
Ultrahyperhumid Uhh > 24.0

Figure 3. Ombrotype horizon values and acronyms of Tropical, Mediterranean, Temperate, Boreal and Polar macrobioclimates, expressed in Ombrothermic Index (Io).