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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In our Bioclimatic approach (Rivas-Martínez, in progress), which is almost a new generic climate worldwide classification, only easily available statistical and meteorological data have been used. Those data have been treated as climatic parameters (single and summatory) or as bioclimatic indexes (combinations of parameters) both obtained through intentionally easy arithmetic formulas. Both parameters and indexes have only been used when some thresholds have shown an accurate relation with the vegetation changes (boundaries of potential natural plant communities) and have a high predictive value. The attached table summarizes the latest approach (16.12.98) to the Worldwide Bioclimatic Classification System (see pg. 9).

  • T: Yearly average temperature in centigrade degrees (Celsius).
  • Ti: Average temperature for every month of the year.
  • M: Average maximum temperature of the coldest month of the year (Tmin).
  • m: Average minimum temperature of the coldest month of the year (Tmin).
  • Tmin: Average temperature of the coldest month. (also m')
  • Tmax: Average temperature of the warmest month. (also M')
  • P: Yearly average precipitation in mm.
  • Pi: Average precipitation any month of the year.
  • Ps: Summer Precipitation. Precipitation of the driest quarter of the year in mm (Pd). Total average precipitation of the three summer months measured in mm: In extratropical zones (north of 23ºN and south of 23ºS) precipitations of June + July + August for the Northern Hemisphere, and for December + January + February for the Southern Hemisphere. Within tropical belts, this is understood as the "estival precipitation", that is: the sum of precipitation occurring during the three driest consecutive months of the year, regardless of their average temperature.
  • Pw: Winter Precipitation (mm). In extratropical zones (north of 23ºN and south of 23ºS) precipitations of December + January + February for the Northern Hemisphere, and of June + July + August for the Southern Hemisphere.
  • Pp: Yearly Positive Precipitation. In mm, total average precipitation of those months whose average temperature is higher than 0ºC.
  • Tp: Yearly Positive Temperature. In tenths of degrees Celsius, sum of the monthly average temperature of those months whose average temperature is higher than 0ºC.
  • Tps: Positive Summer Temperature. Positive temperature of summer quarter of the year.
  • Athermic: The average temperature of every month of the year is 0ºC or lower than 0ºC: consequently, Tp is null.
  • Ic: Continentality Index (yearly thermic interval). Ic = Tmax - Tmin. In degrees Celsius, the number expressing the range between the average temperatures of the warmest (Tmax) and coldest (Tmin) months of the year. The simple continentality index-types and subtypes are: hyperperoceanic (0-11) [extremely hyperperoceanic (0-3), euhyperperoceanic (3-7), barely hyperperoceanic (7-11)], oceanic (11-21) [euoceanic (11-18), semicontinental (18-21)], and continental [subcontinental (21-28), eucontinental (28-45) and hypercontinental (45-65)].  
Types  Subtypes  Ic
(Ic 0-21)
Extremely hyperoceanic   0-3
Euhyperoceanic   3-7
Barely hyperoceanic   7-11
(Ic 11-21)
Euoceanic   11-18
Semicontinental 18-21
(Ic 21-65)
Subcontinental 21-28
Eucontinental 28-46
Hypercontinental 46-65

In the extratropical zones of the World (northern and southern 27ºN and 27ºS parallels, respectively), the Compensated Thermicity Index is designed to equilibrate the cold "excess" that occurs during winter in the continental climates (average tenperature of the coldest month of the year minimum), or the excessively mild winter in the marked oceanic territories, so that these index values can be significantly compared.

If the Continentality Index (Ic) lies between 9 and 18, the Itc value is considere equal to the It value, that means that there is no modification. In the other hand, if the Continentality Index do not reach, or surpass, the mentioned values, it is needed to compensate the Thermicity Index adding or subtracting a figure called Compensation Value (C). In the extratropical markedly hyperoceanic zones (Ic < 9.0), the Compensation Value (C) is calculated by multiplying by ten the result of the difference between 9.0 and the simple Ic: C = (9.0-IC) 10. This Compensation Value is then substracted from the Thermicity Index corresponding value: Itc = It - C.

In the extratropical continental climates (Ic > 18.0) the Compensation Value (C) is a summatory to the Thermicity Index corresponding value: Itc = It + C. This Compensation Value (C) must be calculated in dependence of the Continentality Index figure (Ic). So, if the continentality is moderate (18.0 < Ic <= 21.0) the Compensation Value (Cl) is obtained by multiplying by fl (fl = 5) the result of the difference between the Ic of the station and 18. When the continentality is high (Ic > 21.0), the Compensation Value must be calculated by means of a total whose partial values (Cl, C2, C3, C4) are proportionally higher due to the increment of the multiplier value (fi) as a function of the increasing continentality. Therefore: Itc= It+ (C1+C2+C3+C4).

The compensation values of application, as a function of the Continentality Index values (Ic) and of the multiplication factor (fi), are obtained as follows:

Ic fi Ci Ci highest
18<Ic<=21 f1=5 Ci=C1;
21<Ic<=28 f2=15 Ci=C1+C2
28<Ic<=46 f3=25 Ci=C1+C2+C3
46<Ic<=65 f4=30 Ci=C1+C2+C3+C4
C4=f4(Ic- 46)

By definition, the Mediterranean macrobioclimate is an extratropical macrobioclimate characterized by, at least, two consecutive dry months during the summer (the warmest period in the year). A month is defined as dry if the precipitation (mm) is less than twice the temperature (centigrade degrees) (Pi < 2Ti). Obviously, if the ombrothermic bimonthly quotient of July + August (Ios2) or of January + February, depending on the hemisphere, (Ps July+August)/(Ts July+ August), is higher than two, the territory is not Mediterranean; but if that quotient is less than 2.0, the territory may or may not Mediterranean, as the bimonthly deficient hydrical balance may or may not compensated with the previous month"s precipitation (June or December: Ios3). A quarterly quotient Ios3 of less than 2.0 could even be compensated in some cases (see "Compensation Table") with the previous month"s precipitation (May or November: Ios4). These summer compensated ombrothermic indexes are very useful as they discriminate the frontier between Mediterranean-Temperate and Mediterranean-Boreal territories. In some extratropical places, the warmest months in the year are not those that follow the summer solstice, but those near the next autumnal equinox and, in these cases, those late summer hottest month must be used in calculations.  

Io  Ios2  Ios3  Ios4
2.0-3.6  > 1.9  > 1.9  > 2.0
3.6-4.8  > 1.8  > 1.9  > 2.0
4.8-6.0  > 1.7  > 1.8  > 2.0
6.0-8.0  > 1.5  > 1.8  > 2.0
8.0-10.0  > 1.2  > 1.6  > 2.0
10.0-12.0  > 0.7  > 1.4  > 2.0
> 12.0  -  -  > 2.0

Figure 1.    Summer ombrothermic compensation values (Ios2, Ios3) from Mediterranean to Temperate macrobioclimates (Submediterranean bioclimatic variant). It will be Temperate if Ios2 > 2.0 or Iosc3 and Iosc4 > 2.0, with Ios2 and Ios3 inside the threshold values expressed in the table.