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close this bookThe Improvement of Tropical and Subtropical Rangelands (BOSTID)
close this folderPart I
close this folderCriteria for plant selection
View the documentProject planning
View the documentSocioeconomic and management considerations in feasibility studies
View the documentAdaptation to ecoclimatic conditions
View the documentAdaptation to soils
View the documentAdaptation to physiography, geomorphology, topography, slope, and aspect
View the documentAbility of introduced species to compete with native vegetation
View the documentUse regimes
View the documentAvailability of seeds and plant materials
View the documentMaintenance of biological diversity
View the documentPlant improvement
View the documentReferences

Adaptation to ecoclimatic conditions

In all cases, the planner must match site characteristics with plant ecological requirements. In many cases, the selection of revegetation sites is imposed by local conditions in response to a pressing need for protection. The only possibility then left to the project planner is to select plants that are able to meet the ecological requirements of the site while having a growth rate rapid enough to enable the project to fulfill its role in a reasonable period of time. Some ecologically well-adapted plant materials may have an intrinsic growth or expansion rate that is too slow to achieve any practical result in a reasonable period of time; the reproductive rate may be insufficient or unknown, or in many cases, seeds may not be available in sufficient quantity. Hence, the introduction of a faster growing species may be preferred. In the Mediterranean Basin and in Africa, Australian or American shrubs and trees are commonly used instead of native species.

Latitude, Day Length, Photoperiodism

Some species may have a wide area of distribution, such as:

Atriplex

20° Lat. N. to 45° Lat. N.

Atriplex halimus

25° Lat. N. to 55° Lat. N.

Eucalyptus camaldulensis

15° Lat. S. to 45° Lat. S.

Cenchrus ciliaris

35° Lat. N. to 35° Lat. S.

Cynodon dactylon

50° Lat. N. to 40° Lat. S.

Each degree in latitude results in a difference in day length of 12 minutes at the time of the solstice. Thus, the day-length requirements between the extreme northerly and southerly populations of Atriplex canescens or A. halimus may differ by as much as five hours, making southerly populations unfit for the northern part of the geographic area of distribution and vice versa. For this reason, most species of Atriplex do not produce flowers under intertropical latitudes, even though they can still thrive vegetatively under these conditions. Therefore, in photoperiodically sensitive species, care must be taken to address latitudinal compatibility between the zone or origin of the plant material and the site to be revegetated.

Rainfall and Rain-Use Efficiency

The amount, distribution, and variability of precipitation must be as similar as possible from the sites of origin to the revegetation site. For example, plant material from areas with a predominantly summer rainfall regime should not be expected to perform well in sites with a winter rainfall regime. Within a given rainfall regime, it is usually safer to take plants from dry areas to more humid sites, rather than from humid to dry sites. There are some exceptions, however: Acacia saligna, for example, thrives on sand dunes in southwestern Australia under a Mediterranean climatic regime with rainfall ranging from 700 to 1,200 mm per annum; yet, in the Mediterranean Basin, it grows successfully when annual rainfall is as low as 200-300 mm. Similar cases are known with some Eucalyptus species. Such situations, however, are the exception rather than the rule. Rain-use efficiency (kg of dry matter per ha per year per mm) may be a good general indicator for evaluating environmental suitability (Le Houu, 1984).

Temperature

Tolerance of low or high temperature is usually a limiting factor to plant growth. The easiest criterion to use for assessing tolerance of temperature conditions in a first approximation of potential adaptability to a site is to compare mean minimum and absolute temperatures of the coldest month on the one hand and the mean maximum and absolute maximum temperature of the hottest month on the other. The monthly number of freezing days or the number of hours or days below or above given thresholds may be useful comparisons for some tangential or marginal cases. A certain margin of security must be included, particularly in cases of rough topography, since there may be rather large differences in temperatures over short distances. Temperature inversions may occur in any given elevation according to local topographical conditions; for example, valleys may be colder in winter and hotter in summer than the surrounding slopes above them.

Evaporation, Evapotranspiration, and Water Budget

The site water budget is perhaps the most important single ecoclimatic parameter to be considered in any revegetation project. Water budget must be considered in a space and time perspective with due consideration for topographic, hydrological, and edaphic (soil) factors.

The regional water budget can be schematically shown comparing mean monthly rainfall and mean monthly potential evapotranspiration. When potential evapotranspiration is not known and cannot be easily calculated, a good substitute is the ombrothermic diagram (Bagnouls and Gaussen, 1953), which shows mean monthly rainfall expressed in millimeters on a scale double that of mean monthly temperatures expressed in degrees Celsius (see figure 8-1). The dry season is defined by P<2t, where "P" is mean monthly rainfall in mm and "t" is mean monthly temperature in °C. It has been shown that the rainy/dry season threshold P< 2t is very close to P < 0.35 PET, where PET is mean monthly evapotranspiration using Penman's or Turc's method of calculation (Le Houu, and Popov, 1981).

The distribution of the P/PET ratio over the year shows the average length of dry and rainy seasons. In combination with mean monthly minimum temperature distribution, this allows for the evaluation of the length of growing season, the latter being defined by (1) a positive water balance (including water reserves in the soil), and (2) a mean monthly temperature above 10°C (50°F) or a mean monthly minimum temperature above 5°C (42°F).


FIGURE 8-1 Typical ombrothermic diagram.

It may be useful or necessary at times to assess the probability of drought. In a first approximation, this may be evaluated from the coefficient of variation of monthly and annual rainfall. A good indicator of drought probability is the coefficient of variation (c.v.) of annual rainfall (standard deviation over mean). In the Mediterranean Basin, for instance, a c.v. of annual rainfall equal to 25-30 percent would indicate semiarid conditions with a drought probability of 20-25 percent; a c.v. of 50 percent, arid conditions with a drought probability of 40-60 percent; and a c.v. of 100 percent or above, desert conditions with drought probability of 80 percent or above (table 8-1, and Le Houu, and Popov, 1981).


TABLE 8-1 Relationship Between Mean Annual Precipitation and Variability/Probability of Annual Rains in Arid Zones

TABLE 8-1 Relationship Between Mean Annual Precipitation and Variability/Probability of Annual Rains in Arid Zones

P (mm)

Africa North of the Sahara

West African Sahel

South Texas USA

Northeastern Brazil


Coefficient of Variation of P (percentage)

10-100

80-200+

50-00+



100-200

60-100

40-60



200-300

40-70

30-46



300-400

56-55

26-36



400-600

30-50

20-30

30-40

30-50

600-800

20-35

16-25

26-36

26-66

800-1000

20-26

16-20

20-30

30-60

1,000-1,500

16-20

12-20



Mean annual precipitation = P
CVP = Standard deviation x 100/P
SOURCE: Le Houu and Norwine, 1985

However, the local water budget at the site level may be different from the regional bioclimatic water budget because of physiographic and soil factors variously inducing infiltration or runoff. Runoff zones are obviously drier than the regional average and therefore more drought-prone, whereas zones with satisfactory infiltration may exhibit only temporary water deficits, or no deficit at all, because of the presence of a temporary or permanent groundwater table within reach of the surface vegetation.

The water budget at a site may also be assessed in a nonquantitative manner using the natural vegetation: for example, the presence of xerophytes, mesophytes, hygrophytes, phreatophytes, and other vegetation indicators.