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Relations of the Vegetation and Climatic Types in Nuevo Leon, Mexico

Cornelius H. Muller
The American Midland Naturalist
Vol. 21, No. 3 (May, 1939), pp. 687-729
DOI: 10.2307/2420526
Stable URL: http://www.jstor.org/stable/2420526
Page Count: 43
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Relations of the Vegetation and Climatic Types in Nuevo Leon, Mexico
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Abstract

The dependence of plant growth and the development of vegetation types upon climate has been shown by numerous writers. On the basis of this dependence, edaphic influences having been taken into account, it is assumed that vegetation types and climatic types of Nuevo Leon, Mexico, coincide in range. The variations of climatic factors with altitude, topography, etc. follow a set of laws and result in the occurrence in general of cooler and more humid climates at higher altitudes and on more abrupt slopes. This coincides with the occurrence of vegetation types in the Sierra Madre of Nuevo Leon. The lowland climates of Nuevo Leon are fairly well known. The climatic factors of the various vegetation types in the mountains above the existing meteorological observatories are extrapolated by application of the rules of climatic variation in mountains to the data collected by government observatories in the lowlands. Eight major vegetation types are recognized and on them are based seven climatic types. a. Central plateau desert scrub.-This vegetation type is one of widely spaced thorny shrubs with few herbs save in the rainy season. It occurs west of the Sierra Madre up to altitudes of 2,000 m. The climatic type is called warm and arid (BSkwg) and is characterized by mean annual temperatures ranging from 15⚬ to 20⚬ C. and mean annual precipitation ranging about from 360 to 485 mm., the higher figure occurring near the mountains. b. Eastern coastal plain scrub.-This type differs from the plateau type in having fewer thorny species, the shrubs more luxuriant and closely spaced, and a greater abundance of herbs (particularly grasses). It occupies the lowlands to windward (east) of the mountains and reaches an elevation of 500 m. This climatic type is called warm and semi-arid (BShw). It is characterized by mean annual temperatures ranging about from 21⚬ to 25⚬ C. The mean annual precipitation ranges roughly between 425 and 775 mm. The lower precipitation values occur in the north where the mountains are farther from the Gulf Coast. c. Piedmont scrub.-This shrub type incorporates some of the species of the coastal plain with additional nonthorny species many of which reach smalltree size and are more densely grouped. It occurs on the low rise which occupies the northwestern sector of the state and functions to the south as a narrow transition zone between the coastal plain and the lower slopes of the mountains. d. Montane low forest.-At altitudes of 750 to 1,000 m. on the east slopes of the Sierra Madre the piedmont scrub gives way to a low forest composed principally of Quercus, Juglans, Carya, and numerous shrubs. The trees are low, gnarled, and widely spaced so that herbaceous ground cover, especially of grasses, is abundant. This type persists usually to an altitude of 1,500 m. The piedmont scrub and the montane low forest have been combined in a single climatic type designated as mild and semi-arid (Cwan). This climate is characterized by mean annual temperatures ranging between 17.5⚬ and 22⚬ C., while its mean annual precipitation is estimated in the northern piedmont to be 500 to 1,000 mm.; northern lower mountain slopes, 550 to 1,200 mm.; southern lower mountain slopes, 700 to 1,200 mm. e. Montane mesic forest.-This forest consists of pines, Pseudotsuga, mesic oaks, and shade-tolerant shrubs densely spaced and developing tall, slender trunks on which sometimes large epiphytic bromeliads are abundant. The dense shade limits the growth of any but a few shade-tolerant herb species save in more open woodland or in clearings where the herbs become luxuriant. This is the first zone so far discussed in which the summer drought does not produce a cessation of herb growth. The altitude range is roughly 1,500 to 2,800 m., but the type is developed at the lower altitude only on the east slope of the mountains, while on the west slope it does not occur below 2,500 m. The climate of the area occupied by mesic forest is designated as cool and subhumid (Cwbn). Its mean annual temperature ranges between 9.5⚬ and 13⚬ C., while the mean annual rainfall approximates 1,200 to 1,800 mm. f. Western montane chaparral.-This vegetation type is suggestive of the California chaparral in the many evergreen species which comprise it. It is exceedingly heterogeneous but is constant in the shrub habit of its usually evergreen species and the density of its growth. Its altitudinal range is from 2,000 m. at the edge of the plateau desert up to the western or leeward slopes of the Sierra Madre to the 2,800 m. level. The climate of this area is designated as cool and semi-arid (BSk'w). Its mean annual temperature range is from 11⚬ to 16.5⚬ C. while its mean annual precipitation is calculated to range between 750 and 1,000 mm. g. Subalpine humid forest.-This forest type is developed over limited areas on the slopes of the higher mountains ranging between 2,500 and 3,600 m. altitude, though it seldom descends below 2,800 m. The forest is characteristically dominated by a single species, Pinus Montezumae, and has an exceedingly dense and luxuriant herb layer. Shrubs are sparse and few in numbers of species. The climate of this forest is termed cold and humid (Cwbn). The mean annual temperature ranges between 4.5⚬ and 13⚬ C., while the mean annual precipitation is calculated to be between 1,800 and 2,200 mm. h. Alpine meadow and timberline.-The alpine meadow consists of numerous low herbs, most of them rosette-forming, which densely cover the peak and slopes of the tallest mountains above an altitude of about 3,600 m. Its lower extremities alternate with matted, scrubby Pinus flexilis and Juniperus mexicana and stunted Pinus Montezumae which constitute the upper limit of tree growth. The climate of this zone is designated as alpine (ETn). The rainfall is the highest in the region. The obvious xeric character evidenced by the stunting of the trees is the result of the tremendous evaporating power of the freely moving, rarified atmosphere and the higher percentage of run-off of the peaks. The mean annual temperature is calculated to be about 3⚬ to 4.5⚬ C., while the mean annual precipitation is estimated at 2,250 mm. The area occupied by the new climatic types here proposed for this region comprises more than one-fourth the total area of the state of Nuevo Leon.

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