The change of air pressure in northwestern Mexico and the southwest US is considered the major cause of seasonal variation. The mountain has a mild climate with summer temperatures maintained at constant levels. Precipitation also varies with the annual differences associated with El Nino -Southern Oscillation. The Sierra Madre Occidental creates different sets of condition that are favorable for a wide variety of flora and fauna.
The pine-oak forests found at an elevation of approximately 4, to 9, feet throughout the range has high biodiversity and endemism. The dominant plant species vary across the range, varying from scattered woodland to coniferous and deciduous forests.
The belt of tin deposits lies mostly within the eastern slopes from Durango to Guanajuato. In these areas, tin occurs in rhyolitic ignimbrites in many, possibly over , small deposits of cassiterite. Typically these deposits are divided from overlying ignimbrites by breccia, which may also contain ore. Many of these are thought to be hydrothermal deposits To the east of these units is a belt of more mercury deposits, followed by another belt of manganese deposits, although these deposits are mostly limited to Chihuahua and Hidalgo.
Iron occurs in three different regions placed by different mechanisms. Along the Pacific there the numerous skarn and replacement deposits mentioned above with relation to gold. Iron occurs in Durango in the form of magnetite lava flows, which are often surrounded by smaller hematite deposits, thought to be ash flows.
There is also a belt of iron deposits further east. Most soil in the mountains is covered by an organic -rich layer over layers rich in clay. Soils are commonly phaeozems that are up to centimetres 39 in. Some soils are cambisols that are less than 2 metres 6. Due to the high clay content, which blocks water from being absorbed into the soil, larger rock particles in the soil help reduce runoff and soil losses from erosion. Areas that are covered in stone are known as lithosols, and make up most of the remaining surface in the mountains, especially on upper slopes.
The last major soil type are vertisols, which are thin mineral-poor soils. Soils at higher elevations experience more leaching, due to increased rainfall. Acidic soils may have lower calcium content, leading to lower plant soil quality. Soils along the eastern slopes have increased organic content and a clay heavy layer that is developed more than those of the more arid eastern lands.
The thickness of soil layers also increases. Problems with soil degradation have been aggravated by the cultivation of drug crops, leading to deforestation of many areas. Other problems include overgrazing , which has led to terracing from cattle paths and the formation of a soil crust, and soil acidification, which poses a risk to some grasslands as of The climate varies considerably between the northern and southern extents of this long mountain range. Seasonal variations also occur due to the presence of large bodies of warm water delivering warm damp air from each side of the range.
This region undergoes a seasonal variety with two wet seasons, including a summer monsoon , and two dry seasons each year. Air pressure changes in northwestern Mexico and the southwest United States are the main causes of seasonal variation in the Sierra Madre. A high pressure area that rests over the mountains in the winter begins to move north of the mountains into New Mexico during the month of June.
The high pressure begins to break down and move south during September or October; however, the breakdown is usually at a slower pace than the movement north. This high pressure belt is associated with the mechanics that form the Bermuda High. The climate in the mountains is mild. Summer temperatures maintain a constant level. Maximum temperatures are typically in July at around 31 C with average temperatures of the order of 16 C during this same period.
Below-freezing temperatures can occur in the higher mountains, with precipitation occurring as snowfall certain times of year. During the summer monsoon, wind patterns undergo large-scale changes. In May there is very little large-scale wind current through the mountains, most circulates around the mountains, although onshore flow is present. As the high pressure moves north, air is pulled off the Gulf of Mexico, bringing easterly winds aloft to the mountains.
On the east slope of the mountain, daytime upslope winds flow to the crest, where the flow joins westerly winds.
The mountains cause convection during the following monsoon season, during which nocturnal winds at low levels in the atmosphere move moisture overnight.
After the high pressure breaks down, wind patterns return to those prior to the monsoon season. Precipitation varies on both annual and seasonal scales in the Sierra Madre. Monsoon rains come to the Sierra Madre in June as the high pressure area moves north, leading to wet summer seasons.
This causes east winds bringing moisture from the Gulf of Mexico. On the western side of the mountains, which are affected by the largest change in precipitation between the dry season and monsoon season, in the monsoon season rainfall can exceed mm 12 in in a single month. The southern region receives more of the rainfall than the northern. Much of this precipitation occurs as tropical storms. As the high pressure over New Mexico breaks down, rainfall in the Sierra Madre ends. Because of the monsoon, the summer accounts for the majority of rainfall in the area.
The spring and fall dry season separate out a weaker wet season in the winter. In addition to increasing in the southern ranges of the mountains, rainfall increases in the higher elevations of the Sierra Madre Occidental.
Humidity in the mountains varies at wide levels as does rainfall. Lower in the range, air is not as near saturated in the morning. Also during the day, relative humidity drops and cloud levels rise as temperatures rise. The humidity of the range is also influenced by the same seasonal changes as rainfall. The range allows higher dew points throughout these periods than the surrounding low lands.
This rise in humidity accompanies the monsoon. The mountains create a diverse setting for plants and animals by creating a different set of conditions from the surroundings. The Sierra Madre Occidental pine-oak forests are found at elevations of 1,—3, m 4,—9, ft throughout the range, but the forests in the mountains vary by elevation and place in the range.
This ecoregion is noted for its high biodiversity and large number of endemic species, and for allowing some species to extend their range.
The dominant plants vary across the range, and the habitat that they present varies from scattered woodland to both deciduous and coniferous forest. The elevation and latitude in the range are the major determining factors in the dominant plants. Oaks dominate the lower reaches of the mountains, where stands grow down to about 1, m 3, ft. Some oak species even continue into the surrounding ecosystems. In the western slopes of the range, cloud forest also occurs, mostly on mid-elevation slopes with little exposure to wind or sun in the southern end of the range.
At higher elevations pines begin to grow alongside the oaks, and pines begin to dominate the oaks at higher elevations. At high elevations and in the north, mixed conifer forests become the dominant ecosystem. These forests are composed primarily of pines and firs , which begin to grow at these elevations. There are also grasslands on some mountain tops, surrounded by the forest that occur in those areas.
Plants in the area typically have large responses to the change in climate with each season. Monsoon season is the largest of these effects, with leaf buds following only a few months after the rains begin. Flowering season occurs in the months preceding the summer monsoon. This is only the peak flowering season, with plants flowering throughout the year. Due to the early flowering, fruit is ripe and seeds are dispersed at the beginning of the rains. Much of the range is covered by the Sierra Madre Occidental pine-oak forests ecoregion, which covers an area of about , square km 86, sq.
This ecoregion is an amalgamation of different ecosystems. At lower elevations, it is dominated by very hot, dry deserts and xeric shrublands as well as tropical dry forests. The Sonoran Desert is located to the northwest of the range and extends partly into its lowest and westernmost regions. Meanwhile, the Chihuahuan desert is located to the northeast of the range, and covers part of its lowlands.
The mid elevations of the peaks in the Sierra Madre Occidental are home to a mix of oak wooded grasslands and pine-oak forests. There are dozens of species of conifers and deciduous trees in the range, some of which are endemic to the region.
Additionally, the range contains over bird species, some of which are endangered, such as the thick-billed parrot and the military macaw. As far as mammals go, black bears and jaguars are found in the range. Moreover, both Mexican wolves and Mexican grizzly bears were once found in the Sierra Madre Occidental, though they are now believed to be extinct.
Like much of what is now Mexico , the region in and around the Sierra Madre Occidental has been home to humans for thousands of years. Major groups in the region include the Tarahumara, Huichol, and Tepehuanes, among countless others. Perhaps one of the first Europeans to arrive in the region was Francisco de Ibarra,who traveled along the western coast of Mexico and crossed the Sierra Madre in the s and s.
The range extends from northern Sonora state near the Mexico-U. The high plateau that is formed by the range is cut by deep river valleys. This plateau is formed from volcanic rock overlying a basement of metamorphic rock. This uplift has caused changes in weather patterns; increased rainfall occurring in the mountains has provided areas where ecosystems can form in wetter areas than surrounding land.
This water source forms watersheds that provide the arid surroundings with water that makes it possible to irrigate and farm crops. The wet ecosystems are islands of biodiversity, differing significantly from what would otherwise be a desert landscape. Oak forests are the predominant plant life, and extend into the lowland deserts. Mexico's Gulf Coastal Plain lies to the east of the range, between the mountains and the Gulf of Mexico coast.
The Mexican Plateau , which averages 1, m 3, ft in elevation, lies between the Sierra Madre Oriental and the Sierra Madre Occidental further west. The climate of the Sierra Madre Oriental is drier than the rainforest areas further south. This long range of tall mountains is noted for its abundant biodiversity and a large number of endemic species of plants and wildlife, from the dry north to the wetter south. The Sierra Madre Oriental pine-oak forests are found at high elevations in the range 1, — 3, m or 3, — 11, ft above sea level.
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