Included here are the mountain systems of Mexico including the southern slopes of the Sierra Madre del Sur, the northern parts of the Sierra Madre Occidentale and Sierra Madre Orientale and the altiplane of central Mexico.

Mexican Highlands (Tehuacán Valley) Matorral

This distinctive cactus and thorn-shrub vegetation extends from Sierra de Zongolica in the Sierra Madre Orientale in the north to Sierra de Juárez in the south and has been regarded as a floristic province in its own right. The flora has four endemic angiosperm genera (Gypsacanthus, Oaxacania, Pringleochloa and Solisia) and a multitude of endemic species - in the Asteraceae alone there are 50 endemic species. In addition 55% of all Mexican columnar cacti, 30% of which are endemic, occur here, and the area is considered to be a centre for the origin and diversification of the genus Agave. The vegetation has a maximum height of about 8 m but it is often much less than this. In fact, the structure can be quite varied, but it is only when plants are in full bloom that the tremendous variety can be appreciated. The trees are usually umbrella shaped with contorted branches and often have smooth boles or peeling bark typical of arid zones. Common trees include Acacia subangulata, Cassia pringlei and the endemic Mimosa luisana (Fabaceae). The endemic Morkillia mexicana (Zygophyllaceae) can also be particularly evident when bearing it large magenta flowers. Another distinctive species is the endemic Thouinidium insigne (Sapindaceae) with its inflated, winged clusters of fan fruit. But among the most unusual trees are the endemic Jatropha neopauciflora (Euphorbiaceae) with its thick branches, and the endemic Manihot pauciflora (Euphorbiaceae) with its Oxalis-like foliage. Rising above the trees are various columnar and candelabra cacti. One other most conspicuous of the latter is the endemic Escontria choitilla (Cactaceae) with its distinctive yellow flowers. The shrub layer includes compact forms, such as the endemic Bursera arida (Burseraceae) and others that are tortuously intertwined. Many have inconspicuous flowers, but others such as the endemic Ayenia fruticosa (Malvaceae) and Perymenium ovata (Asteraceae) are much more showy. Also at shrub heights are numerous rosettes of Agave plants such as the endemic Agave rubescens (Agavaceae). The herbaceous flora is also surprisingly varied with species such as the endemic Argemone mexicana and Sanvitalia fruticosa (Asteraceae), but another fascinating feature of this vegetation is the presence of the giant horsetail Equisetum giganteum, which can reach heights of 4 m. It is confined to moist areas and can often be found adjacent to streams. Among the many other endemic species of this unique community are Beaucarnea gracilis (Agavaceae), Echeveria subsessilis (Crassulaceae), Ferrocactus robustus (Cactaceae), Fouquieria purpusii (Fouquieriaceae), Grabowskia geniculata (Solonaceae), Jacquemontia smithii (Convolvulaceae), Lasiocarpus ovalifolius (Malpghiaceae), Neobuxbaumia tetetzo (Cactaceae), Pachycereus hollianus (Cactaceae) Pedilanthus cymbifera (Euphorbiaceae), Pterostemon mexicanus (Pterostemonaceae) and Yucca periculosa (Agavaceae).


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