The plant associations of the phytogeographic province of Tehuacán-Cuicatlán Valley are described and classified according to the species dominance and physiognomy based on field samplings of 500 m2. The altitudinal distribution of the associations was determined conducting 250 m2 samplings located every 100 m altitude in throughout five environmental gradients. A total of 29 plant associations were recognized including nine associations dominated by columnar cacti, four types of deciduous forests, three types of Izotal, and six type of shrublands. Mountainous arboreal vegetation includes five types of forests, as well as a Mountainous lzotal. The vegetation associated to aquatic environments along rivers and springs include Galery forests and "Tulares". The pronounced heterogeneity of Tehuacan-Cuicatlán Valley determines the diversity and the distribution of the vegetation associations.
The evergreen-sclerophyllous vegetation associated to the mediterranean-type ecosystems shares common characteristics that have been explained invoking an evolutionary convergence driven by the mediterranean climate. Mediterranean climate originated in the Quaternary but the plant 'convergent' characteristics are also present in tropical-like lineages that evolved along the Tertiary, before the mediterranean climate appeared. Because evergreen-sclerophyllous vegetation was broadly distributed across the world in the Tertiary, current trait similarities among the mediterranean taxa may be due to historical and phylogenetical constraints and not to evolutionary convergence. We tested historical and phylogenetical vs. convergence hypotheses to explain present ecological attributes found in woody plant species in mediterranean areas. Multivariate analyses were performed on the matrix of genera ¥ life-history reproductive characteristics in three mediterranean-type ecosystems and a tropical system as an outgroup, the Mexical shrubland. These analyses indicate that character syndromes in mediterranean plants may largely be explained in relation to the age of the lineage (Tertiary vs. Quaternary). We also found that the similarities shown among mediterranean vegetations are due to Tertiary (pre-mediterranean-) and not to Quaternary (true mediterranean-) taxa. Furthermore, the similarities among mediterranean taxa are due to phylogenetical inertia because similarities in the character syndromes disappear when common genera are excluded from the analysis.
A comparative study between an unburned evergreen sclerophyllousvegetation located in south-central Mexico under a wet-summer climate,with mediterranean regions was conducted in order to re-analyzevegetation and plant characters claimed to converge under mediterraneanclimates. The comparison considered floristic composition,plant-community structure, and plant characters as adaptations tomediterranean climates and analyzed them by means of a correspondenceanalysis, considering a tropical spiny shrubland as the external group.We made a species register of the number of species that resproutedafter a fire occurred in 1995 and a distribution map of the evergreensclerophyllous vegetation in Mexico (mexical) under nonmediterraneanclimates.The Tehuacán mexical does not differ from the evergreensclerophyllous areas of Chile, California, Australia, and theMediterranean Basin, according to a correspondence analysis, whichordinated the Tehuacán mexical closer to the mediterranean areasthan to the external group.All the vegetation and floristic characteristics of the mexical, aswell as its distribution along the rain-shadowed mountain parts ofMexico, support its origin in the Madrean-Tethyan hypothesis of Axelrod.Therefore, these results allow to expand the convergence paradigm of thechaparral under an integrative view, in which a general trend to ariditymight explain floristic and adaptive patterns detected in theseenvironments.
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