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Universidad Autónoma de Nuevo León (UANL) 2013

Costras biológicas del suelo en ecosistemas de pastizales del Altiplano del norte de México

Molina Guerra, Víctor Manuel

Titre : Costras biológicas del suelo en ecosistemas de pastizales del Altiplano del norte de México

Auteur : Molina Guerra, Víctor Manuel

Etablissement de soutenance : Universidad Autónoma de Nuevo León (UANL)

Grade : Doctorado en Ciencias con Especialidad en Manejo de Recursos Naturales 2013

Résumé
Semiarid grasslands in northeastern Mexico contain many endemic plants and animals and are an important refuge for resident and migratory animals. Here, as in many other semiarid areas, biological soil crusts are a key component of the ecosystem. However, findings about their effect on the germination of vascular plants are in contrast. In this study we determined the effect of seven dominant biological soil crusts (Psora cerebriforme, Placidium sp, Endocarpom pussillum, Nostoc commune, Xanthoparmelia chlorochroa, Psora decipiens, and Oxymitra sp.) from the area, on the germination percentage and germination rate of three native plant species : Frankenia gypsophila, Muhlenbergia arenicola, and Sartwellia mexicana. Germination tests were carried out in an environmental chamber at 26°C and at constant 60% moisture with 12 by 12 hours light and darkness. Biological soil crusts did not affect germination percentage of the three evaluated plant species when compared to bare soil. Germination rate ranged between 3.5 and 5 days and there were no differences between substrates. Results from this research suggested that, with high soil moisture, biological soil crusts do not have an effect on germination of vascular plants. In this thesis, I also assessed the diversity of biological soil crusts in three areas with different soil characteristics, and the relationship of the abundance of each species of crusts with some environmental factors (mean temperature, precipitation, altitude, vegetation cover, electric conductivity, gypsum, bulk density, pH and calcium carbonate) that can determine their presence. The study was conducted in Mexican Plateau grasslands within the Priority Terrestrial Region 80 "El Tokyo" where human activities are affecting vegetation and soil. There were seven species of lichens (Endocarpom pussillum, Placidium sp., Placidium lacinulatum, Psora cerebriforme, Psora crenata, Psora decipiens and Xanthoparmelia chlorochroa), a species of cyanobacteria (Nostoc commune) and liverwort (Oxymitra sp.). The species of biological soil crust that showed the highest cover was Endocarpon pussillum with 57.55% relative coverage, followed by Placidium sp. with 20.62% and Nostoc commune with 7.94%. The first two components of the redundancy analysis (RDA) explained 92.3% of the variance and the analysis allowed visualization of the association of each kind of crust with environmental variables. The RDA indicated that the presence of gypsum and calcium carbonate in the soil were the main variables affecting the distribution of biological soil crusts.

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