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Accueil du site → Doctorat → Mexique → Costras biológicas del suelo en ecosistemas semiáridos : composición, rendimiento fisiológico y efecto en la germinación de plantas

Universidad Autónoma de Nuevo León (2014)

Costras biológicas del suelo en ecosistemas semiáridos : composición, rendimiento fisiológico y efecto en la germinación de plantas

Mendoza Aguilar, Dinorah Ofelia

Titre : Costras biológicas del suelo en ecosistemas semiáridos : composición, rendimiento fisiológico y efecto en la germinación de plantas.

Auteur : Mendoza Aguilar, Dinorah Ofelia

Université de soutenance : Universidad Autónoma de Nuevo León

Grade : Doctor en Ciencias con especialidad en Manejo de Recursos Naturales UANL, 2014

Résumé
The biological soil crusts (BSC) are nonvascular plants that interact in the topsoil and are usually found in areas with no or sparse vegetation. They act on key processes of ecosystem functioning from hyper-arid deserts to ice free areas in Antarctica and are formed of lichenized fungi, bryophytes including mosses and liverworts, algae and free-living cyanobacteria closely interacting with soil particles. This research started by identifying the components of the BSC of an halophytic grassland in the southern part of the Chihuahuan Desert in Mexico. Identification was supported on keys developed by several authors to identify genera and species which involve comparison of the morphological characteristics, reproductive structures and chemical tests of the organisms. In total, eleven species of scuamulose lichens belonging to the Collemataceae, Parmeleaceae, Psoraceae, Thelochistaceae and Verrucariaceae families were identified, two genera of liverworts from the group of the bryophites (Oximitra sp. and Riccia sp.) and two cyanobacteria (Scytonema sp. and Nostoc commune). Ten out of the eleven lichens have a green algae as photobiont and only one lichen has a cyanobacteria (Collema coccophorum). In this study we also evaluated whether some of the species of lichens of the area have higher photosynthetic capacity that could give them an advantage under a climate change scenario where light intensity would increase. Variations in solar radiation can alter climate, such variation includes changes in intensity and changes in the wave length that make the electromagnetic spectrum that could trigger climate changes. Photosynthetic capacity of the lichens was assessed through the chlorophyll fluorescence. Five lichens (Psora decipiens, Psora russellii, Psora crenata, Clavascidium lacinulatum and Endocarpon pusillum) all having green algae photobiont, were evaluated from dark to saturation light intensity. Under drought conditions, there was no photosynthetic activity in the studied lichens. Similar results have been reported from other studies. Upon hydration, the function of the photosystem II was reestablished. Values of photosynthetic activity were between 0.56 and 0.68 at 0.33 µE m-2 s-1 PPFD and declined as light intensity increased. ETRmax and PPFDsat cardinal points were calculated by a regression using ETR and PPFD data and determining the coordinates of the inflexion points which are needed to adequately compare values of electron transport rate (ETR) between lichen species. PPFDsat for the studied lichens was between 1699 µE m-2 s-1 and 2108 µE m-2 s-1. Saturation point for Endocarpon pusillum was the lowest at 1699.83 µE m-2 s-1, followed by P. decipiens with 1788.68 µE m-2 s-1 and P. russellii, P. crenata and Clavascidium lacinulatum in values of PPFD > 2000 µE m-2 s-1. Our results imply that Psora decipiens, Psora russellii, Psora crenata and Clavascidium lacinulatum would have more chances of surviving than E. pusillum under high solar radiation in a climate change scenario. Other aspect addressed in this study was the relationship between BSC composition and soil surface conditions (surface roughness and hydrophobicity) as well seed germination and seedling rooting in a S. tenacissima steppe in southeastern Spain. We explored the causal factors of seed germination at two spatial scales and used SADIE index to represents the soil surface heterogeneity. The complexity of the processes involved prevents generalizations on the outcomes of the interactions between BSC and vascular plants. This complexity is partly the result of heterogeneity in BSC cover and composition. After analyzing the BSC composition in open areas or soil under S. tenacissima tussocks where BSC are scarce compared to open areas with high BSC cover, we found that mosses cover was higher in open areas and lichens were more abundant in open areas with high BSC cover that in soil under S. tenacissima tussocks, which is contradictory to reports from other authors. As a result of differences in surface features, microsites also differed in physical soil properties. Thus, hydrophobicity was lower and roughness higher in open areas with scarce BSC than in the other two microsites. In this investigation germination was weakly affected by microsite but this had a strong effect on seedling rooting.

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