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Centro de Investigación Científica y de Educación Superior de Ensenada Baja California (CICESE) 2017

Biophysical controls of ecosystem fluxes of carbon in a semiarid Mediterranean shrubland

Cueva Rodríguez, A. H.

Titre : Biophysical controls of ecosystem fluxes of carbon in a semiarid Mediterranean shrubland

Controles biofísicos de flujos de carbono a nivel ecosistema en un matorral semiárido con clima mediterraneo

Auteur : Cueva Rodríguez, A. H.

Université de soutenance : Centro de Investigación Científica y de Educación Superior de Ensenada Baja California (CICESE)

Grade : Doctorado en Ciencias. 2017

Résumé
It has been recognized that arid and semiarid ecosystems might play a pivotal role in the global carbon cycle. Nonetheless, there are still challenges to the understanding of the temporal and spatial variability of ecosystem scale carbon fluxes, that goes from standard procedures to perform plot-scale measurements, to the parameterization of empirical- and process-based models. Thus, enhancing the knowledge of the response of arid and semiarid ecosystems to environmental forces will improve our understanding on how these ecosystems could feedback the Earth system. Thus, the main aim of this thesis was to understand the temporal and spatial variability of the principal ecosystem carbon fluxes in a semiarid shrubland with a Mediterranean climate. To address the overreaching objective of this research, a set of micro-meteorological, edaphological, and near-surface remote sensing techniques was employed. I explored how two abnormal years in terms of precipitation, one that was excessively humid, and another extremely dry, influenced the physical controls of the net ecosystem exchange of CO2 (NEE) and the strength and duration of the ecosystem carbon sink. My results suggest that the physical controls of NEE changed when water is not a limiting factor, as an excess of water availability within the ecosystem can extend and enhance the ecosystem carbon sink. In addition, I developed a semi-empirical model to estimate daily gross primary production (GPP) that uses meteorological data and a vegetation index derived from consumer-grade digital cameras as inputs. Daily estimates of this model were comparable with the estimation of GPP derived from the eddy covariance technique, and these estimations improved when including a senescence parameter of foliage, especially in late-summer and autumn. Furthermore, I tested for the effect of temporal discrepancies in spatial surveys of soil respiration (Rs), in a 50x 100 m plot. These results showed that Rs does not change spatially, providing support for temporal representation of Rs based on plot-scale measurements ; however, its biophysical controlling factors changed depending on the sequence of measurements. Finally, the potential biases due to temporal sampling in 24 hours cycles in soil respiration were tested. It was found that customary and convenient morning hours could overestimate Rs, while during nighttime underestimations could occur ; thus, it was proposed a simple correction factor to take into account this pot

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