UCLA

Drylands cover more than 40% of the earth land surface and are home for more than 2 billion people. Despite the harsh environment and the extreme aridity conditions, drylands are among the most diverse ecosystems that contribute to more than 30 % of the global terrestrial net primary production. Assessing vegetation dynamics through remote sensing for dryland surfaces remain a challenge due to the appearance of soil and non-photosynthetic material that usually cause non-linear scattering of light. This aspect of remote sensing is investigated in this dissertation using mechanistic and unmixing remote sensing approaches to assess net primary productivity of the Chihuahuan Desert. The unmixing approach was demanded by the difficulties associated with assessing vegetation productivity with mechanistic remote sensing. We found that including soil and non-photosynthetic surface covers contribute to the predictions of NPP with Multiple Endmember Spectral Mixture Analysis, Random Forest and stepwise regression.

Erosion is considered one of the main complex drivers that contribute to vegetation cover change. The feedbacks between wind erosion and other environmental, and biological driver have not been fully studied. Wind erosion remains one of the most understudied drivers of shrub encroachment despite the evidence of the association of aeolian transport with grass cover decline and shrub encroachment. In this research, we introduce wind erosion processes as major disturbances to vegetation covers in drylands affecting the health and mortality of vegetation species. We investigated the damaging effects of wind transport on major vegetation community types in the Chihuahuan Desert using a linear wind tunnel. We quantified the damaging effects of sandblasting using plant mapping methods to emphasize leaf loss, color change, stem loss and plant height change. Our sandblasting experiment shows some similarities between the grasses and the shrubs in the response to sandblasting. However, the grasses were more sensitive to sandblasting than the shrubs due to their growing point and growth form. Our finding might explain shrub encroachment phenomena in drylands

Aeolian and fluvial processes are fundamental drivers of arid land dynamics because of their effects on soil surfaces and microtopographies. In this research we investigate the coupling effects of wind and water transport in erosion and deposition in ephemeral streams in Moab, UT covering dry and wet periods. We used structure from motion and drone technologies to survey stream over 16 months period to investigate the soil surface elevation change due to wind and water activities. We performed differencing analysis and quantified soil erosion and deposition volume for 5 survey periods. The streams show a significant net soil erosion and deposition over the survey periods which indicate direct interaction between aeolian and fluvial processes in development and changes of channel morphologies.