Riparian Zones are considered biodiversity and ecosystem services hotspots. In arid environments, these ecosystems represent key habitats, since water availability makes them unique in terms of fauna, flora and ecological processes. Simple yet powerful remote sensing techniques were used to assess how spatial and temporal land cover dynamics, and water depth reflect distribution of key land cover types in riparian areas. Our study area includes the San Miguel and Zanjon rivers in Northwest Mexico. We used a supervised classification and regression tree (CART) algorithm to produce thematic classifications (with accuracies higher than 78%) for 1993, 2002 and 2011 using Landsat TM scenes. Our results suggest a decline in agriculture (32.5% area decrease) and cultivated grasslands (21.1% area decrease) from 1993 to 2011 in the study area. We found constant fluctuation between adjacent land cover classes and riparian habitat. We also found that water depth restricts Riparian Vegetation distribution but not agricultural lands or induced grasslands. Using remote sensing combined with spatial analysis, we were able to reach a better understanding of how riparian habitats are being modified in arid environments and how they have changed through time.

Transformation or modification of vegetation distribution and structure in arid riparian ecosystems can lead to the loss of ecological function. Mexico has 101,500,000 ha of arid lands, however there is a general lack of information regarding how arid riparian ecosystems are being modified. To assess these modifications, we use eight sites in the San Miguel River (central Sonora) to analyze (1) riparian vegetation composition, structure and distribution using field sampling and remote sensing data from Unmanned Aerial Vehicles (UAV); (2) productivity (proxies), using vegetation indices derived from satellite data; and (3) variability posed by riparian vegetation and vegetation adjacent to riparian habitats. The development of a simple yet informative Anthropogenic-disturbance Index (ADI) allowed us to classify and describe each study site. We found sharp differences in vegetation composition and structure between sites due to the absence/presence of obligate-riparian species. We also report significant difference between EVI (Enhanced Vegetation Index) values for the dry season among vegetation types that develop near the edges of the river but differ in composition, suggesting that land cover changes form obligate-riparian to facultative-riparian species can lead to a loss in potential productivity. Finally, our tests suggest that sites with higher disturbance present lower photosynthetic activity.

We analyze the importance of riparian ecosystems (RE) as critical areas for carbon storage and productivity in semi-arid regions of Northwest Mexico. We calculated the carbon storage by land cover and compared temporal trends of basal productivity (MODIS) and pre-monsoon productivity (Landsat) of RE, to other land cover types. We used land cover maps generated previously for the region (years 1993, 2002, and 2011), assigning values of carbon stored in aerial and root biomass, as well as organic carbon stored in the soil. To estimate productivity (proxy), time series were generated using the Normalized Difference Vegetation Index (NDVI) values of Landsat 4-5 TM and MODIS for each land cover type. We found that RE stores 93,147 tC/ha, about 1.5 times the estimated storage for oak forest (65,048 tC/ha). Productivity of RE was similar to highly productive land cover types, such as agriculture and oak forest, and higher than in the rest of the ecosystems of the region. We also found that changes from RE to agriculture and cultivated grasslands represented a decrease in productivity (p < 0.001). Finally, we report a gradual decrease in basal productivity (p = 0.0151) and pre-monsoon productivity (p = 0.031) in the RE. These results help us understand that changes in land use, intensive use of water, and climate can influence the ecosystem services of productivity and carbon storage offered by RE in semi-arid areas.

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