Date of Award

2026-05-01

Degree Name

Doctor of Philosophy

Department

Environmental Sciences

Advisor(s)

Vanessa Lougheed

Abstract

Ephemeral stream networks, locally known as arroyos, are among the most ecologically significant yet least protected habitats in arid landscapes. Despite supporting a disproportionately high plant biodiversity compared to surrounding uplands, these riparian systems lack regulatory protection in most arid U.S. states and face increasing pressure from urban expansion and intensifying climate change. There is an urgent need to understand how these factors influence riparian plant communities and hinder their ability to recover, enabling effective conservation and management strategies. In this dissertation, I explored the interplay of biogeographic gradients, precipitation variability, and urbanization in shaping arroyo riparian plant communities across the El Paso-Ciudad Juárez region of the northern Chihuahuan Desert. I combined field-based vegetation community surveys, monitored plant functional groups, and utilized 24 years of satellite remote sensing data examine how environmental filters structure community composition across a land-use gradient, how monsoon precipitation drives functional group resilience under dry and recovery conditions, and how urbanization alters the long-term structural and hydrological trajectories of riparian shrub communities at landscapes scales. The first research-focused chapter (Chapter 2) examined the key drivers of plant community structure across natural and urban arroyos spanning a strong east-west environmental gradient. I documented 103 vascular plant species across four streams and found that biogeographic region, shaped by elevation, precipitation, and soil parent material, was the dominant factor driving plant species presence in the community, explaining 63.2% of variation compared to only 8.1% for urbanization. ANOSIM confirmed near-complete species turnover between eastern and western communities in both annual and perennial assemblages, with SIMPER dissimilarities 83.3% and 81.4%, respectively. Urbanization produced a weaker but significant secondary signal, most notably driving a decline in species richness at the most urbanized stream over just two years. The second research-focused chapter (Chapter 3) delves into the role of precipitation in shaping plant cover dynamics and determining the capacity of various plant functional groups to recover and maintain their cover after a long-term drought. I conducted a three-year study (2020-2022) in two contrasting natural arroyos, monitoring six different plant functional groups. I observed that monsoon precipitation significantly influenced all functional groups, but its effectiveness varied depending on the timing of precipitation relative to the plants' response. Eastern stream perennial communities responded to a longer window of antecedent precipitation (ranging from 30 to 90 days) and demonstrated strong recovery and positive maintenance capacity. In contrast, all functional groups in the western stream responded within 7 to 14 days of precipitation events and failed to sustain post-dry season recovery, regardless of their life-history classification. This study revealed a strong resistance-recovery trade-off in the eastern stream, which was entirely absent in the western stream. This suggests that precipitation alone does not determine resilience; rather, its interaction with site conditions determines whether rainfall translates into vegetation cover. The third research-focused chapter (Chapter 4) extended this analysis to landscape scales, using a 24-year Landsat time series (2000-2023) to track shrub community dynamics across three nested spatial scales, Riparian, Site, and Catchment, in natural and urban ephemeral streams. I found that Normalized Difference Vegetation Index (NDVI), a spectral proxy for above-ground green canopy biomass, was consistently higher at natural sites than urban sites at Site and Catchment scales, indicating that natural shrub communities produce greater greenness per unit of rainfall than their urban counterparts. Normalized Difference Infrared Index (NDII), a proxy for canopy water content and subsurface soil moisture status, remained precipitation responsive across both land-use types but was dissociated from shrub cover in urban sites, where elevated moisture signals reflected surface runoff rather than plant-available water. This dissertation highlights the importance of examining riparian plant communities across multiple scales, from individual reaches to watershed networks, and from seasonal dynamics to multi-decade trajectories, to fully understand how urbanization and climate variability interact to erode their structure and function. It also demonstrates how combining field surveys, plant functional response analysis, and remote sensing can reveal patterns of riparian community degradation and indicates an urgent need to regulatory reform and protection and conservation of ephemeral stream networks in urbanizing desert landscapes

Language

en

Provenance

Received from ProQuest

File Size

164 p.

File Format

application/pdf

Rights Holder

Luis Miranda

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