Remotely Sensed Changes in Vegetation Cover Distribution and Groundwater along the Lower Gila River

Kyle Hartfield,Willem J.D Van Leeuwen,Jeffrey K Gillan

doi:10.3390/land9090326

Abstract

Introduced as a soil erosion deterrent, salt cedar has become a menace along riverbeds in the desert southwest. Salt cedar replaces native species, permanently altering the structure, composition, function, and natural processes of the landscape. Remote sensing technologies have the potential to monitor the level of invasion and its impacts on ecosystem services. In this research, we developed a species map by segmenting and classifying various species along a stretch of the Lower Gila River. We calculated metrics from high-resolution multispectral imagery and light detection and ranging (LiDAR) data to identify salt cedar, mesquite, and creosote. Analysts derived training and validation information from drone-acquired orthophotos to achieve an overall accuracy of 94%. It is clear from the results that salt cedar completely dominates the study area with small numbers of mesquite and creosote present. We also show that vegetation has declined in the study area over the last 25 years. We discuss how water usage may be influencing the plant health and biodiversity in the region. An examination of ground well, stream gauge, and Gravity Recovery and Climate Experiment (GRACE) groundwater storage data indicates a decline in water levels near the study area over the last 25 years.

Highlights

Salt cedar (Tamarix chinensis) is invading riparian corridors in the semiarid southwest at an alarming rate, affecting ecosystem processes and services such as water availability, carbon sequestration, and nutrient cycling
Coverage of salt cedar increased from about 4000 ha in the 1920s to more than 600,000 ha in 1998 [1]. It is unclear how much area salt cedar currently occupies in the southwestern United States, but it is clear that the species is present within most areas suitable to its growth [13]
Salt cedar continues to expand along rivers in the western United States at a rate of around 20–25 km per year [13,14]

Summary

Introduction

Salt cedar (Tamarix chinensis) is invading riparian corridors in the semiarid southwest at an alarming rate, affecting ecosystem processes and services such as water availability, carbon sequestration, and nutrient cycling. The aggressive spread of salt cedar is decreasing resources for native cottonwood and willow species, ideal habitats for native wildlife [4,5]. Some species, such as the southwestern willow flycatcher, have adapted to the salt cedar influx and the changes aiding its expansion [6,7]. Introduced, salt cedar spreads aggressively along the flood plains of rivers in the southwestern United States [8–12]. Coverage of salt cedar increased from about 4000 ha in the 1920s to more than 600,000 ha in 1998 [1] It is unclear how much area salt cedar currently occupies in the southwestern United States, but it is clear that the species is present within most areas suitable to its growth [13]. Salt cedar continues to expand along rivers in the western United States at a rate of around 20–25 km per year [13,14]

Methods

Results

Discussion

Conclusion