Do Framont cottonwoods differentially experience drought stress across a small, restored reach of the Lower Truckee River?
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Authors
Villegas-Fregoso, Andrea
Issue Date
2023
Type
Thesis
Language
Keywords
Ecology , Hydrologic Sciences
Alternative Title
Abstract
River floodplains experience frequent flooding and deposition. Although flooding can be a disturbance to vegetation, it can also lead to nutrient-rich soils and abundant water making them hotspots for biodiversity and productivity. Consequently, floodplains provide ecosystem services by retaining the nutrients, sediment, and floodwaters brought on by flooding events. However, human activity can threaten floodplain conditions through river control structures, flow regulation, intensive use of land, and bank stabilization. Such threats have limited the establishment and vigor of Frémont cottonwoods, a pioneer tree species that often grows in riparian floodplains and are valued because they support biodiversity and stream stabilization. This study focuses on the Frémont cottonwoods located at Mustang along the Lower Truckee River, east of Reno, NV, where restoration efforts (led by The Nature Conservancy) began in 2009, seeding in 2010, irrigation and restoration work continued through 2012 until irrigation was removed in 2014 according to Chris Sega (personal communication, 2023) . During the 2022 growing period, cottonwood stand structure, ecophysiological processes, and hydropedological conditions were studied across 25 plots in the restored floodplain in order to improve basic understanding of hydrological controls over cottonwoods, with implications for future restoration design. Our findings did not suggest that a) collocated Frémont cottonwoods of different life stage (mature trees and saplings) experienced water stress differently, and b) access to subsurface water availability was the main driver of variation among Frémont cottonwoods there; this likely reflects that all trees within the study maintained substantial connectivity to groundwater, regardless of tree age or position within the floodplain. Instead, a combination of factors was associated with variations in tree-level and stand-level vigor such as floodplain position, and those variations were apparent across short distances of 10s-to-100s of meters. While evidence suggested that microsite differences in soil characteristics (bulk density, field capacity, gravel content, and soil moisture) may matter, especially with respect to controlling stand structure, there was no direct evidence of established trees exhibiting water stress over the study period. This study contributes to the overall knowledge of micro-scale variation in floodplain tree physiology and provides a key step in measurements and record-keeping for this restoration site. A multi-year study would be beneficial in tracking the geomorphological changes following extreme restoration efforts and changing weather conditions.