Patterns of Riparian Tree Physiology and Growth During Dry and Wet Years

EPA Grant Number: U915220
Title: Patterns of Riparian Tree Physiology and Growth During Dry and Wet Years
Investigators: Horton, Jonathan L.
Institution: Northern Arizona University
EPA Project Officer: Hahn, Intaek
Project Period: September 1, 1997 through September 1, 2000
Project Amount: $102,000
RFA: STAR Graduate Fellowships (1997) RFA Text |  Recipients Lists
Research Category: Academic Fellowships , Ecological Indicators/Assessment/Restoration , Fellowship - Ecology and Ecosystems


The objective of this research project is to understand the responses to inter-annual fluctuations in water availability caused by the El Niño Southern Oscillation (ENSO) in two native riparian species, Populus fremontii, and Salix gooddingii, and one invasive exotic species, Tamarix chinensis. Specifically, I will address the following questions:

1. Do water sources used by riparian trees differ between dry and wet years along a gradient of groundwater availability?

2. Do differences in water sources affect tree physiological condition and growth?

3. What groundwater depths are necessary to maintain healthy native riparian trees maintained?


I am conducting research at the Hassayampa River, an unregulated river in central Arizona. In the spring of 1997, seven transects were established along a gradient of depth to groundwater. At each transect, two to four groundwater monitoring wells were installed. Depth to groundwater and soil water content were monitored at each transect for two growing seasons (1997, and 1998). Leaf -gas exchange, leaf delta 13C, water potential, canopy condition, branch elongation, and stem radial growth were measured on mature individuals of P. fremontii, S. gooddingii, and T. chinensis. Ground, stream, soil, and xylem waters were collected for stable isotope analysis in order to determine water sources used by riparian trees. Xylem water -isotope ratios will be compared to those of possible water sources in the ecosystem to assess the water sources used by each species. The relative proportion of soil, stream, and/or groundwater being used by each species will be estimated by using scatter plots and linear mixing models. A three-way analysis of variance will be usedconducted to compare leaf physiological characteristics, xylem isotopic ratios, tree growth, and crown condition among sample locations, species, and dry and wet years (1997 a dry year and 1998, a wet yearrespectively). Relationships between these characteristics and depth to groundwater will be assessed withusing scatter plots, regression, and correlation techniques.

Supplemental Keywords:

fellowship, riparian ecology, groundwater availability, Arizona, AZ, water stress, stable isotopes, Populus fremontii, Salix gooddingii, Tamarix chinensis., RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Monitoring/Modeling, Ecology and Ecosystems, environmental monitoring, canopy-level model, climate, forest ecosystem, riparian trees, invasive species

Progress and Final Reports:

  • 1998
  • 1999
  • Final