PHOENIX – The adjudication of water rights claimed by the U.S. Bureau of Land Management (BLM) for the San Pedro Riparian National Conservation Area (SPRNCA) continued Wednesday with more testimony from riparian ecologist Mark Dixon.
Dixon, now a professor of biology at the University of South Dakota, conducted two studies of the health of the SPRNCA in 2016 and 2017 of some 300 cottonwood and willow trees and other riparian vegetation. During his site visits, Dixon testified, he did find indications of possible water-related stress on some of the trees. These signs included dead branches, tips of branches dying off, yellowing leaves and smaller-than-normal leaf sizes.
He told U.S. attorney Dave Gehlert and the court how important the depth to groundwater is to the cottonwoods, which can send roots down nearly 10 feet deep. Willows’ root systems reach down a little more than three feet. To sustain the galleries of cottonwoods and willows along the San Pedro River, Dixon suggested it was wise to provide a claim allowing a safe margin to ensure their health.
Both species need an open, moist environment to germinate seeds and grow from seedlings to saplings, he added. Open areas are often scoured by winter floods, which can provide the flat, damp environment for the seeds when disbursed.
He also agreed with the warning of the National Riparian Service Team (NRST), hired back in 2012 by the BLM to assess the health of the riparian area. These specialists in botany, birds, fish, hydrology, geology and ecological balance, stated in their report that when evidence of severe decline begins to show, it is probably too late to do much about it.
However, when Dixon was cross-examined by Freeport McMoRan Inc. attorney Sean Hood, some irregularities in the science began to show.
Dixon said he assessed trees which looked stressed, but did not investigate if the trees had been infected by insects or if there was some sort of damage to the root system caused by other mechanisms. He did not inspect other trees within the same canopy. He made judgment calls on the yellowing and smaller leaves and did not use a scientific measure to determine his findings.
Hood also asked about the effect on stream flow due to evapotranspiration of the trees and vegetation. Evapotranspiration is defined by Webster’s as “the process by which water is transferred from the land to the atmosphere by evaporation from the soil and other surfaces and by transpiration from plants.” Hood asked Dixon if more trees meant more evapotranspiration and Dixon said it did, though Dixon could not say definitively the trees drawing water from the ground would affect the river flow.
A communication revealed by Hood from Gehlert to Dixon indicated the subject of the BLM using retired agricultural wells to assist river flow should be “ignored.”
Dixon said, “I can’t make a determination on that.”
Hood also referred to the NRST and the members completed. The team determined all but one of the reaches surveyed along the 40-mile river and uplands were in proper functioning condition, with the exception of the St. David reach. It was noted as functioning at risk.
Dixon noted the St. David reach was deemed to be a Class 3 reach, meaning it had healthy tree stands, although they were the invasive, non-native tamarisk, also known as salt cedars. Tamarisk can survive in low-water or drought conditions, while cottonwoods and willows cannot.
Hood also asked if a decline in trees could be caused by climate change and increasing drought which cycles every 7 to 10 years rather than by groundwater pumping. Dixon said, “No.”
Dixon was asked what role disease could play in forest decline and said he did not know.
Trial testimony resumes Thursday morning, before the sides are given a break Friday through Sunday.