Even during periods of drought, Waller Creek on The University of Texas at Austin campus remains hydrated. This is due to a significant proportion of the water that flows through it — ranging from 25% to 50% — originating from wastewater leaking from city pipes.
However, new research has discovered a beneficial aspect to this flawed plumbing system. The water flowing through the creek nourishes the trees growing along its banks, enabling them to flourish during drought conditions that adversely affect trees growing alongside streams in more rural areas.
The study underscores how urbanization can lead to unexpected positive effects, alongside more widely recognized negative impacts such as pollution and increased levels of disease-causing bacteria. "Those negative effects are not canceled out," said Jay Banner, a professor in the Jackson School of Geosciences and the director of the Environmental Science Institute. "One has to weigh the unintended positive consequences with the expected and long-shown negative consequences."
The findings were published in the Nature Partner Journal Urban Sustainability.
Researchers compared the growth record of bald cypress trees along Waller Creek and Onion Creek, a rural stream approximately 12 miles from the UT campus, with the region's drought record.
A tree's growth is documented in its wood's tree rings, with thicker rings indicating wetter periods and robust growth and thinner rings reflecting drier times and minimal growth. Banner and his students extracted this record from living trees by taking cores — narrow rods of wood pulled from tree trunks — which were then sent to the Laboratory of Tree-Ring Research at The University of Arizona for analysis.
The oldest trees from Waller Creek date back to 1933, while those from Onion Creek go back as far as 1844. The cores revealed that since their inception, trees along Waller Creek have been protected from drought, with tree growth showing only a weak correlation to drought severity.
"[The trees’] needs are being met. They’re not being stressed by the climate," said co-author Bryan Black, an associate professor at The University of Arizona who led the core analysis. "It fits right in line with the hypothesis that they’re receiving supplemental water through this urban infrastructure leakage."
Even during the state's most severe recorded drought, which lasted from 1950-1957, the Waller Creek trees exhibited only a minor decline in growth. In contrast, the trees of Onion Creek faced a steep decline.
In addition to helping the trees survive droughts, the constant flow of wastewater also seemed to cause Waller Creek trees to adopt individual growth patterns, with different trees exhibiting varying growth rates throughout the year. The researchers believe that once their water needs were met, other factors influencing tree growth — such as genetics, competition from other trees, nutrients or pest presence — took on a greater role. While this would be an unusual result in a rural setting, researchers were not surprised to see Waller Creek trees behaving independently.
"That’s kind of what we expect in an urban environment," Banner said. "We would expect the natural system to be, well, for lack of a better term, messed up."
Current research is exploring how tree ring chemistry may reflect the timing and quantity of this municipal water input and differentiate between municipal water sources such as wastewater, tap water and irrigation.
Leaky infrastructure is common across cities worldwide. This suggests that there are likely pockets of trees worldwide being sustained by wastewater similar to those along Waller Creek. Understanding the extent of infrastructure leakage and its ecological impact can assist policymakers in comprehending the broader effects of urbanization — both positive and negative — and planning accordingly.
The research was funded by the National Science Foundation, the Cynthia and George Mitchell Foundation, the Jackson School of Geosciences and the UT Planet Texas 2050 initiative.