Trees along riparian corridors are both vital biodiversity hotspots and incredibly important to the health of California's dryland ecosystems. But a recent, multi-year study led by scientists in the Department of Environmental Science, Policy, and Management (ESPM) suggests the state's worsening weather whiplash could push creekside woodlands past their ecological tipping point.
Willow, cottonwood, and valley oak trees located along intermittent stream segments that ran dry during summer had shorter and more variable growing seasons than those growing near segments that had water year-round, according to the study. In some cases, the researchers found that certain tree species shortened their growing seasons by up to five weeks. The findings were first published in the October issue of Global Change Biology.
"California is experiencing more frequent and intense swings between extremely wet and extremely dry years," said lead author Rose Mohammadi, a PhD student in the lab of ESPM Associate Professor Albert Ruhí. "We wanted to investigate whether trees by drying streams were able to tolerate such high volatility, or if these intensifying climatic cycles are pushing them beyond their physiological limits."
To conduct their analysis, the authors combined six years of high-resolution satellite imagery (which tracked the greenness of tree leaves) with high-frequency groundwater measurements collected along the Chalone Creek watershed of Pinnacles National Park. The data includes measurements collected during the second- and third-driest years in California history (2021 and 2022), as well as exceptionally wet ones (2019, 2023, and 2024). Since 2018, the Ruhí lab has maintained a continuous network to monitor for stream intermittency in the park. This effort is supported largely by the National Science Foundation—through a CAREER award to Ruhí and a Graduate Research Fellowship to Mohammadi—and enabled the researchers to better observe changes in trees' growing seasons and water use patterns.
In perennial stream areas where water was readily available, the researchers found that tree greenness generally followed seasonal light cues: it usually peaked in Spring and faded in fall as leaves took on the brown, yellow, and orange hues usually associated with autumn months. However, in areas with more intermittent streamflow, greenness was determined to be more strongly influenced by the region's groundwater levels. During drought periods when the water table dropped below approximately 5 meters (16 feet) in depth, the researchers found that tree roots could not reach the water supply, prompting them to shed their leaves ahead of schedule to prevent dehydration.
Mohammadi said this highlights the intense reliance of some trees on groundwater, a difference that was most apparent during the 2020-2022 California drought. During that period, the authors observed a reduction in tree greenness in both intermittent and perennial stream areas. And while the start of the growing season was delayed everywhere, only trees at the intermittent sites experienced a substantially earlier end of their growing season. Cottonwood trees, for instance, shortened their growing season by as much as 37 days.
Ruhí, who is currently a Berkeley Miller Professor and served as the study's senior author, said their findings highlight a future risk: increasingly long droughts could reshape riparian ecosystems not just across California and the American Southwest, but also along 60% of river miles worldwide that intermittently run dry.
"Folks had assumed that riparian trees by intermittent streams should be resilient to droughts because they are already subject to water stress every summer," he said. "However, our study shows that in some cases these trees are operating dangerously close to their limits—the longer droughts that we are seeing over the last two decades, and that will continue to worsen in the future, may be the last straw".
Additional co-authors include UC Berkeley Professor Todd Dawson; Claire Tiedeman, an emeritus research hydrologist from the United States Geological Survey; and State University of New York College of Environmental Science and Forestry Professor John Stella, PhD ESPM '05.
