Such practices also hasten the destruction of an important and dwindling habitat. Melissa Rohde of the State University of New York College of Environmental Science and Forestry (SUNY-ESF) and colleagues analyzed five years of high-resolution satellite and water resource data showing vegetation greenness along California rivers. Trees growing alongside the 30 percent of state rivers with natural flows decreased in greenness from the wet spring through the dry summer months, the scientists report in the Proceedings of the National Academy of Sciences, demonstrating they rely on groundwater to make it through the dry season. By contrast, woodlands along the 70% of streams receiving water from dams, wastewater treatment plants, and other human sources kept up the same vigorous levels of photosynthesis and water use throughout the summer.

 “One would initially think, well, that’s great, the extra water is really helping the environment,” says Dr. John Stella, a professor of ecohydrology at SUNY-ESF. But while water subsidies enable established riparian trees to grow bigger faster, that benefit comes at a high cost: earlier mortality and poor regeneration. In nature, spring snow-melt floods normally cause riverbanks to erode and channels to migrate across the floodplain, exposing bare, moist sandbars ideal for nurturing the seedlings of riparian trees. Managed river channels, however, seldom migrate much, eliminating spaces for new forests to take root. Meanwhile, on regulated water supply streams such as the Sacramento River, the stable elevated flow during summer followed by severe draw-downs in early fall often leave new seedlings high and dry, with few trees successfully establishing compared to the pre-dam era.

“These riparian forests are what we call disturbance-dependent communities. When you eliminate the disturbance, in this case the spring floods, you eliminate the conditions to which they’re adapted,” Stella says.

Many trees along managed streams today date back to the big era of dam building between the 1930s and 1960s. Nearing the end of their lifespans, many of those trees are being succeeded by different, often non-riparian species. The looming loss of pioneer riparian trees such as cottonwood and willow will have serious ecological repercussions. They provide habitat for threatened and endangered species such as the yellow-billed cuckoo and Swainson’s hawk. Their foliage cools the water, and their fallen, submerged wood provides salmonid habitat. “These niches cannot be provided for by later successional species to the same degree,” Stella says.

Considering riparian forests in water management practices can go a long way toward protecting what little remains. For example, when pumping groundwater, “we need to be cognizant of where, how far, and how fast we draw” to ensure riparian trees can still access a supply of moisture, Stella says. Unfortunately, at present, “if we change water deliveries on managed streams, there’s no backup plan for these trees; their dependence on the altered flow is a vulnerability”. KW

Related Prior Estuary News Story

SOS For Finicky Native Sycamores

Pearls in the ocean of information that our reporters didn’t want you to miss
 

Modern water management practices damp down natural river patterns and produce streamside forests that "live fast and die young."

Such practices also hasten the destruction of an important and dwindling habitat. Melissa Rohde of the State University of New York College of Environmental Science and Forestry (SUNY-ESF) and colleagues analyzed five years of high-resolution satellite and water resource data showing vegetation greenness along California rivers. Trees growing alongside the 30 percent of state rivers with natural flows decreased in greenness from the wet spring through the dry summer months, the scientists report in the Proceedings of the National Academy of Sciences, demonstrating they rely on groundwater to make it through the dry season. By contrast, woodlands along the 70% of streams receiving water from dams, wastewater treatment plants, and other human sources kept up the same vigorous levels of photosynthesis and water use throughout the summer.

 “One would initially think, well, that’s great, the extra water is really helping the environment,” says Dr. John Stella, a professor of ecohydrology at SUNY-ESF. But while water subsidies enable established riparian trees to grow bigger faster, that benefit comes at a high cost: earlier mortality and poor regeneration. In nature, spring snow-melt floods normally cause riverbanks to erode and channels to migrate across the floodplain, exposing bare, moist sandbars ideal for nurturing the seedlings of riparian trees. Managed river channels, however, seldom migrate much, eliminating spaces for new forests to take root. Meanwhile, on regulated water supply streams such as the Sacramento River, the stable elevated flow during summer followed by severe draw-downs in early fall often leave new seedlings high and dry, with few trees successfully establishing compared to the pre-dam era.

“These riparian forests are what we call disturbance-dependent communities. When you eliminate the disturbance, in this case the spring floods, you eliminate the conditions to which they’re adapted,” Stella says.

Many trees along managed streams today date back to the big era of dam building between the 1930s and 1960s. Nearing the end of their lifespans, many of those trees are being succeeded by different, often non-riparian species. The looming loss of pioneer riparian trees such as cottonwood and willow will have serious ecological repercussions. They provide habitat for threatened and endangered species such as the yellow-billed cuckoo and Swainson’s hawk. Their foliage cools the water, and their fallen, submerged wood provides salmonid habitat. “These niches cannot be provided for by later successional species to the same degree,” Stella says.

Considering riparian forests in water management practices can go a long way toward protecting what little remains. For example, when pumping groundwater, “we need to be cognizant of where, how far, and how fast we draw” to ensure riparian trees can still access a supply of moisture, Stella says. Unfortunately, at present, “if we change water deliveries on managed streams, there’s no backup plan for these trees; their dependence on the altered flow is a vulnerability”. KW

Related Prior Estuary News Story

SOS For Finicky Native Sycamores

About the author

Bay Area native Kathleen M. Wong is a science writer specializing in the natural history and environment of California and the West. With Ariel Rubissow Okamoto, she coauthored Natural History of San Francisco Bay (UC Press, 2011), for which she shared the 2013 Harold Gilliam Award for Excellence in Environmental Reporting. She reports on native species, climate change, and environmental conditions for Estuary, and is the science writer of the University of California Natural Reserve System.

Related Posts

Atmospheric Rivers Intensifying as World Warms: How the West Will Know What’s Coming

In just a few years, tracking the West Coast’s atmospheric rivers by airplane has gone from what one hydrologist called “really wild-eyed stuff” to a Congressionally-funded operation. This Atmospheric River Reconnaissance program, which wrapped up its latest season in March, monitors these increasingly powerful storms as they shoot across the...

Scientists in the Central Valley are honing a novel way of giving young salmon the nourishing benefits of wintertime floodwaters without undertaking costly floodplain restoration work.

The method, being practiced along the Sacramento River, mimics the flood patterns of natural Sacramento Valley wetlands by diverting water onto floodplain farm fields, retaining it there for three weeks, and finally flushing the water—now rich with zooplankton and invertebrate protein—back into the river. Onsite studies have shown that salmon...

Hot off the press, Sacramento County Breeding Birds: A Tale of Two Atlases and Three Decades of Change raises red flags for some of the county’s wetland species.

Breeding bird atlases use field observations to record possible, probable, or confirmed nesting in uniform-sized blocks within a county or state. Biologist/artist Tim Manolis led a Sacramento County atlas project in 1988-93, but the results were never published. When Edward Pandolfino of Western Field Ornithologists heard about it he suggested...