(CN) --- Riparian forests, those tree-filled regions running next to rivers and streams, host a breadth of important wildlife --- but water management practices focused on meeting the needs of growing communities and agriculture may be putting their future in jeopardy.
These woodlands serve to protect water quality and stream integrity, host wildlife and control flooding along water ways, but the ecosystems they support are in danger of failing in the coming decades. While water management practices have provided a short-term boon to these ecosystems by providing water “subsidies,” the dependence on those artificial supplies could damage their long-term viability.
At first glance, older trees in these ecosystems appear to be thriving, but the forest floors have become devoid of new growth, meaning when those older trees inevitably die off there won’t be anything coming up to replace them. Researchers describe this predicament in a new study published Monday in the journal PNAS.
"We need to be more intentional in incorporating ecosystem water needs when we manage water--both for aquatic organisms and species on land," said lead author Melissa Rohde, a Ph.D. candidate at State University of New York College of Environmental Science and Forestry, in a related statement. "These forest ecosystems are in a precarious state because we have disrupted the natural hydrologic processes that these plant species rely upon to support and sustain key life processes."
Researchers employed NDVI (normalized difference vegetation index) data taken from satellites to gauge the health of riparian forest growth. NDVI data measures the health of plants based on how they reflect light; healthy plants contain higher levels of chlorophyll, which in-turn absorbs larger amounts of visible light than do sickly plants. The technology allows scientists and farmers to gauge the health of large swaths of flora without ever setting foot on the ground.
Based on five years of NDVI measurements taken between 2015 and 2020, along with groundwater elevation and stream flow data, the authors found that riparian woodlands across California exhibit a stress response to deeper groundwater supplies and are decreasing in reflectivity, indicating their health is in decline.
These areas rely on seasonal flooding and variable groundwater and streamflow throughout the year to propagate and thrive. By artificially subsidizing and standardizing their water flow, the trees in these regions are being prevented from reproducing naturally, and much like their human counterparts of late, there are fewer and fewer saplings on the way to replace elder trees as they die off.
Rohde referred to these forests as the “living dead.” Because those older trees have become accustomed to regular water supplies provided by humans, they now have a reduced ability to draw from groundwater sources during dry periods, such as the droughts that are becoming all-too common in California.
“Altered flow regimes, which stabilize streamflow throughout the year and artificially enhance water supplies to riparian vegetation in the dry season, disrupt the seasonal cycles of abiotic drivers to which these Mediterranean forests are adapted,” explained the authors in their study. “Consequently, our analysis suggests that many riparian ecosystems have become reliant on anthropogenically altered flow regimes, making them more vulnerable and less resilient to rapid hydrologic change, potentially leading to future riparian forest loss across increasingly stressed dryland regions.”
A number of endangered aquatic and terrestrial species call these biodiversity hotspots home and rely on their continued health to survive. By altering these ecosystems to become reliant on artificial water management practices, many are no longer able to sustain themselves when those water sources cease to flow. These trees may appear momentarily healthier in the NDVI data, but that may be masking their underlying vulnerability to water management changes, according to the authors.
Rohde and her colleagues will use the insights gleaned from their research to advise California’s natural resource agencies on how best to sustainably manage groundwater-dependent ecosystems around the state going forward.
"California is one of the most biodiverse regions in the world, containing more species than the rest of the United States and Canada combined,” said Rohde in a related statement. “In the midst of the sixth mass extinction, the long-term sustainability of California's river ecosystems and the preservation of the rare and endemic species that live within them now rely on the deliberate, coordinated management of resource and government agencies."
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