by Dante Capone, PhD Student at the Scripps Institute of Oceanography and the California Current Ecosystems LTER
Invasion of woody shrubs into grasslands disrupts the water cycle, with cascading effects on the ecosystem and conservation.
A Prairie Transformed: The Puzzle of Vanishing Water
In the tallgrass prairie of Kansas’ Konza Prairie LTER, rain filters through soil. The precipitation nourishes the streams, replenishes underground reservoirs, and satiates vegetation above the surface and roots below. Grassland ecosystems, often overlooked, store carbon, harbor biodiversity, and regulate the movement of water. However, long term measurements at the research station have revealed a puzzling paradox and disruption of the hydrologic balance: the stream flow in Konza has been gradually declining, yet the climate has been getting wetter.
Motivated to solve the mystery of the weakening streams, a team of scientists led by postdoctoral researcher Dr. Rachel Keen leveraged long term monitoring data of precipitation and streamflow from both Konza Prairie and the nearby city of Manhattan, Kansas. The data stretched back 40 and 120 years, respectively. The team also employed innovative stable isotope techniques to track the origin of prairie water, comparing the ratios of oxygen isotopes in precipitation, groundwater, and streamflow from over a decade. Their findings, recently published in The Journal of Hydrology, reveal the culprit behind the hydrologic decoupling: the relentless encroachment of woody plants—shrubs and trees—into the prairie landscape.
Credit: Rachel Keen, CC BY-SA 4.0.
Hydrologic Sleuthing: Tracking the Fingerprint of Water
To uncover how woody encroachment altered water systems in the Konza Prairie, researchers turned to the unique fingerprints of water—the stable isotopes of oxygen (δ18O) and hydrogen (δ2H). While the isotopic signature of precipitation remained constant, stream and groundwater isotopes showed a steady decline in δ18O, pointing to significant changes in water movement through the system.
The isotope data reveal that water reaching streams and aquifers increasingly originated from dormant-season precipitation, which is typically depleted in δ18O. This finding aligns with the distinct water demands and rooting patterns of woody plants. Historically, during the growing season between April and September, the shallow roots of grasses allowed rain to seep into upper soil layers, replenishing streams and aquifers. In contrast, the deeper root systems of woody plants enhance infiltration during winter, when plants are less active, but consume more water during the growing season. These changes disrupt the seasonal balance of water recharge, leaving streams more reliant on winter rains and further decoupled from the growing-season rainfall that historically replenished them.
Thirsty Invaders
Woody encroachment isn’t a simple shift where one plant species slots into the place of another. Shrubs and trees represent a completely different functional group from grasses with different life history traits and water demands. These fundamental differences help explain how the changes in vegetation are leading to changes in hydrology in mesic (wetter) grasslands like Konza.
“Woody species transpire, or use water, at much higher rates than the grasses they are replacing. That leads to changes in soil moisture that eventually impact the amount of water making it into the groundwater and stream system,” Dr. Keen explains.
Aboveground the extensive leaves and canopies of woody plants increase water lost to evapotranspiration. But changes in the relationship between vegetation and water also extend belowground. Historically, during the growing season between April and September, the shallow roots of grasses allow rain to seep into upper soil layers, replenishing streams and aquifers. In contrast, the larger roots of woody plants penetrate deeper underground and divert the water away from storage and runoff.
The proliferation of shrubs influences both quantity and timing of water recharge. During the growing season when 75% of rainfall occurs, enhanced water consumption associated with these woody plants creates a temporal shift in water availability, and leaves streams increasingly more reliant on winter rains.
Credit: Kansas State Plant Ecophys Lab, used with permission.
Cascading Effects and a Tipping Point
Woody plants ecosystems have lower biodiversity and don’t provide as much food for important grazers like bison. Beneath the prairie, drying soils in woody systems disrupt microbial activity, nutrient cycling, and changes in the pore sizes of the clay. Deeper still, recent work suggests that these aboveground shifts in vegetation can alter the oldest part of the ecosystem by changing the weathering rates of bedrock.
Together, these steady and subtle changes in grazing and climate have created a new ecosystem state that favors rapid woody encroachment. Researchers at Konza have noted that the year 2000 marked the critical tipping point in the push-and-pull competition between grasses and shrubs. It was then that prairie conditions shifted to favor woody plants. The decoupling of hydrologic connectivity that Keen and her team found in this study aligned with the tipping point around 2000.
This change is an example of an ecosystem state transition, where there is an abrupt shift from one stable state (grassland) to another (shrubland). Once a threshold is crossed, new feedback loops take hold which make a return to a grass-dominated system an uphill battle. Even through active management efforts to restore streamflow and groundwater levels a reversal of the state transition could take decades or even centuries. Yet knowing where these tipping points lie is key to preventing other parts of the prairie from ending up in the same shrub-dominated fate.
Credit: Dante Capone, CC BY-SA 4.0.
A Burning Ally
Another important feedback at Konza is an altered fire regime. Although notorious for its destructive capacity, fire is one of the prairie’s strongest allies in combating woody encroachment. The natural fire regime has evolved to thrive off the fuel subsidies from grasses, resulting in small frequent burns which preserve most of the vegetation biomass beneath the soil. However, shrubs seek to alter the frequency and intensity of fire.
“When given sufficient time between fires, the shrubs grow large and shade out grasses, which reduces fuel loads and increases the intensity of future fires,” Keen notes. With fewer fires, shrubs solidify their dominance, making it increasingly difficult to reverse these changes.
Woody plants like the eastern redcedar still succumb to fire, but when these less frequent burns occur, they can be dangerous and devastating. Crown fires in trees and shrubs are much more intense than grassland fires which threaten local communities and result in a greater emission of CO2 from the large aboveground biomass.
In this study, researchers found that higher burn frequency helped to combat the spread of woody cover but despite this, streamflow declined across all watersheds. A potential explanation lies beneath the surface, as the connectivity between groundwater reservoirs across the ecosystem may extend well beyond the established watershed boundaries.
Grasslands for Grasslands’ Sake
While the loss of grasslands is devastatingly apparent to Kansas natives like Dr. Keen, the issue extends well beyond Konza. Grassland ecosystems make up 20-40% of land area on Earth.
“The shift from grassland to woodland is a much greater threat because of the global extent of woody encroachment. These changes are not confined to Kansas—they’re happening in grasslands and savannas worldwide”.
Growing efforts to combat climate change by planting trees, while well-intentioned, could risk exacerbating these issues in grasslands. “Grassland researchers push back against afforestation efforts that replace functional, historical grasslands with forests,” Keen warns. Such initiatives can disrupt water cycles, reduce grassland biodiversity, and fundamentally alter ecosystem services. Protecting these landscapes means recognizing their value as grasslands, not seeing them as prospective planting grounds for carbon offsets.
