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ABSTRACT Woody encroachment—the expansion of woody shrubs into grasslands—is a widely documented phenomenon with global significance for the water cycle. However, its effects on watershed hydrology, including streamflow and groundwater recharge, remain poorly understood. A key challenge is the limited understanding of how changes to root abundance, size and distribution across soil depths influence infiltration and preferential flow. We hypothesised that woody shrubs would increase and deepen coarse‐root abundance and effective soil porosity, thus promoting deeper soil water infiltration and increasing soil water flow velocities. To test this hypothesis, we conducted a study at the Konza Prairie Biological Station in Kansas, where roughleaf dogwood (Cornus drummondii) is the predominant woody shrub encroaching into native tallgrass prairie. We quantified the distribution of coarse and fine roots and leveraged soil moisture time series and electrical resistivity imaging to analyse soil water flow beneath shrubs and grasses. We observed a greater fraction of coarse roots beneath shrubs compared to grasses, which was concurrent with greater saturated hydraulic conductivity and effective porosity. Half‐hourly rainfall and soil moisture data show that the average soil water flow through macropores was 135% greater beneath shrubs than grasses at the deepest B horizon, consistent with greater saturated hydraulic conductivity. Soil‐moisture time series and electrical resistivity imaging also indicated that large rainfall events and greater antecedent wetness promoted more flow in the deeper layers beneath shrubs than beneath grasses. These findings suggest that woody encroachment alters soil hydrologic processes with cascading consequences for ecohydrological processes, including increased vertical connectivity and potential groundwater recharge. 

Abstract Riparian zones and the streams they border provide vital habitat for organisms, water quality protection, and other important ecosystem services. These areas are under pressure from local (land use/land cover change) to global (climate change) processes. Woody vegetation is expanding in grassland riparian zones worldwide. Here we report on a decade‐long watershed‐scale mechanical removal of woody riparian vegetation along 4.5 km of stream channel in a before–after control impact experiment. Prior to this removal, woody plants had expanded into grassy riparian areas, associated with a decline in streamflow, loss of grassy plant species, and other ecosystem‐scale impacts. We confirmed some expected responses, including rapid increases in stream nutrients and sediments, disappearance of stream mosses, and decreased organic inputs to streams via riparian leaves. We were surprised that nutrient and sediment increases were transient for 3 years, that there was no recovery of stream discharge, and that areas with woody removal did not shift back to a grassland state, even when reseeded with grassland species. Rapid expansion of shrubs (Cornus drummondii,Prunus americana) in the areas where trees were removed allowed woody vegetation to remain dominant despite repeating the cutting every 2 years. Our results suggest woody expansion can fundamentally alter terrestrial and aquatic habitat connections in grasslands, resulting in inexorable movement toward a new ecosystem state. Human pressures, such as climate change, atmospheric CO₂increases, and elevated atmospheric nitrogen deposition, could continue to push the ecosystem along a trajectory that is difficult to change. Our results suggest that predicting relationships between riparian zones and the streams they border could be difficult in the face of global change in all biomes, even in well‐studied sites. 

Abstract Leaf trait variation enables plants to utilize large gradients of light availability that exist across canopies of high leaf area index (LAI), allowing for greater net carbon gain while reducing light availability for understory competitors. While these canopy dynamics are well understood in forest ecosystems, studies of canopy structure of woody shrubs in grasslands are lacking. To evaluate the investment strategy used by these shrubs, we investigated the vertical distribution of leaf traits and physiology across canopies of Cornus drummondii, the predominant woody encroaching shrub in the Kansas tallgrass prairie. We also examined the impact of disturbance by browsing and grazing on these factors. Our results reveal that leaf mass per area (LMA) and leaf nitrogen per area (Na) varied approximately threefold across canopies of C. drummondii, resulting in major differences in the physiological functioning of leaves. High LMA leaves had high photosynthetic capacity, while low LMA leaves had a novel strategy for maintaining light compensation points below ambient light levels. The vertical allocation of leaf traits in C. drummondii canopies was also modified in response to browsing, which increased light availability at deeper canopy depths. As a result, LMA and Na increased at lower canopy depths, leading to a greater photosynthetic capacity deeper in browsed canopies compared to control canopies. This response, along with increased light availability, facilitated greater photosynthesis and resource-use efficiency deeper in browsed canopies compared to control canopies. Our results illustrate how C. drummondii facilitates high LAI canopies and a compensatory growth response to browsing—both of which are key factors contributing to the success of C. drummondii and other species responsible for grassland woody encroachment. 

Abstract Intermittent streams that regularly dry up constitute over half of the world's river network. They exhibit biogeochemical processes distinct from those of continuously flowing perennial rivers. In perennial rivers, discharge is often perceived as predominantly driving water chemistry, as demonstrated by the widespread use of concentration–discharge (CQ) relationships. Does discharge similarly drive water chemistry in intermittent streams? Given its extended periods of no flow, we hypothesized that stream chemistry depends less on discharge alone but more on the granularity of dry‐wet transitions, including their direction (drying or rewetting), history (antecedent conditions), and intermittency. We tested this hypothesis by analyzing three decades of streamflow and solute chemistry data from an intermittent stream (N04D) in the Konza Prairie Biological Station, a Long‐Term Ecological Research site in Kansas, USA. Results showed that concentrations are generally higher at no flow compared to flow conditions and depend on dry‐wet transitions. Geogenic solutes were predominantly chemostatic (relatively constantCwithoutQdependence), contrasting primarily dilution patterns (decreasingCwithQ) in perennial rivers. Biogenic solutes did not exhibit pronounced discharge‐dependent patterns commonly observed in perennial rivers at decadal scale; at monthly scale, however, they exhibited a transition from highly variableCQpatterns at low flows to consistent flushing patterns (increasingCwithQ) at flows higher than 2.5–5 mm/day. These observations support our hypothesis of weaker chemistry dependence on discharge in intermittent streams. We further hypothesize that the emerging discharge thresholds signal a tipping point at which intermittent streams switch from a dry state governed by intermittency‐driven biogeochemistry to a wet, discharge‐driven state resembling perennial rivers. The hypothesis calls for intensive data collection at dry‐wet transitions to develop theories and models for intermittent streams that have become increasingly prevalent globally. 

Abstract Plants allocate significant amounts of carbon belowground to support water and nutrient acquisition, and a large proportion of that carbon is allocated to the production of root exudates. One utility of exudates is to aid in phosphorus acquisition, but the amount of phosphorus that can be liberated by different exudates is largely unknown. We used three common exudates—oxalic acid, malic acid, and acid phosphatase (APase)—to extract phosphate from 25 soil profiles across the United States. These extracts represent the “maximum efficiency” of specific exudates at liberating phosphorus from a wide range of soil types, providing an upper boundary to constrain biogeochemical and land‐surface models aimed at representing exudate‐related nutrient fluxes. Our findings reveal significant variability in the efficiency of exudates to liberate phosphate across locations and soil depths. Oxalic acid exhibited the highest potential extraction efficiency overall, and its efficiency often increased in deeper horizons—likely driven by higher proportions of mineral‐bound phosphorus in deeper soil layers. APase had the lowest overall potential extraction efficiency, and its efficiency typically declined with depth and had stronger associations with soil organic carbon. Malic acid potential extraction efficiency was intermediate, and depth trends were often more similar to APase than oxalic acid. Malic acid and APase showed increasing extraction efficiency at sites with higher mean annual temperature and precipitation. This study highlights the varying efficiencies of exudate compounds in liberating soil phosphorus across ecosystems and provides a framework for understanding nutrient dynamics under changing climate or land‐use conditions.