Understanding the effects of grazing and prescribed fire on hydrology of Kentucky bluegrass–dominated rangelands in the northern Great Plains

Abstract

According to National Resources Inventory data, Kentucky bluegrass (Poa pratensis L.) is now present in over 85% of the areas sampled. This invasive, perennial, cool season grass can serve to stabilize soils and increase site stability; however, it also alters nutrient flows, soil structure, and plant community composition, ultimately degrading biotic integrity. In addition, Kentucky bluegrass alters the hydrologic function of an area by changing root structure and the way in which water flow is captured and released back into the ecosystem. To clarify the effect of Kentucky bluegrass on hydrological characteristics of invaded sites, rainfall simulation experiments and hydrophobicity measurements with water drop infiltration time and molarity of ethanol droplet tests were conducted at three locations all within the same ecological site in the northern Great Plains. Rainfall simulation experiments were performed on 24 large plots (6 × 2 m) at 63.5 mm h⁻¹ and 127 mm h⁻¹ intensities and on 16 small plots (0.7 × 0.7 m) at 63.5 mm h⁻¹ and 103 mm h⁻¹. Rainfall was maintained on the large plots until 10 minutes of steady-state runoff was measured or for a maximum of 30 minutes in the absence of runoff, while on the small plots, rainfall duration was set at 25 minutes. The soil layer was divided into four strata (litter, thatch, root mat, and mineral soil), which were physically separated for their hydrophobic behavior in laboratory and field water drop penetration tests and molarity analyses. Our results indicate that on dry soil strata, water drop penetration time increased by 20 seconds on litter and 3 seconds on thatch for every percentage point increase in Kentucky bluegrass in the vegetation, confirming the close association between this grass species and the development of soil hydrophobicity. Rainfall simulation on dry soils (less than 20% volumetric water content) also revealed that the time needed to initiate runoff was shortened by 5 minutes, and the runoff ratio increased by 0.004 for every percentage point increase of Kentucky bluegrass in the vegetation cover. Hydrophobicity dramatically declined in the thatch layer by a factor of 4 and was completely absent from the litter layer after wetting. In contrast to the rainfall simulations on dry soils, wet runs (volumetric water content ≥20%) showed a beneficial effect of Kentucky bluegrass on hydrologic response with delayed runoff by 5 minutes and reduced runoff ratios by 0.003 for 1% increase in Kentucky bluegrass in the vegetation cover. Prescribed fire increased litter hydrophobicity, but this did not adversely affect hydrologic response. This study highlights the need for further research contrasting detrimental effects of Kentucky bluegrass on hydrologic response in dry soil conditions with the beneficial effect of this grass on infiltration under wet conditions to better predict the overall ecohydrological outcome of an invasion by this grass species.