Was Weaver wrong? Soil moisture dynamics and rooting depths of sandhills grasslands

Classic research by J.T. Weaver documented rooting depths of Great Plains grasses and is still widely cited as the benchmark for grassland root studies. Weaver summarized the roots of Nebraska Sandhills grasses as spreading widely near the surface and “penetrating deeply, mostly 4 to 7 feet” (Weaver 1958). We measured soil moisture to 300cm depth for up to 5 yrs at approximately 100 locations in the Nebraska Sandhills. Although measurements came from separate experiments in grazed and ungrazed grassland, experimental disturbances, and 70 yr old tree plantings, all were made with the same methodology. In most cases, measurements were made year round and were stratified by topographic position. Soil moisture depletion zones under grassland typically reached 60-80cm depth by late summer, but did not exceed 100 cm depth. While late summer soil moisture commonly dropped to <2.0 % (volumetric moisture content) in the top 60 cm, soil moisture contents beneath 100cm depth were commonly 10-20%. Data from experimentally devegetated plots indicate that these patterns are driven by plant uptake and not by soil properties or bare soil evaporation. In contrast to grasslands, established stands of Juniperus virginiana and Pinus ponderosa depleted soil moisture to depths >300cm. 3 month old stands of weeds dominated by Chenopodium album had deeper soil moisture depletion zones than undisturbed grassland. Our measurements of grassland root mass and length by depth support the conclusion that the rooting zone of these semi-arid grasslands is shallow. This has important consequences for regional groundwater recharge and ecohydrology.