Effects of post oak (Quercus stellata) and smooth brome (Bromus inermis) competition on water uptake and root partitioning of eastern redcedar (Juniperus virginiana)

Abstract

Eastern redcedarJuniperus virginianais encroaching into new habitats, which will affect native ecosystems as this species competes with other plants for available resources, including water. We designed a greenhouse experiment to investigate changes in soil moisture content and rooting depths of two-year-oldJ.virginianasaplings growing with or without competition. We had four competition treatments: 1) none, 2) with a native tree (Quercus stellata), 3) with an invasive grass (Bromus inermis), and 4) with bothQ.stellataandB.inermis. We measured soil moisture content over two years as well as root length, total biomass, relative water content, midday water potential, and mortality at the end of the experiment. WhenJ.virginianaandB.inermisgrew together, water depletion occurred at both 30–40 cm and 10–20 cm. Combined with root length results, we can infer thatJ.virginianamost likely took up water from the deeper layers whereasB.inermisused water from the top layers. We found a similar pattern of water depletion and uptake whenJ.virginianagrew withQ.stellata, indicating thatJ.virginianatook up water from the deeper layers andQ.stellataused water mostly from the top soil layers. When the three species grew together, we found root overlap betweenJ.virginianaandQ.stellata. Despite the root overlap, our relative water content and water potential indicate thatJ.virginianawas not water stressed in any of the plant combinations. Regardless,J.virginianasaplings had less total biomass in treatments withB.inermisand we recorded a significantly higher mortality whenJ.virginianagrew with both competitors. Root overlap and partitioning can affect howJ.virginianaperform and adapt to new competitors and can allow their co-existence with grasses and other woody species, which can facilitateJ.virginianaencroachment into grasslands and woodlands. Our data also show that competition with bothQ.stellataandB.inermiscould limit establishment, regardless of water availability.