Water availability may alter species competitive interactions, resulting in different outcomes as plants compete for available water. Eastern redcedar Juniperus virginiana (hereafter ERC) will likely continue to encroach into new habitats, which will affect soil and water budgets. We designed a greenhouse experiment to investigate changes in water uptake and rooting depths of two-year old ERC saplings in the presence of an invasive grass (Bromus inermis) and a native tree (Quercus stellata). We measured soil moisture content over two growing seasons. When grown together, ERC took up water from the deeper layers (21–40 cm) whereas B. inermis used water from the top layers of the soil (0–20 cm). Similarly, when ERC grew with Q. stellata, ERC took up water from the deeper layers and Q. stellata used water mostly from the top layers. This root partitioning can allow the co-existence of ERC, grasses, and other trees, which can facilitate ERC encroachment into grasslands and woodlands. However, when the three species grew together, we found root overlap between the ERC and Q. stellata. This overlap can affect ERC establishment and encroachment in habitats where grasses and trees co-occur. A major factor that affected ERC performance was higher mortality in treatments with B. inermis than in the ERC-alone or with Q. stellata. This indicates that competition with grasses is a major factor affecting ERC sapling establishment.

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