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Eastern redcedar Juniperus virginiana is 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-old J . virginiana saplings 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 both Q . stellata and B . 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. When J . virginiana and B . inermis grew together, water depletion occurred at both 30–40 cm and 10–20 cm. Combined with root length results, we can infer that J . virginiana most likely took up water from the deeper layers whereas B . inermis used water from the top layers. We found a similar pattern of water depletion and uptake when J . virginiana grew with Q . stellata , indicating that J . virginiana took up water from the deeper layers and Q . stellata used water mostly from the top soil layers. When the three species grew together, we found root overlap between J . virginiana and Q . stellata . Despite the root overlap, our relative water content and water potential indicate that J . virginiana was not water stressed in any of the plant combinations. Regardless, J . virginiana saplings had less total biomass in treatments with B . inermis and we recorded a significantly higher mortality when J . virginiana grew with both competitors. Root overlap and partitioning can affect how J . virginiana perform and adapt to new competitors and can allow their co-existence with grasses and other woody species, which can facilitate J . virginiana encroachment into grasslands and woodlands. Our data also show that competition with both Q . stellata and B . inermis could limit establishment, regardless of water availability. 

The expansion of woody plants into grasslands and old fields is often ascribed to fire suppression and heavy grazing, especially by domestic livestock. However, it is also recognized that nutrient availability and interspecific competition with grasses and other woody plants play a role in certain habitats. I examined potential factors causing range- and niche expansion by the eastern redcedar Juniperus virginiana , the most widespread conifer in the eastern United States, in multifactorial experiments in a greenhouse. Historical records suggest that the eastern redcedar is a pioneer forest species, and may be replaced as the forest increases in tree density due to shading. Another possible factor that affects its distribution may be nutrient availability, which is higher in old fields and other disturbed lands than in undisturbed habitats. In its historic range, eastern redcedars are particularly abundant on limestone outcrops, often termed ‘cedar barrens’. However, the higher abundance on limestone could be due to reduced interspecific competition rather than a preference for high pH substrates. I manipulated shade, fertilization, lime, and interspecific competition with a common dominant tree, the post oak Quercus stellata . In a separate experiment, I manipulated fire and grass competition. I measured growth rates (height and diameter) and above- and belowground biomass at the end of both experiments. I also measured total non-structural carbohydrates and nitrogen in these plants. Shade was the most important factor limiting the growth rates and biomass of eastern redcedars. I also found that there were significant declines in nitrogen and non-structural carbohydrates when shaded. These results are consistent with the notion that the eastern redcedar is a pioneer forest species, and that shade is the reason that these redcedars are replaced by other tree species. In the second experiment, I found that a single fire had a negative effect on young trees. There was no significant effect of competition with grass, perhaps because the competitive effect was shading by grasses and not nutrient depletion. Overall, the effects of shade were far more apparent than the effects of fire. 

One of the tenets of introductory biology courses is learning how the form or structure of any unit determines its function, and that form and function has long been used to understand patterns throughout the natural world (Grew, 1682; Wainwright, 1988; D􏰀ıaz et al., 2016). Form predicts function at the cellular and organ level in plants; for instance, xylem cell diameter and pit pore size are great indicators of stem xylem hydraulic conductance and vulner- ability to embolism (Pittermann & Sperry, 2005). Additionally, both theoretical treatments and experimental evidence indicate that root hydraulic function is coupled to leaf function (Cruiziat et al., 2002; Domec et al., 2009) but few studies have investigated how the anatomy and morphology of the roots may influence leaf function. In this issue of New Phytologist, Zhou et al. (2021; pp. 1481–1491) present results from a long-term precipitation experiment, showing that root form predicted leaf physiology. This study highlights that the form of one organ can have effects on function across the entire organism, providing a unique perspective in the study of form–function relationships.