Demand for large (>40 mm) caliper trees to create instant landscapes is greater than for smaller trees (Arnold, 2005). These larger trees are typically harvested BB from wholesale field production. Harvesting roots with soil increases weight and shipping costs and risks root system damage during handling (Koeser et al., 2009). BR trees harvested without soil are widely produced by the nursery industry, largely for use as liners in wholesale BB field production. Occasionally, BR landscape trees are sold retail and transplanted directly into the landscape as a low-cost alternative to BB trees (Buckstrup and Bassuk, 2000).
Production of BB and BR trees results in substantial root loss at harvest (Watson and Himelick, 1982), disrupting the balance between transpiring leaf area and root surface area needed for water uptake. Truncated root systems from both methods means fewer fine roots to take up water, and so increased risk for water stress until new roots grow into surrounding ambient soil (Barton and Walsh, 2000; Kjelgren and Cleveland, 1994). Until established, transplanted trees maintain a delicate balance among leaf area necessary for photosynthesis, root regrowth into ambient soil, and rooting volume necessary for water uptake (Griffin et al., 2010; Shober et al., 2010; Watson and Kupowski, 1991). Transplanted deciduous trees balance transpiring leaf area against root loss by reducing leaf number and size (Dostalek et al., 2009; Riikonen et al., 2011), particularly BR trees with greater root loss (Abod and Webster, 1990; Gunnel et al., 2008). Leaf area can be managed to accommodate root system reduction, again particularly for BR trees, with careful pruning to reduce water stress after transplanting (Dagit and Downer, 2002; Hipps et al., 2014; Ranney et al., 1989).
Transplanted BB and BR trees with truncated root systems in most climates require careful scheduling of irrigation volume and frequency to avoid stressful water deficits, reduced growth, or death (Griffin et al., 2010; Montague and Fox, 2008; Shober et al., 2010). Total leaf area largely determines irrigation volume for trees transplanted BR and BB, and irrigation frequency depends on evaporative “pull” on water from the truncated root zone, greater on hot days for trees with more leaves, less for trees with fewer leaves and cool days (Barton and Walsh, 2000; Gilman et al., 1998; Kjelgren and Cleveland, 1994).
Landscape trees are routinely irrigated in the U.S. Intermountain West (IMW) high desert. Routine irrigation means that properly handled BR trees could be just as easy to establish (Gunnel et al., 2008) as BB trees. Previous work has shown that BR trees transplanted into landscapes often establish as well as BB trees in humid climates (Buckstrup and Bassuk, 2000; Hensley, 1993). In the arid IMW, hot, dry air (high vapor pressure deficits) translates into high evaporative pull that may trigger stomatal closure in many tree species (Montague et al., 2004). Stomatal closure reduces transpiration—at potential cost of less carbon uptake—and slows root zone water depletion and irrigation frequency (Kjelgren et al., 2016). Hot, dry conditions may increase stress on BR trees through greater stomata closure than those harvested with more roots (BB), thus delaying establishment (Anella et al., 2008). In a dry climate, such as the IMW with routine irrigation, how water use and establishment of BR vs. BB has not been studied. Here we compared during establishment first year water use, 2 years of gS and water potential, and 3 years of total leaf area and shoot elongation of Fraxinus pennsylvanica ‘Patmore’ transplanted as BB and two BR sizes in a high desert climate.
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