Field-grown (FG) trees lose a significant portion of their root system (Gilman and Beeson, 1996) and when transplanted are subjected to great stress. Water deficits often develop (Montague et al., 2000a) because the natural balance between root absorptive area and transpiring leaf area is disrupted (Kramer and Kozlowski, 1979). In addition, roots often do not grow rapidly enough to absorb sufficient water to meet transpirational losses (Kozlowski, 1982). Because of increased water deficits, gas exchange and growth are reduced in transplanted FG trees (Kjelgren and Cleveland, 1994). In addition, as a result of partial stomatal closure, transpiration (E) and photosynthesis (PN) are reduced for newly transplanted FG trees (Montague et al., 2000a). Until a balance between root absorptive area and transpiring leaf area is restored, apical growth for newly transplanted FG trees will be limited (Montague et al., 2000a).
Research on establishment of transplanted FG trees has been conducted primarily in regions of high humidity. Evaporative demand (leaf-to-air vapor pressure difference) on transplanted trees is often lower in such regions when compared with arid regions such as the southwest United States. To reduce E during periods of high leaf-to-air vapor pressure difference (LVPD), many plant species reduce g S (Monteith, 1995; Mott and Parkhurst, 1991). High LVPD can result in partial stomatal closure (Farquhar, 1978; Turner et al., 1984) and reduce PN rates (Hinckley et al., 1978) and growth. Transplanted trees are considered to be established when leaf water potential (ψL), gas exchange, and growth are similar to those of nontransplanted trees (Beeson and Gilman, 1992). However, research on gas exchange and growth of transplanted FG trees in arid and semiarid climates has been minimal and establishment rates for transplanted FG trees in such regions need to be determined.
Reduced soil moisture evaporation is one of the benefits organic mulch (pine bark, pruning chips, pine needles, and so on) provides in landscapes (Kraus, 1998; Montague et al., 2000b). Several researchers indicate transplanted trees surrounded by organic mulch have enhanced apical (shoot elongation, leaf area increase, and so on) (Green and Watson, 1989; Hensley et al., 1988) and root growth (Green and Watson, 1989; Watson and Kupkowski, 1991) when compared with trees grown without mulch. However, many authors report transplanted woody plant growth is not influenced or is reduced by organic mulch (Arnold et al., 2005; Gilman and Grabosky, 2004; Hild and Morgan, 1993; Montague et al., 2000b). Despite lack of evidence from replicated research and potential long-term adverse consequences, organic mulch continues to be recommended as a cultural method to assist establishment of transplanted trees (Herms et al., 2001).
An additional concern is placement of organic mulch around established landscape trees. Little research has been published that documents if gas exchange and growth are altered when organic mulch is placed on soil surrounding established trees. Watson (1988) reports root growth of several established tree species that had been surrounded by organic mulch (wood chips placed on soil 20 years after trees were planted). When compared with trees not surrounded by mulch, root density of mulched trees was greater for some species but less for others. In a similar study, Fraedrich and Ham (1982) placed organic mulch (wood chips) around established maple species (planted 4 years before initiation of mulch study). They report growth was affected by mulch treatments, but once again, response varied with species.
Early and successful growth of a transplanted FG tree's root system after transplanting would greatly increase survivability and establishment. However, environmental factors found in arid climates can increase plant stress and delay establishment (Montague et al., 2000a, 2000b). Although mulch is reported to affect gas exchange and growth of bare root and containerized tree species, little research is available on whether organic mulch influences establishment of newly transplanted, large-caliper FG trees in arid regions. In addition, research documenting if gas exchange and growth are altered when organic mulch is placed on the soil surface surrounding existing trees has been conducted on a limited number of species in mesic climates. If organic mulch reduces physiological processes and growth on transplanted tree species, it is logical to conclude organic mulch may also inhibit these processes on established trees. Therefore, objectives of this experiment were to compare the influence of organic mulch on gas exchange and growth of transplanted and nontransplanted, large-caliper FG Shumard red oak trees grown in a semiarid climate.
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