Several studies have demonstrated that pruning roots can negatively impact growth and survivability of trees (Struve et al., 2000; Watson, 2004; Watson et al., 2000). A review of several studies on transplanted trees reveals that canopy growth can be stunted for several years after trees are transplanted (Haase and Rose, 1993; Johnson et al., 1984; Struve, 2009; Watson, 2000, 2005; Watson and Sydnor, 1987). In some cases, 85% to 95% of the rooting area is removed when trees are dug to be transplanted (Watson and Sydnor, 1987). The inability of trees to absorb water and nutrients and the loss of carbohydrate storage due to the removal of roots are largely responsible for the decrease in growth rate (Larimer and Struve, 2002; Struve et al., 2000). A similar phenomenon may be observed when trenching severed roots on established trees in urban environments.
Various techniques have been suggested to improve survivability and promote recovery of root-pruned trees in nurseries and urban environments. Partial root pruning in field-grown trees before transplanting has been recommended as a cultural practice to improve the success of transplanting (Harris et al., 2004). Previous partial root pruning encouraged the growth of new fibrous roots within the root ball that was transplanted along with the tree (Watson and Sydnor, 1987). Although some studies have recommended removing a portion of the canopy to compensate for the root loss (Watson, 1998), other studies have suggested that it is better to avoid canopy pruning to take advantage of the leaves and storage tissues on the branches, which may produce carbohydrates necessary for the regeneration of new roots (Struve et al., 2000). It is believed that survival and recovery of newly planted trees or large trenched trees greatly depend on rapid root growth, so that the plant regains the ability to absorb water from the soil (Harris et al., 2004).
It might be beneficial for trees if the retained canopy could be treated to reduce transpiration. Antitranspirants have been used with varying levels of success (Harris et al., 2004). However, most antitranspirants clog the stomates in the leaves, which could reduce photosynthesis and the production of carbohydrates necessary for root growth (Pallardy, 2008). Instead of pruning the canopy to balance the root:shoot ratio or using antitranspirants to conserve moisture, the use of growth regulators might be a viable option to improve the health of damaged or root-pruned transplants as they can reduce transpiration, promote photosynthesis, conserve carbohydrates, and promote root growth (Watson, 2000, 2001).
Paclobutrazol is a gibberellin-inhibiting tree growth regulator that can be applied as a basal soil drench, which is absorbed by roots and translocated through the vascular system to the canopy (Couture, 1982). Paclobutrazol has been shown to reduce the growth of canopy and trunk diameter of several tree species (Bai et al., 2004; George and Nissen, 2002; Grochowska et al., 2004; Keever et al., 1990; Singh, 2000; Sperry and Chaney, 1999; Williams et al., 2003). This growth reduction appears to conserve carbohydrates that can be stored or used for other physiological functions in the plant. Paclobutrazol also increases production of chlorophyll, which may improve or prolong carbohydrate production (Percival and Salim-AlBulushi, 2007). Additionally, PBZ has been shown to increase the ability of trees to resist both abiotic and biotic stresses (Navarro et al., 2007; Percival and Salim-AlBulushi, 2007). Root growth of some trees can be enhanced by PBZ (Watson, 2000). An increase in fine roots as a result of PBZ application may improve tree recovery after roots have been cut (Watson, 2004).
The objective of this research was to examine the impact of PBZ, when applied with root pruning, on the overall growth and vitality of field-grown live oak tree roots pruned at two distances from the tree.
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