Transplanted trees are exposed to numerous stresses from the time they are harvested until their eventual establishment within the landscape (Davies et al., 1972; Kozlowski and Davis, 1975; McKay, 1996). The digging process often separates a balled-and-burlapped tree from the majority of its root system. After this initial stress, trees are typically brought to a loading site, placed on trucks or trailers, and shipped to their intended destinations. At each stage of the transplanting process, trees are exposed to mechanical shock and vibration that can further disrupt the root system and cause considerable damage to trunks and crowns. Water is typically not available during transport; tissue desiccation can greatly affect tree survival if tarps or other coverings are not used to limit evaporative and transpirational losses. In addition, transplanted trees may be subjected to rapidly changing temperatures and humidity levels as they are moved from sunny to shady sites, from low to high elevations, in and out of box trailers, and across plant hardiness zones, all of which may occur in the course of a few hours (Watson and Himelick, 1997).
Water stress (Gilbertson and Bradshaw, 1985), mechanical damage (Nowak et al., 2004), or extreme temperatures (McKay, 1996) alone may be sufficient to severely damage or kill a transplanted tree. When these stressors are combined, the prospect of survival in the landscape is greatly reduced (McKay, 1996). Individual stress factors affect the growth and survival of trees in different ways. Stresses such as tissue desiccation may have an additive effect, in which many minor water-limiting events may continuously weaken a tree (McKay, 1996). However, this additive property is not universal among stress factors. Prolonged periods of mild mechanical stress generally do not equal one lethal event. In the case of stress caused by rough handling, it appears a threshold must be crossed before damage occurs (McKay, 1996).
Transplant shock describes the period of reduced growth that follows the digging and planting of a tree. Many consider water stress to be the key factor contributing to transplant shock and planting failure (Davies et al., 1972; Kozlowski and Davis, 1975; McKay, 1996). Water stress during the transplanting process is often the result of digging practices that greatly reduce the root system of nursery trees (Gilman, 1988). The undersized root system is incapable of replenishing water lost through transpiration within the crown (Kozlowski and Davis, 1975). Transplanted trees remain in this state of diminished growth until regrowth of the root system is sufficient to balance transpiration loss (Beeson and Gilman, 1992).
Severe root loss significantly affects survival and establishment rates of transplanted trees (Gilman, 1988; Watson, 1985). Root growth after transplanting has been estimated at 30 to 68 cm per year (Watson and Himelick, 1982b). Because smaller trees typically retain a greater percentage of their original root system compared with larger trees, the time needed to regenerate the roots they have lost is substantially reduced. Research has shown when planted at the same time, it is possible for a 10-cm-caliper tree to outgrow a 25-cm-caliper tree in height and stem diameter within a few years (Watson, 1985).
Although root severance may have a negative impact on survival and growth of transplanted trees, pruning of lateral roots to stimulate regrowth of fine absorptive roots is common in the nursery trade (Watson and Himelick, 2005). This root-pruning may be done several times before actual harvest with each new cut made further away from the trunk. When the tree is finally dug for transplanting, the root ball is made large enough to contain the subsequent fibrous regrowth.
Gaining a greater knowledge of the stress factors that influence transplant success is imperative if the process is to be improved. Reduced growth and survival associated with transplanting stress can be a significant financial loss to a consumer. Although costs associated with tree death are easily noted, effects of transplant shock and growth reduction are not as obvious. Unless overly mature, trees typically increase in their economic and environmental worth as they grow in size in an urban setting (Maco and McPherson, 2003). With this in mind, hindering tree growth slows the financial return associated with the initial investment of tree planting. This diminished return is quantified as a reduction in carbon sequestration rates, storm water retention, energy savings, and in potential gains in property value (Maco and McPherson, 2003).
We hypothesized that stresses associated with handling and transporting nursery stock during transplanting could be directly linked to reduced tree growth and survival within the first year of establishment. We also postulated that identifying the specific stress factors responsible for transplant shock and when they occur is crucial if the process is to be improved. Our objective was to determine the short-term impact (i.e., within 1 year) of transplanting on health and establishment of woody plants in the landscape by examining impacts of desiccation and mechanical damage on survival and growth of balled-and-burlapped trees at defined stages in the continuum from harvest to transplanting. This was our rationale to use two commonly used cultivars of Acer L. (maple). Our intent was not to look at potential treatment differences between the two maple species, but rather be able to generalize our results to transplanting events of several woody species. We then used this information to identify key stages in the transplanting process that diminish transplanted tree growth and survival.
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