Survivorship of high-quality landscape field-grown trees is a particular challenge as a result of differences in post-transplant recovery between species. During bare-root tree transplanting, a major part of the root system is severed, the tree is held in storage, and then replanted into a new location. Not surprisingly, the loss of a large proportion of biomass usually results in major physiological changes within the tree until an adequate root system is rebuilt. Meanwhile, poor root–soil contact resulting from the loss of a majority of fine roots (the sites of water and nutrient uptake) often results in water stress in newly transplanted trees (Grossnickle, 1988). This large loss of plant biomass coupled with exposure to dry conditions is referred to as “transplant shock,” in which a plant shows less shoot growth, smaller “scorched” new leaves, and a general lack of vigor (Watson and Himelick, 1983).
Root hydraulic conductance describes the ability of roots to take up water from a growing medium and transport the water to other parts of a tree. Water stress during transplant shock greatly disrupts normal water transport capacity of a tree. Under drought, reduced xylem water potential may cause embolisms, air-filled xylem conduits, leading to a reduction in hydraulic conductance [K (the mass flow rate through the segment divided by the pressure difference)] (Schultz and Matthews, 1988; Sperry and Saliendra, 1994) and thus transpiration (Sperry and Pockman, 1993). Root K, as a major component of whole-tree hydraulic architecture (Cruiziat et al., 2002), has been neglected in previous studies despite its reduction at transplanting and its probable influence on post-transplant recovery. Hydraulic conductance of the entire root system of Corylus colurna was reported to be reduced significantly after transplanting (Harris and Bassuk, 1995). Regarding trees, transpiration pulls water from soil to leaves and to the atmosphere and creates a variable gradient of water potential throughout the tree. Water potential distribution along the root system is thus very dependent on root architecture such as fine root and coarse root branching (Cruiziat et al., 2002). This makes it reasonable to assume fine roots and coarse roots may have different hydraulic responses during transplant shock and may play different roles in post-transplant recovery.
Transplant timing is important in post-transplant recovery. Spring and fall are usually considered to be the appropriate times for temperate tree transplanting, but the question about which season is optimal is highly disputable. The major advantage of fall transplanting is to allow root regeneration before shoot growth in the spring and provide more time for roots to acclimate to the new soil environment, whereas spring provides ample soil moisture and allows transplanted trees to avoid cold weather (Richardson-Calfee et al., 2004). However, species vary in their survival as a function of transplant seasons (Harris et al., 2002). Fine root traits, including physiology and morphology, largely determine maximum potential growth rate of tree seedlings (Comas et al., 2002). By understanding the physiological basis of root behavior during fall vs. spring transplanting, better decisions regarding transplant timing can be made.
Tree transplant size may affect post-transplant recovery. Although large-caliper trees are often more desired to produce an immediately mature landscape, it was often found that large-caliper trees have a slower growth rate than small-caliper trees (Gilman et al., 1998). After root pruning, the artificial imbalance in the proportion of roots to shoots reduces vigor for a period of time with small-caliper trees often returning to the original shoot-to-root ratio sooner and thus having higher survival rates (Watson, 1985). However, there are some factors that may affect the rate of post-transplant recovery of small- and large-caliper trees that when accounted for can increase large-caliper tree recovery to a similar rate as small-caliper trees. For example, genetic variation between small- and large-caliper trees has often been overlooked: the more vigorous trees tend to be harvested at smaller caliper sizes in nursery production practices because they are the first to reach salable size and thus the last ones harvested from a nursery block are usually slower-growing large-caliper trees, which may be genetically inferior to the earlier harvests (Struve, 2009; Struve et al., 2000).
In this study, we used two Quercus species that differ in their transplant ability, Q. bicolor and Q. macrocarpa. Although these species are closely related, Q. bicolor easily survives transplanting, whereas Q. macrocarpa often does not (Buckstrup and Bassuk, 2009). We transplanted both small- and large-caliper trees of the two species and examined K before and after transplanting in fine roots, coarse roots, and the entire root system of the trees. Additionally, we assessed how transplant timing affected post-transplant recovery in large-caliper trees. The objective of this study was to determine whether root K is related to post-transplant recovery of the two Quercus species and whether tree size and transplant timing may affect transplant recovery.
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