Root systems of most nursery-grown landscape trees extend well beyond the edge of the canopy. As a result, when large (greater than 2 m tall) field-grown trees are harvested for transplanting, only a small portion of the root system is moved and a high percentage of fine, absorbing roots are lost (Gilman, 1988; Gilman and Beeson, 1996a). As a consequence, newly transplanted field-grown trees must be supported by a small fraction of the original absorptive root surfaces. Tissue water deficit resulting from an inability to absorb sufficient soil moisture to support the tree is thought to be the primary cause of transplant stress (Larson, 1984), and the rate of recovery from post-transplant water stress is directly proportional to the rate of regeneration of new roots (Nambiar et al., 1979). Thus, the more quickly a tree can regenerate a root system, the sooner the tree will establish. In contrast to the significant loss of roots that occurs when trees are dug from the field, all roots of container plants are within the confines of the container; thus, little root weight or length will be lost during transplant unless root balls are altered. As a result, container-grown trees may be less likely to suffer moisture stress when transplanted if adequately irrigated (Gilman and Beeson, 1996b).
For all transplanted trees, the production and maintenance of new roots is costly. Root turnover, an indicator for the cost of root production, refers to the portion of the root system that dies and is replaced (Burton et al., 2000) and is a measure of annual production and/or mortality relative to standing crop (Jones et al., 2003). In natural forests, root turnover is responsible for a substantial flux of carbon to the rhizosphere (Tierney and Fahey, 2002) and is, therefore, a substantial metabolic cost for trees (Psarras et al., 2000). In a sugar maple-dominated forest, 40% to 60% of the net primary productivity was allocated to fine root production, and annual root biomass production and mortality were estimated to be 7300 to 8000 kg·ha−1 per year and 4800 to 6700 kg·ha−1 per year, respectively (Hendrick and Pregitzer, 1993b).
Bloomfield et al. (1996) proposed three categories of processes that affect the magnitude and timing of root turnover. Processes in the first category are a result of aboveground demands that affect carbon fixation (e.g., foliage production, thinning of branches, and prolific fruiting) and carbon allocation. The second category relates to carbohydrate storage capacity and assumes that roots die when there is insufficient carbon for maintenance. Larger roots usually have larger carbon reserves and thus often live longer. The third category of processes encompasses factors that affect soil microsite quality (e.g., nutrient, water, and oxygen availability, soil temperature, toxic elements, and fungal/microbial populations).
Plasticity in the response of root turnover rates to changes in soil environment may be considered a fitness attribute because plants that respond to changing conditions can better compete for resources in a heterogeneous environment (Espeleta and Donovan, 2002; Hutchings, 1988). As such, trees exhibit a morphological plasticity that results in proliferation of roots in favorable environments and shedding of roots after a region of soil has been depleted of resources (Pregitzer et al., 1993). For example, addition of nitrogen to a mixed hardwood forest resulted in a rapid increase in localized fine root production (Hendrick and Pregitzer, 1992, 1993a), increased longevity, and reduced turnover rates (Burton et al., 2000). Burton et al. (2000) suggested that roots are maintained as long as the benefit they provide outweighs the carbon cost of keeping them alive.
The rapid regeneration of a new root system is essential for the survival of a newly transplanted tree. Thus, basic information concerning the dynamics of how root systems develop after transplanting will improve our understanding of how trees establish and survive. The work described in this article is an exploration of aspects of root turnover in a horticulture system as opposed to a forest ecology system. In what may be the only other report on production and mortality in regenerating roots systems of transplanted landscape trees, Wiseman and Wells (2009) monitored balled-and-burlapped (B&B) Magnolia virginiana L. (sweetbay magnolia) and Acer buergerianum Miq. (trident maple) for 1 year after transplanting with minirhizotrons. However, their study did not address this phenomenon when comparing production methods or transplant dates. The objectives of this research were therefore to characterize the patterns of root production and mortality (turnover) of newly expanding root systems of recently transplanted sugar maples and to determine the effects of transplant timing and production system [B&B or pot-in-pot (PIP)] on these processes. The B&B system was predicted to have greater overall root activity as a result of the loss and subsequent replacement of fine roots when B&B plants are harvested for transplanting. The timing of transplanting was predicted to have a major influence on both root production and mortality reflecting physiological links between above- and belowground growth.
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