184 DEEP STRUCTURAL ROOTS: NURSERY PRODUCTION ISSUES Tree root development in a natural forest environment differs markedly from that in a nursery production environment. When a seed falls to the ground and germinates, the primary root emerges from the seed and grows down in response to gravity. Growth of the primary root slows when it encounters the denser, less aerated conditions of deeper soil regions. Such growth cessa- tion may occur quite close to the surface in compacted or poorly drained soils, especially in species with weak primary roots. Roots may grow somewhat deeper in strong taprooted species and on well-drained sites. As growth of the primary root slows, small lateral roots form near the soil surface (Eshel and Waisel 1996; Stokes 2000), eventually forming the root flare or collar. Though many nursery trees and rootstocks are also propagated by seed, the rooting environment is utterly different from most natural ecosystems. The common production process is to ma- nipulate the root system repeatedly by successive pruning and transplanting events in order to produce a more compact and thus more easily transplanted tree. In field production, trees are fre- quently grown for one or two seasons in seedbeds, after which they are mechanically harvested and the primary root is pruned to a depth of about 10 cm (4 in). In response to primary root pruning, new roots generate predominantly from the cut end, but occasionally higher up on the primary root (Harris et al. 2001). Most of the natural lateral roots above the cut end do not per- sist; thus the newly-generated roots become the primary lateral roots, forming an “adventitious root flare” (Hewitt and Watson 2009) (Figure 1). These early changes in root architecture are im- portant because the major roots established during the first few years constitute the main structural root system (Coutts 1983). Day et al.: Deep Structural Roots in Urban Trees Seedlings are then planted in rows, maintaining the same root depth as in the seedling bed. Typically after one year of growth in the field, the top of the tree is cut back to a low lateral bud so that next year’s growth will be vigorous, resulting in a straight trunk, except where the shoot originated. This area where the young vigorous trunk meets the rootstock ultimately becomes the “dogleg” visible on young nursery trees. This characteristic also holds true for the graft union on grafted cultivars. Nursery opera- tors prune young shade trees in this way to take advantage of the fact that the root system has already been established for a year before cutting back and the new shoot growth is rapid. The stem can be trained into a single straight trunk in one year to produce a “whip” or “liner,” which is then transplanted into the field or con- tainers for growth to saleable size. This above- and belowground pruning can thus create two identifiable portions of the trunk/root system not normally seen in undisturbed seedlings: the “dogleg” or jog in the lower trunk and the “root shank,” a carrot-like por- tion of the upper primary root that has few or no lateral roots. During production, the length of the “root shank” and morphol- ogy of main lateral roots can be affected by root pruning practices. For example, Harris et al. (2001) found that severity of root prun- ing during early growth of pin oak seedlings (Quercus palustris) was positively correlated with the number of main lateral roots formed. However, very severe root pruning (to 5 cm below soil) also reduced both root and shoot dry weights. Because most roots generate from the cut end, choice of root pruning depth must bal- ance the effects on root shank length and lateral root formation against maintaining vigor in the plant. In container production of very young trees (liners), growth of primary roots is usually stopped by air pruning, in which exposure to air kills back the growing root tip, rather than by mechanical pruning. When the tip is killed, limited regeneration will occur from the tap root or radicle because of these same unfavorable conditions, although intact lateral roots will continue to proliferate. Similar artificial stagnation of taproot growth in experiments has encouraged growth of lateral roots sooner than on seedlings where radicles were allowed to continue growing (Eshel and Waisel 1996). This may lead to a less pronounced adventitious root flare and more lateral roots closer to the soil surface. Rathjens et al. (2008) found that the depth of the main lateral roots of nursery trees varied by nursery, but not by production method, i.e. seed vs. cutting vs. budding. This suggests that variations in production techniques early on may influence structural root depth. Whether these dif- ferences were related to the length of the root shank is unknown. A long root shank can create the situation at planting where, Figure 1. Excavated root system of approximately four-year-old nursery tree showing root and trunk regions resulting from prop- agation and nursery production practices. ©2009 International Society of Arboriculture if the structural roots are placed near the soil surface, a portion of the root shank will be above ground. This raises the question, “Is it root or shoot?” Perhaps a more direct question is, “Does the root shank need to be gradually acclimated to exposure or pro- tected from the elements?” This is a controversial question among many growers and arborists and to the knowledge of this paper’s authors, has not been addressed by published research. The tran- sition from root tissue to shoot tissue can be a gradual one. Bark characteristics can change gradually and may depend upon ex- posure to soil. For example, a comparison of root and shoot bark anatomy in English oak (Quercus robur), revealed that there were relatively few structural or anatomical differences between root and stem bark overall: the zone of cell differentiation was wid- er in root bark, the root phelloderm was more distinct, sclereid quantity decreased with increasing distance from the stem, and
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