Arboriculture & Urban Forestry 35(1): January 2009 Establishment There are several measures of plant establishment: re-establis- hment of the tree height-to-root spread ratio (Watson 1985; Gilman 1997), resumption of a pretransplant shoot elongation rate (Struve and Joly 1992), or restoration of shoot xylem water potential relative to untransplanted control trees (Beeson 1994; Beeson and Gilman 1992; Gilman 1992). The re-establishment of the branch-to-root spread ratio is most dependent on the rate of root elongation because shoot growth is typically suppressed during the establishment phase. The rate of establishment, under nonlimiting edaphic conditions, is dependent on the length of the growing season, which affects the root elongation potential. Maintaining adequate soil moisture will also increase the root elongation potential. Because of the longer growing season in Florida (USDA Plant Hardiness Zone 9), a 5.1 cm (2 in) cali- per tree will establish in 6 months, whereas in northern Illinois (USDA Plant Hardiness Zone 5), establishment would take 24 months (Gilman 1997). The establishment period in USDA Plant Hardiness Zone 5 is based on the assumption of 18 in of root elongation per year. Missouri Gravel Bed System To determine the root elongation potential of transplanted 7.6 cm (3 in) diameter red oak in USDA Plant Hardiness Zone 5, red oak trees were balled and burlapped in April and heeled-in beds of pea gravel or wood chips in a modified Missouri gravel bed system (Starbuck et al. 2005). Before heeling-in, half the trees were bare-rooted by washing the soil from the root sys- tems resulting in four treatment combinations: bare root or balled-and-burlapped trees combined with heeling-in either wood chips or pea gravel. In September, the plants were removed from the heeling-in areas. Regenerated root lengths were mea- sured and shoot growth for the year before transplanting and for the next 3 years was measured. Of the 24 trees transplanted, 19 survived. All mortality occurred in the bare root trees heeled-in the fresh wood chips. High temperatures resulting from the composting wood chips killed the bare root trees. However, under the benign conditions of the heeling-in beds, root growth of the five longest roots per tree ranged between 52 and 61 cm (21 and 24 in) for the bare root and balled-and-burlapped trees, respectively, heeled-in pea gravel. All plants had well-established mycorrhizal associa- tions, which formed despite weekly fertilization with 100 mg/L nitrogen water-soluble fertilizer. After measuring, the trees were transplanted to a lawn panel on campus and growth followed for the next 3 years. Based on the lengths of root regenerated between April and September, the trees will have re-established the 3:1 root spread-to-height caliper ratio the second year after transplanting, which is significantly faster than that esti- mated for 3 in caliper trees in USDA Plant Hardiness Zone 5. Alternatively, if establishment was based on the resumption of the pretransplant shoot elongation rate, then the trees were estab- lished within 1 to 3 years of transplanting. Another benefit of the heeling-in period was development of a root system that neces- sitated digging a wide planting hole with gently sloping sides, the type recommended in the Tree Planting Best Management Practices (Watson and Himelick 2005). A Missouri Gravel Bed system may aid in the establishment of difficult-to-transplant trees, in which the critical stages of root regeneration occur under benign conditions. Tree Size It is accepted that large-caliper trees take more time to establish than small-caliper trees, in part as a result of the greater time required to re-establish the preharvest root diameter-to-tree height ratio. However, there are confounding factors to consider when comparing the time to establishment of small- and large- caliper trees. These factors include the following. Large-caliper trees are typically the genetic runts in a nursery block. Faster growing trees are harvested at smaller caliper sizes because they are the first to reach salable size. This is true for both clonal material in which within-clone differences in tree size can be attributed to differences in the degree of graft incompat- ibility, in root stock vigor, in root system quality among rooted cuttings, or in pathogen load. Large-caliper trees are in a reduced state of vigor from assaults associated with previous harvesting of the faster growing trees harvested at smaller caliper size. These assaults include com- pacted soils, root pruning, and mechanical damage (Struve et al. 2000). Additionally, irrigation systems are typically removed (or damaged) and cultural practices such as pest control and fertiliza- tion may be curtailed after the initial harvests have begun. Also, if large-caliper trees have not been transplanted every 3 to 4 years during production, survival and establishment may be compro- mised because larger diameter roots are severed. Large-diameter roots regenerate roots slower that small-diameter ones (Johnson et al. 1984). Regular root pruning increases root system density relative to unroot-pruned trees (Watson and Sydnor 1987). In an experiment comparing the survival and posttransplant growth of 7.6 to 10.2 cm (3 to 4 in) and 3.8 to 5.1 cm (1.5 to 2 in caliper trees, it was found that larger-caliper trees had higher mortality than smaller caliper trees (Struve et al. 2000). However, based on the prediction equations estimating caliper growth (using the data from the 4 years after transplanting), the surviving large-caliper trees have greater caliper increase than transplanted small-caliper trees. The untransplanted small-caliper control trees are predicted to reach similar size as the large-caliper trans- planted trees 11 years after transplanting. In this study, many of the confounding effects associated with large-caliper tree pro- duction were avoided. Thus, at least in this study, there was no evidence that the surviving large-caliper trees established slower than the small-caliper trees. Biostimulants Biostimulants are proprietary nonnutritional products that are marketed to mediate the effects of plant stress. Most products are mixtures of plant hormones, humates, manures, and/or sea kelp extracts. Their benefit as transplanting aids has been mixed (see Sammons and Struve 2004). In a summer digging study with Goldenraintree ( Koelreuteria paniculata Laxm.), field-grown trees were treated with Bioplex (Mt. Joy, PA) as foliar spray, soil drench, a combination of foliar spray, and soil drench 3 days before har- vestings as balled-and-burlapped trees or left untreated (Sammons and Struve 2005). Bioplex treatment reduced transpiration for 3 days after application, relative to untreated plants. After 3 days, the transpiration rate of treated plants was similar to untreated plants. Three days after treatment, some of the plants were dug, whereas others were left untransplanted. Digging Bioplex-treated and untreated plants significantly reduced transpiration relative to nondug control trees. Despite the undersized balls (55.9 cm [22 in] diameter balls on 6.1 cm [2.4 in] caliper trees), minimal aftercare, ©2009 International Society of Arboriculture 11
January 2009
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