Pruning is the cutting away of vegetation from plants for horticultural purposes. In apple production, trees are pruned to open the canopy to sunlight, facilitate fruit bud formation, promote fruit ripening and color development, facilitate the movement of air and sprays of protective chemicals, keep tree size within desirable limits, and manipulate the natural balance between vegetative and reproductive structures (Ferree and Schupp, 2003; Fumey et al., 2011; Jonkers, 1982). Pruning has long been recognized as a dwarfing practice, resulting in trees that are smaller than unpruned trees, with effects on vegetative growth, flowering propensity, fruit quality, size, and yield (Gardner et al., 1922; Seleznyova et al., 2013). The effects of pruning are cumulative, influencing tree growth, tree size, and fruiting habit for years to come. Pruned apple trees are smaller, produce higher quality fruit, and have physical structure that is more conducive to high density orchard systems than nonpruned trees.
Pruning manipulations are categorized as either heading or thinning cuts. Heading cuts involve shortening existing structures by partial removal of a shoot or limb, leaving another portion from which new growth can develop (Ferree and Schupp, 2003). A specific type of thinning cut called a renewal cut involves removal of whole limb structures, except for a short stub. Renewal cuts are used to alleviate limb crowding and shading by removing large limbs while promoting the regrowth of replacement shoots near the point of origin (Wertheim, 1968).
Methods of studying the effects of varying degrees of severity of pruning treatments have ranged widely. Shoots were cut to specified lengths during the dormant season (Gardner et al., 1922), or were cut to specific fractions of their original length (Jonkers, 1982). Gardner describes whole-tree studies in which pruning severity was classed with qualitative rather than quantitative terms, such as heavy, moderate, and light pruning (Gardner et al., 1922). Howe (1923) compared growth and fruiting of young apple trees over a 10-year period. Trees were pruned from years 2 to 4 with either light corrective pruning or corrective pruning plus heading of one half to two thirds of the scaffolds. Lightly pruned apple trees developed a larger canopy, and were more precocious, but pruning effects on yield were not consistent across varieties. Some studies compared pruned with nonpruned trees without varying the levels of severity or severe shoot heading in dormant pruned vs. summer pruned trees (Marini and Barden, 1982a, 1982b; Fumey et al., 2011). Elfving and Forshey (1976) reported that heading successively longer sections of 1-year-old wood from vigorous 8-year-old ‘Delicious’/ ‘M.7’ apple trees increased shoot growth and decreased subsequent fruiting because of reduced bearing surface. Although the benefits of pruning on growth, yield, fruit size, and fruit quality have long been known, the large, complex tree architecture of apple trees on vigorous rootstocks, coupled with fruits being born on spurs originating on 2-year-old wood and older created difficulties with establishment of simple repeatable pruning severity thresholds for whole trees.
Apple trees on dwarfing rootstocks planted at high density have become the industry standard and have gained wide acceptance (Robinson, 2003). In high-density “spindle” systems, every primary limb (a limb that originates from the main trunk or “spindle”) is expected to be removed at some point, with large limbs preferentially removed, and the central leader left as the only permanent part of the canopy. The productivity and simplicity of the tall spindle system has led to wide acceptance (Robinson et al., 2006), in part, because it leads to ease of automation and simplification of management. In this system, removal of the largest limbs is most important. Large limbs tend to be more vigorous and vegetative, and less fruitful than smaller limbs. Removing large limbs stimulates smaller, more fruitful, and renewal limbs that better fit the tall spindle orchard system.
In the renewal pruning of a spindle system during dormancy, excess side branches are removed preferentially by size, leaving a short beveled stub at the base to stimulate renewal growth (Wertheim, 1968). Renewal branches develop and replace the current side branches when these have grown too large and are removed by pruning. Renewal pruning is key to maintaining fruit quality through renewing spurs and creating a favorable light environment. The optimum number of fruiting laterals that should remain after pruning to optimize crop production was described for peach (Marini, 2003) but is not well defined for apple. Unlike peach, apple bears fruit from mixed buds on different ages of wood, with the primary bearing surface comprising spurs on wood that is at least 2 years old.
In the past, the volume and branching complexity of apple tree canopies made it difficult to create a simple, predictable, and repeatable whole tree metric of pruning severity. Much of the literature addressing pruning severity of apple is for heading cuts made on young trees in training systems with permanent scaffolds. The effects of varying renewal pruning severity in spindle systems seems little studied, yet this style of pruning and orchard system are becoming predominant. The adoption of smaller, simplified tree canopies should allow an accurate and repeatable method for establishing pruning severity levels to be developed. To our knowledge, no whole-tree studies that quantify pruning severity have been attempted.
Trunk cross-sectional area (TCSA) has a positive linear relationship with total aboveground weight, can be used to estimate the bearing surface of a tree (Westwood and Roberts, 1970) and is frequently used by pomologists to standardize fruit number per tree based on tree size (Lombard et al., 1988). Similarly, calculating crop density of limbs (LCSA) is an effective subsampling technique for estimating crop density (Forshey and Elfving, 1979). As part of the centrifugal training system (Lauri et al., 2004) a hand-thinning gauge was developed to estimate the target crop load of a limb, based on limb cross-sectional area (Equilifruit; INRA, Montpelier, France). Kon and Schupp (2013) demonstrated that use of the hand-thinning gauge was effective in tall spindle trees, but suggested that yield and final crop density would be a function of total limb cross-sectional area. Thus, identifying the optimal ratio of LCSA to TCSA seemed to be a logical approach to establishing pruning severity thresholds.
Goals for cropload adjustment are partially met through removal of potential fruiting sites, the most drastic of which is whole-limb removal. We hypothesized that combining the per-branch crop load goals indicated by Lauri et al. (2004), with the whole-tree cropload goals given in Kon and Schupp (2013), appropriate levels of pruning severity could be specified to achieve specific cropload potentials for given tree sizes. Removal of fruiting structures could be carried out by whole-limb pruning alone to achieve a favorable balance between limb area and tree size.
Here we propose and implement a method for quantifying renewal pruning severity in the tall spindle system that involves measurement of all limb structures as they emanate from the spindle and sequential removal of largest branches until a required pruning severity index value is reached. The goals of this study are to quantify renewal pruning severity, implement treatments of varying levels of severity, assess the vegetative and fruiting responses of trees to these severity levels, and provide consistent quantifiable guidelines for pruning severity. Such guidelines could provide engineers and horticulturists with measurable and sound rules for designing automated pruning devices and systems.
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