Macadamia is an evergreen tree grown in subtropical regions around the world for its edible kernel. The tree grows by recurrent flushing and flower racemes develop in leaf axils between 40 and 140 cm from the branch apex depending on cultivar and season. Anthesis occurs in early spring (September in Australia) and fruit mature ≈30 weeks later, abscise over six months, and are harvested regularly from the orchard floor. Trees may grow to 18 m (Cull, 1983) and leaf density is high at over 400 leaves/m3 (Olesen et al., 2011). Australian orchards tend to be planted at 7 or 8 m between rows and 3 or 4 m between trees within rows. The trees take several years to come into production, about three years for the most precocious cultivars and six years for the least precocious. Once in production, yields tend to increase with increasing tree size, up to very high levels of orchard light interception [≈94% (McFadyen et al., 2004)]. At even higher levels of light interception, there is some indication that crowding leads to a decline in yields (McFadyen et al., 2004), although the evidence depends heavily on the yields from one site in a regional study. One challenge in confirming yield decline is the large seasonal variation in yield that reflects a tendency to alternate bearing (Wilkie, 2009).
In addition to the potential for yield decline, tree crowding presents a host of management problems, including loss of groundcover and consequent soil erosion, slow orchard floor drying after rain resulting in harvest delays and deterioration in nut quality, greater pest and disease risk, and a reduced ability to spray trees effectively.
Tree removal has been used in a bid to maintain production in crowded orchards and to alleviate the management problems. However, it is expensive, results in an initial reduction in yield per hectare of 15% to 50%, and it can take up to 10 years for yield to recover to that of an unthinned orchard (McFadyen et al., 2003, 2005; Robertson et al., 2012). Regularly hedging the sides of small trees maintains light and ventilation to the lower canopy and orchard floor between tree rows and, for nine-year-old trees, caused less yield loss than tree removal (McFadyen et al., 2004). However, as tree height continues to increase, the orchard floor and lower canopy become shaded for most of the day, diminishing the benefit of the open interrow.
Manual pruning of mature trees has been trialed in other tree crops, where orchard crowding is a problem, to reduce intra- and intertree shading to increase or maintain consistent productivity, and in some cases to reduce tree height. Generally, this has involved the removal of several whole limbs as close as possible to the main stem(s) or a lateral branch. In pecan (Carya illinoinensis), the one-off removal of limbs that competed for light with adjacent limbs or trees increased yield in two of three cultivars but only for one season out of three (Lombardini, 2006). In avocado (Persea americana), reducing the number of main scaffold branches in trees from 8–12 to 6–8 increased productivity on the remaining branches but yield per tree was similar to that of unpruned trees (Thorp and Stowell, 2001). In sweet orange (Citrus sinensis), removal of two or three large scaffold branches reduced yield with the effect increasing with the weight of fresh wood removed (Kallsen, 2005). In macadamia, manual pruning of six-year-old trees reduced yield in proportion to the severity of pruning (Olesen et al., 2011), but the response to pruning older, more heavily shaded trees has not been reported.
In pecan, annual removal of one or three limbs per tree to reduce tree height from 18 to 9 m did not reduce yield (Worley, 1991). However, in a companion study, removal of one to three limbs per year to reduce tree height from 18 to 9 or 12 m decreased yield in both treatments by an average of 21% over 19 years (Worley and Mullinix, 1997). In avocado trees that were 5 to 8 m tall before treatment, branches were pruned at their junction with strong lateral growths, at 4 or 6 m. Yields were reduced in trees pruned to 4 m but not in those pruned to 6 m (Thorp and Stowell, 2001).
Mechanically pruning the tops of trees or “topping” is an easier and cheaper means of reducing tree height than manual removal of whole limbs. However topping, like side-hedging, involves numerous small cuts that stimulates more post-pruning shoot growth than a few large cuts as accomplished with manual pruning (Mika, 1986). This enhanced shoot growth has the potential to inhibit flowering and fruit development (McFadyen et al., 2011; Mika, 1986; Olesen, 2005). Topping has been extensively tested in citrus (Citrus sp.) and had either no effect on average yield in the seasons after topping (Fucik, 1977, 1988; Kallsen, 2005) or reduced yield (Eissenstat and Duncan, 1992; Fucik, 1977; Morales et al., 2000; Whitney et al., 1983). In some hedging and topping studies in citrus, there were no differences in average yield between pruned and unpruned trees for the years following pruning because pruning mitigated alternate bearing (Bevington and Bacon, 1978; Fucik, 1977; Raciti et al., 1981). Topping in pistachio (Pistacia vera) also mitigated alternate bearing, with topped trees producing similar average yields over six years compared with untopped trees (Ferguson et al., 1995). Hedged and topped pecan trees had lower yields than unpruned trees (Worley, 1985), but there is no information on the impact of topping alone for pecan.
The first aim of this study was to better characterize the long-term trends in macadamia production by continuing to monitor the yields at the two most crowded sites established by McFadyen et al. (2004) and at two other sites that were at a similar stage of crowding. The second aim was to examine the effects of manual and mechanical pruning on yield, nut characteristics, tree size, and economics in crowded orchards. Three pruning experiments were conducted. In the first experiment, the effects of either selective limb removal or side-hedging were quantified, relative to control trees. In the second experiment, mechanically topped trees were compared with untopped control trees. In the third experiment, the effects of either mechanical topping, selective limb removal in the upper canopy to reduce tree height, or the removal of a codominant leader were quantified, relative to control trees.
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