296 treatments and concluded that defoliation above 40% has long- term effects on the health of Cocos nucifera. Their meth- od, however, differed from the practices of arborists in that Bailey et al. cut various percentages of pinnae (leaflets) from all leaves in the crown, rather than starting from the bottom of the crown and removing whole leaves. Magat and colleagues have been carrying out studies on CLP at the Davao Research Center in the Philippines for a number of years. Initially, they pruned palms to retain 23, 18, or 13 young- est leaves out of 31 total, and found no significant decrease in yield after one year of experimentation for all three treatments (Magat and Habana 1991). Later, Magat et al. (1994) tried maintaining CLP for a longer period of time and got different results: Although there was once again no decrease in yield dur- ing the first year, trees retaining the 13 youngest leaves showed a 29% loss in production of nuts in the second year followed by a further 20% reduction in the third year (a nearly 50% de- cline in nut production after three years). Treatments retain- ing the 18 youngest leaves caused no significant decline and even showed an improved yield in the third year. Thus, it was concluded that maintaining 18 functional leaves in the crown was sufficient to provide optimum yield in Cocos nucifera. Contrarily, an experiment conducted at the Davao center be- tween 1993 and 2001 recorded a 20% to 25% decline in the yield of palms pruned to retain 18 youngest leaves, except where coffee was used as an intercrop. It was suggested that the lack of significant results in the case of coconut + coffee may have been due to coffee’s high concentration of nutrients in leaf litter (Secretaria et al. 2003). The 1991 and 1994 reports by Magat and colleagues are not the only studies to have found Cocos nucifera unresponsive to pruning during the first year of treatment. Aterrado and Abad (1998) pointed out that no changes in yield occurred within the initial year of pruning. After that, palms deprived of 25% or more of their foliage exhibited decreased fruit set. Eroy et al. (2001) re- ported that CLP did not significantly affect yield in the first year, but nut yield was reduced by an average of 21% per tree after two years. The method and amount of pruning were not mentioned. Extensive research has also been carried out on the effects of pruning on fruit yield in oil palm because older leaves of this spe- cies are customarily removed to facilitate the fruit harvesting pro- cess. Oil palm is similar to coconut palm in form, although it has more leaves (as many as 64) in the crown (Tajudin and Yeoh 1987; Meerow 1992). Calvez (1976) reported that pruning oil palm to retain the 17 youngest leaves on just one occasion, allowing the crown to recover immediately afterwards, caused significant fruit abortion. A summary of the results of several oil palm studies was provided by Henson (2002). He concluded that fruit yield was maximized under maximum retention of leaves, that pruning of older/lower leaves was less damaging to fruit yield than prun- ing of younger/higher leaves, that the effects of pruning did not show up until after 8 to 10 months, and that younger specimens recovered from pruning effects more quickly than did older trees. EFFECTS ON SUSCEPTIBILITY TO PESTS, DISEASE, AND COLD It has been suggested that the interweaving of leaf bases sur- rounding the stem at the crown acts as a buffer in palms to pro- tect the meristem (palm heart) from cool temperatures, so leaf Rosenfeld: Effects of Pruning on the Health of Palms pruning may leave palms more vulnerable to cold (Bailey 2002). Broschat and Meerow (2000 p. 221) wrote, “In the severe 1989 freeze that struck Florida and other areas of the United States, specimens of Sabal palmetto that were over-trimmed in north- central Florida suffered damage, while those left with a normal canopy and full complement of leaf bases were unscathed.” When palm leaves are lopped off at the petiole base, as is stan- dard practice among arborists, plant tissue become exposed and are said to be highly susceptible to infection from disease, such as Phytophthora, which threatens coconut palm in Hawaii (Uchida et al. 1992). Fusarium oxysporum has spread among ornamental palms in the U.S. partly by means of improperly cleaned prun- ing saws coming into contact with this sensitive tissue (Pfalzgraf 2002). Calvez (1976) did not find a higher incidence of disease or insect attack in oil palm subjected to pruning treatments, al- though pruning was applied only once and not maintained. SUMMARY OF MAJOR POINTS * Pruning increased the rate of production of new leaves, but size of new leaves decreased as a result of higher levels of pruning in all experiments in which leaves were measured. * Nutrient concentrations in retained leaves were not much affected by leaf pruning in healthy palms, but nutrient- deficient palms experienced a worsening of deficiency symptoms when older leaves were removed. * Studies with oil palm found that heavily-pruned palms were more susceptible to crown fracture under the force of strong winds than those left unpruned. * Coconut leaf pruning was shown to have significant nega- tive effects on fruit production when fewer than 18 young- est leaves were retained. * Pruning of younger/higher leaves has shown a stronger negative impact on fruit yield than has pruning of older/ lower leaves for both coconut and oil palm. * For all studies, there was an 8 to 12 month delay before maintained leaf pruning began to show its effects on fruit yield for coconut palm and oil palm. * Scientific research is lacking on the question of whether or not leaf pruning causes reduction of trunk diameter (stem tapering) in palms. * Uncontrolled observation of Sabal palmetto following the 1989 freeze in Florida lends some support to the hypothesis that leaf pruning renders palms more susceptible to cold temperatures. DISCUSSION The information gathered in this literature review lends some support to the hypothesis that routine green leaf pruning struc- turally weakens and reduces the size of the crown of palm trees over time and compromises their productive capacity, but there is still a need for research tailored to ornamental palms. Palms that are regularly pruned often exhibit leaves that ap- pear shorter and less turgid than those of unmaintained speci- ©2009 International Society of Arboriculture
November 2009
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