Effect of pruning time on growth, wound closure and

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Effect of pruning time on growth, wound closure and physiology of Sycamore maple (Acer pseudoplatanus L.). Article in Acta horticulturae · June 2013 DOI: 10.17660/ActaHortic.2013.990.9

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Effect of Pruning Time on Growth, Wound Closure and Physiology of Sycamore Maple (Acer pseudoplatanus L.) A. Fini and F. Ferrini Dept. of Plant, Soil and Environmental Science University of Florence 50019 Sesto Fiorentino (FI) Italy

P. Frangi, R. Piatti, M. Faoro and G. Amoroso Fondazione Minoprio - Centro MiRT 22070 Vertemate con Minoprio (CO) Italy

Keywords: reduction cut, removal cut, SPAD, winter pruning, woundwood coefficient Abstract The aim of this work was to evaluate different periods of pruning during the dormant season in order to determine which pruning period promote a fast wound healing and least affects tree regrowth and physiology. In spring 2005, 28 uniform 1012 cm circumference sycamore maples (Acer pseudoplatanus L.) were planted in an experimental plot at the Fondazione Minoprio. Trees were allowed to establish and grow undisturbed for three growing seasons. Thereafter, in winter 2007/2008, plants were pruned according to the following treatments: 1) pruning in December; 2) pruning in January; 3) pruning in February; 4) pruning in March. In each time of pruning, two cut types were carried out (removal and reduction). All plants were pruned in order to reduce leaf area by 1/3. Trees were pruned again in winter 2009/ 2010 with the same methods and treatments as in 2007/2008. The following biometric parameters were measured: stem diameter at 1.3 m, wound closure area, percentage of wound closure, base diameter and length of sprouts released or developed within 20 cm of the cut. Leaf chlorophyll content was measured three times during the vegetative season with a SPAD-meter. Results pointed out slight differences regarding wound closure, diametric growth of pruned branch, diameter of watersprouts and leaf chlorophyll content, therefore no strict indication of the preferred pruning time for sycamore maple emerged from this study. INTRODUCTION Pruning is one of the best things an arborist can do for a tree but one of the worst things we can do to a tree (Shigo, 1989). In fact, pruning is a “double-edged” sword either helping or hurting, depending on where, when and how it is applied (Gilman and Lilly, 2002). The major issues which can arise from improper pruning include the disruption of normal physiological processes and growth patterns and wounding (Shigo, 1990; Grabosky and Gilman, 2007; Fini et al., 2011). Wounds resulting from tree pruning cuts are an overriding point of access for several disease organisms like wood decay fungi (O’Hara, 2007). These pathogens reduce plant vitality and weaken its structure, thus endangering people and assets in the tree surrounding area (Loreti and Pisani, 1990). Plants can react to infections and minimize the pathogenic attacks through compartmentalization (Shigo and Marx, 1977; Oven, 2008). The effectiveness of tree reaction is related to the pathogen strength, to plant species and health and to the time of wounding (Dujesiefken and Stobbe, 2002). Several authors reported that the dormant season - late fall or winter - is the best time to perform formative structural pruning cuts on deciduous young trees, but a particular care has to be reserved to conclude pruning operations before bud swelling and sprouting occur (Coder, 2003). The specified period is however very long and extremely variable in terms of environmental conditions; temperatures in particular can vary a lot during these months. Changes in temperature may affect both pathogens cycle and callus formation, therefore affecting the duration of wound exposure to air before wound healing. Moreover, the release from dormancy is temperature-dependent (van der Schoot, 1996) and, from leaf fall to budbreak, buds go through different stages of dormancy. These include endodormancy (which in the experimental area is broken in late February in Acer, see Saebo et Proc. IInd IS on Woody Ornamentals of the Moderate Zone Eds.: J. Van Huylenbroeck et al. Acta Hort. 990, ISHS 2013

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al., 2006) and ecodormancy (Horvath et al., 2003). Therefore, in the light of the ongoing climatic change, which could anticipate or delay the phenological phases of the plants, there is need of new research aimed at identifying whether there is a preferential period for pruning shade trees during the winter. The aim of this trial was to evaluate different pruning time during the dormant season in order to determine which enables the fastest recovery from wounding and promotes the highest plant quality. MATERIALS AND METHODS In spring 2005, 28 uniform 10-12 cm circumference sycamore maples (Acer pseudoplatanus L.) were planted in an experimental plot at the Fondazione Minoprio. Trees were allowed to establish and grow undisturbed for three growing seasons. Thereafter, in winter 2007/2008, plants were pruned according to the following treatments: 1) pruning in mid December; 2) pruning in late January; 3) pruning in late February; 4) pruning in late March. A parallel forcing experiment confirmed that, buds were endodormant in December and January, ecodormant in late February, and swollen in March. In each time of pruning, two methods were compared: removal cut (branches were pruned back to their attachment to the stem) and reduction (branches were shortened by removing the distal end to a smaller lateral branch) (Fini et al., 2011; Gilman, 2012). All plants were pruned in order to reduce leaf area by 1/3. 6 cuts per plant (3 reduction cuts and 3 removal cuts) were marked with paint to be recognizable throughout the experiment and used for successive measurements. Trees were subjected to a second pruning cycle in winter 2009/2010 with the same methods and treatments as in 2007/2008. The experimental design was a split plot with 7 replicates, where pruning period was the main plot and the type of cut the subplot. Stem diameter at 1.3 m, base diameter of the pruned branches, base diameter and length of sprouts released or developed within 20 cm of the cut were measured 12 and 24 months after the first pruning cycle was done and 12 months after the second. Wound healing was measured using the woundwood coefficient (Schwarze, 2008; Fini et al., 2011). Leaf greenness index (a parameter directly correlated to leaf chlorophyll content, R2 = 0.9295 in sycamore maple, see Percival et al., 2008) was measured three times per year during the growing season with a chlorophyll meter (SPAD-502; Minolta, Osaka, Japan). Three fully expanded leaves per plant were used for this assessment. Stem diameter and leaf greenness index were analyzed with one-way ANOVA. Other parameters were analyzed with two-way ANOVA (where pruning period and type were the main plot and the sub-plot respectively). Means were separated using Duncan’s multiple range test. RESULTS AND DISCUSSION Stem diameter was not affected by pruning period throughout the experiment (Table 1). During the first cycle of pruning both wound area and wound healing were not influenced by the time of pruning (Table 2). However, after the second pruning cycle, cuts done in December and January healed faster than those done in March (Table 2). The pruning method affected wound size and healing in both the pruning cycles, as removing a branch directly in its insertion with the stem (removal cut) caused a larger wound than the reduction cut (Table 2). Despite of the larger wound area, wound healing was faster in removal cut than in reduction cut. No significant interactions between time of pruning and pruning method were found. Diameter of the branches selected and marked to be pruned was, at the beginning of the trial and before the second pruning cycle, homogeneous among treatments (Table 3). After one year from the first cycle of pruning, branches pruned in March showed a higher diameter increase in comparison with branches pruned in February, while the lowest diametrical increase was recorded on branches pruned in December. However the observed differences were not confirmed in the second year after the first cycle of pruning and after the second pruning cycle. Both number and diametrical growth of watersprouts developed within 20 cm from pruning cuts in the first cycle of pruning were not influenced by the period and by 100

the method of pruning (Table 4). A high production of watersprouts has to be considered a negative trait, because they may result in codominant branches (especially with reduction cuts) and give rise to a potential plant structural weakness (Fini et al., 2011). As reported in other studies, watersprouts and other adventitious shoots are in general less strongly inserted on a branch for their insertion at cambium level (Dahle et al., 2006). The absence of a significant effect of pruning period on the number of watersprouts was confirmed also in the second cycle of pruning. In this cycle a significant effect of the pruning period on watersprout dimension was found: February pruning resulted in a higher diametrical growth in comparison with pruning cuts done in the other periods of the winter. The number of watersprouts in the second cycle of pruning was influenced by the pruning method, resulting higher when reduction cut was used. Leaf chlorophyll content before pruning (2007) was uniform in all the treatments (Table 5). During the first vegetative season after the first cycle of pruning it was noted that plants pruned in January had a higher chlorophyll content in comparison with plants pruned in December or in March, while the lowest leaf chlorophyll content was recorded in plants pruned in February. No significant differences between the treatments were instead observed in the second year after pruning. But after a new pruning cycle during winter 2009-2010 differences in leaf chlorophyll content came out. In particular, pruning in February reduced leaf chlorophyll content in comparison with December and January prunings, while no difference emerged with pruning in March. CONCLUSIONS In this experiment we evaluate the effect of four periods of winter pruning and two pruning methods on tree growth and wound healing in sycamore maple. The pruning times were selected to be representative of different dormancy stages, since plants pruned in December and January were endodormant, those pruned in February were ecodormant, and those pruned in March were in the “swollen bud” phenological stage. However, despite of slight differences between treatments in wound closure, diameter growth of the pruned branch and of watersprouts developed or released after the cut, and leaf chlorophyll content, no strict indication of the preferred time to carry out winter pruning emerged from this study. Therefore, results of this study indicate that any time prior to bud-break is suitable for pruning a deciduous tree with high chilling requirements like sycamore maple. ACKNOWLEDGEMENTS This work has been done under the research project “Miglioramento delle tecniche produttive e della qualità del prodotto nel vivaismo ornamentale – TECPRO” financed by Regione Lombardia – Agricultural Department, according to the Plan of Research and Development 2009. Literature Cited Coder, K.D. 2003. Pruning shade trees. Acts of Cooperative Extension work, University of Georgia, College of Agricultural and Environmental Science and USDA, circular 628. Dahle, G.A., Holt, H.H., Chaney, W.R., Whalen, T.M., Cassens, D.L., Gazo, R. and McKenzie, R.L. 2006. Branch strength loss implications for silver maple (Acer saccharinum) converted from round-over to V-trims. Arboricult. Urban For. 32:148154. Dujesiefken, D. and Stobbe, H. 2002. The Hamburg tree pruning system – a framework for pruning of individual trees. Urban For. Urban Green. 1:75-82. Fini, A., Faoro, M., Amoroso, G., Piatti, R., Frangi, P. and Ferrini, F. 2011. Effect of pruning type on growth, physiology and breaking stress of maple trees. Proc. 87th ISA Conf. and Tree Climbing Championship, Paramatta, Australia, 23-27 July. Gilman, E.F. and Lilly, S.J. 2002. Principles, objectives, and pruning types. Arborist News 11(4):17-22. 101

Gilman, E.F. 2012. An illustrated guide to pruning (3rd edition). Delmar Cengage Learning, Independence, KY. 496p. Grabosky, J.C. and Gilman, E.F. 2007. Response of two oak species to reduction pruning cuts. Arboricult. Urban For. 33(5):360-366. Horvath, D.P., Anderson, J.V., Chao, W.S. and Foley, M.E. 2003. Knowing when to grow: signals regulating bud dormancy. Trends in Plant Sci. 8(11):534-540. Loreti, F. and Pisani, P.L. 1990. Structural manipulation for improved performance in woody plants. HortScience 25(1):64-70. O’Hara, K. 2007. Pruning wounds and occlusion: a long-standing conundrum in forestry. J. Forestry 105:131-138. Oven, P. 2008. Structural response of tree tissues to mechanical wounding. Proc. 84th ISA Annual Conf. and Trade Show. St. Louis, MO, 26-30 July. Percival, G.C., Keary, I.P. and Noviss, K. 2008. The potential of a chlorophyll content SPAD meter to quantify nutrient stress in foliar tissue of sycamore (Acer pseudoplatanus), English oak (Quercus robur) and European beech (Fagus sylvatica). Arboricult. Urban For. 34(2):89-100. Saebo, A., Fini, A. and Ferrini, F. 2006. Release from winter dormancy in trees used in the urban green areas in northern and southern Europe. Proc. 9th European Forum of Urban Forestry, Florence, 22-26 May. Schwarze, F. 2008. Diagnosis and prognosis of the development of wood decay in urban trees. Enspec, Rowville, Australia. 366p. Shigo, A. 1989. Tree pruning: a worldwide photo guide. Shigo and Trees, Associates, Durham, NH. Shigo, A. 1990. Tree branch attachment to trunks and branch pruning. HortScience 25(1):54-59. Shigo, A. and Marx, H. 1977. Compartmentalization of decay in trees. Agricultural Information Bulletin, No. 405. USDA Forest Service. Van der Schoot, C. 1996. Dormancy and symplastic networking at the shoot apical meristem. p.59-81. In: G.A. Lang, (ed.), Plant dormancy: Physiology, Biochemistry and Molecular Biology. CAB Intl., Wallingford, U.K.

Tables Table 1. Stem diameter measured at start of the trial and stem diameter increase (20082011) in sycamore maple plants pruned from December to March. Time of pruning December January February March Significance

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Stem diameter (cm) December 2007 5.9 6.0 6.0 6.0 n.s.

Stem diameter increase (cm) 2008-2011 8.0 7.9 8.1 8.6 n.s.

Table 2. Effect of time and method of pruning on wound area and wound closure after one or two years from pruning operations in sycamore maple plants.

Treatment

December January February March Significance Reduction Removal Significance Significance

Wound area (cm2)

First cycle Wound closure (%) after 1 year

after 2 years

Second cycle Wound Wound area closure (%) (cm2) after 1 year

Effect of time of pruning 44.6 72.8 2.32 52.6 78.7 2.06 54.3 87.5 2.47 46.5 71.3 2.13 n.s. n.s. n.s. Effect of pruning method 2.93 b 41.9 b 69.5 b 2.15 b 4.12 a 66.2 a 96.4 a 2.54 a ** ** ** ** Time of pruning x pruning method n.s. n.s. n.s. n.s. 3.30 4.49 2.99 3.40 n.s.

18.7 a 16.6 a 14.6 ab 8.4 b * 11.2 b 17.9 a ** n.s.

For each factor values in columns followed by the same letter are not significantly different according to Duncan’s test (n.s.: non significant; **: significant at P≤0,01; *: significant at P≤0,05).

Table 3. Diameter of the pruned branches measured before pruning operations and its increase after one or two years after pruning as affected by the time of pruning.

Time of pruning December January February March Significance

First cycle Branch diameter increase (cm)

Branch diameter (cm)

after 1 year

after 2 years

1.17 0.95 0.80 0.82 n.s.

0.51 c 0.70 bc 0.95 b 1.17 a **

0.30 0.34 0.33 0.30 n.s.

Second cycle Branch Branch diameter diameter increase (cm) (cm) after 1 year 1.33 0.66 1.22 0.69 1.07 0.87 1.21 0.69 n.s. n.s.

Values in columns followed by the same letter are not significantly different according to Duncan’s test (n.s.: non significant; **: significant at P≤0,01).

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Table 4. Effect of time and method of pruning on number and diameter of watersprouts developed within 20 cm from pruning cuts after one or two years from pruning operations in sycamore maple plants. First cycle

Treatment

December January February March Significance Reduction Removal Significance Significance

Second cycle Diameter of Watersprouts Diameter of watersprouts Watersprouts watersprouts within 20 cm within 20 cm within 20 cm within 20 cm from cut from cut from cut from cut (No.) (cm) (No.) (cm) after 1 year after 1 year after 2 years after 1 year Effect of time of pruning 0.63 0.37 0.05 0.74 0.41 b 0.61 0.47 0.13 0.71 0.41 b 0.91 0.67 0.41 0.77 0.74 a 0.83 0.37 0.17 0.54 0.46 b n.s. n.s. n.s. n.s. ** Effect of pruning method 0.52 0.41 0.21 0.97 a 0.51 0.43 0.55 0.33 0.42 b 0.50 n.s. n.s. n.s. ** n.s. Time of pruning x pruning method n.s. n.s. n.s. n.s. n.s.

For each factor values in columns followed by the same letter are not significantly different according to Duncan’s test (n.s.: non significant; **: significant at P≤0,01).

Table 5. Leaf chlorophyll content measured at start of the trial (2007) and in the following 3 years on sycamore maple plants pruned from December to March in winter 2007/ 2008 (first cycle) and in winter 2009/2010 (second cycle). Time of pruning December January February March Significance

Before pruning (2007) 43.2 43.9 41.2 44.0 n.s.

Chlorophyll content (SPAD) First cycle 2008 2009 38.0 b 39.2 40.2 a 41.9 36.0 c 39.2 38.6 b 42.0 ** n.s.

Second cycle 2010 40.1 a 40.6 a 38.4 b 39.3 ab *

Values in columns followed by the same letter are not significantly different according to Duncan’s test (n.s.: non significant; **: significant at P≤0.01; *: significant at P≤0).

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