Effect of pruning type on growth, physiology and breaking stress of maple trees

The aim of this work was to evaluate the effects of different pruning cuts on succeeding growth, physiology and breaking stress of sycamore maple (Acer pseudoplatanus L.). In spring 2005, 28 uniform 10-12 cm (4-5 in.) circumference maples were planted in the field at the Fondazione Minoprio (Vertemate con Minoprio, Como, Italy; 45°44’ N, 9°04’ E). Trees were allowed to establish and grow undisturbed for 3 years. In February 2008, plant were pruned according to the following treatments: • Topping (T, 7 plants): pruning cuts were done in the in the internode of two-years old branches • Removal cut (RM, 7 plants): branches were cut at their insertion with the stem, having care not to damage branch collar. • Reduction cut (RD, 7 plants): branches were cut back to a lateral with sufficient size to become a new leader. • Control (C, 7 plants): plants were left unpruned. All plants, except controls, were pruned in order to reduce leaf area by 1/3. Pruning material was weighted immediately after pruning. 6 pruning cuts per plant (42 per treatment) were marked with paint, in order to be recognizable and monitored for 2 years. In control trees, paint was used to draw 6 imaginary cuts per plant, which were monitored as well through the experiment. Imaginary cut where drawn close to a lateral of sufficient size to become new imaginary leader. In February 2010, trees were pruned again using the same treatments. The experimental design was a one-tree per replicate complete randomized design with 7 replicates. Leaf gas exchange (photosynthesis, A, mol m-2 s-1; transpiration, E, mmol m-2 s-1, stomatal conductance, gs, mmol m-2 s-1 and Water Use Efficiency, WUE) were measured 7 times with a portable infrared gas analyzer (Ciras-2, PP-System). Assimilation to internal leaf CO2 concentration (A/Ci) curves were drawn by varying external CO2 concentration from 0 to 1800 ppm. Leaf chlorophyll content was measured three times with a SPAD-meter (SPAD, Minolta). In July 2008, 2009 and 2010, ten leaves per tree (70 per treatment), were scanned with A-3 scanner to determine leaf area. Leaves were then put in a oven to determine dry weight. LMA was then calculated as the ration between dry weight and leaf area. Starch content was measured from 1 year old excised shoots using standard methods. Shoots for starch determination were harvested after leaf fall (November) and after leaf expansion (June). In winter 2008, 2009 and 2010, wound closure was measured on all cuts using the Woundwood Coefficient. Biometric parameters were measured during the dormant season in 2008, 2009 and 2010. Measured parameters were: 1) stem diameter (at 1.3 m); 2) number of suckers originated after pruning; 3) number, diameter and lenght of watersprouts originated within 20 cm from pruning cuts; 4) Length and diameter of the pruned branches; 5) Length and diameter of the leader shoot of the branch (in topping treatment, where no apical shoot was left in the branch, the longest of the epicormic shoots originated after the cut was considered as the new leader). The stress required to cause the failing of the attachment between the primary branch and 1) the lateral branch which was selected as new leader (RD); 2) the lateral branch originated after topping which was selected as new leader (T); 3) the lateral branch normally attached to a primary branch which was selected as imaginary leader (C) was measured 2 years after pruning. Results will be discussed in the presentation.