4 Buhler et al.: Effect of Different Irrigation Regimes on Growth and Phenology of Tilia cordata average stem circumference at 1 m (3.3 ft), however, was 499 mm (19.6 in) (S.D. ± 54 mm [2.1 in]). The soil between the park trees was covered with herbaceous vegetation. Irrigation The street trees were treated with 5 different irrigation regimes including a control receiving no irrigation. Both the total amount of water supplied (640, 320, or 280 L [169, 84.5, or 74 gal], 140 L [37 gal], no irrigation) and the irrigation frequency varied (Table 1). Water was supplied manually with a water hose equipped with a sprayer and a water meter. We attempted to spread the water evenly over the whole surface of the planting pit. Curbstones surround- ing the planting pit prevented runoff. Growth Response Stem circumference was measured weekly at a stem height of 1 m (3.3 ft) in the period from 5 April until 30 September using a measuring tape. From these measurements, stem cross-sectional area, absolute stem area increment, relative stem area increment, and annual ring width were calculated. Statistics and Data Analysis The 5 treatments (Table 1) were assigned to the 100 street trees according to a completely randomized block design consisting of 4 blocks with 25 trees each. Thus, each block contained 5 replicates of each treatment, amounting to 20 replicates altogether. Two of those blocks were situated on the northern side of the street, and the other two blocks on the southern side. Based on the stem circumference measurements, stem cross-sectional area (CSA) values were calculated on tree level for each assessment time. For each tree, CSA at the first and at the last assessment time of the year was used to calculate the total CSA increment during the entire growing season of 2004. For each of the 22 measurements, the absolute increment was calculated. Finally, the percentage of total yearly CSA increment was calculated for each tree and assessment time. These data illustrate the relative distribu- tion of increment over the growing season, ranging from 0 to 100%. The distribution of these data is S-shaped, and a range of models was tested for their suitability to describe the relative increment curves. The Gompertz growth function (Equation 1) proved to be most suitable for the present data set and was fitted to the stem growth data of every single tree. I =100*e−e −b* day−c () () (1) Systematic model errors were found to be similar for all treatments around day 130 and day 270. It was therefore decided to estimate the day for onset and cessation of growth around these to days in order to minimize the influence from model bias on treatment effects. These two days correspond to an average of 4% and 98% of the yearly increment, respectively. Thus, the days for onset and cessation of stem diameter growth were estimated for every single tree using Equations 2 and 3 (parameters b and c are from the Gompertz curve fit). day of onset = c + ln(ln( 100 ) − ln( 4)) −b day of cessation = c + ln(ln(100 ) − ln( 98)) −b (2) (3) Data were analyzed by means of SAS version 8.02 (SAS Institute, Cary, NC) and Microsoft Excel 2000. CSA incre- ment was subjected to an analysis of variance (ANOVA) using the GLM procedure in SAS, including block and treatment effects. The beginning and end of the growing season were determined using the NLIN procedure in SAS. Table 1. Frequency and time of irrigation and amounts of water distributed during each irrigation. “Equivalent precipitation” denotes the amount of precipitation corresponding to the amount of water applied by irrigation. Date May 26/27 June 10 July 15 July 26 August 2 August 11 August 26 Whole period Equivalent precipitation (mm) ©2006 International Society of Arboriculture Day of year 147/148 162 197 208 215 224 239 Treatment 1 (L/tree) 100 100 80 80 100 100 80 640 100 Treatment 2 (L/tree) 50 50 40 40 50 50 40 320 50 Treatment 3 (L/tree) 50 — — 40 — 50 — 140 22 Treatment 4 (L/tree) 100 — — 80 — 100 — 280 44 Sum (L) 6,000 3,000 2,400 4,800 3,000 6,000 2,400 27,600
January 2006
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