Arboriculture & Urban Forestry 32(1): January 2006 5 Subsequently, stem increment and growth period means were compared using least significant difference (LSD) comparisons at P < 0.05. Stem increment means were further separated with a Duncan test. Relevant climate data were obtained from the Danish Meteorological Institute (DMI 2004). Table 2. Stem increment data for the growing season of 2004, means of 20 replicates (± standard deviation). Irrigation treatment 1 (640 L) 2 (320 L) 3 (140 L) 4 (280 L) Control RESULTS Precipitation Precipitation distinctly surpassed the average during the period April to Septem- ber 2004. Altogether, 320 mm (12.6 in) (corresponding to 2,048 L [540.9 gal] per planting pit) of precipitation were recorded for the capital area in this period, exceeding the average values of 1961–90 by 55 mm (2.2 in), corre- sponding to 352 L (98 gal) of water per planting pit (DMI 2004). As is valid for the irrigation water, runoff was supposedly minimal due to the elevated curb. However, unknown amounts of precipitation were intercepted by street tree crowns, parking vehicles, and buildings, so the actual amount of water infiltrating the planting pit does not correspond precisely to the recorded precipitation. Annual ring width, 2004 (mm) 4.0 (±1.27) 3.8 (±1.5) 3.2 (±1.45) 3.9 (±1.23) 3.0 (±1.19) Reference (park) 6.3 (±1.03) Absolute stem area Relative stem area increment (mm2 ) 1,432 (±520.05) 1,439 (±593.66) 1,168 (±548.30) 1,461 (±537.87) 1,101 (±448.98) 3,275 (±755.66) *Values in columns differ significantly when followed by different test, P < 0.05). 13.64 (±6.29) ab 11.81 (±5.89) bc 14.26 (±5.19) bc 10.88 (±5.01) c 16.51 (±2.61) c letters (according to LSD area increment (%) 15.14 (±5.40) a* Growth Response Significantly augmented relative stem area increment was registered after irrigation with 280, 320, and 640 L (74, 84.5, and 169 gal) throughout the whole growth period (comparisons of LSD at P < 0.05). The lowest relative stem increment was yielded by the untreated control (10.88%), and the highest relative stem increment was yielded by the trees receiving most water (15.14%). The relative stem area increment of the nontreated park trees surpassed any of the street trees, albeit no significant differences to the street tree treatment irrigated with 640 L (169 gal) of water could be determined (Table 2, Figures 1 and 2). If separating means with a Duncan test (P < 0.05), significantly augmented stem area increment was achieved by the trees receiving most water (640 L [169 gal]). Phenology Because irrigation started well after initiation of stem increment, treatment effects were absent in regard to the onset of growth. However, considerable and significant (P < 0.001) block effects were revealed by an ANOVA of the days for growth initiation. Thus, the trees in blocks 1 and 2, positioned on the northern side of the east–west- going street, started stem growth 7 days earlier than the trees in blocks 3 and 4 (Table 3). These results are supported by cursory observations of budbreak on day 118 (April 27), when 96% of the trees in blocks 1 and 2 had flushed, while this was the case for only 41% of the trees situated in blocks 3 and 4. Figure 1. Growth curves for all treatments and reference trees (park trees), illustrating relative stem area increment at 1 m stem height. Each point denotes the mean value of 20 replicates. Figure 2. Relative stem area increment in response to total irrigation over the whole growth period. Error bars denote prediction errors; different letters denote significant differ- ences between means (P < 0.05). ©2006 International Society of Arboriculture
January 2006
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