184 Ferrini et al.: Effect of Fertilization and Backfill Amendment on Oak RESULTS AND DISCUSSION Plant Growth and Leaf Gas Exchange, First Year All plants had little growth when compared to the already minimal growth, which is typical the first year after plant- ing, probably due to a period of drought and high tempera- tures that lasted the entire summer, but they all survived. This compares well with the average survival rate in Italy which is about 80% to 85%. Leaf area was greater in the fertilized plants, while no effects were detected for the other parameters (Table 1). Photosynthesis and transpiration, as determined by leaf gas exchange data, were increased in the compost treatment, especially in the first part of the season (June and July and measurements; data not shown). However, in assembling the four sampling dates, no significant data were found (Table 2). Beginning in the middle of July, the plants showed symptoms of water stress, even with weekly irrigation. Numerous studies show that nitrogen applied later than about a month before budbreak seldom affects early season tree growth and may not be effective until the following season. Specific research on deciduous oaks (Quercus spp.) showed that leaf size and color were improved the next season, but little or no increase in shoot growth was evidenced until the following year (Harris et al. 2004). Plant Growth and Leaf Gas Exchange, Second Year During the second year, growth of the shoots was minimal. Compost and leonardite treatments registered significantly larger averages (Table 1). Leaf area and leaf dry weight were higher for fertilized trees. Fertilization increased photosyn- thesis and water use efficiency (Table 2). Contrary to the previous year’s findings, addition of compost did not stimulate photosynthesis, probably because the greater amount of rainfall during the growing season masked any possible effect of the compost in terms of higher water retention ability. As for transpiration rate (E), it is difficult to identify a definite trend for the single treatments because the data were quite variable along the season. Considering the average data of four sampling dates, however, leaf evaporation rate was greater in the leonardite treatments. WUE was lower later in the season, when transpiration values were similar to those in June and lower photosynthesis values were recorded (data not shown). This finding suggests the importance of ensuring an adequate supply of water for the trees, in the first part of the season when vegetative potential is very high and water use efficiency is clearly greater. In the second part of the season, lower photosynthetic potential of the plants (i.e., senescent leaves, shorter day length, and lower PAR values accompanied by greater probability of cloud cover) makes it less necessary to turn to external water sources, even if irrigation must be maintained in cases of drought and for species with a longer vegetative period. More chlorophyll was present in the fertilizer treatment than any other treatment for both sampling dates (Table 3). Control plants had less total chlorophyll compared to the fertilizer and compost ones, while, when compared to the leonardite treatments, the differences were statistically significant only for the June measurement (90 days after budbreak, Table 3). Table 1. Effect of backfill composition on shoot elongation (SE) (cm), leaf area (LA) (cm2 (DW) (g) of Quercus robur trees. Treatment Compost SE Fertilization 4.40 Leonardite Control 4.02 3.89 1st year LA DW SE 4.73 ns* 20.85 b 0.14 ns 4.84 a 25.74 a 0.18 22.2 b 0.16 20.63 b 0.14 4.19 b 4.82 a 3.93 b 2nd year LA DW 20.91 b 0.16 c 25.10 a 0.22 a 22.67 b 0.18 b 22.18 b 0.17 bc *Values differ significantly when followed by different letters at P ≤ 0.05 (LSD test); ns = not significant. Table 2. Effect of backfill composition on net photosynthesis (Pn), evaporation rate (E), and water use efficiency (WUE) of Quercus robur trees (Pn: µmol m–2 s–1 Treatment Compost Fertilization Leonardite Control Pn 1st year E 1.00 1.10 1.05 rate). Data are the average of four sampling dates in the first and second year and six sampling dates in the third year. CO2; Evaporation rate: µmol m–2 WUE 7.69 6.45 7.16 Pn 8.15 ns* 1.50 ns 5.43 ns 9.92 c 7.69 7.61 7.52 ©2005 International Society of Arboriculture 2nd year E 2.54 c 12.24 a 2.64 c 11.10 b 3.06 a 11.02 b 2.85 b H2 WUE 3.90 b 4.63 a 3.62 b 3.86 b *Values differ significantly when followed by different letters at P ≤ 0.05 (LSD test); ns = not significant. Pn 3rd year E WUE 15.03 a 5.30 ns 2.83 ns 16.05 a 5.40 13.32 b 5.21 13.18 b 5.04 2.97 2.55 2.62 s–1 O; WUE: Pn/evaporation SE ), and leaf dry weight 3rd year LA 32.53 33.33 DW 6.98 bc 30.88 ns 0.22 b 14.62 a 37.43 5.11 c 8.37 b 0.29 a 0.23 b 0.24 b
July 2005
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