96 Ferrini and Baietto: Response to Fertilization of Tree Species in the Urban Environment Table 5. Effect of fertilization on net photosynthesis (Pn), evaporation rate (E), on water use efficiency (WUE) of Liquidambar styraciflua L. trees (Pn = µmol m−2 s−1 of CO2; Evaporation rate = mmol m−2 Treatment Control 1 Fertilization 2 Fertilization Pn 5.8 c* 7.19 b 10.52 a 2002 E 1.26 b 1.37 b 1.98 a 2003 WUE Pn 4.60 b 5.25 a 5.31 a E 4.93 a 5.50 a 5.53 a *Values differ significantly when followed by different letters at P 0.05 (LSD test). ration (E), while WUE was not affected (Table 1) in both years in the Japanese pagoda tree. In the European ash no significant effects were found in any of the parameters con- sidered following fertilization treatment (Table 2). This is not consistent with what was found by Brix and Mitchell (1986), who reported that nitrogen fertilization increased WUE, pre- sumably by stimulating root growth in Douglas-fir. However, it is difficult to draw conclusions because net photosynthesis was highest in the fertilized trees the first year of measure- ments, whereas the results were the opposite in the following year, contrary to what was observed in a similar study with oak trees (Ferrini et al. 2005). In 2004, because of the reduc- tion in the irrigation volume, leaf gas exchange was lower compared to the first year. This confirms that water is the most important factor for the trees in the establishment phase. Chlorophyll Content No significant trend emerged from the analysis of total chlo- rophyll content in both years and in both species (Tables 1 and 2). This is not consistent with other research (Ferrini et al. 2005) that showed a higher chlorophyll content per unit weight in fertilized plants compared to control ones. Plant Growth No significant differences were found between the fertilized trees and the control ones regarding shoot growth and leaf dry weight in both years (data not shown), thus confirming find- ings by other authors who did not find an appreciable increase in plant growth from fertilizer applications on trees after transplanting (Perry and Hickman 1998; Gilman et al., 2000; Struve 2002). Leaf Mineral Content The foliar nitrogen level did not increase consistently in trees receiving soil-applied fertilizer in both species (Tables 3 and 4) in 2003, whereas a higher foliar nitrogen content was found in Fraxinus in 2004. Similarly, no statistically signifi- cant increments in phosphorus and potassium levels were detected in the different years and species. This partly agrees with Perry and Hickman (1998), who observed that fertiliza- tion did not affect leaf mineral content and tree growth. In conclusion, no correlation was found between leaf mineral content and growth parameters in this study. ©2006 International Society of Arboriculture Experiment 2 Gas Exchange In the first season after planting, instantaneous net photosyn- thesis (Pn) was higher in the trees that were fertilized twice. Differences were also found between the plants fertilized only once and the control trees (Table 5), showing a marked effect of fertilization on this parameter. Trees that received a second fertilization also showed a higher evaporation rate (E) and WUE, but only when compared to control trees. In the second year, only WUE was higher in the fertilized plants, regardless of the quantity of fertilizer. In the third year, opposite results were obtained: Pn and E were higher in the control trees, and a statistical difference was also found between the two levels of fertilization. Trees fertilized only once showed a higher level of leaf gas exchange compared to those fertilized twice. No differences in WUE were observed. It seems reasonable to hypothesize that sweetgum trees responded to fertilization in the first year, but the addition of fertilizer failed to produce any response in the following years. The dry climate of 2003 and 2004 might have pre- vented nutrient uptake and utilization, especially on these plants, which grow in a disturbed and compacted soil. Tom- linson (1993), in a work carried out on oak species, postulated that increased temperature and reduced rainfall led to nutrient deficiency, fine root mortality, and, eventually, crown die- back as nutrient cations were leached from the soil by acids formed from mineralization and nitrification in excess of tree needs. Although the research by Tomlinson (1993) was con- ducted in forest stands, the high concentrations of nitrates may have triggered a decline process in our plants as well, Table 6. Effect of fertilization on chlorophyll content (mg * cm−2 Treatment ) of Liquidambar styraciflua L. trees. 2002 2003 Control 1 Fertilization 2 Fertilization 19.40 b* 20.44 a 20.65 a 18.18 b 19.09 a 19.81 a 1.95 a 1.94 a 2.12 a WUE Pn 2.53 b 2.84 a 2.68 a 7.49 a 5.97 b 4.79 c s−1 of H2 O; WUE = Pn/Evaporation rate). Data are the average of three sampling dates in the first and in the second year and five in the third year. 2004 E 3.07 a 2.61 b 2.22 c WUE 2.44 a 2.29 a 2.16 a 2004 17.56 a 17.66 a 17.94 a *Values differ significantly when followed by different letters at P 0.05 (LSD test).
May 2006
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