Arboriculture & Urban Forestry 39(6): November 2013 263 Table 3. Foliar nutrient status of Pyrus calleryana trees grown in the three pit types. Leaves were collected on August 15, 2011, and August 29, 2012 approximately 12–14 weeks after the full bloom. Planting pit technique 2011 and 2012 Nutrients Mean (%) Mean (µg/g) Small covered 1.27b 0.10b 1.23b 2.14 0.23 21.73 5.25 0.25 0 Large covered 1.60b 0.13ab 1.31 ab 2.16 0.24 22.73 9.05 0.40 0 Open tent in the upper 20 cm of the soil profile (Rahman et al. 2011). Consequently, trees grown in small and large covered pits de- veloped a shallow root system and showed reduced growth and physiological performance despite not being under water stress. Leaf water potential corresponded generally well with rain events during the study. Following dry springs, both midday and pre-dawn leaf water potential of tree leaves grown in the three different types of pits were more negative. The sharp contrast in terms of planting techniques are that the trees grown in the open pits showed a significantly more negative midday leaf water po- tential than those in covered pits (small and large), but in terms of pre-dawn leaf water potential, trees grown in large covered pits were more negative. This might be due to the pavement ef- fect, which reduces evaporation during the daytime but at night releases absorbed heat and evaporates more water from the soil relative to the open pit. Nevertheless, Nielsen et al. (2007) ar- gued that soil hydrology is not always the driving mechanism of tree transpiration in street planting pits. Reduced soil pore vol- ume and continuity due to sealing—in the case of both the small and large covered pits—might affect the root respiration and nutrient uptake of trees (Herbauts et al. 1996; Horn et al. 2007) leading towards reduced growth and stomatal conductivity. Moreover, topsoil in the open pits might have more opportunities to retain and accumulate nitrate by way of atmospheric deposi- tion, although they are more vulnerable to leaching compared to the sealed pits. Lower C and N content of soils under paving surfaces were predictable and are in line with other research; in- cluding work by Raciti et al. (2012), who reported around 66% and 95% decrease in C and N content in the soil (0–15 cm) under impervious surfaces in New York City, New York, U.S. Macronutrients, such as nitrogen, are also a dominant factor affecting the plant Chl a and are generally related to productivity (Filella et al. 1995). This might explain the higher chlorophyll content for trees grown in open pits. The higher Chl a+b contents in the leaves of trees grown in open pits suggest that no dam- age had occurred in the chlorophyll biosynthetic pathways (De Nicola et al. 2011). Chlorophylls not only absorb light but also funnel the excitation energy from Photosystem I to Photosystem II, and the depression of Fv/Fm has consequently been used as an indicator of nutrient stress or imbalance (Kruskopf and Fly- nn 2006). Trees grown in the open pits also showed efficient use of light captured with their higher chlorophyll content in terms of better photochemical efficiency of PS II (Fv/Fm). Moreover, chlorophyll fluorescence of the trees grown in the open and large covered pits were within the specified range of the healthy, non- stressed, deciduous and evergreen trees (0.78–0.85) through- out the summer, indicating no sign of stress among those trees (Demmig and Bjorkman 1987; Maki and Colombo 2001; Per- cival 2004). Although there are reports that stomatal conduc- tance is not directly determined by photosynthetic capacity (e.g., von Caemmerer et al. 2004), Matsumoto et al. (2005) showed N P K Ca Mg Al B Co Cu Fe Mn Mo Na 52.58b 14.20b 0 67.23a 16.20ab 0 Ni Zn 165.20 8.00 23.60b 184.40 8.95 29.85ab 1.96a 0.15a 1.49a 2.31 0.22 22.38 10.30 0.30 0.15 72.13a 21.15a 1.30 169.15 12.80 36.60a that the stomatal conductance variability depended markedly on chlorophyll function and that the degree of dependence was al- most equal to that on solar radiation or vapor pressure deficit. All these factors together contributed towards a 40% in- crease in stomatal conductance of trees grown in open pits com- pared to those grown in small covered pits. Combined with a higher leaf area index and canopy spread, this meant that trees grown in open pits provided around 1 kW of cooling in June 2012. Although, the total energy loss per tree was calculated based on the transpiration rate of sunlit leaves, there is a chance of overestimation, since many of the leaves would have been shaded at the time of measurements. Ansley et al. (1994) com- pared the whole-tree transpiration rate based on porometer measurements with stem flow using sap flow gauges. They re- ported that the values were comparable; however, during the peak transpiration time (midday), porometer measurements might overestimate the whole-tree transpiration rate. But, con- sidering the size of the canopy of those trees, it is arguable that most of the leaves would have sunlight at some point of the day. Projecting the cooling ability of those trees grown in open pits over the next three growing seasons using existing stomatal conductivity and LAI but extending the canopy at the previous rate of growth gives a figure for cooling rate of around 7 kW. This is comparable to a previous study in which P. calleryana trees were grown for six years in open pits with a non-compacted and sand-based soil (Rahman et al. 2011). Simi- larly, the study authors predict that the cooling ability of those P. calleryana grown in the small covered pits after three more growing seasons would be around 1.1 kW between June and August. This value is comparable, if somewhat smaller than the previous findings for P. calleryana grown in highly compacted pits six years after planting (Rahman et al. 2011; Rahman et al. in press). In this way, trees grown in small covered pits are losing the advantages of growing in non-compacted soil due to sealing. The experimental design for this study was opportu- nistic, based on three commonly used planting techniques by Red Rose Forest in Manchester, UK. There is a chance of con- founding effect of soil composition and pit design. However, the consistency of the results across the studied years negated the advantages of better soil composition (both in terms of topsoil filled in root cell systems or mixing with sand) by the pit de- sign. In this way, the study further emphasizes the importance of aeration, which might be equally or even more important for growth and the cooling ability of trees as soil compaction. However, it must be stressed that these trees are all still very young in their establishment age, and the pattern might change as the trees mature, getting closer to their natural lifes- pan of 30–50 years. Once they start to break out of the con- fines of the pit, soil condition in the road site situation and the impervious pavement over root zones can confound other urban stresses on tree growth and physiology, since the trees on ©2013 International Society of Arboriculture
November 2013
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