Arboriculture & Urban Forestry 35(3): May 2009 117 percentage of total plant dry weight in total root, fine, and coarse root dry weights and the greatest leaf area ratio (Table 7). There were significant differences among the sources in percentage of total plant dry weight in leaves, stems and root tissue, leaf area- to-fine root dry weight ratio and leaf area ratio (Table 8). One of the most sticking differences among the sources was the range in the leaf area-to-fine root (the water transpiring-to-water absorb- ing) ratios. The greatest difference among the sources exceeded 250%, 321.3 versus 128.3 cm2 g-1 g-1 , for the Q. robur Cascine versus Q. cerris Amiata, respectively. Almost as great was the difference in the leaf area-to-fine root dry weight ratio between the two Q. cerris sources, 340.5 versus 128.3 cm2 , a 265% difference. It is not known whether these differences reflect differences in root system water absorbing efficiency or in leaf water use efficiency. Quercus robur seedlings had a resource allocation pattern of a species capable of rapid growth: a greater investment in aboveground biomass at the expense of belowground biomass: more leaves of smaller size, a greater percentage of plant dry weight in stem and leaf tissues and larger shoot-to-root ratio. As a consequence of the relatively high investment in aboveground biomass, it had the greatest water use per seedling, but had the compensating adaptive traits of relatively lower water use cm-2 leaf area and high leaf area-to-fine root dry weight ratio. Quercus pubescens seedlings, relative to Q. robur seedlings, had a low- er investment in aboveground biomass and a relatively greater investment in root tissue. They also had the highest water use per leaf area and lowest leaf area-to-fine root dry weight ratio. Quercus cerris seedlings had a higher investment root biomass than Q. pubecscens and Q. robur seedlings. However, there were significant differences among the sources in their growth hab- its and water use characteristics; the Bazzano, Cerreta, Cascine and Pineta (Q. pubescens and Q. robur, respectively) had simi- lar growth and water use characteristics while the Sellano and Amiata (Q. cerris) sources had different growth habits and water use characteristics from each other and the other seed sources. Additionally, there were significant differences in height and water use characteristics. The results illustrate the remarkable degree of variation between and within these seed sources and the great potential for genetic improvement to address a chang- ing world climate and the harsh environmental conditions as- sociated with urban forests. The great degree of variation oc- curred whether the seeds were collected from an individual tree (Q. robur Cascine), mixed-species stands (both Q. pubes- cens sources) or single-species stands (both Q. cerris sources). From a nursery production aspect, where crop value is de- Figure 3. Height-adjusted water use (g water per cm seedling height per day) for two sources from each of three Quercus species. there were significant differences between the sources within a species (Table 6). The extremes among the sources were Quercus robur seedlings from the Cascine source which had the greatest leaf area and dry weights, except for coarse and total root dry weights which were greatest in seedlings from the Q. cerris Amiata source. Quercus robur seedlings had a similar percentage of total plant weight in leaf tissue as Q. pubescens seedlings, but the highest percentage of total plant dry weight in stem tissue, the greatest leaf area-to-fine root dry weight ratio and the lowest per- centage of total plant dry weight in total, fine and coarse root dry weights (Table 7). Quercus cerris seedlings had the greatest termined by the rate of height and caliper growth, Q. robur is the most desired species, but the species with the highest water use per seedling. The Q. pubescens Bazzano source is a possible alternative to the two Q. robur sources. Those seedlings grew nearly as tall, but the seedlings of this source used 30% less wa- ter per seedling than the Cascine seedlings. The Bazzano source seedlings also had a more efficient root system than the Cascine seedlings absorbing 0.00038 and 0.00017g water day-1 g-1 fine root dry weight, respectively. Seedlings of the Q. pubescens Cer- reta source had the most efficient root system, 0.00065 g water per day per g fine root dry weight. As a species, Q. pubescens can maintain high rates of hydraulic conductance during drought which was attributed to a deep rooting habit (Naraini et al. 1998; Nardini and Pitt 1999; Lo Gullo et al. 2003; Poyatos et al. 2005; Zweifel et al. 2006). The apparent water absorbing efficiency of ©2009 International Society of Arboriculture
May 2009
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