210 Struve et al.: Growth and Water Use Characteristics of Oak tion: Q. velutina seedlings in experiment 1 had the lowest root area and highest shoot/root ratio. There was no consistent pattern between water use seed- ling−1 and relative drought resistance. For all species in ex- periments 2, 3, and 4, water use seedling−1 loaded positively on principal component 1, as did seedling height, leaf area, root area and shot, root and total plant dry weight (factors that describe seedling size). Thus, not unexpectedly, larger seed- lings tended to used more water than shorter seedlings. The one exception was Q. prinus seedlings, in which water use seedling−1 loaded negatively on principal component 1. How- ever, the other measures of water use:water use cm−1 height, cm−2 leaf or cm−2 root surface area, loaded negatively on principal component 1, indicating that smaller seedlings with less leaf area, root area, and total dry weight had higher water use cm−1 height, cm−2 leaf area, and cm−2 root surface area than larger seedlings. In general, the most drought-resistant species had the high- est water use cm−1 height and cm−2 leaf surface area. Water use cm−2 root surface area (a measure of water absorbing efficiency) was not related to relative drought resistance. For instance, the most drought-resistant species in experiment 1, Q. velutina, and the most drought-resistant species in experi- ment 3, Q. prinus, had the highest water use cm−2 root surface area. In contrast, the least drought-resistant species in experi- ment 2, Q. palustris, had a more efficient water-absorbing root system than the most drought-resistant species, Q. mac- rocarpa. Our expectation was that xeric site-adapted species would have both high water use cm−1 height and cm−2 leaf surface area and an efficient water-absorbing root system. However, there was not a consistent relationship among these factors. They loaded differently for each species on the prin- cipal components (Tables 4 and 5). It is logical that sympatric species (species that inhabit a common area) would have evolved different responses to water stress. Dickson and Tomlinson (1996) argue that because of the wide range of sites occupied by oaks, there would not be a common re- sponse to water stress. In experiment 4, there were sufficient numbers of seedlings per family to develop correlations between seedling height and water use. Three correlations were developed: seedling height versus seedling water use, seedling water use versus height-adjusted water use, and seedling height versus height- adjusted water use. For all species in experiment 4, the cor- relations between seedling water use and height-adjusted wa- ter use were not significant (water use seedling−1 and water use cm−1 height). The correlations between seedling height and water use seedling–1 were positively and significantly correlated. Not surprisingly, taller seedlings used more water day–1 than short seedlings. In contrast, the correlations be- tween seedling height and water use cm−1 height were nega- tively and significantly correlated. These correlations are graphed for Quercus rubra family 23 (Figures 1A and B and ©2006 International Society of Arboriculture 2A). We plotted height-adjusted water use (g water cm−1 height) against seedling height as a way of identifying appar- ently efficient and inefficient water use seedlings. Efficient water use seedlings were those with low height-adjusted wa- ter use: tall seedlings with low water use cm−1 height. We felt that seedlings with low height-adjusted water use were not experimental artifacts. Efficient water use seedlings could have been falsely identified because they transpired so much substrate moisture that substrate moisture limited transpira- tion before the 48 hr water use period ended. However, at the end of the 48 hr water use period, the substrate in containers with large seedlings was still moist to the touch. Furthermore, we estimated that half the plant available water had been transpired by the end of the 48 hr water use period (see Drunasky and Struve 2005). Experiment 4 also showed simi- lar relationships between seedling height and height-adjusted water use for both Quercus macrocapra families and in two of three Q. palustris families. The one exception was for Q. palustris family 20 in which the correlation between seedling height and height-adjusted water use was negative (Table 6). More Q. palustris families need to be tested to determine the nature of the relationship between seedling height and height- adjusted water use for Q. palustris. We considered short seedlings with relatively high height- adjusted water use to have a xeric water use pattern. Bunce et al. (1977) suggested that “inefficient” water use characteristic of slower growing, shade-intolerant early successional seed- lings adapted to xeric sites may be a mechanism to increase its competitive ability. Xeric site-adapted species have a con- servative growth habit; they invest proportionally more in root growth than shoot growth, thereby increasing their drought resistance. Also, xeric site-adapted species tend to have superior drought tolerance (Abrams 1990). The benefit of preferential investment in root mass is the ability to main- tain net photosynthesis at lower soil moisture potentials than species adapted to mesic sites. Thus, on xeric sites, seedlings with a xeric growth habit survive in stressful environments, whereas mesic species die or suffer reduced grow rates; low soil moisture potential reduces leaf growth of mesic site- adapted species such as Q. rubra (Dickson and Tomlinson 1996). By actively lowering soil moisture, xeric-adapted spe- cies alter the soil environment to their competitive advantage. This phenomena has been termed competitive exploitation of soil moisture (Bunce et al. 1977). We found that within the half-sib families studied, there was a range of mesic to xeric site-adapted seedlings. We propose that those Q. rubra seedlings with a xeric morphol- ogy and water use pattern represents introgression of drought resistance genes from more a drought-resistant species such as Q. veltuina. Quercus rubra is genetically diverse (Burger 1975; Jensen and Eshbaugh 1976; Jensen 1977; Houston 1983; Manos and Fairbrothers 1987; Guttman and Weig 1988; Kriebel et al. 1988; Schwarzmann and Gerhold 1991;
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