202 Struve et al.: Growth and Water Use Characteristics of Oak Arboriculture & Urban Forestry 2006. 32(5):202–213. Growth and Water Use Characteristics of Six Eastern North American Oak (Quercus) Species and the Implications for Urban Forestry Daniel K. Struve, Petra Sternberg, Nick Drunasky, Kurt Bresko, and Rico Gonzalez Abstract. Seedling growth and water use of six North American oak species were studied in a series of four experiments to determine inter- and intraspecies water use characteristics. Xeric-site adapted species (chestnut oak, Q. prinus [L.] and black oak, Q. velutina [Lamb.]) had slower growth (height and dry weight accumulation and lower shoot:root ratios) than mesic-site adapted species (bur oak, Quercus macrocarpus [Michx.]; pin oak, Q. palustris [Muenchh.], northern red oak, Q. rubra [L]; and Shumard oak, Q. shumardii [Buckl.]). Principal component analysis (a statistical technique used to identify correlated variables) using 11 variables found that seedling water use loaded positively with seedling growth factors (taller seedlings tended to have higher dry weights and greater leaf and root areas and used more water than shorter seedlings, which tended to have lower dry weights, smaller leaf, and root areas) in the first principal component. However, in the third experiment, seedling growth factors loaded negatively with seedling water use for Q. prinus. Tall Q. prinus seedlings tended to use less water than short seedlings. However, other measures of water use (g water cm−1 cm−2 height and seedling waters use were significant and positive, but great within-species and within half-sib family differences in height-adjusted water use (g water cm−1 leaf and root area) loaded negatively on the first principal component. Correlations between seedling heights and height) were found. By plotting height-adjusted water use against seedling height, efficient and inefficient water use seedlings could be identified. Inefficient water use seedlings were shorter and had higher water use cm−1 height than efficient water use seedlings. Inefficient water use seedlings were described as having a xeric-water use habit, whereas tall seedlings had a mesic-water use habit. Potentially, height-adjusted water use could be used as a method for selecting individual oak seedlings better adapted to stressful urban planting sites. Key Words. Dry weight; water use efficiency; xeric- and mesic-site adapted species. Species used in this study. Bur oak (Quercus macrocarpus (Michx.)); pin oak (Q. palustris (Muenchh.)); chestnut oak (Q. prinus (L.)); northern red oak (Q. rubra (L)), (Q. shumardii (Buckl.)), (Q. velutina (Lamb.)). Northern red oak (Quercus rubra L.) is an important and widely distributed tree species in eastern North American forests (Fowells 1975; Harlow et al. 1979). It is considered a keystone species by Spetich et al. (2002) and occurs on a variety of sites and soil types. Optimal development occurs on mesic sites, those sites where soil moisture is least likely to limit to tree growth. In contrast, xeric sites are those char- acterized soils with limited water availability. Provenance tests have revealed significant genetic varia- tion for Q. rubra growth rate (Farmer et al. 1981; Schlarbaum and Bagley 1981; Kriebel et al. 1988). Provenance tests are planted with seedlings raised from seeds collected in different parts of a species range. Because the environment at the test site is relatively uniform, differences among individuals are attributed to genetic differences. Under horticultural condi- tions (container production, transplanting to an agricultural ©2006 International Society of Arboriculture site followed by intensive cultural practices) designed to minimize environmental effects, low age–age correlations for height growth were found (Struve and McKeand, 1993). In the genetic studies, significant within provenance variation was observed (Farmer et al. 1981; Kriebel et al. 1988). It was recommended that further selection efforts should concen- trate on phenotypic selection without particular regard to geo- graphic origin (Kriebel et al. 1977). A later study provided the biologic basis for the recommendation; there was weak family clustering within stands (Schwarzmann and Gerhold 1991), although near-neighbors are genetically similar (Sork et al. 1993). That is, trees from different parts of a stand tend not to be closely related, although clusters of trees (near- neighbors) are. The great phenotypic variability associated with red oak is in part the result of its reproductive biology. It is a wind-
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