Black oak
· Black oak is a common name for several species of tree. These include:
Quercus kelloggii, the California black oak, from the western U.S.
Quercus velutina, the eastern black oak, from the eastern U.S. and Canada
Casuarina pauper, an Australian tree species
Wind-pollinated forest trees usually have high outcrossing rates, but allogamy does not necessarily translate into high pollen movement. The goal of this study was to determine the outcrossing rates, pollen pool genetic structure, and the size of the effective pollination neighborhood in a population of black oak, Quercus velutina, in a Missouri oak-hickory forest. Based on 6 allozyme loci, 12 maternal trees, and 439 progenies sampled along a transect of 1300 m, we found complete outcrossing (tm = 1.000, P < 0.001) and small amounts of biparental inbreeding. Using a TwoGener analysis of the pollen gene pool, we found significant structure across maternal plants (ΦFT = 0.078, P < 0.001), which when corrected for adult inbreeding translates into ΦFT = 0.066 that corresponds to an effective number of pollen donors of 7.5 individuals. Assuming a bivariate normal distribution and an adult density of 16.25 trees ha−1, we estimated that the effective pollination neighborhood area had a radius of 41.9 m. Even assuming that our estimates may be conservative, these findings join a growing body of evidence that suggest that the local neighborhood of wind-pollinated forest tree populations may be relatively small creating opportunities for local selection and genetic drift.
Wind-pollinated trees typically have high levels of genetic variation within populations, low levels of differentiation among populations, and high outcrossing rates, which are all traits that indicate high levels of gene flow (Hamrick and Godt 1996). Despite these patterns, if the movement of a large portion of pollen is local, these same populations could be genetically subdivided creating local genetic structure (Smouse and Sork 2004). Recent evidence in oaks suggests that local pollen flow occurs within moderate distances, originating from neighboring trees, including relatives. For example, an analysis of pollen pool structure in forest populations of Quercus alba from the Missouri Ozarks indicates that the effective mean pollen dispersal area could have a radius as small as 16 m (Smouse et al. 2001). Results in Pinus echinata indicate high pollen pool structure and localized pollen flow (Dyer and Sork 2001), possible due to vegetative density (RJ Dyer and VL Sork, in preparation). At the same time, oaks in sparse landscapes have also shown both significant pollen pool structure despite high outcrossing rates (Sork et al. 2002; Fernández-Manjarrés and Sork 2005). Thus, locally heterogeneous pollen pools are not simply attributable to high forest cover.
Here, we examine the mating patterns of Quercus velutina Lam. in a secondary oak-hickory forest in southern Missouri, USA, using the mixed mating system model as implemented in the software MLTR (Ritland 2002) and the pollen pool structure model, TwoGener (Austerlitz and Smouse 2001; Smouse et al. 2001). MLTR is a widely used method of estimating the outcrossing rates of populations, but increasingly, we are seeing that tree species often show such high outcrossing rates that this parameter alone is not an informative indicator of gene flow patterns. The TwoGener model provides additional information about effective pollen movement contributing to the genetic neighborhood (sensuWright 1943), without the need of high genetic resolution (Smouse and Sork 2004). In this paper, we present a case study of pollen movement for a forest oak population based on a previously generated data set (Idol 1991) to determine the size of the pollination neighborhood. We asked the following questions: 1) What is the population outcrossing rate? 2) Does the pollen pool have significant genetic structure? 3) What are the effective number of pollen donors and the size of the effective pollination neighborhood?
· Materials and Methods
· Study Species
Quercus velamina Lam. (Fagaceae) is a common tree found in eastern and central North America growing on dry, sandy, or rocky ridges and upper slopes (Fowells 1965). It is commonly associated with hickories (Carya spp.) and many other oak species, such as post oak (Quercus stellata Wangenh.), scarlet oak (Quercus coccinea Muenchh.), southern red oak (Quercus falcata Michx.), blackjack oak (Quercus marilandica Muenchh.), chestnut oak (Quercus prinus L.), white oak (Q. alba L.), and northern red oak (Quercus rubra L.). Acorns mature in 2 years, and seeds need a light covering of leaves for germination. Seedlings cannot survive under a dense understory.
· Study Site
The study site was located in an oak-hickory forest in Tyson Research Center, an ecological reserve owned and managed by Washington University of St Louis. This 2000-acre reserve is located in Eureka, St Louis Co., MO, at the northeastern end of the Ozark Plateau (38°31′N, 90°33′W). Oak-hickory forest covers approximately 75% of the research area. Three dominant species of oak inhabit this area: Q. rubra, Q. alba, and Q. velutina. Hence, black oak is common in the reserve but is not the dominant species. The largest trees in the area range in age from 120–160 years and show spatial aggregation (Hampe 1984) as shown in Figure 1. Study trees are located within continuous forest along a ridge top accessed by a dirt road, which approximates a transect of 1.3 km. Study trees were separated by distances ranging from 10 to 1320 m.
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