Fine-scale spatial genetic structure of two red oak species, Quercus rubra and Quercus ellipsoidalis

Abstract

Peripheral populations located at their range edge, may be at risk due to geographical isolation, environmental changes, human disturbances or catastrophic events such as wildfires. Fine-scale spatial genetic structure (SGS) investigations provide a way to examine the spatial arrangement of genetic variation within populations. SGS can result from restricted seed and pollen dispersal and might be affected by geographic isolation and environmental changes and disturbances even in outcrossing wind-pollinated species like oaks. Studying the SGS of peripheral populations provides information that can be used to develop improved conservation and management plans at the species’ range edge. We assessed the level of genetic variation and SGS in twelve range edge populations in northern Wisconsin and the Upper Peninsula of Michigan (USA): eight Quercus rubra and four Quercus ellipsoidalis populations that were subject to different management regimes and natural disturbances. In contrast to Q. rubra populations, the drought tolerant Q. ellipsoidalis populations are isolated from the species’ main distribution range. These populations are not actively managed but are especially prone to recurring fire events. The four managed and four old growth (“unmanaged”) Q. rubra populations displayed similar levels of genetic variation. Likewise the Sp statistic showed similar SGS levels in managed and unmanaged Q. rubra populations (Sp = 0.005) comparable to other Quercus species (European Q. robur: Sp = 0.003). Q. ellipsoidalis populations showed similar or more pronounced SGS than neighboring Q. rubra populations extending up to 83 m in one population. A significant excess of homozygotes across markers in two of the Q. ellipsoidalis populations suggests potential inbreeding. In summary, diverse management activities combined with various natural disturbances are likely both influencing SGS patterns. Outcrossing forest trees like oaks hold large amounts of genetic diversity allowing adaptation to environmental changes over their long life spans. Reductions of these genetic stores, through inbreeding for example, can inhibit a species’ ability to adapt to changing environmental conditions.