Climate change or naturalized ornamental genotypes? Factors controlling range expansion in an evergreen temperate tree
Anne-Marie Thonning Skou
dept. Agriculture & Ecology, Copenhagen University
Holly (‘Kristtorn’) (Ilex aquifolium) is a dioecious evergreen tree or shrub that grows as a sparse but usually gregarious understory element in deciduous forests of western and southern Europe. In Denmark, the plant is at its northeastern range margin in forests of eastern Jutland, and it seems to be controlled by winter frost, expanding its range eastwards (ca. 100 km within ca. 50 years). However, numerous (frost-hardy) cultivars have been introduced in this region, and the species is an invasive alien plant in Australia and naturalized in New Zealand and western North America. Factors controlling its range expansion include: higher winter temperatures in eastern Denmark (permitting factor), trade and plantations (driving factor), dispersal by birds (100 m per year?); leaf mining; time between planting and fruit maturation (limiting factors). The objectives of my PhD thesis are: to determine propagule pressure of I. aquifolium cultivars in Denmark over time, to investigate genetic differences between native and introduced genotypes of Ilex aquifolium; to analyze the degree of genetic introgression from garden cultivars into native populations; to study the frost tolerance of native and introduced genotypes, and survival in field transplant experiments; to determine whether thrushes have preferences for the fruit of different cultivars; and to assess the effects of introduced genotypes on plant-animal interactions of different specificity.
