Hawthorn (Crataegus monogyna) Populations in Mid-Canterbury

Hawthorn (Crataegus monogyna) is a naturalised tree that is classified as a noxious plant in several counties in the eastern South Island. It is locally abundant in lowland forest remnants, and with the indigenous spiny shrub matagouri (Discaria toumatou), on grazing land in Canterbury. Two scrub sites near Porters Pass, where the original hawthorn trees still existed, and a forest site near Kowai Bush were sampled by measuring stem diameters and counting growth rings, to determine the age structure and dynamics of hawthorn. Matagouri was sampled similarly at one of the sites. There is a close positive relationship between age, stem diameter and plant height for both hawthorn and the slower growing matagouri. Hawthorn growth is impeded by grazing, but it is spreading near Porters Pass in clumps of matagouri and in pastures that are only lightly grazed. If existing management continues, hawthorn is predicted to increase as more of the bushes reach fruiting age. In the forest site, less hawthorn is establishing now than when the forest was more disturbed, whereas seedlings and saplings of native trees are abundant. Hawthorn is predicted to decrease here. The management of hawthorn should recognise these different situations.