Age structure and growth of the woody legume weed Cytisus scoparius in native and exotic habitats: implications for control.
To be successful, integrated weed management (IWM) requires sufficient knowledge of the ecology of the weed and the invaded system to allow prediction of the outcome of control efforts. We studied the age and size structure of both native and exotic populations of Scotch broom Cytisus scoparius to test predictions regarding why it is invasive in exotic habitats and, consequently, to determine management priorities. Particular emphasis was given to the efficient selection of biological control agents. A minimum of 30 plants was harvested per site in Europe (seven sites), New Zealand (20 sites) and Australia (11 sites). Age, determined by counting annual growth rings, stem diameter and height were recorded for each plant. The area cleared was recorded to calculate plant population density. Stem diameter and height were significantly correlated with age, but there was considerable variation in size for any given age and no major differences were apparent between the growth rates of plants from native and exotic populations. This may be because population densities of exotic stands were significantly higher than native stands, and intraspecific competition has previously been demonstrated to limit plant growth. The age structures of many populations were described by regressions between loge(number of plants m-2) and loge(age). However, the proportion of populations described by this regression varied significantly between countries and was highest in Australia and lowest in Europe. Departure from this relationship was most common at roadside sites and recently cleared commercial forestry plantations, indicating that large-scale artificial disturbance can promote pulses of seedling establishment. Disturbance-related establishment was found to be most common in Europe, indicating that large-scale disturbance is unlikely to explain increased weediness in exotic habitats. The maximum age of plants sampled did not vary between native and exotic habitats, indicating that, as for the related weed Ulex europaeus, increased longevity of exotic plants is unlikely to explain why exotic populations are weedier than native populations. Significantly higher population densities recorded in exotic habitats may be a consequence of a paucity of specialist insect herbivores. Such higher density populations are more likely to exclude competing vegetation so, in the absence of major disturbance, exotic broom stands are more likely to regenerate than European populations, and therefore to persist for generations. Synthesis and applications. The data support the hypothesis that natural enemies regulate native populations of Scotch broom and, consequently, that invasive exotic populations are suitable targets for biological control. Agents should be sought that (i) reduce plant fecundity and therefore seedling densities and (ii) defoliate established plants, enabling competing vegetation to establish beneath stands and reduce seedling survival. These results are potentially applicable to other invasive introduced legume shrubs.