Aerial photographs as a tool for assessing the regional dynamics of the invasive plant species Heracleum mantegazzianum.
The initiation of an invasion event is rarely dated in studies of alien plants. Data from aerial photographs documenting the invasion from the outset facilitate the quantification of the rate of spread, allowing researchers to analyse species' population dynamics and providing a basis for management. For 10 sites invaded by Heracleum mantegazzianum in the Slavkovský les, Czech Republic, aerial photographs from 11 sampling dates between 1947 (before invasion started) and 2000 were analysed. The area covered by the invader was measured digitally in a 60-ha section of landscape, and information obtained on invaded habitats, year of invasion, flowering intensity and structure of patches. Invaded area was regressed on residence time (time since the beginning of invasion) and regression slopes were used to measure the rate of spread. Data were analysed by ANCOVA, multiple regression and path analysis. Pastures and fields contributed 84.7% to Heracleum total cover, forest and scrub 13.7% and human settlements 1.6% at the later stage of invasion. The direct effect of the rate of invasion on invaded area (0.82) was greater than that of residence time (0.22), but the total effect (direct and indirect) of residence time was only slightly less (0.79) than that of the rate of invasion (0.82). As invasion proceeded, the populations spread from linear habitats to the surrounding landscape. Mean rate of areal spread was 1261 m2 year-1 and that of linear spread 10.8 m year-1. Flowering intensity did not exhibit any significant trend over time. Synthesis and applications. The strong effect of the rate of spread on the invaded area indicates that local environmental conditions hardly limit the spread of Heracleum. The species is easily detectable on aerial photographs taken at flowering and early fruiting times, from June to August. Knowledge gained from aerial photographs allows managers to identify dispersal foci and to focus control efforts on linear landscape structures with developing populations. Knowledge of the rate of spread and habitat vulnerability to invasion facilitates the identification of areas at highest risk of immediate invasion.