Long-term impact of Heracleum mantegazzianum invasion on soil chemical and biological characteristics

Exotic plant invasions often change ecosystem properties with subsequent impacts on the structure of invaded communities. Despite an increasing knowledge of post-invasion ecosystem changes, these modifications are only rarely studied within the temporal context of ongoing invasions. In this study we investigated 19 soil chemical and biological characteristics, as well as light conditions, in uninvaded grassland sites and compared them with those from sites invaded by giant hogweed (Heracleum mantegazzianum) for different times (from 11 to 48 years). We further related variation in these soil and light characteristics to richness and productivity of native plant species and hogweed cover measured in the field, and to common-garden performance of hogweed grown in soil inocula from the same sites. d presence significantly reduced red/far-red light ratios but increased soil pH. Longer invasion history was associated with increasing soil conductivity and content of extractable phosphorus. There were also parameters that displayed opposite trends in different periods of invasion such as fungal/bacterial ratios or relative amount of photosynthetically active radiation (ΔPAR). These parameters initially increased (fungal/bacterial ratio) or decreased (ΔPAR), but after reaching a certain breakpoint they tended to return to pre-invasion conditions. ences in native species richness were best correlated with light availability and soil pH, and productivity with composition of soil microbial communities. Differences in hogweed cover were associated with soil pH and conductivity. The variation in hogweed performance in a common garden was related to the composition of soil microbial communities, soil conductivity and light availability of sites from which soil inocula were collected. tudy documents that ecosystem properties can be altered not only by an invasion event but are further modified as the invasion proceeds. These ecosystem changes likely underlie long-term impacts of invasive plants on native communities.