Four Ambrosia species in Israel: invasive, naturalized and casual alien plants.
Four Ambrosia species are currently identified in Israel: the invasive perennial species Ambrosia confertiflora DC., the naturalized perennial species Ambrosia tenuifolia Spreng. and Ambrosia psilostachya DC., and the casual annual species Ambrosia artemisiifolia L. In this study, we performed a comparative analysis of three species: A. confertiflora, A. tenuifolia, and A. artemisiifolia. We found differences in morphology, growth and flowering phenology and physiology that may explain, besides climatic constraints, the factors involved in the differing invasion status of these species in Israel. A. artemisiifolia and A. confertiflora use strategies typical of invasive species: rapid growth and regeneration, phenotypic plasticity and reproductive ability. These characteristics have enabled the spread of A. confertiflora in Israel. However, A. artemisiifolia has not become established in Israel due to the country's dry summers. The naturalized populations have survived in ecological niches in Israel for many years, but due to their slow growth and low numbers of viable achenes, they have not spread.