Ailanthus altissima is an alien tree that aggressively invades old-fields, roadsides, and other disturbed sites.  Recent investigations suggest allelopathy may be one factor that contributes to its invasive ability.  Extracts of roots, bark, and leaves from A. altissima trees inhibit root growth of indicator species in laboratory bioassays.  The major phytotoxin in A. altissima (ailanthone) exhibits strong herbicidal activity on many weeds and crop species.  Established A. altissima stands expand mainly by producing new plants at the tips of their wide-ranging lateral roots, but the colonization of new sites at considerable distances from established stands depends on seeds and seedlings.  The allelopathic effects of older A. altissima trees have been investigated in some detail, but little is known about the allelopathic effects of seeds and young seedlings.  Therefore, because the establishment of seedlings is critical for the invasion of new sites, the purpose of this investigation was to evaluate the allelopathic potential of seeds and young seedlings of A. altissima.  Allelopathic effects were quantified using aqueous extracts prepared from oven-dry A. altissima tissues and bioassays using garden cress (Lepidium sativum) seeds.   

Results/Conclusions

Extracts made from A. altissima seeds separated from the surrounding pericarp tissue (wing of the samara) were extremely phytotoxic.  IC₅₀ values (extract concentration needed to inhibit cress radicle growth to 50% of control) were 0.08 g/L for new seeds collected in September at the end of the growing season and 0.18 g/L for old seeds collected in March, after hanging on the trees and being leached by precipitation most of the winter.  In contrast, extracts made with the surrounding pericarp tissue had very low toxicity (IC₅₀ = 2.5 g/L for new samaras and > 4 g/L for old samaras).  Investigations using soil in petri dishes indicate the phytotoxic compound is able to diffuse from the seeds into surrounding soil, where it inhibits cress radicle growth. The allelopathic activity of 5-week-old, 14-week-old, and 1-year-old A. altissima seedlings was lower than that of seeds, but was comparable to that of mature trees.  For all three age groups of seedlings and mature trees, the IC₅₀ was 0.3 to 0.7 g/L for roots or root bark, 0.6 to 0.8 g/L for stems or stem bark, and 1.3 to > 4 g/L for leaves or leaflets.  These results show that A. altissima seeds and young seedlings have strong allelopathic activity that is comparable to, or even greater than, that of mature trees.