Responses of the biennial forest herb Alliaria petiolata to variation in population density, nutrient addition and light availability


B. C. McCarthy (tel. 740-593-1615; fax 740-593-1130; e-mail


1Alliaria petiolata, a European biennial herb, is an important pest in temperate North American deciduous forests. Habitat resource structure has probably been important for invasion and proliferation of this species.

2Alliaria was grown in an experimental garden at two densities (equivalent to 17 and 170 plants m−2), three nutrient levels (no, low, or high nutrient addition) and three light levels (ambient sunlight and two shading treatments) to determine the effects of environmental heterogeneity on growth, reproduction and resource allocation in both mature and rosette plants.

3 Overall, rosette growth and allocation patterns were significantly affected by all three variables tested. Low plant density, nutrient addition and high light availability resulted in plants with more leaves and greater dry weight biomass. Biomass allocation to shoots was greatest for plants in high-density and low-light treatments. Leaf chlorophyll content was significantly greater for plants in the two shaded treatments.

4 Mature plants also responded to environmental manipulation with significantly greater total dry weight biomass at low plant density, high light availability and with nutrient addition. Low density and high light availability resulted in significantly higher seed production. Plants in the lowest light treatment allocated significantly more biomass to shoot production and less to root production. Leaf chlorophyll content was lowest for plants in the highest light treatment and increased with nutrient addition at the two highest light treatments.

5 Plants growing at high density responded to nutrient addition and light attenuation in the same direction but with greatly reduced magnitude compared with plants growing at low density. The effect of irradiance was the most important determinant of all facets of growth and allocation patterns in both rosettes and mature plants.

6 Our results indicate that although density and site fertility may play important roles in Alliaria invasion and establishment, light availability may be the most important factor affecting subsequent growth and proliferation.