Effects of Agathis australis (New Zealand kauri) leaf litter on germination and seedling growth differs among plant species

Agathis australis (A. australis, New Zealand kauri, Araucariaceae) exerts a substantial influence on soil properties and nutrient cycling, and mature specimens form an acidic organic soil layer beneath them that can be up to 2 m deep. We investigated whether phytotoxic compounds occurred in A. australis leaf litter and organic soil, and whether allelopathy may explain the distinctiveness of plant communities surrounding A. australis. We extracted water-soluble compounds from fresh litter, and conducted bioassays of seed germination and seedling growth in these extracts on both A. australis-associated and non-associated species. Germination of all species except A. australis was inhibited by extracts from A. australis litter, which probably contains phytotoxic compounds. Germination of a forest species that is not associated with A. australis was inhibited by the low pH organic soils collected from beneath mature A. australis, but when these soils were neutralised using lime, its germination was not inhibited. Lactuca sativa, a species highly sensitive to phytotoxic compounds, was negatively affected by both the low pH of the organic soil and the presence of phytotoxic compounds. In contrast, there was no effect of the organic soil on the germination and growth of A. australis-associated species. These results suggest that the high acidity of A. australis organic soil plays a considerable role in structuring the composition of plant communities associated with A. australis, and also that A. australis litter probably contains unidentified phytotoxic compounds that may exert additional direct allelopathic effects on sensitive species.