Strong below‐ground competition shapes tree regeneration in invasive Cinnamomum verum forests

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Abstract
Summary: Plant species invading nutrient‐poor ecosystems are likely to have their greatest impact on the native plant community by competing for resources below‐ground. We investigated how root competition by an invasive tree, Cinnamomum verum, affects regeneration in nutrient‐poor tropical secondary forests, in the Seychelles. We performed three trenching experiments to investigate the effects of severing the root systems of mature trees on the growth of juveniles. These experiments had the following objectives: (i) to compare the responses of native and invasive saplings to release from root competition; (ii) to compare how seedlings (< 50 cm tall) and saplings (> 50 cm tall) of C. verum respond to trenching; and (iii) to compare the response of C. verum seedlings to trenching in forest stands with and without C. verum as the dominant species. The results indicate that the dense topsoil root mat produced by mature C. verum trees suppresses the growth of young trees, mainly by increasing competition for scarce nutrients. Growth responses to trenching were stronger for seedlings than saplings, and stronger for juveniles of invasive than of native species. We conclude that stands of C. verum exert a strong below‐ground filtering effect on seedling regeneration. This effect is likely to influence secondary forest succession by selectively reducing the establishment of invasive and small‐seeded species. Because of the bias in invasion biology towards relatively nutrient‐rich, productive ecosystems, few studies have investigated the role of below‐ground resource competition in plant invasions. Our results for an infertile, phosphorus‐poor ecosystem show that root competition by an alien species can exert a strong influence on forest regeneration. We suggest that this mechanism may be of general importance in nutrient‐poor tropical forests invaded by alien tree species.

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