Relations between the invasive Cinnamomum verum and the endemic Phoenicophorium borsigianum on Mané island, Seychelles

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Abstract. In the Seychelles the endemic palm Phoenicophorium borsigianum and the aggressively invasive alien Cinnamomum verum are competitors in a series of habitats. The extent to which demographic variables (relative growth rates of leaf area (RGRAL) and height (RGRR) together with mortality and recruitment) were important determinants of the competitive ability of the above species was investigated.It was studied whether competitive relations between Cinnamomum and Phoenicophorium can change with light intensity as a result of different morphological responses and to what extent native plants such as Phoenicophorium can establish, regenerate and potentially outcompete Cinnamomum in important biodiversity sites in mountain mist forest and intermediate forest habitats. The light intensity in terms of diffuse site factors (DifSF) was estimated at 32 microsites from hemispherical photographs using a computerized image analysis system. A new plant growth analysis function, the so‐called leaf area height index (ALHI), has been introduced and a principal components analysis (PCA) was carried out to evaluate the seedling performance of Cinnamomum and Phoenicophorium. Competition between Cinnamomum and Phoenicophorium was greatly influenced by the amount of available photosynthetically active radiation (PAR). For both species there was a negative correlation between mean leaf area of seedlings and light levels. Phoenicophorium responded significantly better in terms of RGRAL and ALHI to low gap light levels than Cinnamomum. Both species showed no further response to direct sunlight with levels of 98.7 % diffuse site factor. Phoenicophorium and probably other palms endemic to the Seychelles such as Deckenia nobilis and Roscheria melanochaetes act as a filter affecting the distribution and abundance of establishing Cinnamomum seedlings. Mortality of Cinnamomum was strongly negatively correlated with levels of PAR while Phoenicophorium showed no such correlation. Unlike Phoenicophorium, Cinnamomum could establish and regenerate only in light levels of >7.5 % diffuse site factor.