Background. Biological facilitation is a type of relationship between two taxa that benefits at least one of the participants and harms neither. Although native species are widely known to compete with native taxa, recent studies suggest that invasive and native species can have positive relationships. This study aims to examine the biological facilitation of the germination of invasive Miconia calvescens by giant tree fern Angiopteris evecta , native to French Polynesia.
Methods. Field surveys were conducted to measure A. evecta and M. calvescens by applying the 10 × 10 m 2 quadrat survey method. The density of seedlings, saplings, and matures of M. calvescens growing on the rhizomes of A. evecta and on bare soil was compared, and the correlation between the size of the rhizomes and the number of M. calvescens growing on them was checked. Comparative soil nutrient experiments were done for the substrates of the rhizomes of A. evecta, soil under the rhizomes, and bare soils to check whether the rhizomes are chemically different than other microenvironments.
Results. Field surveys show that there is a greater number of seedlings and saplings of M. calvescens growing on the rhizomes of A. evecta as compared to bare soil. Furthermore, there is a positive correlation between the size of rhizomes and the number of M. calvescens growing on them. Substrates of A. evecta had higher phosphorus and potassium contents compared to other soils and substrates.
Discussion. Angiopteris evecta is facilitating the germination of M. calvescens, supported by the much higher number of seedlings and saplings growing on the rhizomes and the positive correlation between the size of the rhizome and the number of M. calvescens growing on it. Microslopes on the rhizomes of A. evecta prevent leaf litter from accumulating on the rhizomes, and enable more sunlight to reach and facilitate the germination of M. calvescens seeds. Also, field observations suggest that A. evecta has fast decomposing leaves, which can induce the germination and the recruitment of M. calvescens in forest understories dominated by A. evecta by preventing the accumulation of leaf litter on the ground and enabling more sunlight to reach to the seed bank. The germination of trees in tropical rainforest is known to be limited by the availability for potassium, suggesting that the rhizomes of A. evecta might be favored chemically. However, further studies are needed to verify whether potassium is a limiting factor of the germination of M. calvescens. Biological facilitation of the germination of invasive M. calvescens by native A. evecta can give better understanding on the invasion success and the relationship between the native and invasive species. Invasion of A. evecta can induce and promote further invasion of M. calvescens. Therefore, thorough management of ongoing invasion of A. evecta in some places is especially required.