Does release from natural belowground enemies help explain the invasiveness of Lygodium microphyllum? A cross-continental comparison

Lygodium microphyllum (Cav.) R. Br., a climbing fern native to the Pantropics of the Old World, is aggressively colonizing natural ecosystems in the Florida Peninsula. Here, we examined soil factors that might affect the fern’s invasiveness, specifically addressing the hypothesis that a release from natural belowground enemies contributes to its vigorous growth in Florida. We also investigated phenotypic differences of sporophytes raised from spores collected in Florida and the fern’s native range in Australia, hypothesizing that the Florida population would possess traits resulting in faster growth and superior competitive ability than the two Australian populations. We tested our hypotheses in parallel greenhouse experiments—one in Australia using soil from the fern’s native habitat, and another in Florida, USA, with soil from a recently colonized ecosystem. Fern growth rate and its principal determinants were expressed relative to the optimal growth with a common sand culture in each experiment and compared among treatments in which soil was altered through either sterilization or nutrient amendment, or both. Contrary to the expectation, the optimal growth rates in the sand culture were higher for Australian populations than the Florida population, while the comparatively poor growth of all populations in unaltered soil was stimulated by nutrient amendment and sterilization. The overall effect of sterilization, however, was muted under high-nutrient conditions, suggesting that the effect of soil sterilization may be due to greater nutrient availability in sterilized soils. The only exception was the local population from the site where the soil was collected for the experiment in Australia, which grew significantly faster in sterilized than in non-sterilized soil, and also more rapidly in response to soil insecticide application. Our results indicate that the invasiveness of L. microphyllum in Florida is not a simple phenotypic difference in inherent growth rate as predicted by the evolution of increased competitive ability hypothesis, but it may be mediated in part by release from soil-borne enemies that vary in their effectiveness even within the native geographical range of the fern.