AimsCentral and northern Europe experienced extremely dry conditions in 2018, which caused dieback events in many ecosystems, including coastal heathlands. Our aim was to determine the causes of the varying drought responses observed in Calluna vulgaris on the Baltic island of Hiddensee after this extreme drought.LocationIsland of Hiddensee, Baltic Sea, NE Germany.MethodsWe assessed heathland community damage in autumn 2018 and 2019 with drone-based remote sensing. In addition, we measured water table depths and excavated root samples of C. vulgaris. In 2019, we sampled neighboring C. vulgaris individuals showing contrasting vitality statuses (dead, weakened or healthy), measured their height and canopy width, and prepared cross-sections of the main stem at the root collar or soil surface level to count and measure their xylem rings. We also assessed climate-growth correlations in these individuals.ResultsAn 8.0% of all heathland plants showed damage in 2018, while this value decreased to 6.6% in 2019. Only 18% of the plants showing damage in 2018 recovered in 2019. Plant damage was positively related to elevation. Groundwater measurements showed that water table depth dropped below C. vulgaris rooting depth during 2018 and 2019. Healthy plants were taller and had a larger crown area than neighboring weakened and dead individuals, but they did not differ in age nor in previous years' growth. C. vulgaris growth was positively correlated with June and July precipitation.ConclusionsOur study evidences the negative impacts of extreme droughts on C. vulgaris in coastal heathlands, especially in individuals growing in elevated areas and/or relatively small-sized a, and emphasizes the need for adapting heathland management planning accordingly.

Cassytha filiformis is a hemiparasitic plant that causes severe effects in its host plants. Since this decade, this alien species has been increasing its distribution towards the coastal areas of the Peninsula of Yucatan, parasitizing shrub species that play a crucial role in the containment of soil erosion. Here we studied the current distribution of C. filiformis along the coastal dune in northern Yucatan, recording the frequency of parasitism and the identity of its host plants. In addition, we evaluated the effect of C. filiformis on the sexual reproductive success of the main host plants and the effect of host species identity on C. filiformi's reproductive success. We found that the distribution of C. filiformis occurs throughout the coastal dunes of Yucatan (covering approximate to 250 km), parasitizing 15 species. However, ca. 70% of C. filiformis plants occur on three common shrub species: Suriana maritima, Scaevola plumieri, and Tournefortia gnaphalodes. The frequency of parasitized plants by C. filiformis was not dependent on host plant abundance. Tournefortia gnaphalodes suffer a higher proportion of parasitism. The reproductive success of the three host plants was lower in the presence of the parasitic plant. On the other hand, C. filiformis showed higher reproductive success when parasitizing S. maritima. Our results suggest that C. filiformis has extensively invaded the Yucatan coastal dunes, significantly reducing the sexual reproduction of its host-plant species. Overall, our results suggest that C. filiformis has the potential to cause significant damage in the Yucatan coastal dune community.