Hemiparasite abundance in an alpine treeline ecotone increases in response to atmospheric CO2 enrichment

Populations of the annual hemiparasites Melampyrum pratense L. and Melampyrum sylvaticum L. were studied at the treeline in the Swiss Alps after 3 years of in situ CO₂ enrichment. The total density of Melampyrum doubled to an average of 44 individuals per square meter at elevated CO₂ compared to ambient CO₂. In response to elevated CO₂, the height of the more abundant and more evenly distributed M. pratense increased by 20%, the number of seeds per fruit by 21%, and the total seed dry mass per fruit by 27%, but the individual seed size did not change. These results suggest that rising atmospheric CO₂ may stimulate the reproductive output and increase the abundance of Melampyrum in the alpine treeline ecotone. Because hemiparasites can have important effects on community dynamics and ecosystem processes, notably the N cycle, changing Melampyrum abundance may potentially influence the functioning of alpine ecosystems in a future CO₂-rich atmosphere.