Decomposition of sub-arctic plants with differing nitrogen economies: A functional role for hemiparasites

Although hemiparasitic plants have a number of roles in shaping the structure and composition of plant communities, the impact of this group on ecosystem processes, such as decomposition and nutrient cycling, has been poorly studied. In order to better understand the potential role of hemiparasites in these processes, a comparison of leaf and litter tissue quality, nitrogen (N) resorption, and decomposability with those of a wide range of other plant groups (involving a total of 72 species and including other groups with access to alternative nutrient sources, such as nitrogen fixers and carnivorous plants) was undertaken in several sub-arctic habitats. The foliar N concentration of hemiparasites generally exceeded that of co-occurring species. Further, hemiparasites (and N fixers) exhibited lower N resorption efficiencies than their counterparts with no major alternative N source. As a consequence, annual and perennial hemiparasite litter contained, on average, 3.1% and 1.9% N, respectively, compared with 0.77–1.1% for groups without a major alternative N source. Hemiparasite litter lost significantly more mass during decomposition than many, but not all, co-occurring species. These results were combined with those of a litter trapping experiment to assess the potential impact of hemiparasites on nutrient cycling. The common sub-arctic hemiparasite Bartsia alpina was estimated to increase the total annual N input from litter to the soil by 42% within 5 cm of its stems, and by 53% across a site with a Bartsia alpina stem density of 43 stems/m2. Our results therefore provide clear evidence in favor of a novel mechanism by which hemiparasites (in parallel with N-fixing species) may influence ecosystems in which they occur. Through the production of nutrient rich, rapidly decomposing litter, they have the potential to greatly enhance the availability of nutrients within patches where they are abundant, with possible consequent effects on small-scale biodiversity.