Potential advantages of highly mycotrophic foraging for the establishment of early successional pioneer plants on sand.

Adaptive traits ensuring efficient nutrient acquisition, such as extensive fine root systems, are crucial for establishment of pioneer plants on bare sand. Some successful pioneer species of temperate, European sand ecosystems are characterised as obligate mycorrhizals, thus likely substituting fine roots with arbuscular mycorrhizal fungi (AMF). However, it is not clear whether AM fungal-mediated acquisition of scarce and immobile nutrients such as phosphorus (P) is an advantageous strategy on bare sand over foraging via roots. We compared the foraging performance of three obligately mycorrhizal forbs and two facultatively mycorrhizal grasses, regarding the influence of AMF on their capacity to acquire P from bare sand. Comparison of mycorrhizal and non-mycorrhizal individuals revealed a markedly higher AM fungal-dependency for P acquisition and growth in the forbs than in the grasses. Periodical soil core sampling, allowing for assessment of root and hyphal growth rates, revealed hyphal growth to markedly enlarge the total absorptive surface area (SA) in the forbs, but not in the grasses. Correlations between SA growth and P depletion suggest an AM fungal-induced enhanced capacity for rapid soil P exploitation in the forbs. Our study showed that AM fungal-mediated foraging may be an advantageous strategy over root-mediated foraging in sand pioneer plants.