Exploitation of pollen by mycorrhizal mycelial systems with special reference to nutrient recycling in boreal forests

Very large quantities of pollen are released annually by wind-pollinated tr ees, which dominate northern forest ecosystems. Since pollen is enriched in both nitrogen and phosphorus, this recurrent pulse of deposition constitut es a significant potential source of these elements in what are known to be severely nutrient-limited systems. Here, we demonstrate for the first time ? to our knowledge, that an ectomycorrhizal fungus, Paxillus involutus, is able to scavenge effectively for nitrogen and phosphorus in pollen and to r eturn a significant proportion of each nutrient to its autotrophic host, Be tula pendula. More than 75 and 96%, respectively, of the nitrogen and phosp horus were removed from pollen in microcosms containing the mycorrhizal fun gus, 29 and 25%, respectively being transferred to the plants. In contrast, in microcosms without the mycorrhizal fungus only 42 and 35%, respectively , of nitrogen and phosphorus were lost from the pollen, presumably as a res ult of export by saprotrophs, and only 12 and 7%. respectively were transfe rred to the plants. We hypothesize that this process of resource recapture, by contributing significantly to the ability of the trees to sustain the n ecessary annual investment in pollen production, will have a major impact u pon their reproductive capabilities and hence 'fitness'.