On the ecology of ectomycorrhizal networks between overstorey trees and seedlings in a New England forest stand

The importance of soil organisms in structuring terrestrial plant communities and in maintaining ecosystem functions is becoming plain. Of these, mycorrhizal fungi, with which most vascular plants associate in obligatory symbioses, may be especially influential in mediating plant and plant species interactions. I report on a series of studies in which I investigated the role of common mycorrhizal networks, wherein plants of the same or different species are linked by shared fungal symbionts, in limiting the negative effects of overstorey tree competition on seedlings recruiting in a forest understorey. That common mycorrhizal networks (CMNs) should mitigate competitive effects on small or disadvantaged competitors by larger or stronger ones (whether or not they are of the same plant species), has been widely discussed for decades. But prior to the research described here, this phenomenon had not been demonstrated experimentally, nor had hypothesized mechanisms of such facilitation been linked to it empirically.; I present evidence from a two year field study in a New England forest that suggests that CMNs offset somewhat the negative effects of competition by overstorey trees on compatible pine, birch, and hemlock seedlings. Moreover, mycelia of overstorey trees accounted for all of their competitive effects on the survivorship of incompatible maple seedlings. I also discuss tests of hypotheses about effects CMNs on pine seedling tissue chemistry. Frequently cited hypotheses about facilitation in CMNs state that networks preferentially transmit carbon and soil nutrients to poorer plant competitors, but results of my C and N analyses (including analyses of C and N isotopes) suggest that neither C transfer nor increased N availability were related to positive CMN effects on pines. They suggest that another mechanism, perhaps dedicated water allocation, produced the observed results of CMNs on pines. Finally, I consider the role of CMNs in the colonization of birch seedlings by mycorrhizal fungi during secondary succession. Results of the experiment suggest that CMNs were essential for maintenance of diversity and community structure of mycobionts on birches. Insofar as overstorey CMNs have different effects on different species of seedlings, they may influence plant community dynamics during and following secondary succession in forests.