Forests trapped in nitrogen limitation--an ecological market perspective on ectomycorrhizal symbiosis.
Ectomycorrhizal symbiosis is omnipresent in boreal forests, where it is assumed to benefit plant growth. However, experiments show inconsistent benefits for plants and volatility of individual partnerships, which calls for a re-evaluation of the presumed role of this symbiosis. We reconcile these inconsistencies by developing a model that demonstrates how mycorrhizal networking and market mechanisms shape the strategies of individual plants and fungi to promote symbiotic stability at the ecosystem level. The model predicts that plants switch abruptly from a mixed strategy with both mycorrhizal and nonmycorrhizal roots to a purely mycorrhizal strategy as soil nitrogen availability declines, in agreement with the frequency distribution of ectomycorrhizal colonization intensity across a wide-ranging data set. In line with observations in field-scale isotope labeling experiments, the model explains why ectomycorrhizal symbiosis does not alleviate plant nitrogen limitation. Instead, market mechanisms may generate self-stabilization of the mycorrhizal strategy via nitrogen depletion feedback, even if plant growth is ultimately reduced. We suggest that this feedback mechanism maintains the strong nitrogen limitation ubiquitous in boreal forests. The mechanism may also have the capacity to eliminate or even reverse the expected positive effect of rising CO2 on tree growth in strongly nitrogen-limited boreal forests.
IIASA- International Institute for Applied Systems Analysis, A-2361, Laxenburg, Austria.
Oskar
O
Franklin
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83, Umeå, Sweden.
Torgny
T
Näsholm
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83, Umeå, Sweden.
Peter
P
Högberg
Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-901 83, Umeå, Sweden.
Mona N
MN
Högberg
D004463:Ecology; D025461:Feedback, Physiological; D065928:Forests; D008962:Models, Theoretical; D038821:Mycorrhizae; D009584:Nitrogen / Q000378:metabolism*; D063245:Plant Development; D012988:Soil Microbiology; D013559:Symbiosis
D009584:Nitrogen
below-ground; elevated carbon dioxide; game theory; mutualism; optimization; pine; trade; tragedy of the commons