The role of life-history traits and regeneration strategies in the emergence, distribution and timing of nitrogen fixation in tropical vs. temperate forests

Background/Question/Methods

Symbiotic N₂-fixation by plants is the main source of nitrogen input to most natural ecosystems.  Yet, why N₂-fixers are ecologically and evolutionarily favored in ecosystems where they ultimately act to remove soil N constraints on plant growth (mainly tropical forests), whereas, in other ecosystems fixers are either rare and/or quickly excluded by competition with others even though limitation persists (mainly temperate or boreal forests) remains a poorly understood mystery. Our objective is to understand the factors involved, and specifically the role of life-history traits, in controlling the emergence and maintenance of symbiotic N₂-fixers in different types of woody plant communities globally.  We explored how symbiotic N₂-fixing and non-fixing plants interact in the context of the competitive forces that control community organization and what determines the emergence of individual strategies. Using game-theoretic adaptive dynamics models (mathematical and simulation models), we examined how plant regeneration and traits related to nitrogen acquisition determine spatial and temporal patterns of growth of the fixers and non-fixers and their coexistence within a community in temperate and tropical forest ecosystems.  We also tested how community composition is affected by the disturbance regime, and how the life-histories of fixers are adapted to disturbance regimes in different ecosystems.

Results/Conclusions

Results of our simulations show the dynamics of alternative strategies, and how different combinations of parameters for fixers and non-fixers change the emergence and maintenance of N₂-fixation. N₂-fixation is favored only in recently disturbed areas, where soil N availability is low and does not allow non-fixers to achieve dominance. We found that the presence of fixation depends on a set of essential life-history traits that are related to the adaptation of N₂-fixer to local disturbance dynamics. Depending on disturbance type, N₂-fixation displays two alternative strategies: a large seedling bank for small-scale disturbances (e.g. forest gaps), as typically found in legume-tree fixers in tropical forests, vs. widely dispersed seeds for large-scale disturbances, representing the way actinorhizal trees disperse after infrequent large fire events in temperate forests. These different regeneration strategies also imply different seeds, i.e. few, large, locally dispersed vs. many, small, long-dispersed seeds, respectively.  Our results link the biogeochemistry of N₂-fixation and N cycling with the evolution and ecology of individual life-history. Understanding of how individual traits and community organization impacts processes of N cycling at the ecosystem level has important implications for the study of ecosystem functioning and could help forecast the impacts of man-made disturbances to N-cycling.