Role of mycorrhizal associations in structuring forest communities across environmental and climatic gradients

Background/Question/Methods

Forest species composition can influence forest ecosystem functions.  Species can be classified into function guilds to simplify communities for more efficient modelling.  Mycorrhizal association encapsulates a suite of functional traits that can be useful for understanding forest dynamics.  Shared mycorrhizal networks and symbiotic affinity for nutrient cycling rates could facilitate co-occurrence of species with similar mycorrhizal associations, resulting in spatial clustering of species across landscapes.  Alternately, differences in resource foraging and pathogen susceptibility may result in co-occurrence of species with different mycorrhizal associations, resulting in spatial over-dispersion of species across landscapes. 

We examined spatial patterns of woody plant species with contrasting mycorrhizal associations at several large-scale forest dynamics plots across North America. We utilize nearest-neighbor and bivariate point-pattern analyses to investigate the spatial scales at which small individuals (<10 cm DBH) associate with larger individuals of similar or opposing mycorrhizal association (arbuscular-mycorrhizal or ectomycorrhizal species). In each forest, we tested whether species with similar mycorrhizal associations are more spatially clustered or over-dispersed than expected by chance.

Results/Conclusions

Arbuscular mycorrhizal species were more abundant than ectomycorrhizal species at all forests. Nearest-neighbor distances differed among species with similar mycorrhizal associations, but the type of spatial pattern varied among forests. Small arbuscular mycorrhizal stems tended to be spaced further from large arbuscular- and ectomycorrhizal stems than expected by chance. Small ectomycorrhizal stems tended to be further from large arbuscular- and ectomycorrhizal stems than expected. Bivariate point-pattern analysis revealed a more complex relationship between mycorrhizal association and distance from large stems. At two sites, small arbuscular-mycorrhizal stems were significantly over-dispersed from large arbuscular-mycorrhizal stems at distances of less than 10m. In contrast, ectomycorrhizal stems exhibited random spatial patterns in the bivariate analysis, even though they tended to be over-dispersed near large arbuscular-mycorrhizal stems and clustering near large ectomycorrhizal stems. Although mycorrhizal association can influence local ecosystem function, our results suggest that these mutualistic relationships do not necessarily have a consistent effect on regeneration dynamics or spatial patterns of host species across forests. This lack of consistency may be due to species specificity of mycorrhizal associates. Alternately, other key ecological processes, such as dispersal limitation, may be having a stronger contribution to spatial patterns than mycorrhizal association.