Soil legacy effects of Scotch broom invasion on Douglas-fir mycorrhizae

Background/Question/Methods

Plants affect the microbial communities in the soil in which they grow.  Exotic invasive plants that have not coevolved with the mycorrhizal species in the introduced environment may decrease the abundance and richness of the native mycorrhizal fungi.  After two consecutive years of wholesale failure of Douglas-fir (DF) reforestation efforts in Scotch broom invaded sites in western Washington, we conducted a series of greenhouse experiments to better understand the mechanisms responsible for the astounding seedling mortality within these invaded sites.  Scotch broom is a nitrogen-fixer that affects soil nutrients and may shift soil microbial communities to the detriment of native species.   However, studies examining nitrogen enrichment on ectomycorrhizal fungi (EMF) abundance have reported inconsistent results. We hypothesized that soil N would be greater in the Scotch broom invaded soils compared to the uninvaded soils; and moreover that the EMF community would be degraded either because broom does not associate with the local EMF species or that the EMF are deterred by N enrichment.

We compared the abundance of EMF on DF seedlings grown in broom-invaded and uninvaded soils and compared total soil N and carbon (C) in both soil types.  To measure EMF abundance we examined root tips microscopically to determine presence or absence of a fungal mantle. We then used ITS sequencing methods to compare the kinds of ectomycorrhizal fungi on DF grown in invaded and uninvaded soils.  

Results/Conclusions

We found that DF seedlings grown in broom-invaded forest soils had lower rates of ectomycorrhizal colonization than seedlings grown in soils not invaded by Scotch broom and that the EMF communities were different.  We also found that DF growth was strongly and positively correlated with the degree of EMF colonization.  The soil C:N ratio in broom-invaded forest soils was significantly lower than in non-invaded soils. However, we did not find that total soil N predicted EMF colonization rates, primarily because uninvaded forest soils have a much larger soil organic matter pool.