OOS 30-3 - Timing of microbial mutualist arrival has a greater effect on seedling growth than interspecific competition

Wednesday, August 9, 2017: 2:10 PM
Portland Blrm 258, Oregon Convention Center
Kabir G. Peay, Department of Biology, Stanford University, Stanford, CA

While plant-microbe interactions play a critical role in structuring communities, the temporal dimensions of these interactions are generally ignored or considered implicitly in their study. Experimental studies of plant-microbe interactions are generally carried out in greenhouses to adequately control the microbial community, but most greenhouse experiments examine only the effects of microbial presence or absence. For naturally recruiting plants, though, symbiont availability more likely varies as a function of time due to ongoing dispersal of the microbial partners. While presence-absence studies capture the extremes of partner arrival time, they may not reflect the most common interaction scenarios.

In this talk I report results from a growth chamber experiment to test the relative importance of timing and biotic context on mutualistic interactions using an ectomycorrhizal fungus, its pine host, and the pines’ major competitor. Over 9 months I varied the timing of ectomycorrhizal inoculation (Inoculation Time), presence of a competitor (Competition), and measured their effects on pine seedling growth. The experiment was conducted in live field soil from two sites (Soil Origin) differing in their proximity to adult pines. Seedlings were harvested at three time points (Harvest Time) to capture key phases in the temporal dynamics of plant-microbial interactions.


Ectomycorrhizal colonization varied substantially across treatments as evidenced by a significant three-way interaction between Soil Origin x Harvest Time x Inoculation Time (F1,6 = 10.79, P < 0.001). Colonization was uniformly high for seedlings growing in soils collected near pines, but in soils collected away from established pines seedling colonization was determined by the timing of ectomycorrhizal spore inoculation. The treatments also generated substantial variation in pine seedling biomass as evidenced by a significant three-way interaction for Soil Origin x Harvest Time x Inoculation Time (F1,6 = 25.84, P < 0.001). The overall pattern of biomass effects largely mirrored those for ectomycorrhizal colonization. Notably, for seedlings growing in soils collected away from established pines, biomass decreased as delay in ectomycorrhizal inoculation increased. Competition treatments had significant negative effects on pine seedling growth. However, the competition effect was much smaller than the effect of delayed mutualist arrival. These results have two important implications. First, the importance of spore arrival time suggests that plants may experience mutualist limitation more frequently than previously expected. Second, the magnitude of seedling responses to mycorrhizal fungi and competing plants show that mutualism is likely of equal or greater importance compared with interspecific competition during community assembly.