Snowpack influences potential fungal seed pathogens and the emergence of desert invasive and native grass seedlings

Background/Question/Methods

Seedling emergence is a fundamental limitation to ecosystem restoration efforts attempting to revegetate burned ecosystems.  Following seeding, seeds of native plants usually germinate at high levels in fall and must contend with extreme winter soil conditions before establishing in spring.  Many germinated seeds do not emerge from soils resulting in an “emergence bottleneck.”  This bottleneck results from multiple mechanisms, but fungal seed pathogens, in particular, may drastically influence seedling death.  However, our understanding of fungal seed pathogen dynamics in deserts is extremely limited, and it remains unclear if climate-induced changes in wintertime conditions will influence fungal virulence or restoration success.  To address this knowledge gap, we evaluated the potential for snowpack and fungal interactions to influence invasive and native seedling recruitment in a shrub-steppe ecosystem (OR, USA).  We seeded (i.e., broadcast seeding and seed bags) Bromus tectorum (Brte) and Pseudoroegneria spicata (Pssp) into three snow treatments (i.e., snow removed, added, or left in place) and measured seedling densities and seed fungal dynamics through winter and spring.  We tracked fungal biomass and the abundance of specific pathogenic fungal genera in dead and germinated seeds using quantitative PCR of the 18S rRNA gene, and screened for pathogenic fungi with clone libraries.

Results/Conclusions

We found that snow removal altered soil conditions, stimulated fungal dynamics, and reduced the emergence of seeds regardless of grass species.  Snow removal created dry and frequently frozen soils, while snow additions and ambient snowfall produced wetter and warmer soils.  We measured an emergence bottleneck in all the snow treatments; however, emerged seedling densities were 20% and 27% lower under snowfall removal than the other two treatments for Brte and Pssp respectively.  Further, in the snow removal treatment, higher fungal biomass coincided with lower seedling establishment of Brte and Pssp, suggesting that soil freezing and thawing may enhance fungal pathogen success.  We detected the presence of at least nine fungal pathogens on dead seeds and found that Fusarium were more abundant on dead Brte seeds under the snowfall removal than the other two treatments.  Our results suggest snowfall change may alter pathogen dynamics and limit seedling emergence.  Restoration efforts may be maximized if fall seedings occur in years projected to have high levels of snowpack.