Biocrust and seed placement influence emergence rates of perennial grass Elymus elymoides across five North American deserts

1. Dryland vascular plant emergence is often limited by water availability especially with projected climate and precipitation changes. Biological soil crusts (biocrusts) can promote water retention and nutrient availability that benefit germination, and emergence yet can also act as a surface barrier preventing critical seed soil contact and hindering emergence. Alongside these factors, dryland fire frequency has increased with the inclusion of invasive species and vegetation structural changes. With enhanced continuous fine fuel distribution there is a high potential to disrupt biocrust-plant interactions and influence associated management actions.

2. This study explores the dynamic relationship between biocrusts and fire-related heating effects on seedling emergence across five North American deserts: the Chihuahuan Desert, Colorado Plateau, Great Basin, Mojave Desert and Sonoran Desert. We conducted a greenhouse-based seedling emergence experiment using Elymus elymoides (bottlebrush squirreltail), a common perennial grass, with biocrust and bare soil mesocosms in which half were heated to mimic the effects of wildfire temperature.

3. The variables that had the greatest influence on germination rate and germination timing were the presence of biocrust and seed placement (on top of vs within the biocrust/soil matrix). Emergence rate was greatest atop bare soil followed by seeds inserted into biocrust. Emergence timing was faster with biocrust present, but only when seeds were inserted into it. Desert origin of biocrust and soil collection also influenced germination where the probability of any one seed emerging was highest in the Chihuahuan and Mojave desert sites relative to the Sonoran desert site which showed the lowest probability. Heating had mixed effects whereby it increased the likelihood of emergence but did not affect the overall rate or timing.

4. Synthesis and applications. This study underscores the importance of healthy and impaired biocrusts in dryland systems and suggests ways in which the combination of biocrust and seed placement can influence plant establishment, in addition to providing insight into seeding strategies for managers and restoration practitioners working in dryland sites.