Species-specific facilitation of germination may promote regeneration of a post-drought perennial community

Background/Question/Methods

Extensive drought caused unprecedented mortality of adult and juvenile perennials in a long-term study plot in the Colorado Desert, CA.  After nearly a decade, regeneration of this community has been slow to nonexistent. Plant facilitation, a predominant interaction among desert plants, is likely to play a critical role in the regeneration of this community. Ambrosia dumosa, Larrea tridentata, and Tetracoccus hallii are the most common species at this site and adults of these species may facilitate seedling establishment.  Among these three, only A. dumosa, which suffered the greatest adult mortality, showed significant positive spatial associations and improved survival with common perennial juveniles prior to the drought.  It is unclear if these species-specific patterns are due to improved germination or survival.  During March of 2012 we established a controlled experiment to determine if seed germination of common perennials responds to facilitator species. One hundred seeds of the target species, A. dumosa, L. tridentata, Sphaeralcea ambigua and Eriogonum fasciculatum, were packaged in nylon bags and buried under the canopy and at the canopy edge of each facilitator species, and in a control interspace area 1.5 m from any plant canopy using a block design with ten replicates.

Results/Conclusions

Facilitator species differentially affected germination of the target species.  Of the target species Ambrosia dumosa showed species preference demonstrating significant germination under the canopy of A. dumosa (P=0.0294), the canopy edge (P=0.0193) and under the canopy of L. tridentata (P=0.0162) compared to the canopy edge of T. hallii.  Eriogonum fasciculatum did not show facilitator preference; it germinates significantly greater under A. dumosa (P<0.0001), L. tridentata (P<0.0001) and T. hallii canopies (P<0.0001) and the canopy edges of A. dumosa (P<0.0001), and L. tridentata (P<0.0001) compared to interspaces.  In contrast, L. tridentata germinates significantly greater in interspaces than under the canopy of A. dumosa (P=0.0018), and the canopy edge of L. tridentata (P<0.0036), and under the canopy of T. hallii (P<0.0001). S. ambigua germination did not show microsite preference.  These results suggest that each of the dominant species can facilitate seed establishment, even though pre-drought patterns suggest that only A. dumosa is a significant facilitator.  Germination is only one of many factors affecting regeneration. If our controlled responses mirror those that determined the pre-drought community, then species that favor germination may not promote seedling survival or stochastic factors may exert strong control over plant distributions.