Sierra Nevada subalpine meadow plant competition across a moisture gradient

Background/Question/Methods

Sierra Nevada subalpine meadow plant communities are highly diverse and harbor many rare taxa. This diversity is often attributed to the unique patchy hydrology of the geologically young meadow soils.  However, species interactions such as competition and facilitation may play roles in community assembly as well. A number of plant interaction studies have found that as abiotic stress increases, facilitation increases. The converse is also true; when conditions are benign, competition is more prevalent.  This idea is called the stress gradient hypothesis and suggests plant associations are not as individualistic and random as has been believed. This study examines the relative degree of competition and facilitation in five species of common meadow plants across a moisture gradient in three meadows. Two of the meadows are wet meadows and one is dry.  This study was conducted in the Van Vleck Meadow Restoration Area in the Eldorado National Forest of the Sierra Nevada in summer 2009.  We expected to find that facilitation increased as drought stress increased. Our study organisms were Deschampsia cespitosa, Ligusticum grayii, Stachys albens, Mimulus primuloides, and Senecio triangularis.  In each meadow, 8 pairs of each species were selected.  One of each pair was assigned randomly to treatment.  Treatment consisted of the hand removal of vegetation from a radius of 30 centimeters around the plant.  At the end of the growing season, plants were harvested and dried.  The above ground biomass weights were analyzed with nonparametric statistics.

Results/Conclusions

We found that species interactions in the two wet meadows were neutral and in the dry meadow competition predominated.  Apparently the stress of growing in saturated soil is greater than the stress of drought.  These results are surprising and contravened our expectations and previous studies.  This implies that management of Sierra Nevada subalpine meadows should consider species interactions as well as hydrology. Increasingly, ecological studies are finding that facilitation plays a role in plant community assembly. We may soon see a synthesis of Gleason's and Clements' opposing ideas into a modern model of plant community assembly that includes both community and individual dynamics.