Do shrubs improve reproductive chances of neighbors across soil types in drought?

Authors: SWANSON, ELIZABET - Oregon State University; Sheley, Roger; JAMES, JEREMY - University Of California Agriculture And Natural Resources (UCANR)

Submitted to: Oecologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/11/2019
Publication Date: 11/25/2019
Citation: Swanson, E.K., Sheley, R.L., James, J.J. 2019. Do shrubs improve reproductive chances of neighbors across soil types in drought? Oecologia. 192:79-90. https://doi.org/10.1007/s00442-019-04559-x.
DOI: https://doi.org/10.1007/s00442-019-04559-x

Interpretive Summary: Storms and periods of drought are expected to increase in frequency and intensity in the western United States over the next century. Plant reproduction is very sensitive to extremes in weather creating uncertainty for how the will maintain populations in the coming years. Facilitation among plants is becoming more understood and capitalizing on biotic interactions to improve the chances for plant reproduction is an exciting and emerging area of research. In our study we test if sagebrush shrubs can improve the reproductive potential of its neighboring grasses during periods of drought and if this interaction between shrub and grass is different under normal conditions and also high moisture conditions. We found that several species of grasses in sagebrush steppe are strongly facilitated in drought, but that this changes based on soil type. There is likely an interaction between soil texture, soil percolation rates and species-specific life history traits that determine the propensity for facilitation at a given location. This is interesting because it implies that biotic interactions are important for determining where and how plant communities assemble across large spatial scales of soil mosaics. We also found Bottlebrush squirretail to be highly dependent on sagebrush shrubs for improving plant reproduction in times of drought, implying this species is potentially a facilitation obligate species or simply uses its sagebrush neighbor to maintain a competitive niche over other grasses in years of severe weather.

Technical Abstract: Abstract Plant reproduction is highly sensitive to stress from severe weather. While facilitation has been shown to buffer negative impacts along stress gradients, less is known about facilitating plant reproduction in drought periods. Because intensity and frequency of drought are predicted to increase, plant reproductive facilitation has important implications for a species ability to adapt to changes in climate. Our primary study objective is to test if nurse shrubs act as reproductive micro-refugia across soil types, by improving reproductive potential of perennial bunchgrass neighbors subjected to severe drought. To investigate this objective, we designed a fully factored study testing direct interaction between shrub and bunchgrasses in eastern Oregon sagebrush steppe, at two sites with different soil types. The study consisted of six simple effect treatments combining three moisture regimes (moist, ambient, and drought) with two shrub conditions (shrub intact or shrub removed). Our results indicate when facilitation of reproductive potential occurs, it occurs strongly and particularly in drought, consistent with the stress gradient hypothesis (SGH), where several species produced at least 54% more inflorescences in the presence of shrub neighbors. In addition, we found facilitation to be consistent with the SGH at the species level likely reflecting differences in plant strategy and perception of strain, but to follow alternative SGH models more closely at the site level where facilitation declined on the drier soil. Ultimately, our findings highlight the importance of facilitation in improving plant reproductive potential in drought, and support the role of nurse shrubs as micro-refugia in a changing climate.