Abstract
The strength of species interactions often varies geographically and locally with environmental conditions. Competitive interactions are predicted to be stronger in benign environments while facilitation is expected to be stronger in harsh ones. We tested these ideas with an aboveground neighbor removal experiment at six salt marshes along the California coast. We determined the effect of removals of either the dominant species, Salicornia pacifica, or the subordinate species on plant cover, aboveground biomass and community composition, as well as soil salinity and moisture. We found that S. pacifica consistently competed with the subordinate species and that the strength of competition varied among sites. In contrast with other studies showing that dominant species facilitate subordinates by moderating physical stress, here the subordinate species facilitated S. pacifica shortly after removal treatments were imposed, but the effect disappeared over time. Contrary to expectations based on patterns observed in east coast salt marshes, we did not see patterns in species interactions in relation to latitude, climate, or soil edaphic characteristics. Our results suggest that variation in interactions among salt marsh plants may be influenced by local‐scale site differences such as nutrients more than broad latitudinal gradients.
Highlights
The competitive effect of the dominant species, S. pacifica, on subordinate species varied among sites, but was unrelated to latitude, climate, or any of several measured local environmental conditions (Figure 4)
These findings suggest that geographic variation in interaction strength among salt marsh plants is not always predictable based on latitude or aspects of the environment as anticipated by the stress-gradient hypothesis (SGH)
Removal treatments had no significant effect on soil salinity or moisture, these effects may have been more pronounced in the summer when we did not sample the experiment
Summary
Along an estuarine gradient in Oregon, interactions among plants were more competitive at lower salinities as expected by the SGH, unlike in east coast marshes, facilitative effects of neighbors were rarely observed (Keammerer & Hacker, 2013) This test was only within one site, and few studies have examined the variation in interaction strength among multiple sites along a gradient rather than at two ends of it. An improved understanding of the drivers of species interactions across space will allow us to better predict how species interactions will be affected by climate change This can inform conservation efforts which is important in salt marshes as they are already highly threatened (Gedan, Silliman, & Bertness, 2009; UNEP 2006). Facilitation of subordinate species by the dominant may be expected in the south where high temperatures and low precipitation lead to high salinity
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