Positive or negative canopy-understory interactions? A test using the comparative-experimental approach

Background/Question/Methods

With climate change, the US Pacific coast is predicted to face multiple changes in ocean conditions, including sea level rise, increasing storminess, and lower ocean pH. Determining how these changes will affect coastal communities requires understanding the mechanisms behind species interactions and using models to project the consequences.

The low rocky intertidal of this region is dominated by the canopy-forming kelp Saccharina sessile and a diverse understory community, including both calcifying and non-calcifying turf-forming macrophytes. We focused on the interaction between the canopy and understory community, asking, (1) What is the strength and sign of the interaction between canopy algae and understory organisms? (2) Do these effects vary along an oceanographic gradient?

In 2008, we manipulated the interaction between kelp and understory algae at 10 sites nested within 4 capes, spanning 600 km in Oregon and California. The capes experience different ocean conditions driven by shelf topography and ocean upwelling. Five replicate blocks consisted of four treatments: control, kelp removal, understory removal, and kelp and understory removal. Richness, abundance, and recruitment were recorded, and interaction strength was calculated at the end of two field seasons.

Results/Conclusions

Our experiments showed two important results. First, the interaction between canopy and understory algae was insensitive to differences in latitude and ocean conditions. Second, the interaction had both positive and negative components, making it more complicated than originally thought.

In particular, we found that across capes, the effect of canopy algae on the understory was negative. However, the negative effect was not consistent across understory types, with coralline algae showing no net effect of canopy algae. In addition, we found that the effect of the understory on the canopy was positive across all oceanographic conditions, with many Saccharina individuals being ripped off the rocks in the absence of an understory. Additionally, the effect of understory on Saccharina recruits was positive in the absence of a canopy, but had no effect on recruits when Saccharina adults were present.

Overall the understory had positive effects on the canopy via stress amelioration and recruitment facilitation, and kelp had negative effects on the understory. Though we hypothesized that mesoscale variation in oceanographic conditions should be important in determining species interactions, we found that interactions are fairly robust to this variation, suggesting more resilience than expected. How this interaction may vary due to climate is considered.