Stress Gradient Hypothesis in marine forests
Although highly variable, the effect of turfs on the canopy was consistently facilitative in intertidal environments and at higher latitudes. Intertidal communities are characterized by extreme abiotic stress for marine organisms, primarily thermal and desiccation stress due to immersion. As predicted by the SGH (Bertness & Callaway 1994), we found that the effect of the turf species on the canopy shifted from competitive in the subtidal to facilitative in the intertidal. Specifically, coralline turfs increasingly facilitated the canopy at shallower depths in experimental studies (Fig. 4). Among the observational studies (all intertidal), both coralline and non-coralline turfs facilitated the canopy (Fig. 3). For example, coralline turfs facilitated the intertidal kelp, Postelsia palmaeformis , due to the cooler temperatures and higher humidity among turfs than on bare rock (Hutto 2011). Thus, turfs may be critical for ameliorating stressful abiotic conditions, therefore critical for canopy-forming species recruitment and persistence in certain cases (also reported in Bennett et al. 2015).
We also found a suggested facilitative effect of turf algae at higher latitudes. Among experimental studies alone, there was a trend of coralline turf facilitating canopy algae at higher latitudes (Fig. 5, Appendix S3, Table S3). Although non-coralline crusts had a stronger competitive effect at higher latitudes, we discuss our reservations in interpreting the effect of non-coralline crust below (sectionAssumptions and limitations ). Further, observational studies were conducted at higher latitudes and demonstrated overall facilitation by turf species. When combined, these two lines of evidence support the conclusion that facilitation of the canopy may be stronger at higher latitudes. Conflicting with our results, multiple marine and terrestrial studies have demonstrated stronger facilitation at lower, warmer latitudes (Bertness et al. 1999; Pennings et al. 2003; McAfee et al. 2016). A reason for this discrepancy may be the complexity of latitudinal gradients in sea surface temperature. Many of our included studies were conducted along the Northeast Pacific coastline, where upwelling results in little change in sea surface temperature across ~25ยบ of latitude (Schoch et al. 2006). Ultimately, across ecosystems, the functional relationship between temperature and the strength of facilitation remains unclear. Both water depth and latitude in this study are proxies for stress, and more work needs to be done to truly isolate the effect of desiccation stress (or drought stress for terrestrial plants) and temperature on the prevalence of facilitation.