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.