Observational studies
Observational studies of understory effects on canopy algae were mostly conducted on kelp (n Laminariales = 90, n Fucales = 1), at early life history stages (recruits or juveniles). Further, these studies were exclusively carried out in the intertidal, significantly shallower than experimental studies and at a higher mean latitude (botht -test, p obs - exp < 0.001). Thus, these results are comparable only to a subset of the experimental results above. Among observational studies, the absence of an algal understory had a strong negative effect on the canopy, indicating a positive interaction between the two groups (random effects model, mean LNRRobs = -1.46 ± 0.74, p = 0.048). The effect of turfs on the canopy varied among turf functional groups (overall Wald-type test, QM[3], obs = 17.37, p < 0.0001, pseudo-R2 = 0.71). In observational studies, most turf species had no effect on the canopy, but unlike experimental studies, coralline turf facilitated the canopy (ϐcoralline turf = -2.74, p = 0.0016). This effect did not change across depth or latitude (Appendix S3 Tables S5-7).