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).