Experimental Studies
Overall, the experimental removal of an algal understory had a mean
neutral effect on the canopy (random effects model, mean
LNRRexp = 1.55 ± 0.94, p = 0.099), with
interactions ranging from strongly competitive to strongly facilitative
(Fig. 3). The mean interaction did not change over the course of an
experiment, measured as the number of days since the start of the
experiment (ϐdays = 0.0012, p = 0.052). The
effect of turf species on the canopy varied among turf functional groups
(overall Wald-type test, QM[4], exp = 17.16, p
= 0.0018, pseudo-R2 = 0.35, Fig. 3). Specifically,
the overall competitive effect of turfs on the canopy was driven by the
negative effects of coralline turf and non-coralline crust
(ϐcoralline turf = 3.59, p = 0.0009;
ϐnon-coralline turf = 2.91, p = 0.034); all other
turf species had no significant effect on the canopy (all p> 0.05; Appendix S3 Table S1). However, the effect of turf
species differed among canopy types. Among kelps, non-coralline turf
taxa also had a negative effect on the canopy
(ϐnon-coralline turf = 5.04, p = 0.026). Among
Fucales, all turf functional groups had an overall neutral effect on the
canopy (all p > 0.10, Appendix S3 Table S1).
However, these overall effects marked geographical and latitudinal
variation in the effect of turfs on canopies. In line with the
widespread paradigm of kelp forest interactions, the effect of turfs was
overall competitive in subtidal systems. Specifically, the effect of
coralline turf on canopy taxa was competitive in the subtidal, and
increasingly facilitative in the intertidal (Fig. 4,
pseudo-R2 = 0.16, ϐdepth * coralline
turf = -0.61, p = 0.0003). Though found in a narrower depth
range than turfs, non-coralline crust also had an increasingly positive
effect on the canopy at shallower depth (ϐdepth *
non-coralline crust = -1.33, p = 0.009). The effect of coralline
crust and non-coralline turf on the canopy did not change across depth,
and this pattern held for canopy kelps (Appendix S3 Table S2). Studies
of canopy Fucales, conducted primarily in the intertidal zone,
demonstrated no difference in interactions across depth (all functional
groups, p functional group: depth >
0.05, pseudo-R2 = 0.27; Appendix S3 Table S2).
The effect of turfs on the canopy varied across latitude
(QM[8] = 26.69, p = 0.0008,
pseudo-R2 = 0.39). In particular, the effect of
non-coralline crust became more competitive at higher latitudes
(ϐlatitude * non-coralline crust = 0.40, p =
0.027). Further, the effect of coralline turf was more facilitative at
higher latitudes, with marginal significance (ϐlatitude *
coralline turf = -0.35, p = 0.058; Appendix S3 Table S3).
Next, we looked only at variation in the interaction across life history
stage of the canopy, and did not include variation across depth and
latitude. The effect of turfs on the canopy differed among canopy life
history stages, depending on the turf functional group and the identity
of the canopy (QM[31] = 45.13, p <
0.0001, pseudo-R2 = 0.13; Fig. 3). Turf species had
competitive effects on the canopy, primarily at early life history
stages. Coralline crust and all turfs negatively affected germling
canopies (overall effect, p for linear contrasts <
0.01: ϐcoralline crust = 4.77, p = 0.029;
ϐcoralline turf = 9.65, p = 0.00008;
ϐnon-coralline turf = 5.43, p = 0.014). Further,
coralline crusts negatively affected the later canopy “recruit” stage
(overall effect: ϐcoralline crust = 4.77, p =
0.004). All other effects were not modified by life history stage of the
canopy (Appendix S3 Table S4). Finally, considering only studies that
manipulated herbivory (n = 28), the mean effect of turfs on the canopy
did not depend on the presence or absence of herbivores (linear
contrast, ϐ-herbivore - +herbivore = 2.84 ± 2.10,p = 0.18; pseudo-R2 = 0.44, Appendix S3 Figure
S1).