Other factors influencing interactions with marine
canopies
Facilitation is also predicted to be stronger at early life history
stages (Callaway & Walker 1997). The “nurse plant syndrome” has been
widely demonstrated among plants, where adult “nurse” plants
facilitate the recruitment and persistence of seedlings (Miriti 2006; He
et al. 2013). We found conflicting evidence for the presence of this
syndrome in turf/crust - canopy interactions. Experimental studies
generally demonstrated competitive effects of multiple understory
species on canopies at early life history stages, while observational
studies supported strong facilitation of canopy recruits by coralline
turf. Overall, few studies in our meta-analysis quantify effects on the
canopy across multiple life history stages (Appendix S3 Fig. S2), which
precludes any strong conclusions here. Given that canopy-forming brown
seaweeds produce microscopic stages as part of their recruitment process
(Schiel and Foster 2006), these stages must be included in studies as
they are fundamental to population recovery and persistence.
We also found little support for the hypothesis that the effect of turfs
on the canopy is influenced by the presence of herbivores. Experiments
that excluded herbivores demonstrated the same interaction as paired
plots that included herbivores (Appendix S3 Figure S1). Herbivory should
only alter the interaction if herbivores were facilitated or excluded as
a function of turf removal. For example, some understory turf algae
prevent the influx of sea urchins at the kelp forest boundary by
limiting their movement, resulting in a positive indirect effect of turf
on kelp (Konar et al. 2014). On the other hand, recruitment facilitation
by turf algae of canopy herbivores can drive the emergence of kelp
forest - urchin barren alternative stable states (Baskett & Salomon
2010). Marine herbivores generally have strong effects on primary
producers (Poore et al. 2012), and urchin herbivory in particular is a
dominant force structuring global kelp forests (Pearse 2006; Ling et al.
2015). However, few studies have explicitly examined the effect of
herbivory on marine turf-canopy interactions (included in present study,n = 14), with factorial experiments to disentangle these
interactions. Work in the Patagonian steppe has even found that the
interaction among grasses was mediated by the strength of herbivory
(Graff et al. 2007) - a mechanism of the SGH that may be worth exploring
in marine forests. Future studies should examine the interaction of turf
species with canopy herbivores to fully quantify the direct and indirect
effects of competition, facilitation, and herbivory.