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.