Growth and reproductive consequences of photosynthetic tissue loss in the surface canopies of Macrocystis pyrifera (L.) C. Agardh

Abstract

Macrocystis pyrifera (L.) C. Agardh provides biogenic habitat for species of significant commercial, cultural and recreational value; however, the floating surface canopies of M. pyrifera are particularly vulnerable to physical and biological disturbance (e.g., herbivory, storm events and harvesting). Given the key ecological roles of M. pyrifera, it is important to identify the impacts of photosynthetic tissue loss on growth and reproduction. I hypothesized that canopy removal would result in compensatory regeneration of the surface canopy. As life history theory posits a tradeoff between growth and reproduction, I also hypothesized that canopy removal would involve a reduction in reproductive investment, as either: (i) reduced production of reproductive structures; or (ii) changes in reproductive condition from fertile to sterile. To evaluate these hypothesizes I conducted two field experiments. In the first experiment, there was no significant difference in the generation of vegetative fronds or reproductive blades between controls (no loss of photosynthetic tissue from surface canopies) and treatments where photosynthetic tissue in surface canopies was thinned by either 30% or 70%. Relative to controls, the removal of entire surface canopies to a depth of 1.2m (i.e., simulated commercial harvesting) did not affect the generation of new vegetative fronds; however, the generation of reproductive blades was reduced by an average of 86%, suggesting that without the organic production supplied by the canopy, reproduction, but not growth, suffers. Further, the lack of evidence for compensatory growth despite reductions in reproduction suggests that M. pyrifera has little tolerance to canopy loss. The second experiment, which examined the effect of removing surface canopies on rate and longevity of changes in reproductive condition, found that although no control algae became sterile, 89% of algae with their surface canopies removed became sterile 50d after canopy removal, with effects persisting for up to 83d. As the supply of M. pyrifera propagules in the center of kelp forests can be tightly coupled to local reproductive output, induced sterility via the loss of photosynthetic tissue could affect the long-term stability of M. pyrifera beds. Further investigation into the scalability of these results and implications for long-term stability of M. pyrifera beds is warranted.