Limitations of using the canopy to infer the structure and functioning of giant kelp forests

Data obtained from remote sensing of floating kelp canopies are routinely used to assess changes in the distribution, structure and functioning of kelp forests. Such assessments rely on the assumption that changes in surface canopies reflect changes in the structure and functioning of the underlying kelp forests. So far there has been little attempt to validate this assumption, which raises the risk that management decisions place more confidence in what surface canopies reveal about kelp forests than is warranted. To help avoid this pitfall we tested the ability of the floating canopy of giant kelp (Macrocystis pyrifera) to predict the structural complexity of the kelp forest at three sites in southern California using a 23-year monthly timeseries of the density, size and morphology of giant kelp plants. We found that on average the surface canopy of giant kelp accounted for only 12-38% of its total standing biomass, depending on the site and time of year. Canopy biomass was a relatively good predictor of the amount of giant kelp biomass below the sea surface, especially in forests with low densities of large plants, which were most prevalent in late winter to early spring when subsurface biomass was at its minimum. Notable exceptions included numerous instances of high subsurface biomass when there was little to no biomass in the surface canopy. Importantly, we found surface canopy biomass to be a poor predictor of plant density, size and morphology during most of the year making the canopy much less useful for assessing the structural complexity of giant kelp forests and the demographic processes that underlie it. Understanding the degree to which different ecological processes and functions of kelp forests depend on different structural traits is critical when using floating canopies to assess the ability of kelp forests to resist and recover from different forms of disturbance. Our results on the degree to which different structural elements of the kelp forest are related to the surface canopy represents a necessary first step towards advancing this understanding, which is pertinent to their effective conservation and management.