Disturbance structures canopy and understory productivity along an environmental gradient

Abstract

Disturbances often disproportionately impact different vegetation layers in forests and other vertically stratified ecosystems, shaping community structure and ecosystem function. However, disturbance‐driven changes may be mediated by environmental conditions that affect habitat quality and species interactions. In a decade‐long field experiment, we tested how kelp forest net primary productivity (NPP) responds to repeated canopy loss along a gradient in grazing and substrate suitability. We discovered that habitat quality can mediate the effects of intensified disturbance on canopy and understory NPP. Experimental annual and quarterly disturbances suppressed total macroalgal NPP, but effects were strongest in high‐quality habitats that supported dense kelp canopies that were removed by disturbance. Understory macroalgae partly compensated for canopy NPP losses and this effect magnified with increasing habitat quality. Disturbance‐driven increases in understory NPP were still rising after 5–10 years of disturbance, demonstrating the value of long‐term experimentation for understanding ecosystem responses to changing disturbance regimes.

Standing Crop, Turnover, and Production Dynamics of Macrocystis pyrifera and Understory Species Hedophyllum nigripes and Neoagarum fimbriatum in High Latitude Giant Kelp Forests

“…This short-term pattern may have arisen from decreased competition for resources (e.g., light and nutrients) as FSC of canopy-forming conspecifics declined (Gerard 1976. Although increased resource availability following removal of a M. pyrifera surface canopy can enhance understory kelp production in some cases (Miller et al 2011, Castorani et al 2021, we did not observe such a response at our sites. This finding is consistent with research in southern KELP PRODUCTION AT HIGH LATITUDES California showing that high herbivore densities can suppress the response of understory algae to surface canopy loss (Castorani et al 2021).…”

“…While surface-canopy forming kelp genera (e.g., Macrocystis, Nereocystis, Ecklonia, Laminaria) have received the most attention for their carbon production potential, these kelps frequently co-occur with substantial macroalgal subcanopies. Total production by understory algae has been estimated to rival production by M. pyrifera in kelp forests within the center of its range, and it can increase to compensate for production lost if the surface canopy is removed (Miller et al 2011, Castorani et al 2021. In higher-latitude giant kelp forests, understory algal communities are often dominated in biomass by a few species of large, fast-growing stipitate kelps, such as Hedophyllum nigripes, Neoagarum fimbriatum, Agarum clathratum in the north Pacific (Schiel andFoster 2015, Kroeker et al 2021).…”

Global estimates of the extent and production of macroalgal forests

“…This is a major limitation, since macroalgal forests are believed to form the most extensive and productive habitats of all coastal vegetated ecosystems (Duarte, 2017; Duarte & Cebrián, 1996; Smith, 1981). Macroalgal production varies across populations and communities, as well as with resource (light and nutrients) supply, hydrodynamic conditions (waves and currents) and herbivory (Bustamante et al., 1995; Carpenter et al., 1991; Castorani et al., 2021). The resulting net primary production (NPP) per unit area of many macroalgal stands ranks among the highest of any habitat in the world, rivalling that of tropical rain forests (Pace & Lovett, 2013).…”

Tail‐dependent spatial synchrony arises from nonlinear driver–response relationships

“…Here, we demonstrate how tail‐dependent spatial synchrony can arise from threshold‐like relationships between ecological responses and environmental drivers using a general theoretical model; and we also analyse an empirical instance of our theoretical ideas in populations of giant kelp ( Macrocystis pyrifera ). Giant kelp is a superb organism for studying spatiotemporal dynamics (Bell et al, 2015; Castorani et al, 2015, 2017; Cavanaugh et al, 2013), as well as being the foundation species of productive (Castorani et al, 2021) and diverse coastal ecosystems (Castorani et al, 2018). We show that giant kelp exhibits geographically variable tail dependence in spatial synchrony.…”