Disturbance structures canopy and understory productivity along an environmental gradient: evidence from a 10-year experiment at Santa Barbara Coastal LTER

Max C. N. Castorani, Department of Environmental Sciences, University of Virginia, Charlottesville, VA, Shannon L. Harrer, Robert J. Miller and Daniel C. Reed, Marine Science Institute, University of California, Santa Barbara, Santa Barbara, CA

Background/Question/Methods
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 at the Santa Barbara Coastal Long Term Ecological Research project, we tested how kelp forest net primary productivity (NPP) responds to repeated canopy loss along a gradient in grazing and substrate suitability.

Results/Conclusions
We discovered that habitat quality can mediate the effects of intensified disturbance on canopy and understory NPP. Experimental pulse and press 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 7-10 years of disturbance, demonstrating the value of long-term experimentation for understanding ecosystem responses to rapidly changing disturbance regimes.