Canopy-forming seaweeds provide shade for smaller algae and invertebrates in intertidal communities, ameliorating low tide abiotic stressors such as ultraviolet radiation, desiccation, and high temperatures. Conversely, canopies can negatively affect understory organisms by limiting settlement, causing physical disruption, and trapping sand. We are examining the effect of Egregia menziesii, a canopy-forming foundation species, on intertidal communities, using manipulative experiments in two portions of its geographic range (southern California and northern Washington). We hypothesized that plots with Egregia would have higher abundance and richness of other invertebrates and algae than plots without Egregia. At each site, we have set up 25 low intertidal plots, each 0.25m ² in size, where we randomly assigned the following five treatments (five replicates of each): Natural Egregia (no manipulation), –Egregia (Egregia removed), No-Natural Egregia (no manipulation), +Egregia (transplanted Egregia), or +mimic Egregia (plastic Egregia mimic). We have conducted community surveys of sessile and mobile organism cover/abundance for two seasons (Summer 2019 and Winter 2020). We have also measured sand depth and temperature at both sites within all plots. Our data show temperature differences between plots with Egregia and without Egregia, confirming that Egregia can ameliorate heat stress when present. We predicted Egregia plots would trap more sand during the winter at all sites but have seen no strong evidence of sediment trapping in any treatment. We expect differences in understory species composition between southern California and northern Washington but predict plots with Egregia in both regions will have higher richness and abundance than plots without Egregia.