Historical data reveal extirpation of foundation species and kelp forest community deborealization in a coastal hotspot.

Climate change is restructuring ecological communities globally, yet the impacts are often underestimated or poorly resolved due to the lack of historical baselines. In temperate oceans, biologically diverse and socioeconomically important kelp forests are the marine ecosystem most threatened by climate change. However, long-term historical baselines for kelp forests are lacking and the processes driving community-level changes remain poorly resolved. Here, using recently discovered aerial imagery and subtidal quadrat data from 1972, we recreated historical baselines for kelps and associated benthic macroalgae in a global hotspot within the northern Salish Sea (British Columbia, Canada). We resurveyed the same sites in 2023 to quantify community shifts, showing that a half-century ago, bull kelp (Nereocystis luetkeana) formed expansive kelp forests in the region (>550 ha), none of which remain today. Satellite time series of bull kelp show that the majority was lost between 1972 and 1984. These data increase baselines of bull kelp canopy extent in this area by more than 10-fold. Changes to the benthic kelp forest assemblage were mainly driven by loss of the dominant kelp, Saccharina latissima (-78%), across all depths. Historically abundant species of red algae also decreased substantially (e.g., Mazzaella splendens [-98.5%] and Plocamium pacificum, [-62.1%]), largely above three meters depth. Applying the community temperature index (CTI) to this half-century comparison, we show that CTI of the kelp forest community (+1.4°C; 95% CI: 0.43-2.37°C) had tracked increases of summer SST (+1.66°C; 95% CI: 1.20-2.13°C) more closely than winter SST (+0.65°C; 95% CI: 0.46-0.84°C), indicating that temperatures during the hottest summer months are likely driving community shifts. The abundance of cold-affinity species decreased more than warm-affinity species abundance had increased, indicating that the subtidal kelp forest community was predominantly restructured by deborealization, rather than tropicalization. Community deborealization may be prevalent in temperate hotspots that are disjunct from areas with similar climatology, creating colonization barriers for warm-affinity species. Our study underscores the importance of historical data for understanding the true magnitude of climate change impacts and suggests that deborealization of temperate kelp forest communities may be more common than has previously been recognized.