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ABSTRACT ObjectiveApex-predator-initiated trophic cascades occur in many nearshore marine habitats that simultaneously serve as critical habitat and food sources for commercially and ecologically important species, including juvenile Pacific salmon Oncorhynchus spp. Yet the potential relationships among apex predators (e.g., sea otters Enhydra lutris), submerged aquatic vegetation, and juvenile salmonids are not well understood. In Southeast Alaska, we investigated (1) juvenile salmonid abundance in eelgrass Zostera marina meadows and understory kelp beds and (2) potential drivers of juvenile Chum Salmon Oncorhynchus keta and Pink Salmon O. gorbuscha abundance in eelgrass meadows. MethodsWe analyzed historic (1998–2007) beach seine surveys to compare juvenile salmonid abundance in nearshore habitats. We then employed the same survey (2017, 2019) in eelgrass to quantify juvenile salmonid abundance alongside the influence of sea otter density (number/km2), distance from anadromous stream (km), seasonality, sediment categorization, and aboveground eelgrass biomass (g/m2). ResultsWe found greater abundance of Chum Salmon in understory kelp compared with eelgrass, whereas Pink Salmon abundance was similar between habitats. In eelgrass, Chum Salmon abundance peaked in June and was positively associated with sea otter density. Pink Salmon abundance varied seasonally, peaking in May. We found increased Pink Salmon abundance with increasing sea otter density and distance from anadromous stream and decreased abundance with increased eelgrass biomass. ConclusionGrowth and survival while juvenile salmonids are out-migrating from streams and relying on nearshore vegetated habitats can determine if they recruit to fisheries as adults. Here, we highlight the use of multiple habitats, eelgrass and understory kelp, indicating that both should be explored as critical nursery habitat. We present evidence of indirect effects of sea otters influencing the abundance of juvenile salmonids, with potential further implications as sea otter populations expand. Apex predators, quality of vegetated habitats, and their structuring roles in the nearshore are critical for informing adaptive coastal fisheries management. 

Abstract Forest canopy complexity (i.e., the three‐dimensional structure of the canopy) is often associated with increased species diversity as well as high primary productivity across natural forests. However, canopy complexity, tree diversity, and productivity are often confounded in natural forests, and the mechanisms of these relationships remain unclear. Here, we used two large tree diversity experiments in North America to assess three hypotheses: (1) increasing tree diversity leads to increased canopy complexity, (2) canopy complexity is positively related to tree productivity, and (3) the relationship between tree diversity and tree productivity is indirect and driven by the positive effects of canopy complexity. We found that increasing tree diversity from monocultures to mixtures of 12 species increases canopy complexity and productivity by up to 71% and 73%, respectively. Moreover, structural equation modeling indicates that the effects of tree diversity on productivity are indirect and mediated primarily by changes in internal canopy complexity. Ultimately, we suggest that increasing canopy complexity can be a major mechanism by which tree diversity enhances productivity in young forests.