Century-Long Stomatal Density Record of the Nitrophyte, Rubus spectabilis L., From the Pacific Northwest Indicates No Effect of Changing Atmospheric Carbon Dioxide but a Strong Response to Nutrient Subsidy

Triangle Island on Canada's Pacific coast is home to a large, globally important sea-bird breeding colony. The shrub Salmonberry Rubus spectabilis and tussock-forming Tufted Hairgrass Deschampsia cespitosa together form ~70% of vegetation coverage and  contain  the  vast  majority  (~90%) of seabird nesting burrows. Salmonberry has in recent decades greatly expanded its coverage, while that of Tufted Hairgrass has receded.  Seabirds  prefer  not  to  burrow  under  Salmonberry,  making  its  ongoing  ex-pansion a potential conservation issue. We investigated three hypotheses proposed to  explain  Salmonberry's  expansion  (climate  change,  biopedturbation,  and  nutrient  input),  using  comparisons  of  stomatal  density  of  Salmonberry  leaves  sampled  from  Triangle  Island,  other  seabird  colonies,  other  coastal  locations,  and  from  historical  specimens in herbaria. Stomatal density helps regulate photosynthetic gain and con-trol water loss, and responds to light, nutrient, carbon dioxide, and water availability. Differing patterns of stomatal density are expected among sample locations depend-ing on which of the hypothesized factors most strongly affects Salmonberry's perfor-mance. Our data are most consistent with the nutrient input hypothesis. We discuss possible reasons why Salmonberry has expanded so recently, even though Triangle has been a large seabird colony for at least a century and likely much longer.