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.

Ron Ydenberg,Herbert H T Prins,Ben Leyland,Mark Hipfner

doi:10.1002/ece3.8405

Abstract

Triangle Island on Canada's Pacific coast is home to a large, globally important seabird 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 expansion 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 control water loss, and responds to light, nutrient, carbon dioxide, and water availability. Differing patterns of stomatal density are expected among sample locations depending on which of the hypothesized factors most strongly affects Salmonberry's performance. 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.

Highlights

Ecologists have long been interested in the interactions between seabirds and vegetation on breeding colonies (Duda et al, 2020; Mulder et al, 2011)
The coverage of Salmonberry has greatly expanded in recent decades at the expense of Tufted Hairgrass, raising serious concerns that the suitability of Triangle Island as a breeding colony is shrinking (Hipfner et al, 2010)
The data presented here indicate that the mean stomatal density of Salmonberry leaves collected in coastal British Columbia has remained steady since at least 1895, the date of the earliest specimens in herbarium collections

Summary

| INTRODUCTION

Ecologists have long been interested in the interactions between seabirds and vegetation on breeding colonies (Duda et al, 2020; Mulder et al, 2011). Croll et al (2005) provided experimental and comparative evidence from the Aleutian Islands that vegetation around seabird colonies was altered to grassland from its natural tundra state and reverted to tundra when and where Arctic foxes Vulpes lagopus were introduced (for fur) These predators greatly reduced seabird numbers and the nutrient supply. The climate change hypothesis is that the recent expansion of Salmonberry at the expense of Tufted Hairgrass on Triangle Island is attributable to one or more of the atmospheric changes in the past century These include increased carbon dioxide concentration, higher temperature, altered precipitation, and perhaps others such as wind speed or storm intensity. If biopedturbation is the dominant influence, as hypothesized by Rodway et al (2017), we expect stomatal density and the size of the colonies to be negatively correlated, as plants adjust to the reduced nutrient and water supply caused by soil disturbance and root damage

| METHODS

Findings

| DISCUSSION