Burrowing seabirds affect forest regeneration, Rangatira Island, Chatham Islands, New Zealand

The forests of Rangatira Island (218 ha) in the Chatham Islands are a critical breeding site for a number of rare and threatened forest bird species, but are also home to more than three million seabirds, which could significantly affect forest regeneration processes. We surveyed the forests of Rangatira Island by establishing 40 permanent forest plots, estimated seabird density through burrow counts, and analysed soil properties. To determine if seabirds were impacting on forest regeneration, we established exclosures (0.25 m²) in 30 of the forest plots, and examined the role of canopy gaps in forest regeneration. The tallest current forest (c. 15 m), dominated by Plagianthus chathamicus, has mostly regenerated since stock were removed in 1959. Mean burrow density was estimated to be 1.19 per square metre, all soils were highly acidic (pH 3.36–5.18), and burrow density was positively correlated with soil phosphorus. Seedling density of woody species in seabird exclosures measured after 9, 24 and 33 months was significantly higher than in the adjacent non-gap plots, and seedling density was positively associated with reduced canopy cover. Seedling densities were also significantly higher in canopy gaps than in adjacent non-gap plots, but seabird burrow density was significantly lower in gaps. These results suggest that canopy gaps allow forest regeneration despite the negative impacts of seabird burrowing. However, the gap makers, largely senescing Olearia traversii, are slowly disappearing from the forests. The cohort of Plagianthus that has regenerated following farm abandonment may progressively collapse, allowing regeneration to continue in small openings, but there is also the potential for a catastrophic blowdown. This might have serious implications for forest-dwelling birds, invertebrates, and plants.