The effects of burrow-breeding seabirds on the forests of South East Island (Rangatira), Chatham Islands

Abstract

South East Island, (Rangatira), Chatham Islands (44°20'S, 176°10'W) is a globally significant bird sanctuary. Rare and endangered terrestrial and seabird species breed in the forests and are dependent on the quality of the habitat that the forests provide for their ongoing survival. Until now, no permanent measures had been put in place to monitor the quality of habitat or changes in forest structure and composition since the island became a reserve in 1954. It is one of the most heavily burrowed seabird islands in New Zealand (> 3 million birds). Most seabird island research has focused on the positive effects of high marine nutrient inputs by seabirds, but little quantitative work has been undertaken to assess the negative impact of burrowing seabird disturbance on forest dynamics or the role of canopy gaps in regeneration on seabird islands.

This study had four goals: (1) to describe the current forest structure and composition, (2) to examine the impact of seabirds on seedling regeneration (3) to examine the importance of canopy gaps for forest regeneration (4) to establish methods for permanently monitoring the island's forests.

Five forest communities were described from a cluster analysis of 40, 100 m² plots randomly located across the island: Plagianthus Forest (30%), Mixed Forest (30%), Plagianthus/Melicytus Forest (25%), Plagianthus/Myrsine Forest (7.5%) and Coprosma-Olearia Forest (7.5%). The four main canopy species showed very different size-frequency distributions that will have important implications for future forest regeneration patterns. Tree ages from increment cores indicated that most of the present forest had regenerated after farming ceased in the 1960s creating an even-aged, even-canopied forest.

Exclosures (0.25 m²) to exclude seabirds were established in 30 of the forest plots. After nine months, seedling establishment was significantly greater where seabirds were excluded. Canopy cover was not a significant factor in seedling regeneration within the forest, and while there was a trend toward forest community influencing seedling species and density in the exclosures, this was not statistically significant.

A canopy gap survey located 14 small canopy gaps (0.73% of the 6 ha sample area). Seventy-one percent of the gap makers on South East Island were mature Olearia traversii trees, survivors from old forest cleared for farming but now rare in the forest. The most common form of treefall was uprooting (71 %). Exposure to the southerly wind, proximity to sea and forest edge were site factors that affected the frequency of treefall. Woody seedling regeneration in gaps was significantly higher than in adjoining controls. Burrow density was conversely much lower in gaps compared to controls, indicating that seabirds are negatively affected by gaps.

While regeneration is not taking place within the forest, it is within the canopy gaps. Further research is needed to determine whether canopy gaps in fact lead to long-term regeneration, particularly in small gaps where canopy closure may be fast, or whether forest regeneration is confined to the few very large and infrequent gaps recorded on the island.

Given the even-aged forest, dense burrowing and disturbance to seedlings, strategies to encourage regeneration should be put in place. These are needed to ensure over the long-term sufficient quality forest habitat continues to be available for the rare and endangered species dependent on it should a blow down occur. With this in mind the study concludes with eight recommendations for ongoing monitoring and management of South East Island's forests.