Ecological importance and long-term viability of native shrubs on New Zealand gravel beaches

Abstract

Gravel beaches are a naturally rare ecosystem in New Zealand. Often poorly understood and managed, they also support a number of rare and threatened plant species. Extreme environmental conditions and high levels of disturbance combined with close proximity to highly modified landscapes make this a challenging environment for many species to survive in. An observational study showed that facilitation is likely to be occurring in this highly stressed environment. There also appears to be a failure in the recruitment of native woody species. The low levels of native shrub recruitment are likely to have long term impacts on the plant community structure. Two planting experiments were set up to assess which factors may be limiting the successful recruitment of native shrub seedlings. The first experiment tested if established shrubs could facilitate recruitment by planting seedlings under the shelter of shrubs or in bare gravel and monitoring their growth and survival. Fertiliser and water were added as randomized treatments to these planted seedlings to test if there was either water or nutrient limitation. The second experiment tested if an exotic forb could facilitate seedling recruitment by planting seedlings next to the forb, where the forb had been removed, and in bare gravel. Both studies showed that a number of interacting factors impacted on seedling survival including seedling species, the planting treatment and the distance from the sea. Of these factors seedling species appeared to be the dominant factor. The results indicate that while facilitation may influence seedling recruitment, surprisingly it is not the most important factor and there are a number of interacting processes which may be limiting recruitment.