The establishment and survival of native trees on degraded hillsides in Hong Kong

ABSTRACT: Deforestation and land degradation in the tropics and subtropics are proceeding at an unprecedented rate, threatening a massive loss in global biodiversity, comprising a fifth of the global warming potential, and resulting in losses in goods and services to people living in or near the forest. Most of tropical and subtropical China has been deforested and only tiny remnants of the original forest of Hong Kong remain. Tropical and subtropical reforestation is thus an urgent priority. However, both spontaneous forest succession and active reforestation in degraded grasslands are hindered by a succession of filter-barriers, including poor seed dispersal, high seed predation, low seed germination, and poor survival and growth of established seedlings. These successive barriers were investigated at degraded hillside sites in Hong Kong. Seed traps placed in grassland for one year caught 12,600 seeds of 35 woody species, with over 94% of these from traps under perches used, by birds. The seed rain in the open was less than 0.2 seeds per m per year. All woody seeds trapped were from animal-dispersed species but a survey of spontaneous woody invasion of other grassland sites showed that wind-dispersed species were also important, although more patchily distributed. Seeds of 16 species placed in grassland and shrubland sites suffered 60-day predation rates of 6.5 -100 %, with 11 of 12 species suffering 100 % .

loss at one shrubland site. Trapping and laboratory feeding experiments showed that the major seed predators were two species of rat, although the same rat species also disperse some small seeds. Seed predation was independent of seed size but was lower on species with a tough or thick seed coat. Germination rates for cage-protected seeds at grassland sites were generally much lower than under nursery conditions, suggesting that some factors other than seed viability were affecting seed germination. These three studies together show that seed dispersal, seed predation and germination all limit the rate and diversity of woody plant succession at degraded hillsides in Hong Kong. A four-way ANOVA experimental design was used to investigate the effects of species, fertiliser, dry-season irrigation and herbicide control of competing plants on tree seedling survival and growth. None of these treatments resulted in a significantly higher seedling survival over two years, suggesting that low soil nutrients, seasonal drought and grass competition are not limiting factors for survival. Irrigation had little or no effect on seedling growth, but fertiliser and herbicide together led to significantly higher growth. An additional 10 species were planted at 3 sites without any treatments. Survival was high for 9 species and growth rates were high for 4. These two experiments suggest that once seedlings are established, the main barriers to forest succession have been overcome. However, direct planting of tree seedlings is labour intensive and expensive. The promotion of natural seed dispersal, direct seeding, and controlling seed predators may provide cheaper alternatives. Where seedlings are planted, careful attention to post-nursery care and planting precautions can greatly reduce initial mortality and make native species a viable alternative to the commonly-planted exotics.