Abstract
Introduction
Riparian disturbance has recently considered to play important role in regeneration of diverse tree species of riparian forests. We investigated spatial distribution of vascular plants in a temperate riparian forest to elucidate the role of debris flows on the secondary succession and species diversity in a riparian forest.
Study site and methods
1) Study site
The study was conducted in Kanumazawa Riparian Research Forest (KRRF), northern Japan. The Kanumazawa forest has contrasting stands (riparian and an adjacent terrace). The riparian stand contains a mixture of various species, while the terrace is overwhelmingly occupied by Fagus crenata and Quercus crispula. Around KRRF, 315 vascular plant species has been recorded, 81% of which are found in the riparian zone.
2) Spatial distribution of vascular plants
Vegetation survey was performed in August 2000. Sample quadrats of 2x2m were regularly placed with 10-m spacing (totally 2.47ha, n=203 for the riparian stand and 112 for the terrace). All vascular plants in the understorey (<3m in height) in each quadrat was recorded.
3) Tree regeneration on debris avalanche and stable forest floor
Sapling (>30cm in height) census was performed in each of the 2x2m quadrat to compare densities of tree saplings between the riparian and terrace areas. The census has been continued every two years from 1994. On a debris avalanche (680m2) formed in 1988, both secondary succession and tree regeneration have been followed from 1992. We systematically placed 27 2x2m quadrats on the avalanche. In each quadrat we recorded species and the degree of vegetation recovery in August and monitored established seedlings in September. From 1996, we also monitored all saplings on the avalanche and in an adjacent canopy gap. Only canopy and subcanopy tree species were analysed.
Results
The riparian stand contained more species in understorey than the terrace. Species diversity was also significantly higher in the riparian zone than on the terrace.
Vegetation structure contrasted between the two stands. On riparian forest floor excluding debris avalanche, perennials, shrubs and ferns occupied a total of 78% of recorded species in each quadarat, and trees less than 10%. On the debris avalanche, the frequency of tree species accounted for 18%. In contrast, the terrace stand was occupied by trees (canopy or subcanopy) and shrubs by nearly 90%, and herbaceous and fern species was rarely found. Thus riparian vegetation consists mainly of various herbaceous and fern species, while those on the terrace of woody species.
Sapling density was generally low over the KFFR, but in sites with soil disturbance saplings were relatively abundant. Saplings of 26 species (80%) of canopy and subcanopy trees have established on the debris avalanche, though in the canopy gap, only Aesculus turbinata was regenerating. Dominant species on the avalanche were Acer mono, Fagus crenata, Ulmus laciniata, Cercidiphyllum japonicum, and Zelkova serrata. Survival rates of these saplings became lower with increasing years after the debris flow. These results suggest that debris flow provides suitable sites for tree regeneration but its effect on the seedling establishment appears to be short-lived.
