Influence of regional climates on the availability of decaying-log microsite for conifer establishment on Vancouver Island, Canada

We examined the influence of regional climatic conditions on the availability of a decaying-log microsite for the establishment of western hemlock （Tsuga heterophylla） , western redcedar （Thuja plicata） , and Douglas-fir （Pseudotsuga menziesii） . Field research was carried out at three sites located within different climatic zones on southern Vancouver Island, British Columbia, Canada. We investigated whether trees （> 4 m tall） are e stablished on the decaying-logs or not. We also compared the density of saplings （0.3 m ≤ 1 m tall） colonized on the decaying-logs among the sites. Proportion of the established trees growing on the decaying-logs was higher at moist sites than at dry sites for all the examined conifer species. The proportion was the highest for western hemlock at all site and lower as for western redcedar, especially D ouglas-fir. The proportion of Douglas-fir established on the decaying-logs never exceeded that on the ground. The density of saplings on the decaying-logs of western hemlock and western redcedar was the highest at the site located within the intermediate zone in the gradient of climatic dryness. The saplings of Douglas-fir hardly appeared on the decaying-logs at any sites. Our hypothesis that r egional dryness heightens the importance of moisture reserved in decaying-logs for conifer survival was not supported. Intensification of dryness at the sites largely reduced the availability of the decaying-logs for the conifers. The species-specific difference in the dependence of the establishment on the decaying-logs and the site-specific difference in the availability of the decaying-logs m aybe play a key role, which determines the species composition of the coniferous forest in this region.