Global Positioning System reception quality was observed under tree canopies using four different antenna heights in four different forest environments bimonthly in a mountain area in Kyoto, Japan. After post-processing, relationship among GPS reception quality, canopy opening index evaluated from hemispherical photographs, and Signal Interruption Probability (SIP), which indicates segmentation of GPS signal reception, were analyzed considering the horizontal error calculated by using code differential correction. The results show that because of high levels of interference from the canopy, mid-height antennae had the tendency to provide more reliable results as long as the antenna pole could be stabilized. GPS reception quality decreased in higher antennae due to antenna pole bending and wobbling. Although the canopy opening index expressed seasonal changes of canopy structure, especially at the deciduous point, SIP was a more significant factor related to GPS horizontal errors, which are affected mainly by stems or large branches. These results suggest that SIP has a better usability in estimating GPS accuracy than the canopy opening index, and furthermore, SIP may express the forest canopy structure, especially on stems and branches.

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This study aimed to establish a CTL (Cut to Length) system in gentle terrain in Japan. Accordingly, ordinary and line thinning operations were analyzed using a small harvester and forwarder newly introduced from Europe. The productivity of felling and processing operations for ordinary thinning was 15.9 m^3/hour, based on a timber volume of 0.40 m^3/tree and a mean on-site run distance of 269 m, while that for line thinning was 21.1 m^3/hour with 0.57 m^3/tree and 301 m of running. The productivity of the forwarding operation for ordinary thinning was 4.1 m^3/hour with a mean forwarding distance of 406 m (including 135 m running in the site to correct logs), while that for line thinning was 3.8 m^3/hour with 503 m of running (232 m). Accordingly, the labor productivity of this CTL system for ordinary thinning was 3.1 m^3/man-hour, and that for line thinning was 3.1 m^3/man-hour.

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