Abstract: Photosynthetically active radiation (PAR) is an important factor indicating the photosynthetic productive potential. Traditional PAR estimation methods, however, need much more climatic parameters, and are sensitive to weather conditions and low in accuracy. Thus, we provided a new method to estimate PAR duration, which is defined as the duration when PAR exceeds a certain value, e.g., 300 μmol·m^-2·s^-1 in a day of the growing season, using the ratios of observed sunshine duration (the most common index in each meteorological station) to theoretical sunshine duration. We monitored both sunshine duration and PAR duration for 7 years. The paired values of observed and estimated PAR durations based on different thresholds (27, 36, 100, 200, 300, 360, 450 and 900 μmol·m^-2·s^-1) were collected. The effects of monitoring time length, seasons and weather conditions on the accuracy of estimated PAR duration were analyzed. There were no significant differences between estimated PAR durations (mean values) and the observed values (P>0.05) when the monitoring time length was equal or greater than 4 years for PAR>300 μmol·m^-2·s^-1. The accuracy of the estimated PAR duration was higher when the monitoring time length became longer; it was lower in summer than in spring and autumn, and lower in cloudy sky than in sunny and fair weather. We recommended the use of longterm (at least 4 years) observations of sunshine duration to estimate the duration of PAR >300 μmol·m^-2·s^-1 because the overall effects of the weather conditions in whole growing season can be reflected by the long term observations.

Key words: growth, Taxus wallichina var. mairei, form quality, Cunninghamia lanceolata, gap size