Abstract: Spruce-fir forests at their upper distribution (above 1400 m) in Changbai Mountain Nature Reserve showed growth decline and die-off, but the decline pattern and underlying mechanism are unclear. In this study, based on Landsat data from 1994 to 2019, we calculated the net primary productivity (NPP) by CASA model and analyzed the spatiotemporal variation of NPP in sprucefir forests at their upper distribution in Changbai Mountain Nature Reserve. Results showed that: (1) NPP of spruce-fir forest varied within 111.96-182.19 g C·m^-2·a^-1, with a mean value of 141.85 g C·m^-2·a^-1. From 1994 to 2019, the maximum and minimum NPP in spruce-fir forest occurred in 1996 (182.19 g C·m^-2·a^-1) and 2018 (111.96g C·m^-2·a^-1), respectively. (2) ^The NPP of spruce-fir forest was negatively correlated with elevation and slope, and there was no significant difference in its distribution among different aspects. In the range of 1600-1650 m, the change rate of NPP was close to zero, while NPP at lower and higher altitudes was decreasing. The decreases of NPP of spruce-fir forest were the most obvious when the slope was within the range of 0-10°, and the variation trend of NPP decreased with increasing slope. The increases of NPP showed a trend of decline on shady slope and semi-shady slope, but tended to be stable on sunny slope and semi-sunny slope. (3) The NPP of spruce-fir forest was positively correlated with precipitation, but not related with temperature. The decrease in NPP was partly related to tourism activities in the protected areas. Our results indicated that there was no consistent, large area decline in the high altitude spruce-fir forest, and the variation of NPP was mainly affected by site factors, precipitation, and human activities.

Key words: net primary productivity, spruce-fir forest, spatiotemporal pattern, driving factor, Changbai Mountain Nature Reserve.