Effect of growing season degree days on gross primary productivity and its variation characteristics in alpine shrubland at the southern foot of Qilian mountains

In the context of global climate change, a gradual increase in temperature will have an important impact on the carbon cycle of terrestrial ecosystems. Based on CO₂ fluxes measured with the eddy covariance technique, we analyzed the relationship between gross primary productivity (GPP) and growing season degree days (GDD) at different time scales in an alpine shrubland at the southern foot of the Qilian Mountains during the growing seasons from 2003 to 2016. This study has great importance on studying the impact of climate warming on the carbon cycle of alpine ecosystems. The monthly GPP and GDD in the growing season showed a single peak trend over the alpine shrubland ecosystem. The maximum monthly GPP and GDD values all occurred in July or August, whereas the minimum value occurred in May. During the whole growth season scale, GPP and GDD showed high variability, but with a gradually increasing trend (P < 0.05). The mean values of seasonal GPP and GDD for the entire growing season from 2003 to 2016 were 507.11 g·m⁻² and 975.93 ℃. On the daily, monthly, and growing season scales, all GPP values positively correlated with GDD (P < 0.05). However, on comparing GPP and GDD in each month of the growing season, no significant correlation was found between GPP and GDD in May and September (P > 0.05), whereas the correlation was strongest (P < 0.01) in July. Overall, the GPP of the alpine shrubland ecosystem indicated a positive reaction to the thermal condition, suggesting that the photosynthetic capacity of the shrubland ecosystem in the Qinghai-Tibet plateau could be improved in the future under the effect of global warming.