Seasonal dynamics of gross primary productivity and the underlying mechanism in the Qinling-Daba Mountains.

The seasonal variation characteristics of vegetation gross primary productivity (GPP) and its response mechanisms to climatic and vegetation structural factors represent a key scientific issue for assessing regional carbon sink functions. Based on MODIS GPP, leaf area index (LAI), and meteorological data from 2003 to 2020, we comprehensively employed trend analysis, partial correlation analysis, and multiple linear regression to investigate the spatiotemporal variations of GPP in spring, summer, and autumn at the seasonal scale in the Qinling-Daba Mountains. We further explored the synergistic driving mechanisms of vegetation structure and climatic factors. The results showed that, from 2003 to 2020, GPP in spring, summer, and autumn all exhibited significant increasing trends, with the magnitude ranked as summer > spring > autumn, and the proportions of areas with significant increasing trends were 82.9%, 64.1%, and 20.6%, respectively. LAI increased significantly in all the three seasons, with the proportion of increasing areas exceeding 86.9%. In spring, precipitation showed an overall fluctuating upward trend, vapor pressure deficit (VPD) displayed a downward trend, while temperature remained relatively stable. In summer, both precipitation and VPD exhibited overall downward trends, while maximum temperature showed a significant upward trend, indicating a certain warming-drying trend in the climate. In autumn, precipita-tion increased slightly, while temperature and VPD remained relatively stable. Regarding the temporal effects of climatic factors, there were significant seasonal differences. In spring, precipitation was mainly characterized by lag effects, temperature by synchronous effects, and VPD by cumulative effects. In summer, all climatic factors were mainly characterized by synchronous effects. In autumn, precipitation, maximum temperature, and VPD were domi-nated by synchronous effects, while minimum temperature was mainly controlled by cumulative effects. LAI was the primary positive driving factor for the seasonal-scale increase in GPP. Temperature factors predominantly exerted promoting effects. Precipitation showed inhibiting effects in spring and autumn but promoting effects in summer. VPD exhibited inhibiting effects in both spring and summer, but a weak promoting effect in autumn. At the seasonal scale, we revealed the dominant driving mechanism of improved vegetation canopy structure on GPP enhancement in the Qinling-Daba Mountains, providing scientific basis for evaluating the carbon sink function of regional ecosystems.