The impact of key ecological function areas on county-level ecological efficiency: An empirical analysis based on multi-period difference-in-differences model

National key ecological function areas are crucial for ensuring national ecological security. A deep understanding of their influence on county-level ecological efficiency is crucial for informing policy assessment, advancing the high-quality development of national key ecological function zones, and reinforcing national ecological security barriers. Employing Chinese county-level panel data from 2005 to 2022, this study constructs a measurement system for ecological efficiency and applies a multi-period difference-in-differences method to examine how the designation of national key ecological function areas affects county-level ecological efficiency. This helps reveal the internal implementation paths of ecological function areas and ensures national ecological security. Key findings of this study are as follows: (1) Following a series of robustness checks, including parallel trend tests, placebo tests, PSM-DID, sample truncation, and controls for policy interference, the results confirmed that the implementation of the national key ecological function areas policy significantly improved ecological efficiency in the designated regions. Under fixed time and individual effects, the policy increased the ecological efficiency of the treatment group by an average of 3% compared to the control group. Industrial structure upgrading and fiscal structure adjustment are two effective paths for enhancing county-level ecological efficiency. Mediation effect analysis shows they partially mediate between national key ecological function areas and ecological efficiency. (2) Heterogeneity analysis by geographical location shows that national key ecological function areas in the eastern and central regions have a significant positive impact on local ecological efficiency, while those in the western region have a significant negative effect. Analysis by ecological function area type indicates that the establishment of areas for biodiversity maintenance, water conservation, and soil and water conservation significantly improve county-level ecological efficiency. In contrast, windbreak and sand-fixation areas exert no significant negative impact. (3) The spatial spillover effects are examined by employing a geographical distance matrix and a spatial Durbin model with both spatial and temporal fixed effects. The analysis reveals that the national key ecological function areas policy generates positive spatial spillover effects, significantly enhancing ecological efficiency in both local and neighboring counties. These effects remain robust after employing economic geography nested matrices and economic distance matrices.