Environmental matching reduces satellite-derived growing season contrasts between planted and natural forests in Northern and Western China

Spatial and environmental imbalances can distort comparisons between mapped land cover classes. We compared planted and natural forests in northern and western China using 1-km forest origin records and MODIS phenology and flux products for 2001–2023. Observations before first mapped identification were excluded from planted forest records; matching used location, seasonal temperature, terrain, and forest functional type. The growing season length (LOS) contrast was 8.00 d across the landscape, 1.37 d in the primary eligible sample before matching, and 0.42 d after matching and synchronization of valid years. The approximately 95% attenuation combined eligibility restrictions, reference selection, and temporal support; it was not a causal estimate. The residual arose mainly from later dormancy; type-specific LOS intervals included zero, and stricter distance trimming weakened statistical support. In the gross primary production (GPP)-valid sample, LOS was approximately 0.88 d longer and March–October GPP slightly lower. Under strict quality control, seasonal and annual GPP contrasts both had confidence intervals including zero on common support. LOS–GPP associations varied with spatial support, and residuals showed no monotonic pattern with years since identification. Comparisons should distinguish landscape from conditional differences and report sample eligibility, observation years, and spatial support.