Relationship between distribution patterns of Qinghai spruce populations in the Qilian mountains and environmental...

The Qilian Mountains form a critical ecological barrier in western China, with Qinghai spruce (Picea crassifolia) forests as the dominant forest community. Understanding the environmental factors controlling their distribution is essential for forest conservation and ecological restoration under climate change. We combined remote sensing data, DEM-derived topographic variables, long-term meteorological records, and field soil surveys to investigate the distribution patterns of Qinghai spruce in the Dayekou watershed of the Qilian Mountains. Decision tree and random forest models were applied to assess the relative importance of environmental factors. Elevation was the most important topographic predictor (weight = 0.52), followed by slope aspect (0.35), while slope gradient had a weaker influence (0.13). Qinghai spruce primarily occurred between 2900 and 3300 m, favoring shaded to semi-shaded slopes, particularly north-, northeast-, and northwest-facing aspects, which together accounted for 69.73% of the mapped forest area. Under the elevation-corrected framework, the observed distribution corresponded to an annual mean temperature of -3.03°C to 1.45°C and annual precipitation of 394.62-574.69 mm. Soil depth was positively correlated with population density, with denser populations on deeper soils. These findings indicate that Qinghai spruce distribution in the watershed is shaped by the combined effects of topography, hydrothermal conditions, and soil depth. The quantified environmental ranges provide practical guidance for afforestation and ecological restoration in arid mountain watersheds with similar conditions.