Leaf functional traits (LFTs) are highly plastic and essential to plant resource acquisition and utilization. However, the collinearity of environmental factors and differences in the response of distinct functional types generate challenges in clarifying the contributions of environment variables to LFTs. This study selected 28 species from four forests in northern China with similar mean annual precipitation (MAP) to explore how other dissimilar environmental factors influence LFTs after controlling for the most influential factor. Specifically, we examined the effect of geographical (latitude, longitude, and altitude) and non-precipitation environmental factors (mean annual temperature (MAT), aridity index (AI), and soil pH) on leaf dry matter content (LDMC), and leaf carbon (LCC), nitrogen (LNC), and phosphorus (LPC) concentration, and pH. The collinearity of environmental factors on LFTs can be attributed to the regulation of moisture and temperature by geographical factors. Therefore, the remaining geo-environmental factors still played a significant role in LFT variation after excluding the most influential factor, which assist LFTs to maintain the original pattern of change across the environmental gradient. As latitude, longitude, and AI decreased, MAT increased, LCC and LPC decreased, LNC first increased, then decreased, and LDMC and leaf pH did not change significantly. The trends and intensity of LFTs variation varied between functional types. Significant differences in correlation were observed in LPC-to-LDMC and LPC-to-LNC across all regions for each species. Trait-trait relationships were inconsistent between life forms (herbs and woody plants) for leaf pH-to-nutrient concentration (C, N, P), LCC-to-LPC, and LCC-to-LDMC. Environmental variables were larger contributors to LFTs (especially LNC, LPC, and LDMC) variation than plant functional types (PFTs), while LCC and leaf pH were largely dependent of PFTs. These patterns result from plant adaptations to multiple variables, and a single environmental factor (even the core element that generated the collinearity of environmental factors such as MAP) in northern China cannot dominant geographic patterns among LFTs. This study sheds light on plant adaptation strategies to environmental change, deepening our understanding of ecosystem function and functional coordination.

Leaf trait variations and correlations across four forests with similar mean annual precipitation in northern China