Nutrient loads and ratios both explain the coexistence of dominant tree species in a boreal forest in Xinjiang, Northwest China

The resource ratio hypothesis (RRH) and the nutrient-load hypothesis (NLH) state that species coexistence is mediated by the ratios and loads (availability), respectively, of multiple limiting resources. However, few attempts to quantify these effects in mature forest ecosystems have been attempted to date. Data were collected from 300 quadrats (20 m ? 20 m) spread across a 12-ha (400 m ? 300 m) boreal forest dynamics plot in the Kanas Reserve of Xinjiang, Northwest China. Torus translation tests were used to analyze associations between dominant tree species and specific microhabitats. Linear mixed models were used to assess how soil nutrient availability and nutrient ratios affected the abundance and basal area of specific tree species, as well as how these effects varied across life stages. Two shade-tolerant species, Picea obovata and Pinus sibirica, exhibited opposite relationships with soil nitrogen (N) and phosphorus (P) content. Picea obovata was associated with microhabitats with high N:P ratios, while P. sibirica preferred microhabitats with low N:P ratios. The light-demanding Betula pendula was most abundant at low N and P sites. Another light-demanding species, Larix sibirica, did not show any significant habitat preferences. Furthermore, N:P ratios mainly affected species abundance, while N and P contents largely impacted basal area. Soil nutrient effects on species distributions weakened with tree age (from saplings to adults). Different microhabitats in the boreal forest had distinct soil resources. Both nutrient ratios and loads were correlated with the distribution of dominant boreal tree species, though these effects weakened as trees grew, especially for shade-tolerant species. Overall, niche partitioning due to resource variation may alleviate interspecific competition and contribute to species coexistence, and this study provides convincing proof of the importance of both nutrient ratios and loads on maintaining boreal forest diversity.