Owing to the increased need to assess global forest behavior under changing climatic conditions within a long-term context, spatial coherence in tree-ring records is of widespread interest. Spatial networks of tree growth have been widely discussed from regional to global scales, but few studies have reported stable oxygen isotopes in networks of tree rings (δ¹⁸O_tree), thus impeding a comprehensive hydroclimatic interpretation of δ¹⁸O_tree records over large spatial scales, i.e., isoscapes. In this study, a network of four annually resolved δ¹⁸O_tree chronologies (1805–2016) was developed for Southwest China, the transitional zone of the Asian summer monsoon, to test spatial coherence in δ¹⁸O_tree variations and to explore the climatic factors driving synchrony at different spatial scales. The quantitative analyses show that the four chronologies exhibit consistent annual variations, with the first principal component (PC1) accounting for 70% of the total variance, which uniformly respond to local relative humidity (RH) and precipitation δ¹⁸O during the monsoon season. This confirms the spatial coherence in δ¹⁸O_tree records. Furthermore, significant inter-site correlations of local RH and precipitation δ¹⁸O are also detected, revealing that it is the regionally homogeneous year-to-year variations in the hydroclimate that should be responsible for spatial coherence. For a larger spatial perspective, spatial correlation analyses reveal that the most prominent correlations between δ¹⁸O_tree and hydroclimate fields occur in the Indochina Peninsula, in which δ¹⁸O_tree changes are synchronized with those in Southwest China and the synchrony temporally varies with the Indian summer monsoon (ISM) intensity, indicating that coherent δ¹⁸O_tree variations are linked to ISM-related moisture transport. In addition, δ¹⁸O_tree records have shown strong in-phase relationships with the natural variabilities of the ISM and El Niño-Southern Oscillation (ENSO) across interannual and interdecadal timescales over the last 150 years, suggesting that spatial coherence in δ¹⁸O_tree records can be utilized to reflect the long-term history of large-scale atmospheric circulation.