1. The assumption that greater biodiversity enhances ecosystem stability, commonly known as "portfolio effect", has attracted considerable research attention. However, the potential portfolio effects on spatial stability (the similarity of ecosystem functioning among forest communities) are still poorly examined especially at different spatial scales and under varying environmental stress conditions. Accordingly, this study investigates the biodiversity-spatial stability relationship among regional communities across different spatial scales and environmental conditions in a temperate forest region.
2. We define spatial stability as the invariability of the productivity of woody plants among plots within a regional community. To test spatial stability, the N closest plots to a given plot were aggregated to form regional communities representing different spatial scales. Structural equation modeling was used to evaluate how biodiversity (including taxonomic (TD) and phylogenetic diversity (PD)) increases spatial stability via species asynchrony and/or population stability across spatial scales. Hierarchical Bayesian modeling was used to evaluate the environmental dependence of the portfolio effects on spatial stability.
3. TD and PD both increased spatial stability by increasing asynchrony, but decreased population stability. The portfolio effect of TD on spatial stability became stronger and reached saturation at the intermediate scale and then decreased as regional communities became larger. The portfolio effects of TD were weaker under the stressful conditions of drought, high precipitation seasonality and high elevation but unchanged across temperature seasonality and human disturbance. PD showed no discernable effect on spatial stability and did not change across spatial scale and environmental condition.
4. Our results suggest that the positive effect of biodiversity on species asynchrony overcomes the negative biodiversity effect on population stability to buffer the spatial change in productivity in diverse communities. Future research on the biodiversity–spatial stability relationship may thus benefit from incorporating different spatial scales and environmental conditions into the analysis. 

The study region covers a large, natural forest area in northeastern China (39°N–54°N latitude, 119°E–134°E longitude) where coniferous and broadleaved mixed forests occur. The region belongs to a temperate continental climate, characterized by long and cold winters and warm summers. The annual average temperature ranged from -5.6°C to 9.8°C across the regional area. The annual average precipitation ranged from 363.8 mm to 1,073.7 mm, with more rainfall concentrated from June to September. The total study area covered approximately 700,000 km², where the 456 circular field plots with relatively even cover, each measuring 0.1 ha, were used in this study. The elevational range of the study region extends from 79 to 1255 m.

The N plots closest to the focal plot were aggregated to form regional clusters of varying extents. These clusters represented local communities of different sizes, which in turn represented different regional communities in different geographical locations. Each spatial extent included a total of 456 regional communities distributed within the temperate forest regions. We used three spatial scales with 5, 10 and 20 plot-communities. The spatial extent of the three regional communities (N=5, 10 and 20) covers an average distance of 48, 76 and 113 km, respectively.