3.3 Coupling effects of environmental and forest structure attributes on the productivity and stability of planted and natural forests
To clarify the effects of environmental and forest structure factors on natural forest stability, we fitted a piecewise SEM. Our model elucidated 38% of the variation in AGB stability and confirmed that the most important determinant of stability was the direct positive influence of stand age and the direct negative effect of precipitation decline (Fig. 5a). Latitude played a direct significant influence on AGB stability and an indirect influence via precipitation. Deeper soil depth facilitates increases in tree density in natural forest. The SEM model demonstrated that all predictor together clarified 61% of the variation in the AGB stability of planted forest (Fig. 5b). Stand age significantly and actively worked on the stability of planted forest. Richness made a negative impact on the stability of planted forest. Canopy had a direct positive impact on richness. Canopy and stand age had a direct positive effect on tree density. More detailed information in the SEM was presented in Table S2.
In natural forest, the predictors expounded 38% of the productivity change. Soil depth and precipitation had direct positive effects on productivity. Productivity decreased significantly with stand age. Latitude affected productivity directly as well as indirectly via precipitation (Fig. 5c). All predictors together expounded 75% of the productivity change in planted forest. Altitude, richness, and tree density had positive, direct effects on productivity. Stand age made a direct negative impact on productivity and an indirect impact via tree density (Fig. 5d). Canopy played an indirect positive role in productivity via density and richness.