4.2 Impact of forest structure on forest productivity and AGB stability
The major difference in productivity and AGB stability between natural and planted forests came to the prominence of stand age, richness, and density (Fig. 4). As the delta AGB in forest decreases with stand age and time (Chen and Luo, 2015), we found that productivity markedly came down with stand age in planted forest (Fig. S3f), which may stem from tree growth decline (Gower et al., 1996). However, both productivity and AGB stability in natural forests finally achieved a stable state with stand age (Fig. S3 c and f). The longevity of dominant species complicate forest age, especially in planted forests where tree species with shorter lifespans mature more quickly than long-lived ones. Thus, Natural forests have a variety of dominant tree species, some age-controlled appearance may be masked by the complex forest structure. In addition, stand age may affect productivity and stability via changes of density and species richness as forests development (Ouyang et al., 2019).
Our results partly verified that richness increased productivity in planted forest but not in natural forest. The niche complementarity effect (Aussenac et al., 2019) and selection probability effect (Isbell et al., 2018) potentially explain the positive impact of richness on productivity in planted forest. In addition, richness also caused the increase of mean (µ) and standard deviation (SD) of AGB in planted forest; annual variation in the standard deviation (SD) of AGB might be similar to the change amplitude in mean value (µ). This may stem from the high synchrony in the selected tree species in the planted forest, which weakens the effect of richness on stability (Valencia et al., 2020). Nevertheless, temporal stability in biomass was not affected by plant species diversity in natural forest but may be decided by dominant species and asynchronous population dynamics (Ma et al., 2017), especially in late-successional stages (Guo and Ren, 2014). The mean stand age of natural forest was about 80 years, which is significantly higher than the mean stand age of plantations (17 years) in our study. The natural forest may be more mature.