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.