Discussion
Lacking a general understanding of dominant factors driving forest
little decomposition leads to high uncertainties in the estimation of
carbon dynamics at both regional and global scales and its feedback to
climate changes (Davidson and Janssens 2006, Krishna and Mohan 2017).
From the perspective of experimental macroecology, this study took
advantage of highly diversified environmental gradients along elevations
of multiple mountain ecosystems, and analysed the effects and relative
importance of 10 abiotic and biotic factors in decomposition processes
both within and among mountains by using standardized litter materials.
The results demonstrate a high variation of decomposition rates (0.002
– 0.05) and stabilization (0.006 – 0.35) across study sites. Within
mountains, soil microclimate played a dominant role in decomposition
processes mainly in the western high-altitude mountains with colder
environments, a similar magnitude of importance for non-climatic factors
emerged in the western and the northernmost mountain, indicating the
spatial variation of driving factors in decomposition processes. Across
mountains, although tree richness impacted decomposition rates, and
moisture, soil pH, and slope affected stabilization significantly, soil
temperature possessed the strongest association with both decomposition
rates (0.48) and stabilization (-0.46), supporting the importance of
microclimate and environmental range or gradient in controlling
decomposition processes (Bradford et al. 2014, Gallois et al. 2023). Our
study provides a general understanding of microclimate and non-climatic
factors’ effects on decomposition across mountains, shedding light on
the underlying mechanisms for forest carbon dynamics under global
change.
Decomposition rates and stabilization along elevations
Among mountains, the velocity of litter mass loss is generally higher in
the east than in the west due to the premium hydrothermal conditions.
The highest decomposition rate reached 0.24, with no significant
difference between the southeastern mountains. This result is comparable
to a study in Kilimanjaro Mountain which used the same standardized
litter. Their maximum mean decomposition rate is nearly the same as our
observations. The difference is that the stabilization in Kilimanjaro is
generally higher possibly caused by the obvious transition between
warm-dry and warm-wet weather during the decomposition period (Becker
and Kuzyakov 2018). For both studies, high elevations are projected to
promote carbon stocks due to slow decomposition rates and/or higher
capacity in stabilization possibly contributed by large macroaggregates
(Feyissa et al. 2023).
The dominant role of microclimate in
decomposition
Of the 10 mountains, the significant changes in decomposition rates and
stabilization with elevation are nearly in line with mountains that
experienced significant correlations between decomposition
(stabilization) and microclimatic factors. Although non-linear and
linear correlations emerged for the same factor among mountains, there
is a potential convergence of significant effects of microclimate in
mountains located in the west. The western mountains generally possess
higher elevations and cooler environments because of the expanded
monsoon that was caused by the fast uplift of the Tibetan Plateau at
~ 41 million years ago (Wu et al. 2022). Colder
environments are often associated with a higher thermal sensitivity of
microbes or soil animals, resulting in increased decomposition rates
with temperatures (Koven et al. 2017). In contrast, the expected
positive effects from the evident temperature gradient on decomposition
rates in warmer tropical environments can be neutralized by
thermal-adapted microbes (Koven et al. 2017).
Across mountains, we found a significant change in the slope of the
decomposition rate as a function of soil mean temperature across all
mountains (the best breakpoint at 19.79 ℃, N = 998, P< 0.001). Below the breakpoint, decomposition rates increased
with soil temperatures significantly, and the mean soil temperatures of
these western mountains fell into this range. Beyond the breakpoint, the
increase in soil temperature no longer accelerates decomposition rates.
Of the 10 mountains, this ecotone mountain is the only one with an
east-west orientation and acts as a barrier to the southward flow of the
cold air (Wang et al. 2016), which also experienced a mean soil
temperature lower than the breakpoint. Taken together, these
observations stress the key role of microclimate in decomposition
processes and imply that there might be a threshold of temperature
effects on decomposition rates.
Inconsistent effects of non-climatic drivers on
decomposition
Non-climatic environmental drivers (e.g., tree species richness, soil
biochemistry, and topography) emerge as significant factors constraining
litter decay across mountains. For instance, litter decomposition rates
increased with tree species diversity, echoing the facilitation of high
plant species diversity on decomposition rates via complementary effects
caused by multiple functional traits (Handa et al. 2014). Given
temperature and species diversity co-vary at the global scale, i.e.,
tropical forests often possess higher species diversity than boreal
forests, it is essential to consider their interaction to increase the
predicting accuracy of carbon stocks in forest ecosystems (Spohn et al.
2023). In complex terrain, topography emerges as a significant driver
regulating decomposition through the effect on stabilization of
litter-derived organic carbon (Fig. 6). Across the globe, complex
terrain accounts for more than 50% of the land surface (Rotach et al.
2014), and the explanatory power of slope on the variance of soil
CO2 flux exceeds 50% in such complexed ecosystems
(Reyes et al. 2017). Our results thus advocate that the prediction of
ecosystem functions at large spatial scales should take the assumption
of microhabitat (e.g., topography and canopy cover) into account, which
has been currently underrepresented in multiple earth system models
(Bonan et al. 2013, Phillips et al. 2019, Ren et al. 2024).
Forest litter decomposition towards experimental
macroecology
With massively accumulated evidence of different factors controlling
decomposition (Cornwell et al. 2008, Becker and Kuzyakov 2018, Forrester
et al. 2023), the incomparability between findings impeded our general
understanding of drivers in this critical process because of
differentiations in approaches, designs, and methodologies (Spake et al.
2022, Catford et al. 2022). This extent of incomparability can be
partially addressed by statistical methods in meta-analyses, while a
straightforward and efficient solution is conducting a standardized
experimental design at multiple locations at once (“coordinated
distributed experiments”) (Vellend 2016). For instance, a meta-analysis
focusing on surface litterbags at a global scale (a wide vegetation
type) uncovers that 70.2% variation in decomposition rates has been
attributed to litter quality but not climate (Zhang et al. 2008). A
macroecological experiment spanning from Mediterranean forests to boreal
forests in Europe verifies the dominant control of macroclimate in
decomposition (Joly et al. 2023). These contrasting results expand our
view of decomposition but also indicate that there are intrinsic
differences in terms of system characteristics that cause divergent
results, such as surface vs. buried decomposition, and forest vs.
non-forested ecosystem. Our coordinated distributed experiment controls
this difference one step forward to the microenvironment in mountainous
forests where the microclimate has been intensively buffered by dense
canopies and tree diversity is generally high. As tea bags have been
used as standardized litter in many ecosystems (e.g., grassland, alpine
and boreal forests) (Althuizen et al. 2018, Chen et al. 2018; von Oppen
et al. 2024), future macroecological experiments may consider the
transferability of these standardized results to realistic mass loss of
local litters, contributing profoundly to current and future dynamics of
terrestrial carbon and nutrient cycling.
In conclusion, our macroecological experiment across 10 mountains
provides empirical evidence that there is a large spatial variation of
soil microclimate and non-climatic factors controlling decomposition
processes, with a dominant role of microclimate in the western mountains
with colder environments. Across all mountains, drivers differ between
decomposition rates and stabilization, with a positive effect of
temperature-related variables and tree diversity on decomposition rates
and mainly negative effects of microclimate, pH, and slope on
stabilization. This coordinated distributed experiment generates a
general understanding of abiotic and biotic influences in decomposition,
laying underlying mechanisms for carbon dynamics in complex terrain.