Introduction
Mountainous forest ecosystems account for only 1.8% of the Earth’s
surface (Price et al. 2011), but they hold extremely high carbon storage
globally (Cuni-Sanchez et al. 2021), and more than half of them are
returned to soil through litter decomposition (Wardle et al. 2004).
Forest litter decomposition is also considered as an essential ecosystem
process affecting nutrient cycling and species diversity in an ecosystem
(Mayer et al. 2021). However, there exists high uncertainty in
accurately estimating the contribution of litter decomposition to
terrestrial carbon cycle and its feedbacks to ecosystems, which largely
hampers the development and prediction abilities of earth system models
(Jones et al. 2005) and dynamic global vegetation models (Sitch et al.
2003). To enhance the estimates of the role of forest litter
decomposition in global element cycling and multi-taxon biodiversity,
the fundamental understanding of the dominant controls on decomposition
across different mountains and forests is critically needed (Bradford et
al. 2016).
The mass loss of litter components is mainly regulated by temperature,
moisture availability, edaphic factors, and the quality of the plant
litter (Tenney and Waksman 1929, Bradford et al. 2016). Air temperature
and moisture at broad spatial scales are currently used as the
predominant factor controlling decomposition rates due to their direct
and indirect controls on decomposers and litter quality (Wall et al.
2008, Pablo García-Palacios et al. 2013, Steidinger et al. 2019, Ma et
al. 2022). However, this climate-centred evidence relies heavily on air
temperature and moisture at coarse spatiotemporal scales or in open
areas that are not matched with the closely experienced microclimate
within forest litter layers (Steidinger et al. 2019, Joly et al. 2023).
For instance, soil temperature can differ from air temperature up to 10
℃ owing to buffering effects by habitat terrain (Lembrechts et al.
2022). Despite a few recent studies assessing the role of microclimate
on decomposition, its contrasting (little vs. significantly positive)
effects on decomposition rates were reported, with a substantial bias
towards tundra ecosystems and temperate forests (Chen et al. 2018, 2023,
von Oppen et al. 2024). This mismatch and data bias lead to knowledge
gaps in the regulating effects of microclimate variation on litter
decomposition, particularly the underrepresented subtropical and
tropical forests where microclimate is perennially buffered by dominated
evergreen tree species when compared to other terrestrial ecosystems.
Non-climatic biotic and abiotic factors (e.g., soil physiochemistry,
microtopography and vegetation composition) can impact substantially
litter degradation by adjusting directly and indirectly climate and the
activity of soil microbes (Cornwell et al. 2008, Kaspari et al. 2008,
Mori et al. 2020). But the extent and strength of these variables vary
dramatically in different forests and mountains, causing discrepancies
in the general importance of the same factor in decomposition processes.
For instance, variations in soil pH change its association with
decomposition processes via different extracellular enzyme activities
under acidic or alkaline environments (Romaní et al. 2006). Tree
richness ranging from 1 to 4 had no effect on decomposition rates in a
temperate forest (Fujii et al. 2017), while a gradient up to 24 in a
subtropical forest explained 54.3% variation in decomposition
(Seidelmann et al. 2016). Such discrepancy of inconsistent roles of
these variables on decomposition among findings might associate with
insufficient gradients due to sampling bias and the limitation in
studied spatial scales (Keuskamp et al. 2013). Therefore, from a
perspective of experimental macroecology, a distributed and coordinated
study with wide environmental gradients across different mountains could
shed insights into a general understanding of the underlying mechanisms
that regulate forest litter decomposition.
Another advantage of experimental macroecology in forest litter
decomposition is to avoid the incomparability among a large number of
studies in different ecosystems with diverse plant composition, which is
a common issue in ecology (Spake et al. 2022). This top incomparability
can be attributed to distinctive litter quality. The composition and
quality of forest foliar litter are highly location-specific at both
local and regional scales due to diverse species composition. The high
plant diversity determines the diversity of litter, yielding an
overriding influence of litter quality on decomposition processes across
any type of ecosystem (Mori et al. 2020). Therefore, to tease
environmental effects on decomposition apart from litter quality per se,
standard litters, such as a full mix of all plant litters from a study
site or uniform litter type that does not belong to any study sites
(e.g., tea bags and wood stick), are a premium method to control the
intrinsic effects by litter quality (Fanin et al. 2020, Joly et al.
2023). Except for the huge difference in litter quality, the
incomparability also reflects on the absence or presence of the same
environmental factors in different studies (Bradford et al. 2016).
Hence, the use of standardized litter substrate and the inclusion of
multiple environmental factors across multiple mountains are certainly
helpful to reduce the biases induced by incomparability.
In this study, we conducted a macroecological litter decomposition
experiment across 10 mountains spanning a wide range of Chinese
subtropical and tropical forests. These mountains provide high
divergence in abiotic and biotic environmental gradients. We buried
6,864 standardized teabags across 568 elevational sites in 10 mountains
and quantitatively evaluated the influence of soil microclimate, tree
diversity, soil physiochemistry, and topography on decomposition rate
and litter stabilization factor within and among mountains.
Specifically, we addressed three questions: 1) Are there general
patterns of forest litter decomposition rates and stabilization along
elevational gradients in these mountains? We predict that decomposition
rates decrease along elevations and stabilization increase because of
harsh environmental conditions and increased inhibition at high
elevations. 2) Do soil microclimate (temperature, moisture, and their
variations) play the predominant role in controlling decomposition rates
and stabilization across mountains? We predict that temperature and its
variation increase decomposition rates due to their direct and indirect
controls on decomposers (Glassman et al. 2018), and the moisture and its
variations affect stabilization because soil humidity conditions can
modify the absorption and adhering of litter organic carbon on soil
surface or aggregate (Morffi-Mestre et al. 2023, Feyissa et al. 2023).
3) To what degree do non-climatic factors (edaphic factors, tree
diversity, and topography) contribute to explaining the variations in
litter decomposition within and among mountains? We predict negative
effects of high soil pH and slope on stabilization due to increased
cation exchange capacity and water erosion (Zhu et al. 2019), and strong
constraints of low tree diversity on decomposition rates because of
potentially lower microbial diversity (Joly et al. 2017).