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\begin{document}

\title{Effects of Rodent-mediated Dispersal Limitation on Ridge Regeneration}



\author[1]{Chao Ruan}%
\author[1]{Ganggang Zhang}%
\author[1]{Xiao Fan}%
\author[1]{Yang Wang}%
\author[1]{Deqing Meng}%
\author[1]{Xuyang Han}%
\author[1]{Fei Yu}%
\affil[1]{Henan Normal University}%


\vspace{-1em}



  
  \date{May 25, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
Rodent-mediated seed dispersal largely affects the regeneration and
colonization of the forest vegetation. However, due to the steep
topography, complex terrains and the heavy anthropogenic logging from
1970s to 1990s, the secondary succession process of the forest is
greatly inhibited where temperate deciduous broadleaf forests were the
zonal vegetation. Previous studies have ignored the seed dispersal
limitation mechanism among different slope positions in montane forests.
We established 90 sample plots in Taihang Mountains among different
slope positions (i.e., ridge, midslope and valley), and investigated the
characteristics of seed removal rate, seed fate and seed dispersal
distance of Quercus wutaishanica forest according to three slope
positions. The results showed that only one from each of the three
rodent species was captured at the ridge, while 52.1\% and 43.8\% of the
small rodents were found in valley and midslope, respectively. Compared
to the ridge whose almost all released seeds were intact in site, the
seed removal rates were significantly higher in midslope and valley, and
the proportions of scatter hoarded in ridge and midslope were
significantly different, while both has no significant difference with
that in valley. The average seed dispersal distance in midslope was 4.78
m, significantly greater than that in valley, while that of the ridge
was only 2.09 m. Therefore, the midslope had the best seed dispersal,
but the seed dispersal of ridge was severely restricted, providing the
first empirical evidence for the Mid-domain Effect model and the
Resource Availability Hypothesis. These results provide a better
understanding of the dispersal limitation mechanism of the oak forest
and the plant-animal interactions system in mountainous areas.%
\end{abstract}%



\sloppy


\textbf{Effects of Rodent-mediated Dispersal Limitation on Ridge
Regeneration}

Chao Ruan\textsuperscript{1}, Ganggang Zhang\textsuperscript{1}, Xiao
Fan\textsuperscript{1}, Yang Wang\textsuperscript{1}, Deqing
Meng\textsuperscript{1}, Xuyang Han\textsuperscript{3}, Fei
Yu\textsuperscript{1, 2 *}

\textsuperscript{1} College of Life Science, Henan Normal University,
Xinxiang 453007, China

\textsuperscript{2} State Key Laboratory of Urban and Regional Ecology,
Research Center for Eco-Environmental Sciences, Chinese Academy of
Sciences, Beijing 100085, China

\textsuperscript{3}College of International Education, Henan Normal
University, Xinxiang 453007, China

*Corresponding Author, e--mail address: yufei@htu.edu.cn (F. Yu)

\textbf{Acknowledgements}

We would like to thank the Yellow River Ecological Engineering
Technology Research Centre of Henan Province for providing technology
support platform and the researchers in the Beijing Forest Ecosystem
Research Station for helping with field investigations.

\textbf{Funding information}

The work was financially supported by the National Natural Science
Foundation of China (No. 32371609, 32101504).

\textbf{Conflict of Interest}

All authors declare that there is no conflict of interest.

\textbf{Author Contributions}

FY conceived the idea and supervised this work. YW and CR collected and
analysed data. CR and XF wrote the first draft of the manuscript. GZ
provided important comments and perfect the manuscript. DM and HX
conducted a preliminary collation and cleaning of field experiment data.
All authors contributed critically to the drafts and gave final approval
for publication.

\textbf{Data availability statement}

The raw data can be found in ``seed\_data.xlsx'' and the code can be
found in ``seed.R''.

\textbf{Abstract} : Rodent-mediated seed dispersal largely affects the
regeneration and colonization of the forest vegetation. However, due to
the steep topography, complex terrains and the heavy anthropogenic
logging from 1970s to 1990s, the secondary succession process of the
forest is greatly inhibited where temperate deciduous broadleaf forests
were the zonal vegetation. Previous studies have ignored the seed
dispersal limitation mechanism among different slope positions in
montane forests. We established 90 sample plots in Taihang Mountains
among different slope positions (i.e., ridge, midslope and valley), and
investigated the characteristics of seed removal rate, seed fate and
seed dispersal distance of \emph{Quercus wutaishanica} forest according
to three slope positions. The results showed that only one from each of
the three rodent species was captured at the ridge, while 52.1\% and
43.8\% of the small rodents were found in valley and midslope,
respectively. Compared to the ridge whose almost all released seeds were
intact in site, the seed removal rates were significantly higher in
midslope and valley, and the proportions of scatter hoarded in ridge and
midslope were significantly different, while both has no significant
difference with that in valley. The average seed dispersal distance in
midslope was 4.78 m, significantly greater than that in valley, while
that of the ridge was only 2.09 m. Therefore, the midslope had the best
seed dispersal, but the seed dispersal of ridge was severely restricted,
providing the first empirical evidence for the Mid-domain Effect model
and the Resource Availability Hypothesis. These results provide a better
understanding of the dispersal limitation mechanism of the oak forest
and the plant-animal interactions system in mountainous areas.

\textbf{Keywords} : slope position; rodent; dispersal limitation; seed
dispersal; seed predation; Taihang Mountains

\textbf{Introduction}

Dispersal limitation is the failure of a seed to reach all regeneration
sites suitable for its germination after leaving the parent. The failure
reasons include seed factors (i.e., seed dispersal mode, dispersal
ability, seed size, and yield) and environmental factors (i.e., wind,
water flow, and topography) (Han and Wang, 2002; Thomson, 2011).
Previous studies have shown that dispersal limitation played an
important role in Changbai Mountain forests for the Jaccard coefficient
of seed rain and neighboring mature trees decreased sharply with
increasing neighborhood radius (Li \emph{et al.} , 2012) and in the
habitat fragmentation of tropical forest, where species diversity is
rich and seeds are mainly dispersed by animals (Cordeiro \emph{et al.} ,
2009). Considering niche differentiation and dispersal limitation
simultaneously may better explain successional mechanisms in tropical
forests (Dalling \emph{et al.} , 2002), and dispersal limitation may be
more important than ecological niche differentiation in determining
species composition in habitats where dispersers are scarce and forest
cover is low (Dent and Estrada-Villegas, 2021). For example, dispersal
limitation due to the disappearance of seed dispersers such as birds and
mammals in tropical forests has been a major factor in the inability of
later successional species to spread into young forests (Abbas \emph{et
al.} , 2021). Therefore, birds and mammals are believed to play an
important role in seed dispersal (Willson, 1993; Glyphis \emph{et al.} ,
1981). Previous studies suggested that mammals and birds were probably
the main dispersers of palm seeds (Zona \& Henderson, 1989), and mainly
focused on the effects of large mammals and birds on seed dispersal
(Gosper \emph{et al.} , 2005). Seeds from excrement or seeds dropped
during consumption by fruit-feeding primates could survive and germinate
(Corlett \& Lucas, 1990), and these seedlings had a higher survival rate
(Pearson \& Theimer, 2003). The composition and dynamics of large mammal
and bird communities have undergone significant alterations, primarily
attributable to pervasive overhunting and extensive habitat loss on a
global scale (Carpenter \emph{et al.} , 2020; Fern\selectlanguage{ngerman}ández-Palacios
\emph{et al.} , 2021). Furthermore, this has resulted in a significant
reduction in the population of these large herbivores, leading to
extinctions in certain cases, as well as a conspicuous reduction in the
average body weight of large mammalian species. Evidenced by a shift
from an average of approximately 14 kilograms at the 95th percentile to
a mere 4 kilograms (Wenny \& Levey, 1998), this trend underscores a
substantial change in mammalian community structure. Consequently, this
transformation has elevated the ecological significance of smaller
fauna, particularly rodent species, in the process of seed dispersal.

While feeding on plant seeds and fruits, rodents carry some seeds and
fruits to other places for storage to cope with food shortages
(Zhang\emph{et al.et al.} , 2015), and to help seeds escape predators,
ultimately leading to successful seed dispersal and regeneration
(Hirsch\emph{et al.} , 2012). Therefore, dispersal hoarders, mainly
rodents, are prominent seed dispersers of plants (Roth \& Vander Wall,
2005). These rodents can spread viable seeds to different favorable
environments (Jansen \& Forget, 2001), some seeds buried can be
retrieved by animals for food, while others would germinate successfully
and achieve seedling settlement to a large extent. This animal storage
behavior, especially decentralized storage, is profitable in reducing
the competition for resources and space between seedlings and mother
trees, to promote seed germination and seedling formation and to realize
plant regeneration (Xiao \& Zhang, 2016; Yang \emph{et al.} , 2020).
Food storage animals usually have an advantage over general foragers in
finding storage food (Stapanian \& Smith, 1978), this may be related to
not only the difficulty of retrieval and the hidden conditions (Tamura
\emph{et al.} , 1999), but also the directional dispersal preference of
certain storage animals. For example, scatter-hoarding rodents would
transport seeds to high-density microsites (such as the tree canopy) and
redistribute them for storage (Dimitri \& Longland, 2017), resulting in
the seed survival rate and the dispersal pattern, and ultimately
affecting the regeneration pattern of vegetation (Yu \emph{et al.} ,
2017). The repeated movement of seeds and the distance to the habitat as
the two main factors determining seed dispersal in rodents (Perea,
2011), and chemosensory information from predators and parasitoids also
influence foraging decisions of storage rodents, and thus seed dispersal
(Sunyer\emph{et al.} , 2013).

Seed dispersal and seedling establishment are the key stages to the
mountain forest succession and regeneration, and their failure would
limit the recolonization of farmland (Standish \emph{et al.} , 2007).
Some studies exploring seed dispersal constraints are currently
receiving increasing attention from naturalists, especially
topography-specific structures (Muscarella \emph{et al.} , 2020;
Wang\emph{et al.} , 2014; Freitas \emph{et al.} , 2010). The topography
in mountainous and hilly areas plays a significant role in regulating
the distribution of the solar radiation and precipitation, this can
effectively reveal the microclimatic state of the local environment and
showcases variations in the soil depth and nutrient content across
different locations (McDonald \emph{et al.} , 1996). As an important
topographical factor, slope position directly affects vegetation through
geomorphic processes and controls the spatial redistribution of resource
factors through morphological changes, thus indirectly affecting the
distribution of vegetation (Kikuchi, 2001; Yang, 2005). Furthermore,
slope position also has an effect on vegetation cover and its ability to
recover after disturbance, with lower slopes having higher vegetation
cover and height, while upper slopes are more susceptible to
disturbances such as forest fire and deforestation (Pereira \emph{et
al.} , 2016). The higher the slope of the mountain, the more spatially
isolated it is and the more susceptible it is to fire and logging
disturbances (Han \emph{et al.} ,2018). Furthermore, crop yields are
generally lower in the upper slope than those in the lower slope due to
differences in soil types (McPhee, 1997), and acorns on the upper slope
are more susceptible to insects than those on the lower slope (Yu,
2003). Scatter hoarders also weigh the pros and cons of cache difficulty
against the risk of theft, with higher risk of predation often implying
higher seed establishment rates (Muñoz \& Bonal, 2011; Steele \emph{et
al.} , 2014). Compared to the hilltop area, there are usually many
exposed rocks and vegetation providing safe shelter for rodents in
midslope (Meliyo \emph{et al.} , 2014), this may lead to better seed
storage and seedling regeneration. Treeline recruitment was seriously
affected by the high rate of seed theft by rodents (Pansing \emph{et
al.} , 2007). Therefore, the dispersal limitation is more pronounced on
the upper slope than on the lower slope (Han \emph{et al.} ,2018).
However, the relationship between food storage rodents and their
dispersal plants often changes depending on environmental conditions,
with mutual benefits and seed predation occurring (Vander Wall, 2010),
and there is no strict consistency.

As a result, differences in vegetation regeneration exist between
different slope positions, yet the reasons for these disparities remain
unknown. Could rodent-mediated seed dispersal serve as a critical factor
in explaining the regeneration variations among slope positions? To
quantitatively measure variations in seed dispersal across slope
positions, the current study explored rodent abundance and tracked seed
removal rate, seed fate, and seed dispersal distance by establishing
experimental plots at different slope positions of the \emph{Q.
wutaishanica} -rodent ecosystem in the Dongling Mountains situated in
the North of Taihang Mountains. Thus, addressing the following
questions: 1) Do slope positions significantly influence rodent-mediated
seed dispersal? 2) What about the seed dispersal efficiency among
different slope positions? 3) Are there definite seed dispersal
limitations among slope positions? Based on the aforementioned
inquiries, we formulate the following preliminary hypotheses: 1)
Concerning seed dispersal, in accordance with the Mid-domain Effect
(MDE) Model (Colwell and Lees, 2000), the dispersal rate is expected to
exhibit a unimodal pattern as slope position increases, characterized by
an initial rise followed by a subsequent decline. 2) In terms of faunal
and floral abundance, according to the Resource Availability Hypothesis
(Coley, 1985), with increasing slope position, an equilibrium point of
resource utilization is anticipated to exist, resulting in a pattern of
initial abundance escalation followed by a subsequent decline.

\textbf{Materials and Methods}

Study region

The Beijing Forest Ecosystem Research Station (40°00'-40°03'N,
115°26'-115°30' E) was located in Dongling Mountains, the northern part
of the Taihang Mountains. The region belongs to warm temperate
semi-humid continental monsoon climate, and its average annual
temperature is 5\textasciitilde{}10\selectlanguage{english} and mean annual precipitation is
500\textasciitilde{}600 mm, mainly precipitate from July to August. The
main soil type is brown soil and its depth is about 50 cm. The zonal
vegetation is warm temperate deciduous broad-leaved and coniferous mixed
forest, especially the \emph{Q. wutaishanica} forest, which is widely
distributed and plays a significant role in the conservation of water
and soil, the maintenance of biodiversity and the regulation of climate
in the Taihang Mountains. To minimize heterogeneity among forest types
dominated by \emph{Q. wutaishanica} , ten transects were established
from the foot to the top of every mountain's western slope (i.e., each
transect occupied a different elevation segment of the slopes), forming
a single elevational gradient of montane forests (1020 m-1770 m) (Xu
\emph{et al.} , 2017). According to Parker's (1982) classification
method widely used in previous studies (Coppoletta \emph{et al.} , 2016;
Guarin \emph{et al.} , 2005; Clinton \emph{et al.} , 1993) and the
topographic characteristics of Dongling Mountains, the transects were
divided into three slope positions: ridge, midslope, and valley, forming
three continuous sampling lines of oak forest at the different slope
positions.

Seed marking

The seed of \emph{Q. wutaishanica} was collected during the mature
period in 2020, and stored in the refrigerator at 4 \selectlanguage{english} for the
germination rate of the oak seeds stored for seven days only slightly
decreased and there was no significant difference compared to fresh
seeds (Chen and Yan, 2019). Therefore, fresh and healthy seeds were
selected within 48 hours by the water flotation and visual inspection
method. The seed marking was completed by referring to the original
method proposed by Zhang and Wang (2001) and made some necessary
changes. A micro-drill was used to drill a small hole with a diameter of
0.3 mm at the bottom of the seed to avoid damaging the core. Small
consecutively numbered plastic labels were made to track the seed fate,
and 10 cm thick wires were used to pass through the holes in the seeds
to connect the plastic labels with the seeds. Tags are generally exposed
to the surface of the ground in most cases, so there is a high chance of
finding them after the seeds have been buried by rodents. According to
Kempter's (2018) research, labels have some influence on seed dispersal,
but would not change with slope position and other factors concerned.

Seed release

The seeds were released in 10 transects set along the elevation gradient
in the \emph{Q. wutaishanica}forest in the Taihang Mountains from August
26 to October 14 in 2020. According to the elevation gradient, each
transect line was classified into three slope positions: ridge (the
upper 1/3 of each transect), midslope (the middle 1/3 of each transect),
and valley (the lower 1/3 of each transect). In total, there were 90
seed stations, 10 transectsx3 slope positionsx3 seed stations, and there
were 10 seeds released at each 1 m x 1 m seed station. For repeated
experiments, the scale of concern was the community and the factor of
interest was the slope position, with 30 seed stations set for each
slope position. Considering that seed fates changed little after 15
days, observations were made only once a week. Therefore, it is observed
two or three times a week in the early stage and only once a week after
15 days, in order to assess the harvest of the seeds and their removal
by small rodents (Fig.1). Seed fates were classified as six types: (1)
intact in situ (IIS), (2) eaten in situ (EIS), (3) eaten after removal
(EAR), (4) intact after removal (IAR), (5) scatter hoarded (SH) or (6)
missing (M). We recorded the seed code numbers, measured the distance
between the tagged seed and its original seed station where the seed was
removed, and marked the specific cache locations. During March and May
of the following year, we investigated and identified the seedlings
established from seeds scattered by rodents according to the tagged
plastic labels.

Rodent composition structure

Trapping plots were set at about 500 m from the seed dispersal
experiment region at each transect and its three slope positions in the
Taihang Mountains to minimize interference. Three live traps, formed a
trapping plot, were placed at each slope position in August 2019 and
2020, respectively. Rodents were captured using 40 cmx 25 cmx 20 cm live
stell wire traps with peanuts as bait. The trapping experiment lasted
one week per transect and slope position, and the trapping period was
one month per year. From August to October 2020, an infrared camera was
set up in each seed dispersal area of each slope position to monitor
rodent activity.

Seedling surveys

We selected 10 transects to survey seedlings in the Taihang Mountains
from October 3 to October 6 in 2019 before seed release experiment.
Three slope positions (ridge, midslope and valley) were selected for
each transect to investigate the abundance of \emph{Q.
wutaishanica}seedlings. The seedling survey experiment was repeated
three times and a total of 90 (3 slope positions x 10 sample strips x 3
replicates) seedling data were obtained.

Data analysis

All statistical analysis were performed in R version 4.1.1 (R Core Team,
2021). The seed removal time of three slope positions was compared using
Cox Regression Analysis, and seed fate and dispersal distance were
analysed using seed tracking experiments. Differences in seed fate
(i.e., IIS, EIS, EAR, IAR, SH and M) among three slope positions and the
influence of the distance group on the removal distance were examined
using a Generalized Linear Model (GLM). Tukey's HSD post hoc test was
applied for multiple comparisons of seed fates, seed removal rate, and
seed dispersal distance among three slope positions. The number of
seedlings was analysed using One-way analysis of variance (ANOVA) with
post hoc tests using the Least Significant Difference (LSD)method.

\textbf{Results}

Rodent species composition and abundance

There were 38, 32 and 3 captured rodents in valley, midslope, and ridge,
respectively, including \emph{Apodemus peninsulae} , \emph{Apodemus
agrarius} , \emph{Sciurotamias davidianus} , \emph{Tamiops swinhoei} ,
and\emph{Sciurus vulgaris} (Table 1). \emph{A. peninsulae} was the most
common, accounting for 64.4\% of the total, followed by \emph{S.
davidianus} and \emph{T.s swinhoei} , and their percentages were 17.8\%
and 12.3\%. Specifically, the first four rodent species mentioned above
were captured in midslope and their relative abundances in the midslope
were 68.6\%, 3.1\%, 9.4\% and 18.8\%, respectively. All five species of
rodents were captured in valley, and \emph{A. peninsulae} and \emph{S.
davidianus} were common, accounting for 63.2\% and 26.3\%, respectively.
Only three rodents were captured in ridge, including \emph{A.
peninsulae} , \emph{A. agrarius} , and \emph{T. swinhoei} .

Seed removal rate

Most of the seeds released in valley and midslope, except the ridge,
were carried by rodents within 11 days (Fig.2). Cox regression analysis
showed that seed removal rates were significantly higher in midslope
(Z=10.69, \emph{P} \textless{}0.001) and valley (Z=10.21,\emph{P}
\textless{}0.001) than that in ridge. Furthermore, the seed removal rate
was significantly higher in midslope (Z=2.24\emph{P} =0.0252) than in
valley. The seed removal rates in midslope and valley almost had no
change after 11 days and then became basically stable after 22 days.

Seed fate

Almost all seeds were kept intact (ISS) in ridge, while half of the
seeds at valley and midslope were eaten by rodents (EAR, EIS) (Fig.3).
The ratios of IIS, EIS, EAR, IAR and M in valley (IIS: \emph{t}
=14.112,\emph{df} =87, \emph{P} \textless{}0.001; EIS: \emph{t}
=6.504,\emph{df} =87, \emph{P} \textless{}0.001; EAR: \emph{t}
=8.619,\emph{df} =87, \emph{P} \textless{}0.001; IAR: \emph{t}
=2.707,\emph{df} =87, \emph{P} =0.0220; M: \emph{t} =8.800, \emph{df}
=87,\emph{P} \textless{}0.001) and midslope (IIS: \emph{t}
=13.363,\emph{df} =87, \emph{P} \textless{}0.0001; EIS: \emph{t}
=5.488,\emph{df} =87, \emph{P} \textless{}0.001; EAR: \emph{t}
=9.328,\emph{df} =87, \emph{P} \textless{}0.0001; IAR: \emph{t}
=4.410,\emph{df} =87, \emph{P} =0.0001; M: \emph{t} =4.470, \emph{df}
=87,\emph{P} =0.0001) differed significantly from those in ridge. The
proportion of SH in ridge (\emph{t} =1.892, \emph{df} =87,\emph{P}
=0.1470) and midslope (\emph{t} = 3.478, \emph{df} = 87,\emph{P} =
0.0023) had not significant difference with that in valley, while there
was significant difference in the proportion of SH between ridge and
midslope (\emph{t} =3.379, \emph{df} =87, \emph{P} =0.0031).

Seed dispersal distance

The seeds dispersed by the rodent in ridge were all within 3 m and were
mainly concentrated in 1-3 m (Fig.4). The maximum dispersal distance of
midslope was almost 15 m, with a major concentration around 5 m. The
average seed dispersal distance was significantly higher in midslope
(\emph{t} =3.449, \emph{df} =207,\emph{P} =0.002) than that in ridge,
indicating that the seed was dispersed farther in midslope. In contrast,
the dispersal distance of the seeds in ridge was finite.

Seedling survey

The mean value of seedling was 1.43 cm as a whole, while those in ridge,
midslope and valley were 1.13 cm, 1.83 cm and 1.33 cm. The variance test
statistic showed that the number of seedling in different slope
positions were significantly different (F=4.567, \emph{P} =
0.013\textless{}0.05). The number of seedling was significantly less in
ridge than those in midslope (\emph{P} =0.004, SE=0.239) and valley
(\emph{P} =0.039, SE=0.239). There were no significant differences for
the number of seedling between midslope and valley (\emph{P} =0.404,
SE=0.239).

\textbf{Discussion}

Seedling regeneration

According to our investigation, the number of seedlings in ridge was
lower than midslope and valley, this is consistent with the results of
Takahashi \emph{et al.} (2010) and O'Brien \& Escudero (2022),
supporting our hypothesis (2) as mentioned above. The reason is that
soil in ridge is shallower and drier than other slope position, creating
droughty and frost-prone conditions, which can be detrimental to
seedling germination and early establishment (Fenner \& Thompson, 2005;
Takahashi \emph{et al.} , 2010). Furthermore, as the ridge slope
increased near the top, airflow velocity increased, increasing snow flux
and snow transport, leading to an increase in snow accumulation
(Toloui-Semnani \& Johnson, 2019) and intensifying transpiration of
pioneer seedlings (Renison \emph{et al.} , 2015). This further
exacerbates the challenges faced by seedlings trying to establish
themselves in ridge. Seedlings at ridge showed wilting and needle
discoloration due to low photosynthetic carbon gain, reduced soil water
potential, low symbiotic infection of seedlings with mycorrhizal fungi,
limited root growth, and ultimately seedling dehydration, which may
explain the higher than ninety percent mortality rate of seedlings in
the ridge region (Smith \emph{et al.} , 2009). As long as the processes
of freezing, tumbling, and burial persist at ridge, it will be very
difficult to establish seedlings in these areas (Butler \emph{et al.} ,
2009). The ecosystem stability of mid-elevation zones at the altitude
between 500 and 2000\selectlanguage{english} m were particularly stable, while high-elevation
zones (above 2000\selectlanguage{english} m) and low-elevation zones (below 500\selectlanguage{english} m) were
comparatively vulnerable (Geng et al., 2019). This is, the communities
of middle region being more stable compared to the other two regions
(Jin et al., 2022). Furthermore, the forest litter layer may act as a
barrier, making it difficult for seeds to contact the soil, thus
affecting tree regeneration in the Taihang Mountains (Yang et al.,
2014). These natural challenges make ridge areas particularly
inhospitable for seedling regeneration and survival. Therefore,
topographic fragmentation, special climate and soil conditions lead to
poor vegetation regeneration in Taihang Mountains, with simple layers
and low coverage, especially at the ridge of the mountain.

Rodent abundance

Among the five recorded rodent species, the \emph{Apodemus
peninsulae}was the most common in the Taihang Mountains, and the
midslope and valley had more rodents than ridge. According to the
'landscape of fear' theory, vegetated habitats can provide shelter and
improve survival chances for animals, so animals prefer vegetated
habitats over open high-risk habitats (Laundre \emph{et al.} , 2001),
small rodents were found in greater numbers in forested sparse grassland
areas (Afonso, 2021), and nests around shrubs on the slopes could avoid
the negative effects of rainfall and uphill runoff on burrows (Jiang
\emph{et al.} , 2017). Furthermore, plants generally exhibit better
growth in midslope compared to ridge and valley (Liao \emph{et al.} ,
2021; Ohsawa \& Ide, 2008), and the vertical structure of vegetation
distribution on the mountain exhibits a central peak pattern (Lomolino,
2001). A similar feature was also found that the distribution of alpha
diversity of plant community in the Taihang Mountains had unimodal
distribution on the elevational gradient, which was in line with the
theory of ''diversity peaked at the intermediate altitude'' (Liang,
2022). Therefore, the midslope and valley may have more rodents than the
ridge. Significantly, the distribution of small mammals exhibits a
mid-elevation peak, i.e., a single-peaked distribution at mid-elevation
(McCain, 2010) according to the MDE (Colwell and Lees, 2000), meaning
that the midslope should have more rodents than ridge and valley.
Unfortunately, this study showed that there were more rodents in the
valley, which is inconsistent with hypothesis (2), but there is little
difference in rodent abundance between the valley and the midslope. This
may be due to the limitation of plant growth by forest water sources as
vegetation is primarily driven by energy supply (Roebroek \emph{et al.}
, 2020). According to the energy limitation hypothesis (Colwell and
Lees, 2000), light, water and other conditions in mid-slope may be
moderate, but not as abundant as in valley, some rodent species may
adapt to this medium energy limitation environment, but other species
may not adapt well, reducing their diversity and quantity. Our results
of rodent abundance were obtained based on camera trap monitoring and
traditional survey methods, the applicability of the high resolution,
the large breadth, and the high sensitivity of infrared camera may allow
better access to animal behavioral data and may improve the results in
future.

Seed removal rate and seed fate

The seed removal rate and the seed collected proportion by rodents was
lowest in ridges, which is consistent with our hypothesis (1). On the
contrary, we found that the midslope had the highest seed removal rate
and proportion collected. Seed removal rate is a critical metric for
understanding the dynamics of seed dispersal and regeneration within an
ecosystem, reflecting a composite of various ecological activities,
including those of seed predators, primary seed dispersers, and
secondary seed dispersers (Hambuckers \emph{et al.} , 2020; Miloti\selectlanguage{polish}ć
\selectlanguage{english}\emph{et al.} , 2018). So, this rate is not merely a quantitative
measure but also provides insights into the qualitative aspects of seed
dispersal mechanisms and their effectiveness under different
environmental conditions (Carlo \& Morales, 2016). Seeds in ridges
exhibit limited dispersal, encountering challenges in germination
primarily attributed to restricted movement towards suitable germination
sites (Forget, 1990). The limitation in seed dispersal is chiefly due to
the diminished or absent role of key seed dispersers, particularly
rodents and birds. According to the niche limitation hypothesis (Romanuk
\& Kolasa, 2002), the scarcity of resources and lower temperatures at
high elevations would increase the intensity of competition so that only
rodent species adapted to these conditions and survived in these
regions. Ecological niches at high altitudes may only be suitable for
some specific rodent species. Previous studies highlight the importance
of specific seed dispersers for large-seeded plants is underscored
(Moreira \emph{et al.} , 2017). For example, the critical function of
various bird sizes in seed dispersal (God\selectlanguage{ngerman}ínez-Alvarez \emph{et al.} ,
2020), the interspecific interactions of rodents for enhancing seed
dispersal effectiveness in Japanese walnut (Okawa \emph{et al.} , 2023),
and the dual reciprocal roles in pollination and seed dispersal of
dispersal-storage rodents (Xiao, 2021). These studies emphasize the
necessary contribution of vertebrates in overcoming the challenges of
seed dispersal, confirming the integrated part of these animals in
maintaining ecosystem balance and plant reproduction (La Mantia \emph{et
al.} , 2019). Thus, the limiting effects of the absence of dispersers on
seed dispersal are evident, which not only hinders the movement of seeds
to suitable germination sites, but may also has long-term effects on the
balance of the ecosystem as a whole.

An interesting observation is that rodent-mediated seed dispersal was
more active in midslope, instead of the valley and ridge. Within our
study, the EIS of \emph{Q. wutaishanica} on the mid-slope was found to
be lower than the average EIS rate (74\%) reported in previous studies
(Li and Zhang, 2001). Midslope's higher seed removal rate and
scatter-hoarding rate imply greater opportunities for seed dispersal and
establishment. This is inconsistent with the idea that seed release at
the top of the mountain was an order of magnitude greater than at the
bottom of the mountain (Katul, 2012). This is mainly because: 1) the
seeds are likely to roll downhill when they break away from the mother
tree, which can lead to superimposed seed shadows from different mother
trees (Ohsawa \emph{et al.} , 2007; Schupp \emph{et al.} , 2019) on
midslope and valley. Furthermore, some valleys may accumulate seeds at
the bottom or in valley areas, as they may be natural meeting points for
wind or water. (Boland, 2017). 2) Rodents prefer to spread seeds
horizontally or downward because it consumes less energy (Li and Zhang,
2003), but the absence of rodents may limit seed dispersal and seedling
regeneration in ridge. Although birds are capable of dispersing and
establishing seeds in high mountainous or other high-altitude areas
(Watanabe, 1994), Quercus species are only moderately preferred as food
by birds (Wright, 2022). Consequently, the impact of birds on the
dispersal of Quercus seeds is quite limited.

Seed dispersal distance

The dispersal distance reflects rodent foraging strategies and
dispersing effectiveness to some extent (Wang and Yan, 2017). We found
that the dispersal distances of the seeds were longer in midslope and
valley than those in ridge where the dispersal distance in ridge was
extremely limit (Fig.4). This phenomenon further validates the
reasonableness of our hypothesis (1). Although seeds would tend to
accumulate on lower slopes due to gravity and topography, and then
valley had a higher seed density (Yamase and Sekioka, 2006),
rodent-mediated seed dispersal is a central driver of horizontal seed
dispersal as it requires less energy (Wang \emph{et al.} , 2018), while
in the vertical direction is just the opposite. Compared to the ridge
and valley, the better vegetation and less disturbance on midslope
attract more rodents to feed, and provide more diverse habitats for
rodents to meet their requirements of spreading over a longer distance
while avoiding predation (Bergstedt and Milberg, 2001). Furthermore,
long-distance dispersal increases reproduction rates by allowing seeds
to escape the high density of conspecifics in the vicinity of the mother
tree, reducing competition and predators, and providing them with the
opportunity to find suitable colonization sites (Novaes \emph{et al.} ,
2020) according to the negative density-dependent hypothesis
(Metz\emph{et al.} , 2010; Jansen \emph{et al.} , 2014). In our study,
the dispersal distance was concentrated within 15 m, which is consistent
with previous studies (Chen \emph{et al.} , 2022). The dispersal
distance was mostly around and within 5 m, and the seed removal rate
exhibited an initial rapid decline followed by a gradual slowdown,
indicating that the rodents accelerated the possession of food, which
might be caused by competition (Jenkins and Peters, 1992).

These results, as mentioned above, were helpful in understanding the
dispersal of seeds mediated by rodents in the \emph{Q.
wutaishanica}population at different slope positions. Further research
still needs to interpret the seed dispersal and vegetation regeneration
pattern at a larger temporal and spatial scale (Nathan and
Muller-Landau, 2000). For example, \emph{Q. wutaishanica} interannual
variation and its subsequent effects on vegetation structure might have
been overlooked (Wang and Smith, 2002).

\textbf{5. Conclusions}

Slope position did significantly affect rodent-mediated seed dispersal,
leading to differences in vegetation regeneration among different
slopes, which supports our hypothesis that there was a significant seed
dispersal limitation in the ridge region, which may act as an important
influence on forest regeneration in ridge. Also interesting was that
seed dispersal and regeneration were best in midslope, providing the
best evidence for the long standing suppositions named Mid-domain Effect
and the Resource Availability Hypothesis. These findings emphasize the
key role of rodents in seed dispersal, and elucidate to some extent the
mechanism of rodent storage feeding on the regeneration pattern
of\emph{Q. wutaishanica} forest communities from a slope perspective,
providing a scientific basis for further explaining the regeneration
dynamics of mountain forests and improving the theoretical system of
plant and animal interactions. Future research is still needed to
explore the specific causes of seed dispersal limitation and how to
improve the restoration and regeneration strategies of forest
ecosystems.

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\textbf{Figure captions}

Fig. 1 Schematic illustration of the seed dispersal experiment.

Fig. 2 Seed removal rates for \emph{Quercus wutaishanica} after release
at different slope positions. The data were represented as mean +-
standard error (SE).

Fig. 3 Seed fates of \emph{Quercus wutaishanica} after primary dispersal
by rodents. The data were represented as mean +- standard error (SE).
IIS: Intact in situ; EIS: eaten in situ; EAR: eaten after removal; IAR:
intact but not buried after removal to another location; SH: scatter
hoarded after removal; M: missing; Primary dispersal: the initial
movement of seeds from the seed station.

Fig. 4 Seed dispersal distances of \emph{Quercus wutaishanica} after
primary dispersal from the seed stations in the three slope positions.
Note: The data were represented as mean +- standard error (SE). The
number above the boxplot represents the p-value.

Fig. 1\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Fig. 2

Fig. 3

Fig. 4

\textbf{Table captions}

Table 1 Abundance and composition of rodents. Note: ``*'' represents the
number of rodents photographed by infrared cameras or visually observed
during the experience of seed release.

Table 1\selectlanguage{english}
\begin{longtable}[]{@{}lllll@{}}
\toprule
Species & Valley & Midslope & Ridge & Total\tabularnewline
\emph{Apodemus peninsulae} & 24(63.2\%) & 22(68.6\%) & 1(33.3\%) &
47(64.4\%)\tabularnewline
\emph{Apodemus agrarius} & 1(2.6\%) & 1(3.1\%) & 1(33.3\%) &
3(4.1\%)\tabularnewline
\emph{Sciurotamias davidianus} & 5*+5(26.3\%) & 3*(9.4\%) & 0(0) &
13(17.8\%)\tabularnewline
\emph{Tamiops swinhoei} & 2(5.3\%) & 5*+1(18.8\%) & 1(33.3\%) &
9(12.3\%)\tabularnewline
\emph{Sciurus vulgaris} & 1*(2.6\%) & 0(0) & 0(0) &
1(1.4\%)\tabularnewline
Total & 38(52.1\%) & 32(43.8\%) & 3(4.1\%) & 73(100\%)\tabularnewline
\bottomrule
\end{longtable}

\selectlanguage{english}
\FloatBarrier
\end{document}

