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\begin{document}

\title{Vegetation regeneration on natural terrain landslides in Hong Kong:
direct seeding of native species as a restoration tool}



\author[1]{Ying Ki Law}%
\author[2]{Calvin Lee}%
\author[3]{Chun Chiu Pang}%
\author[2]{Billy Hau}%
\author[2]{Jin Wu}%
\affil[1]{The University of Hong Kong}%
\affil[2]{University of Hong Kong}%
\affil[3]{The Hong Kong Bird Watching Society}%


\vspace{-1em}



  
  \date{June 16, 2024}


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\selectlanguage{english}
\begin{abstract}
Landslides are common in tropical and subtropical regions with hilly
terrains and heavy rainstorms, which cause significant economic,
ecological, and social impacts. Natural forest succession is usually
slow on landslide scars due to poor soil structure and the lack of seeds
of woody plant seeds, and often comes with a higher risk of repeated
landslide. Ecological forest restoration has recently been suggested as
an effective alternative to restore the exposed landslide scars,
however, a comprehensive study to identify effective landslide
restoration strategies remains lacking, particularly associated with
seed treatment methods and species selection. Here we evaluated the
effectiveness of different seed coating treatments of both pioneer and
later successional tree species of different seed sizes on seed
germination in a one-year study on three landslides in Hong Kong. Our
results show that bare seeds had germination rates of 17 to 67\% across
all selected species (n=7). Biochar-dominant seed coating formulation
boosted an additional 9.33 (SE= 0.04) in seed germination rate, while
the clay-dominant seed coating formulation did not show significant
effect on germination. Our results also show that medium and
large-seeded non-pioneer species have significantly higher germination
rates than pioneer species. These results collectively suggest that
direct seeding using a biochar seed coat is a manageable and useful
method to enhance tree seed germination---an essential first step to
restore the forests after landslide disturbances in Hong Kong, with
potential to be extended to other humid tropical and subtropical
forests.%
\end{abstract}%



\sloppy


\textbf{Journal:} \emph{Land Degradation and Development}

\textbf{Title:} Vegetation regeneration on natural terrain landslides in
Hong Kong: direct seeding of native species as a restoration tool.

\textbf{Authors:} Ying Ki Law, Calvin K. F. Lee*, Chun Chiu Pang, Billy
Chi Hang Hau, Jin Wu*

\textbf{Affiliation:} School of Biological Sciences, The University of
Hong Kong, Pokfulam Road, Hong Kong, China

\textbf{* Corresponding Authors:}

School of Biological Sciences, University of Hong Kong, Pokfulam, Hong
Kong (email: jinwu@hku.hk and leeckf@hku.hk; phone: +852 2299-0655)

This manuscript is original and has not been published elsewhere, nor is
it currently under consideration for publication elsewhere. We have no
conflicts of interest to disclose and do not involve ethical issue of
human and animals.

\textbf{Figures/Tables Record:}

Number of figures/tables: 8 figures, 3 tables

\textbf{Abstract}

Landslides are common in tropical and subtropical regions with hilly
terrains and heavy rainstorms, which cause significant economic,
ecological, and social impacts. Natural forest succession is usually
slow on landslide scars due to poor soil structure and the lack of seeds
of woody plant seeds, and often comes with a higher risk of repeated
landslide. Ecological forest restoration has recently been suggested as
an effective alternative to restore the exposed landslide scars,
however, a comprehensive study to identify effective landslide
restoration strategies remains lacking, particularly associated with
seed treatment methods and species selection. Here we evaluated the
effectiveness of different seed coating treatments of both pioneer and
later successional tree species of different seed sizes on seed
germination in a one-year study on three landslides in Hong Kong. Our
results show that bare seeds had germination rates of 17 to 67\% across
all selected species (\emph{n} =7). Biochar-dominant seed coating
formulation boosted an additional 9.33 (SE= 0.04) in seed germination
rate, while the clay-dominant seed coating formulation did not show
significant effect on germination. Our results also show that medium and
large-seeded non-pioneer species have significantly higher germination
rates than pioneer species. These results collectively suggest that
direct seeding using a biochar seed coat is a manageable and useful
method to enhance tree seed germination---an essential first step to
restore the forests after landslide disturbances in Hong Kong, with
potential to be extended to other humid tropical and subtropical
forests.

\textbf{Keywords:} forest restoration, direct seeding, seed coating,
biochar, landslide



\textbf{Introduction}

Landslides are ubiquitous in any hilly terrestrial environment,
especially in high rainfall tropical and subtropical regions
(Kirschbaum, Stanley, \& Zhou, 2015). Over the past three decades, many
global studies have documented a nearly 40\% increase in fatal
landslides in terms of both frequency and intensity due to the combined
effects of extreme rainfall events and human activities (Froude \&
Petley, 2018; Haque et al., 2019; Lin \& Wang, 2018; Petley et al.,
2007). Not only do these landslide events have large socio-economic
impacts (Petley, Dunning, \& Rosser, 2005; Sultana, 2020; F. Zhang \&
Huang, 2018), they also have significant ecological consequences.
Landslides can change the morphology of the affected earth's surface,
removing vegetation and habitats, directly affecting wildlife
populations (Schuster \& Highland, 2001). Landslides remove topsoil and
seed banks, hindering vegetation regeneration (Walker \& Wardle, 2014),
with some affected areas maintaining low species richness and diversity
decades after the landslide (Pang, Ma, Hung, \& Hau, 2018; Ren, 2007).
Moreover, these slopes are particularly vulnerable to repeated
disturbances before regeneration and during the pioneer community stage
due to low soil stability, especially in comparison with forests in
later successional stage with high biomass and species richness
(Kobayashi \& Mori, 2017; Osman \& Barakbah, 2011).

The extreme conditions of landslide scars, such as high soil surface
temperature, low water availability, and soil infertility, limit seed
establishment and survival (Aide \& Cavelier, 1994), hindering natural
regeneration (Pickett, 1987). As such, the ability for seeds to
germinate and establish seedlings successfully under these harsh
environmental conditions is an essential first step of forest
restoration after landslides. Apart from seed germination and seedling
establishment, successful restoration should also lead to forests with
high species diversity and complex vegetation structure (Ruiz-Jaen \&
Mitchell Aide, 2005; Wortley, Hero, \& Howes, 2013). Plant species
diversity can benefit soil hydrology (Gould, Quinton, Weigelt, De Deyn,
\& Bardgett, 2016), soil aggregate stability (P\selectlanguage{ngerman}érès et al., 2013; Pohl,
Alig, Körner, \& Rixen, 2009), and enhance the rate of belowground
ecosystem recovery (Klopf, Baer, Bach, \& Six, 2017). However,
naturally, low seed dispersal rate and long distance from neighbouring
seed source means less late successional species that can reach the
landslide sites. Meanwhile, some pioneer species like ferns which first
colonize the bare grounds are highly competitive to woody plant species
by releasing allelopathic chemicals (Walker, 1994; Walker, Landau,
Velazquez, Shiels, \& Sparrow, 2010), preventing the establishment of a
diverse and complex forest. An active and human mediated restoration
programme has thus been increasingly advocated to help boosting forest
recovery on such disturbed sites.

Among human mediated restoration strategies, direct seeding or planting
young seedlings has been most commonly implemented (Dimson \& Gillespie,
2020). Although germination rates when employing direct seeding are
often lower in degraded sites than under ideal nursery conditions,
direct seeding is still more cost-effective than seedling plantation
(Pérez, González, Ceballos, Oneto, \& Aronson, 2019; Palma, 2015). It is
also a useful method for large scale restoration in disturbed areas due
to massive deforestation and climate change (Grossnickle \& Iveti\selectlanguage{polish}ć,
\selectlanguage{english}2017). Moreover, direct seeding is effective in promoting a more
complex-layered forest, which is more similar to resilient natural
regeneration sites than homogeneous seedling plantation sites (Freitas
et al., 2019). Additionally, direct seeding has been proved to be
successful in restoring forests across different habitats such as
abandoned pasture, agricultural fields, Brazilian savanna, gullies, and
young forests all over the tropics (Cole, Holl, Keene, \& Zahawi, 2011;
Doust, 2006; Pellizzaro, 2017; Rodrigues et al., 2019). However, the
effectiveness of direct seeding on landslides remains unclear.

When employing direct seeding, the plants' early establishment has been
shown to depend on both seed coating and species selection. Seed coating
is the process of applying additional materials to the surface of a seed
coat and is widely used in the agricultural sector to improve seed
handling, modification of seed shape and size, and germination
improvement (Kaufman, 1991; Pedrini, Merritt, Stevens, \& Dixon, 2017).
During the past two decades, seed coating has gained increasing
popularity in natural habitat restorations to enhance seed germination
((Lee \& Park, 2006; Madsen, Kostka, Inouye, \& Zvirzdin, 2012),
seedling growth, and to reduce predation risk (D. E. Pearson et al.,
2019; Taylor et al., 2020). Various materials could be used for seed
coating depending on the objective. Fertilizer and clay are commonly
used while insect and rodent repellent or even fungicide and herbicide
could also be added into the formulation to control pathogen infection
(Gornish, Arnold, \& Fehmi, 2019). Coated seeds have higher germination
rates than bare seeds in the greenhouse environment (Brown et al., 2018;
Richardson et al., 2019), benefited by a nutritive, water holding medium
for root development. The coatings enhance plant metabolism and reduce
nutrient loss during germination to help the seeds to withstand drought
stress and salinity stress (Afzal, Javed, Amirkhani, \& Taylor, 2020;
Overdyck, Clarkson, Laughlin, \& Gemmill, 2013).

From the ecological perspective, choosing suitable species for
restoration is equally important. Plants of different successional
groups and/or seed sizes could contribute to different seed germination
success, which could also help to form a diverse forest community and
increase the post-disturbance forest restoration success. Previous
research demonstrated that large or intermediated size seed species
often lead to a higher survival rate (St-Denis, Messier, \& Kneeshaw,
2013; Tunjai \& Elliott, 2012) while non-pioneer species were also
observed to have higher rate of establishment in the habitats of
tropical forests (D. C. de Souza \& Engel, 2018; Mart\selectlanguage{ngerman}ínez-Garza, Pena,
Ricker, Campos, \& Howe, 2005).

To investigate the effectiveness of direct seeding on the seed
germination success in the landslide trails, here we used Hong Kong as
our study area, where rain-triggered landslides are very common and
frequent phenomena, making it a good representative for this
investigation. There are on average 320 landslides per year on natural
terrains in Hong Kong since 1945 (CEDD, 2019). Despite the lack of
seismic activity, Hong Kong experiences frequent landslides due to high
precipitation, steep slope, and vulnerable geology. Located in the
sub-tropical monsoon climate with a mean annual temperature of 23.5 °C
(Hong Kong Observatory, 2021), precipitation, at a mean annual rate of
2431 mm, is the main driver of landslide in Hong Kong (Gao, Zhang, \&
Cheung, 2017). Occasional intense rainstorms and typhoons further making
slopes unstable and susceptible to failure (Ngecu \& Ichang'i, 1999).
Additionally, Hong Kong has a hilly terrain where over 60\% of the land
areas are on slopes with gradients greater than 20° (Au, 1998; Lan,
Zhou, Lee, Wang, \& Wu, 2003; Yao, Tham, \& Dai, 2008). Lastly, 66\% of
Hong Kong's landslides occurred in soil originated from decomposed
volcanic rocks. With this geology under hot and wet climate, Hong Kong's
hillside is vulnerable to weathering (Chau et al. (2004). For these
reasons, Hong Kong is a good trial site to investigate a suitable
strategy on post-landslide forest restoration.

In this study, we investigated the effectiveness of different types of
seed coats and species in promoting germination rate of directly seeded
plants on landslide scars in tropical Hong Kong. With the data collected
from this experiment, we address the following two questions: 1) Does
seed coating affect the germination rate of sowed seeds? 2) Do seeds of
different sizes and functional groups have different germination
success? We hope to offer important insights and recommendations for the
effective human mediated strategies to facilitate post-landslide forest
restoration in Hong Kong and other tropical hilly regions.

\textbf{2. Material and Methods}

We conducted two experiments in Hong Kong over 2020 to 2021, each
subjected to different treatment methods (seed coating and surface
soil). The procedure is described in detail in the sections below, where
we define the study site in Section 2.1, identify the species collection
method in Section 2.2, describe the experimental designs of seed coating
treatment and surface soil treatment, respectively in Section 2.3, and
describe the data analysis methods to evaluate the effectiveness of
treatment measures and species characteristics on seed germination
success in Section 2.4.

\emph{2.1 Study site}

The experiment was conducted on three recent natural terrain landslides
(approximate 200m altitude) in the New Territories of Hong Kong, China
(Figure. 1; 22° 27\selectlanguage{english}' 35" \selectlanguage{english}N, 114\selectlanguage{ngerman}° 06\selectlanguage{english}' 13" \selectlanguage{english}E). The three selected landslide
sites were formed in late August 2018 under a rainfall exceeding 200 mm
within a day in the New Territories (Hong Kong Observatory, 2021). Prior
to the landslides, grassland and shrubland were the most abundant
vegetation types in these sites due to repeated human-induced hill fires
in the surrounding areas. After the disturbances, the landslide scars
remained barren for two years, and were not even occupied by pioneer
grasses or ferns. Soil in these three sites was composed of highly
decomposed crystal tuff with decomposition grade VI to VII (Geotechnical
Engineering Office, 1988). The soil was shallow, with good drainage and
aeration, but low in fertility. The terrain is unstable and easy to
collapse.

\emph{2.2 Species collection and storage}

We focused on 10 native species spanning different successional stages
(pioneer and late successional) and a large variety of seed size with a
diameter ranging from 3 to 13 mm (Table 1). These species were selected
based on the fruiting cycles of local trees and the availability of
sufficient quantities of seeds at the time of the study. The major
fruiting season of Hong Kong's vegetation is in the winter dry season
(September to March), during which we collected seeds of each species
from at least three mother trees. The collected seeds were cleaned and
capsule separated within a week after collection. The successional
characteristics of these species were assigned according to information
in the literature (Zhuang \& Corlett, 1997, Kammesheidt, 2000; Han,
2001).

To minimize the negative effect of long storage time, a suitable storage
method is needed to keep the seeds viable. We stored the recalcitrant
seed species such as \emph{Machilus breviflora, Sterculia lanceolata}and
Fagaceae family in wet sand beds in refrigerators kept at 5 to 10 \selectlanguage{ngerman}°C,
following the storage suggestion from the Chinese Forestry Ministry
(2001). Orthodox seeds were stored in unzipped plastic bags at room
temperature. Summer fruiting species storage did not exceed three weeks
to ensure their quality. The seeds were not subjected to treatments to
break dormancy. To ensure seed viability and quality, we screened out
any floating, unhealthy seeds prior to storage and planting. The
planting experiment was conducted in spring when the climate was wet and
warm. Additionally, we planted plain seeds in an outdoor nursery as
controls of seed viability at the same time with the field experiment.

2.3 \emph{Experimental design}

\emph{Seed coating experiment}

To examine the effect of seed coats on native seed germination success,
we used two formulations of seed coats and one control (i.e., raw seed
without any treatment). The two formulations refer to seeds encrusted
with mixtures of clay powder and biochar powder at ratios of 7:3
(clay-dominant) and 4:6 (biochar-dominant) in weight respectively. The
biochar composed of mixed woods, mainly~\emph{Acacia confusa}
,\emph{Dimocarpus longan} and \emph{Bauhinia} ~spp.~The wood debris is
heated up to approximately 500 °C in a low oxygen environment for 30
minutes to create the biochar. Large seeds (over 10 mm in diameter) were
treated using a sweet dumpling rotating machine (Model YX-65, China),
while small and medium-sized seeds (3-5.9 mm; 6-9.9 mm in diameter) were
treated using a DIY modified centrifuge (EBA 8, Hettich, Germany).

Seeds were planted on three landslide sites between April and June 2020
(Figure. 1a). Within each site, there were nine plots (each 1.6 m by 1.8
m), and the plots were at least 2 m from each other (Figure 1b). Within
each plot, we further divided it into six equal grids as six species
were tested (Table 1). We planted 24 seeds per grid for each species
(Figure 2a). There were three replicate plots for each of the two
treatments and the control, hence nine plots per site.

\emph{Surface soil treatment experiment}

In order to test whether surface soil treatment could improve the
germination rate of native seeds, a pair of biochar treatment and
control plots (3.6 m x 1.8 m each) were set at each of the three
landslide sites. Biochar powder (0.62kg/m\textsuperscript{2}) were mixed
with surface soil (around 3 cm at the top) in each treatment plot. Each
plot was sub-divided into four equal grids as four species were tested
(Table 1). For each species, 60 plain seeds were planted in one grid of
each plot (Figure 1 and 2b).

\emph{Monitoring}

Biweekly field monitoring of seed germination was conducted over a year
from April 2020 to May 2021. During each monitoring session, we recorded
and labelled each seed germinated. Overall, over 90\% of the geminated
seeds were recorded within 1-3 months after seed sowing. Very few seed
germination (\textless{} 5\%) was observed beyond 6 months after seed
sowing.

2.4 \emph{Statistical analysis}

We used generalized linear mixed models (GLMM) to analyse the results of
the seed coating and surface soil treatment experiments. These were
performed using R version 3.6.3 with the `nlme' package (Pinheiro et
al., 2021). We used germination rate as the response variable in both
experiments, while the three seed coating treatments (clay-dominant,
biochar-dominant and control) and two surface soil treatment (biochar
and control) were considered as fixed covariates respectively. In order
to incorporate the dependency among observations in the same site and
nested structure of our experiment, we used site and species as random
effects. We did not include any interaction term due to small sample
size. Moreover, due to the poor seed viability, some of the species such
as \emph{Eurya nitida, Tetradium glabrifolium} and \emph{Zanthoxylum
avicennae} failed to germinate in both nursery and field trials and were
excluded from further analyses.

We separated the analysis into two parts: 1) across all species and 2)
each species independently. For all species, we ran four models by
including different covariates and selected the model with the lowest
Akaike information criterion (AIC) value (Table 2). For the species
models, we modelled the relationship between germination rate and
treatment with site as a random factor. Model assumptions of normality
and homogeneity of variances were verified by plotting model residuals.
Post hoc analyses for pair-wise comparisons of means were undertaken
using least square means using the lsmeans package (Russell, 2016).

Additionally, to test the effect of seed size and successional stage on
germination rate of native seeds on landslide scars, data from the
controls of the two experiments were pooled and analysed. We used linear
mixed models to compare seed germination rate of different successional
groups and seed sizes. All analyses were conducted using R version 3.6.3
(R Core Team, 2020) for GLMM.

\textbf{Results}

\emph{Seed coating experiment}

We observed an increase of 9.33\% in seed germination rate when seeds
were treated with the biochar-dominant seed coat in comparison with the
control at all three sites (SE=0.04, p\textless{} 0.05; Figure. 3). The
clay-dominant seed coat did not significantly improve germination rate
(p=0.38; Figure 3), and there was no significant difference in seed
germination rates between the seed coat treatments (p=0.15; Figure 3).
Exploring the seed germination and seed treatment relationships further,
we found that different species had different effect sizes for the
different seed treatments. 14.35\% of \emph{Celtis sinensis} seeds
(SE=0.07, p\textless{} 0.05) and 10.65\% of \emph{Sterculia
lanceolata}seeds (SE= 0.05, p\textless{} 0.05) germinated more when
treated with a biochar-dominant seed coat than the control seeds (Figure
4). However,\emph{Reevesia thyrsoidea} and \emph{Ormosia semicastrata}
did not have significantly different germination rates between the three
treatments. We noted that heavy rain washed away a substantial amount of
seeds, leading to a small remaining sample size of these two species.

\emph{Surface soil treatment experiment}

We found no significant germination rate difference in the surface soil
treatment compared to the control including \emph{Cyclobalanopsis
myrsinifolia} , \emph{Machilus breviflora,} and \emph{Syzgium
levinei}(Figure 5). The mean germination rate for all species across
sites was 56.10\% (SE = 0.14). The highest germination record was
\emph{Machilus breviflora} in site two (75\%, SE= 0.14) whereas the
lowest germination rate was also \emph{Machilus breviflora} in site
three (40\%, SE=0.02). Germination rates of each independent species
were also observed with no significant difference (Figure 6).

\emph{Influence of successional group and seed size on seedling
establishment}

Combining the control data from the two experiments together, we found
that successional group and seed size greatly affected germination rates
on landslide sites. Late successional group species had a 30\% higher
germination rate than pioneer species by direct seeding on landslide
scars (p\textless{}0.001; Figure 7). Meanwhile, medium and large seeded
species have 31.6\% - 36.3\% higher germination rates than small size
species (p\textless{}0.01; Figure 8). There was no significant
difference of seed germination percentage between medium and large size
seeded species (Figure 8), which might largely be because only one
large-size seeded species was explored here.

\textbf{Discussion}

Landslides are natural disturbances that affect forest ecosystems in
high rainfall tropical and subtropical regions and can incur substantial
socioeconomic loss and ecological damage. We investigated potential
methods of enhancing forest regeneration after landslide events via
human mediated restoration efforts, focusing on ways to increase native
seed germination rate. Our results show that, first, seed coating
technique improves seed germination rate of native tree species. Second,
selecting suitable successional groups and seed size can lead to higher
germination rates and restoration success.

We found that biochar dominant seed coats had a significantly positive
effect on seed germination success across our study sites, and there is
evidence that seed coating can further encourage growth for young
seedlings (Jones, Schwinning, \& Esque, 2014; Lee \& Park, 2006; Scott,
1975). Biochar coating also had differential effectiveness in enhancing
seed germination in landslide scars across the species we investigated,
agreeing with results from other studies showing that seed coating
improves seed emergence under different soil conditions (Liu, 2010;
Madsen, Davies, Boyd, Kerby, \& Svejcar, 2016). Landslide scars are
characterised by soil compaction and lack of organic matter, reducing
soil nutrients availability and limiting vegetation growth and
restoration potential (B\selectlanguage{polish}ł\selectlanguage{english}o\selectlanguage{polish}ń\selectlanguage{english}ska, Lasota, Zwydak, Klamerus-Iwan, \& Go\selectlanguage{polish}łą\selectlanguage{english}b,
2016; Fetcher et al., 1996). Treating seeds with additional coating
helps seeds to overcome this initial barrier to germination due to poor
site properties by serving as a carrier of nutrients and a physical
shield to avoid physical damage to the seeds. Moreover, the seed coat
increases the likelihood of seeds reaching germination favoured
microsites, reducing the chance of the seeds being blown or washed away
and minimises seed loss from predation (Overdyck et al., 2013; Taylor et
al., 2020; A. G. Taylor, Eckenrode, \& Straub, 2001). Moreover, adding
active ingredients in coating formulation boosts the seed germination
rate. In modern seed coating technology, seed coats act as a carrier of
different ingredients including nutrients, plant protectant or even
plant beneficial microbes for several purposes (Afzal et al., 2020;
Rocha, 2019). For instance, in our experiment, biochar dominant seed
coating has a 30\% higher than clay dominant seed coat and raw seeds
which agree with previous findings that biochar coating serves as soil
conditioner and enhances water holding ability of soil (Brown et al.,
2018; Madsen, Davies, Mummey, \& Svejcar, 2014).

In the soil surface treatment experiment, we found no evidence of sowing
biochar powder on top of soil surface affecting native seed germination
on the landslide scars. Previous research has found that biochar powder
applied to topsoil can enhance tree and crop growth through an increase
in soil pH, fertility, and water holding ability, resulting in plants
with higher biomass (Mohamed, 2015; Thomas, 2015). It seems that biochar
soil surface treatment is only able to enhance growth after seed
germination but does not offer direct advantages in enhancing seed
germination rate.

In our study, the overall germination rates of raw seeds were 17\% -
67\% (Table 3) while having biochar dominant seed coat gave an
additional 9.33\% improvement of germination rates. Our finding was much
higher than a meta-analysis of direct seeding indicating that the seed
germination rate was 23.9\% on average (Ceccon, 2016 ). It is also much
higher than many restoration programmes, in which seedling establishment
rates are often observed at less than 10\% (Merritt \& Dixon, 2011). The
observed much higher seed germination rate in our study can likely be
because of the use of native species seed or the test-site in a
favourable condition of humid tropical environment, and a more in-depth
understanding of the underlying difference and mechanism remain needed
in the future attempts.

Our results showed that species selection was an important factor
affecting seed germination rates. First, different successional groups
of plant species had different seed germination rates, as they require
different environmental conditions from the colonizing pioneer species
to establish on landslides (Walker, Vel\selectlanguage{ngerman}ázquez, \& Shiels, 2009). This
also agreed with previous studies showing that species in
late-successional group had an advantage in germination rates in other
landscapes (D. C. de Souza \& Engel, 2018; R. P. de Souza \& Válio,
2001). Late successional species have a germination strategy that
conserves nutrients, allowing them to be more shade tolerant when
compared to pioneer species (Kleijn, 2003). This gives them higher
competitive ability but low colonizing ability (Kleijn, 2003; H. Zhang,
Qi, \& Liu, 2018), which is a potential reason for our observed result.
Moreover, pioneer species require special environmental conditions for
dormancy break to germinate, so food reserves in seeds may be depleted
before these conditions are created, resulting in lower overall
germination rates (Baskin \& Baskin, 1998). Our results showed that
non-pioneer or late-successional species could successfully establish on
degraded hillsides, and that late successional species establishment is
limited by seed dispersal rather than tolerance to harsh site
conditions. Thus, late successional species may be used in direct
seeding restoration in landslide scars.

Second, we found a positive relationship between seed size and
germination rate, implying that a larger seed size improves germination
rate. This also agreed with previous studies that compared the sizes of
different tree species (St-Denis et al., 2013; Tunjai \& Elliott, 2012).
In the natural environment, large and small seeded species have
different establishment strategies. Large seeded species appears to be
more tolerant to stresses such as drought and shade than small seeded
species as they have a larger initial energy reserve (Leishman \&
Westoby, 1994). Their large cotyledons support embryo development when
seedlings compete for limited resources, which may be an advantage in
habitats where gaps in the canopy are regularly created. Moreover, large
seeded species are able to germinate equally in light and darkness and
is more resistant to temperature fluctuation (T. R. H. Pearson, Burslem,
Mullins, \& Dalling, 2002), while small seeded species produce seedlings
that are more susceptible to extreme environmental conditions and do not
resist long periods of unfavourable growing conditions (Camargo, 2002).
Although small seeded species have an advantage in quantity during seed
production (Moles \& Westoby, 2004), in human mediate restoration
efforts, large seeds mask the effect of production by having rich energy
source in cotyledons to enhance the chances of restoration success.

In addition to seed coat treatment and species selection, there are
other methods of enhancing restoration success which were not assessed
in this study. Pre-sowing seed treatments and site treatments are most
commonly used in practice. Pre-sowing seed treatments such as
scarification (Kaye \& Kuykendall, 2001) and sowing agglomerated seeds
(Madsen, Davies, Williams, \& Svejcar, 2012), were shown to give higher
germination rates than sowing singular raw seeds. Field treatments, such
as sowing seeds at different microhabitats and biotopes (Chantal,
Kuuluvainen, Lindberg, \& Vanha-Majamaa, 2005; Doust, 2006), and
creating grass canopy on soil surface before direct seeding (Guarino \&
Scariot, 2014), were also shown to increase seed germination and
establishment rates. In addition to seed and site treatments, the method
of seed sowing can also affect seed germination rates. In this study, we
focused on direct seeding with seed burial, as seed burial can minimise
animal predation on the sowed seeds (Alem, 2020; Garcia\selectlanguage{english}-Orth \&
Martinez\selectlanguage{english}-Ramos, 2008). The difference between seed coating and bare
seeds on germination rate is potentially greater when broadcasting seed
than seed burial as the seed balls themselves can act as physical
protection to the seeds (Leverkus, Rojo, \& Castro, 2015). Broadcasting
native seeds on landslide scars is still under investigation and a
potential avenue for further research. Furthermore, in our current
results, large seeded species only occurred in the late successional
stage. It would be interesting to investigate how small seeded late
successional species and large seeded species in other successional
stages behave on landslide scars.

Landslides are natural disturbances that can cause enormous
social-economic loss and severe ecological consequences. Forest
restoration on landslide scars can contribute substantially to soil
surface protection, reducing future disturbance risk and help to protect
human lives and properties whilst restoring destroyed habitats.
Effective regeneration of these degraded areas requires successful
germination of seeds, which can often be a slow natural process. Human
mediated restoration can help to speed up this process. We were able to
show the effectiveness of biochar seed coating and planting non-pioneer
species with medium to large seeds in improving seed germination rates
on landslide scars. We used Hong Kong in this study, but the findings
could be applied to post disturbance vegetative restoration and recovery
on landslides of other hilly terrain in the seasonal tropics.

\textbf{Acknowledgements:}

The work was primarily supported by the Centre for Slope Safety
(AoE/E-603/18) of the Research Grants Council of the Hong Kong SAR
Government. C.K.F. Lee and J. Wu were supported by National Natural
Science Foundation of China (\#31922090) and HKU seed fund for basic
research (\#202011159154). C.K.F. Lee was also in part supported by the
HKU 45\textsuperscript{th} round PDF scheme.

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