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 (Pérès 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 & Ivetić,
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; Martínez-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.