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.