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 (Błońska, Lasota, Zwydak, Klamerus-Iwan, & Gołąb,
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, Velázquez, & 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‐Orth &
Martínez‐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.