By Kay Y.K. Law, The University of Hong Kong
Landslides are severe natural disasters worldwide which are exacerbated by anthropogenic causes, including deforestation and climate change. With topsoil and soil seed bank removed, the extreme environmental condition on landslide surfaces often hinder natural regeneration. Stabilization and restoration of landslides by engineering or ecological restoration means are often hampered by the steepness and inaccessibility of the landside sites. Hence, the exposed soil surface of landslides is vulnerable to further erosion. Active forest restoration has been regarded as a potential method to repair such landslide scars. Apart from planting tree seedlings, direct seeding is a more cost effective and practical method in large-scale forest restoration. However, the effectiveness of direct seeding on landslides is not well studied. How could seed germination and seedling survival be improved on the poor soil surface on landslides?
Two formular of biochar seed coatings (biochar dominated and clay dominated seed balls) were tested at three landslide sites in Hong Kong (Figure 1) in direct seeding using native tree species for a year. In addition, we have also tested separately at the same three landslides if adding biochar powder to the soil surface on landslides could enhance seed germination and seedling establishment in direct seeding for a year. Landslides are common on degraded hillsides in Hong Kong during the summer wet season. Biochar is often used in agriculture in improving soil quality for crop growth (Ding et al., 2016).
Figure 1. The three landslide sites in Hong Kong in this study and the aerial photo illustrating experimental plots.
The results show that the overall seed germination rate of biochar dominated seed balls are significantly higher (by 9.3 %) than plain seeds (SE = 0.04, p < 0.05). Two of the three species tested have similar results (Figure 2). However, in the soil surface treatment experiment, we found that sowing biochar powder on top of soil surface did not have significant effects on native seed germination. Meanwhile, we did not find any significant relationships between seedling survival and both seed coating and surface soil treatments. Seedling mortality after germination was very low.
Figure 2. Comparison of the seed germination rate of biochar, clay formula and control of seed coating treatments.
Late-successional species had a 30% higher germination rate (p < 0.001) than pioneer species in our test, which is similar to the findings in the literature (de Souza & Engel 2018). Late-successional species were found to be limited by seed dispersal rather than harsh site conditions. Late-successional species may be widely used in direct seeding on landslides for forest restoration.
Like similar direct seeding research (St-Denis et al., 2013; Tunjai & Elliott, 2012), we also found that heavier seeds had significantly higher germination than lighter seeds. An increase of 0.1 g in seed weight has led to 6.56 % higher germination in our study.
The overall germination rates of raw seeds were 17% to 67% in our study. These were higher than a meta-analysis of direct seeding in which the mean seed germination rate was 23.9% (Ceccon et al., 2016). This suggests that direct seeding in Hong Kong, which is in the seasonal tropics, is a promising forest restoration technique.
We were able to show the effectiveness of biochar seed coating (Figure 3) and planting late-successional and large seed species in improving seed germination rate on landslide scars. These findings could be applied to post-disturbance vegetative restoration and recovery on landslides of other hilly terrains in the seasonal tropics.
Figure 3. Procedure of coating seeds in the laboratory. a. and b. preparing the biochar-clay mix (40 : 60 and 60 : 40 respectively). c. The rolling machine for coating the biochar-clay powder on seeds. d. seed balls made.
Reference
Ceccon, E., González, E. J., & Martorell, C. (2016). Is direct seeding a bio- logically viable strategy for restoring forest ecosystems? Evidences from a meta-analysis. Land Degradation & Development, 27(3), 511– 520. https://doi.org/10.1002/ldr.2421
Civil Engineering and Development Department (CEDD), HKSAR (2021). Landslide risk in Hong Kong. Hong Kong Slope Safety. Retrieved 2 January 2022 from https://hkss.cedd.gov.hk/hkss/en/slope-safety- in-hong-kong/landslide-risk/index.html
de Souza, D. C., & Engel, V. L. (2018). Direct seeding reduces costs, but it is not promising for restoring tropical seasonal forests. Ecological Engineering, 116, 35–44. https://doi.org/10.1016/j.ecoleng.2018. 02.019
Ding, Y., Liu, Y., Liu, S., Li, Z., Tan, X., Huang, X., Zeng, G., Zhou, L., & Zheng, B. (2016). Biochar to improve soil fertility. A review. Agronomy for Sustainable Development, 36(2), 1–18. https://doi.org/10.1007/ s13593-016-0372-z
St-Denis, A., Messier, C., & Kneeshaw, D. (2013). Seed size, the only factor positively affecting direct seeding success in an abandoned field in Quebec, Canada. Forests, 4(2), 500–516. https://doi.org/10.3390/ f4020500
Tunjai, P., & Elliott, S. (2012). Effects of seed traits on the success of direct seeding for restoring southern Thailand's lowland evergreen forest ecosystem. New Forests, 43(3), 319–333. https://doi.org/10.1007/ s11056-011-9283-7