Reconstruction and sustainable management of degraded forest based on the combination of inter-planting nitrogen fixation rare tree species and thinning

Project title: Reconstruction and sustainable management of degraded forests based on combination of inter-planting nitrogen fixation rare tree species and thinning [Project ID: 2018P4-CAF]
Executing agency: Experimental Centre of Tropical Forestry, Chinese Academy of Forestry 
Implementing agency: Institute of Forest and Wildlife Research and Development
Budget in USD (total/APFNet grant): 503,000/378,000
Duration: April 2018–March 2021
Target economy: Cambodia
Location: Bos Thom village, Khna Por commune, Soth Nikum, Siem Reap province, Cambodia
Objectives: Explore and trial effective approaches for transforming degraded forests; improve growth and quality of forest stands; and enhance ecosystem services using forest restoration and sustainable forest management; Improve livelihoods of local forest-dependent people; Share information and knowledge of best practices on forest restoration and rehabilitation.
Expected outputs: Demonstration of effective approaches on restoring degraded community forests, optimizing forest structure, and improving forest ecosystem services; Non-forestry activities are developed to improve community livelihoods; Dissemination of information and knowledge of best practices on degraded forest restoration and rehabilitation in Cambodia.

Project background
Restoration of degraded forests in Cambodia remains challenging due to lack of finance and experience in using different restoration practices and technologies. Most degraded forest areas in Cambodia, often community forests, have infertile sandy soils which are low in organic carbon and other nutrients, and have poor water holding capacity. The Bos Thom community forest is located 30 km from Siem Reap city and has 445 ha of degraded forests. The forests have been over-exploited due to lack of management, allowing illegal cutting of valuable tree species and firewood collection. This has resulted in large forest canopy gaps, reduced biodiversity and soil erosion. This site needs effective forest restoration that not only restores the trees, but also the soil. The APFNet-funded project on “Reconstruction and sustainable management of degraded forest based on the combination of inter-planting nitrogen fixation rare tree species and thinning” demonstrates new approaches for integrated forest restoration by restoring the soil through planting nitrogen-fixing tree species while also improving local livelihoods. Building upon the results of a completed project “Multifunction forest restoration and management of degraded forest areas in Cambodia”, also funded by APFNet, this project will showcase new forest restoration approaches in different forest ecosystems and conditions to help Cambodians find a restoration solution that works for their forests.
Project featured topics
Restoring forests by restoring their soil
APFNet has tested several different restoration models across Cambodia, including enrichment planting, agroforestry, assisted natural regeneration and general silviculture techniques in Siem Reap, Kampong Thom and Kampong Speu provinces. The project will adopt a new method with the degraded community forests of Bos Thom with technical support and experience from the Experimental Center of Tropical Forestry under the Chinese Academy of Forestry. The method uses close-to-nature forest management techniques including group inter-planting in forest gaps with rare tree species that have a nitrogen fixation ability such as the rosewood species, Dalbergia cochinchinensis, and Senna siamea. Using nitrogen-fixing trees is one of the most promising and low cost measures for restoring degraded forests above and below ground. The method involves removing poorly growing or damaged trees to make space, and interplanting the stand with nitrogen-fixing species. Two planting models will be used, depending on the degree of forest degradation. 
Figure 1. Belt planting model in severely degraded forest (left) and Cluster planting model in moderately degraded forest (right)
Figure 2. Site preparation (left) and new nitrogen-fixing species plantation (right) in severely degraded forest. Photo: Institute of Forest and Wildlife Research and Development 
Figure 3. Preparing for planting in moderately degraded forest. Photo: Institute of Forest and Wildlife Research and Development 
Gradually, the forest will transform into a resilient, uneven-aged mixed forest, enhancing productivity, stand quality and stability, and ecological function. Forest succession will be accelerated by thinning non-target tree species, which will also provide timber resources. 
Reducing dependence on forests by improving local livelihoods
Successful forest restoration is not just defined by whether the forest itself is restored, but by whether additional economic benefits are provided for local people who are dependent on forest resources.This project will provide benefits and income for the local community by establishing home gardens and providing alternate sources of energy to reduce the pressure on forests. Normally, local households in rural villages live in wooden houses on a small piece of land (less than 1 ha) on which several tree species, crops and vegetables grow. Usually, this land only provides the basic needs of a single household.Transforming these small patches into home gardens will enable families to earn an income instead of barely subsisting on their land. Home gardens combine various fruit trees in multiple stories with crops and sometimes domestic animals. This project will interplant fruit trees that can provide short-term incomes such as banana, papaya, coconut or cashew nut in combination with existing plants around households. It will improve food diversity for the family’s daily consumption as well enable them to sell excess product to the local market. 
In addition, the project will encourage people to plant trees for firewood around their house, to help avoid cutting trees from natural forests. The installation of small-scale solar equipment will provide electricity and reduce energy expenses for households. The combination of alternative sources of income and energy, availability of alternate firewood resources and decrease in living costs will reduce the pressure on the surrounding forests.

What are nitrogen-fixing tree species?
Nitrogen is one of the key nutrients in soil and essential for plant growth. Generally, soil obtains nitrogen through the death and subsequent breakdown of organisms on the forest floor (falling leaves or other plant or animal tissue). Most tree species cannot use atmospheric nitrogen. Forests are fairly good at keeping nitrogen levels stable through internal recycling but the nutrient cycle is vulnerable to soil erosion, which effectively washes nitrogen downstream, leaving the soil stripped of nutrients.
Nitrogen-fixing tree species form a unique symbiosis with certain types of bacteria and archaea called rhizobia within the nodules of their root systems. These organisms are able to transform atmospheric nitrogen into molecules such as ammonia that can then be used by trees. Eventually these trees die and their dead plant material will be added into the overall nitrogen cycle, effectively adding former atmospheric nitrogen into the soil.