Which tree species survive and grow best when restoring a degraded peat swamp forest?

This post is also available in Indonesian here.

Whilst preserving and protecting forests is crucial, tremendous efforts are on-going globally to restore degraded and lost forests, particularly in the tropics. In their latest research, Smith and colleagues review tree survival and growth in tropical peatland reforestation projects across Southeast Asia.

Tropical peatlands cover a small area globally, but are dense stores of carbon. Waterlogged conditions in peatlands slow decomposition of plant matter that allows carbon-rich peat to accumulate, yet the value of peatlands extends beyond carbon: they are home to many endemic and threatened species of flora and fauna, and are important for local community livelihoods and cultures.

Across Southeast Asia, widespread deforestation, drainage and degradation of peatlands is leading to peat loss and higher risk of fires. Peatland fires contribute to regional haze air pollution with costly public health impacts. As in other habitats, disruptions to the tropical peat swamp habitats and wildlife harvesting also increases the risk of outbreaks of zoonotic emerging infection diseases.

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Burnt and degraded peatland in Tahura, Jambi province, Indonesia. Photo: Stuart Smith

Restoring degraded and burned peat swamp forests is challenging and requires consideration of hydrology, vegetation, and sustaining local livelihoods. Our review focused on “active” tropical peatland reforestation – i.e., tree planting. A key first step in this is selecting and sourcing tree species to plant. Knowledge of which species might survive comes from trial and error in pilot studies, local knowledge, and past reforestation project experience.

To-date a quantitative synthesis of monitoring data from trials and past planting projects has been lacking for tropical peatlands, thus limiting access to this knowledge. We carried out the first systematic literature review of planted tree species survival and growth in degraded Southeast Asian tropical peatlands, collating monitoring data from published and “grey” literature, for instance non-government organisation reports, and in multiple languages – Indonesian, English and Japanese.

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Volunteers planting trees in on kerapah (heath forest on peat) in Lumut, Brunei Darussalam. Photo: Rahayu Sukri and team 2015.

Across 94 reforestation projects identified in the review, we found 141 tree species had been planted. This represents one-tenth of potential diversity of the regional peatland tree flora. Yet, despite the need to expand the number of species planted, survival rates were promising averaging 62% survival of planted trees, lasting on average 2.5 years before 50% of them died.

A key finding was that slower growing species survived longer in reforestation projects. Survival rates could also be predicted by some plant functional traits, such as leaf nutrient contents and wood densities, raising the potential of screening species for reforestation based on their traits.

The least consistent results in our review was the impact of site treatments. This was due to a lack of studies adopting control vs. treatment experimental designs. Large sums of money are invested into site treatments, such as mounding seedlings above potential flood levels. However, without controls it is difficult to determine the effectiveness of these treatments.

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One of the successful slow growing tree in the systematic review – Alstonia spathulata planted to reforest Klias Forest Reserve, Sabah, Malaysia. Photo: Reuben Nilson

To advance peat swamp reforestation requires planting more species, involving slower growing species, trialling species screening using plant traits, and testing treatments adequately. In conducting our review, we highlight the value of collecting and sharing monitoring data.

A total of 43 authors participated in the review with many actively involved in tropical peatland reforestation projects. Across Southeast Asian peatlands many more restoration projects exist, and we hope our review will encourage transparency and data sharing for future syntheses of peat swamp forest restoration and other tropical forests.

Read the full Open Access paper Tree species that ‘live slow, die older’ enhance tropical peat swamp restoration: Evidence from a systematic review in Journal of Applied Ecology

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