Research Indicates that Reforestation with Varied Seedling Mixes Speeds up Forest Restoration After Logging

Satellite observations of one of the world’s most extensive ecological experiments on Borneo Island have unveiled that the process of replanting logged forests with a diverse array of seedlings can markedly expedite their recovery.

The study titled “Positive Effects of Tree Diversity on Tropical Forest Restoration in a Field-Scale Experiment” has been published in the journal Science Advances. This comprehensive experiment was initiated more than two decades ago by Professor Andy Hector and his colleagues from the University of Oxford as part of the SE Asia Rainforest Research Partnership (SEARRP). The project aimed to assess the recovery of 125 distinct plots within a region of logged tropical forest, each sown with varying combinations of tree species.

The findings of this study demonstrated that plots replanted with a mixture of 16 native tree species exhibited a swifter recuperation in terms of canopy area and total tree biomass, in comparison to plots replanted with just four or a single tree species. Notably, even plots replanted with a solitary tree species showed a more rapid recovery than those left to naturally regenerate.

Professor Andy Hector, the lead scientist of the study from the Department of Biology at the University of Oxford, remarked, “Our new study reveals that the rejuvenation of logged tropical forests through the introduction of diverse mixtures of native tree species brings about multiple benefits, including the accelerated restoration of tree cover, biodiversity, and essential ecosystem services like carbon sequestration.”

The researchers attribute this phenomenon to the fact that different tree species occupy distinct ecological niches, encompassing their adaptability to physical and environmental conditions as well as their interactions with other organisms. Consequently, diverse mixtures of tree species complement each other, enhancing the overall functioning and stability of the ecosystem. For example, some tropical tree species possess greater drought tolerance due to their production of protective chemicals, imparting resilience to the forest during periods of low rainfall.

Professor Hector further explained, “Diversity within a tropical forest can be likened to an insurance policy, akin to maintaining diverse investment portfolios in financial strategy.”

Furthermore, a diverse assortment of trees can support a broader range of animal species. For instance, large mature trees with nesting holes are essential for hornbills’ survival.

Tropical forests, despite covering only 6% of the Earth’s land surface, are home to approximately 80% of documented species and play a significant role in carbon sequestration. However, these vital ecosystems are rapidly disappearing due to activities such as logging for timber and conversion into palm oil plantations. Between 2004 and 2017, an area equivalent to the size of Morocco was lost, totaling 43 million hectares.

Restoring logged tropical forests is crucial in addressing both the biodiversity and climate crises. Nevertheless, it remained uncertain whether natural regeneration (using dormant soil seeds) or active replanting was the most effective method.

To investigate this, the researchers collaborated with local partners to establish the Sabah Biodiversity Experiment on 500 hectares of logged forest in Sabah, Malaysia, on Borneo Island. The area was divided into 125 experimental plots, some left to regenerate naturally and others planted with mixtures of one, four, or 16 tree species frequently targeted for logging.

The 16 species chosen included endangered species and the world’s tallest tropical tree species, Shorea faguetiana, which can reach heights exceeding 100 meters. The planting commenced in 2002, with nearly 100,000 trees planted over subsequent years.

The progress of these plots was assessed using statistical models applied to satellite images. Within a few years, it became evident that plots with a single species fared worse than those with a mixture of four species, and those enriched with 16 species exhibited the most robust recovery.

Ryan Veryard, the lead author who analyzed the data for his Ph.D. at the University of Oxford, commented, “Importantly, our results indicate that logged forests can recover, as long as they are not converted into agricultural land, such as oil palm plantations. They also underscore the necessity of conserving biodiversity in undisturbed forests so that it can be restored in previously logged areas.”

The Sabah Biodiversity Experiment team is now embarking on a new three-year project to conduct a comprehensive census of all surviving trees within the experiment. This effort will incorporate a wider range of remote sensing methods, including lidar sensors carried by helicopters and smaller sensors carried by drones, to provide a more extensive analysis of forest health.

Source: University of Oxford

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