(NEW PUBLICATION) Effectiveness of Ecosystem Restoration in Tropical & Sub-Tropical Forests in Asia

Photo 2017 Reinout Dujardin. Pixabay.

In recent decades, equatorial forests have seen extensive tropical deforestation and degradation driven by logging and agriculture expansion and have resulted in increasing human-modified forest and secondary growth forests. While forest restoration, both active (e.g., tree planting) and passive (e.g., natural regeneration), is a way to mitigate damage from anthropogenic impacts, its efficacy can vary widely based on contextual circumstances such as land use, intervention used, local priorities and expected outcomes (ecosystem restoration vs ecological restoration). With the limited resources available for forest restoration, this publication sets out to measure the outcomes expected from tree planting as a tool for restoration to improve its use for the future.

We found that 18% of planted trees died within 1 year and 44% after 5 years and ~50% beyond 10 years. Mean mortality rates were highly variable between sites indicating environmental, methodological, and social factors thereby indicating that restoration outcomes are highly context dependent. For example, mortality was typically ~20% higher in open areas than degraded forests indicating that habitat condition to be an important driver of outcomes. Mechanisms such as: (i) shade-tree cover, (ii) proximity to established trees, (iii) competition and suppression by native and non-native plants and (iv) habitat type (peat swamp forest vs forest) may explain this variation. We also found a positive effect of tree height at planting on survival in the open degraded sites. This may be explained by the resilience of larger seedlings in their ability to withstand environmental challenges or maximise opportunities for rapid growth.

Variability in mortality rates may also arise because of species-specific responses and the appropriateness of selected species to the planting environment. Yet, we did not find a strong effect of the number of planted species on survival and the effect of site was stronger than that of species. This may be due to a small fraction of the native species pool being used in restoration at most sites and when higher-level drivers of mortality are affecting more species. Long-term monitoring is necessary to investigate the effects of low diversity plantings on survival in restoration settings.

Growth rates (i.e., height growth) were found to be negatively related to species-level wood density in degraded forest and plantation settings. Wood density being a less important driver of growth may be explained by: (i) smaller plant sizes allocating carbon to growth instead of wood density and (ii) lack of competition from neighbouring trees and a high light environment in open ecosystems. We found that wood density is not determinant of tree mortality. Apart from wood density, other traits may be important in influencing survival, growth and their trade-offs and needs further investigation for contexualising trait-rate relationships for better restoration outcomes.

This study has shown that outcomes of tree planting are highly variable and are context-dependent. Also key is considering how we can overcome the barriers to greater diversity in plantings and long-term monitoring and reporting of large-scale mortality events and causes is crucial.

Read the full paper here published in Philosophical Transactions of the Royal Society B and led by Lindsay Banin at the Centre for Ecology & Hydrology, UK.

https://royalsocietypublishing.org/doi/10.1098/rstb.2021.0090