Achieving ambitious, yet cost-effective, global forest restoration goals requires creative approaches. Nino T. Amazonas, Pedro H. S. Brancalion & Karen D. Holl present a novel strategy from Brazil, using mixed plantations of exotic eucalypts and native tree species as a transitional stage for tropical forest restoration.

Many countries worldwide have committed large portions of their territory to forest landscape restoration, which has been widely advertised as a silver bullet to take up carbon, to conserve toucans and monkeys, and to support the livelihoods of people who use the forest. What we don’t hear about is who is going to pay for forest restoration. Planting and taking care of trees until they grow into a forest is not cheap, and most landowners need to make money from their land. Growing native species that produce valuable timber and non-timber forest products, such as fruits, nuts or oils, is an option, but the lack of knowledge about their cultivation, processing, and trading presents a critical risk for investors. One alternative is to plant widely-cultivated species that have demonstrated potential to provide income to investors and landowners.

In the Atlantic Forest of Brazil, exotic eucalypts (a few tree species in the genus Eucalyptus and Corymbia) are grown by both large corporations and smallholders to provide a variety of products, such as pulp, round logs, sawn lumber, firewood, charcoal, fencing poles, and oil. However, industrial, short rotation eucalypt plantations have been referred to as ‘green deserts’ due to the limited regeneration of native species in their understory, which has been attributed to eucalypts’ high consumption of water and allelopathy – i.e., the release of chemicals in the soil that prevent the regeneration of other plant species. On the other hand, abandoned and long rotation plantations in the Atlantic Forest usually have an abundant and diverse community of native woody species regenerating in the understory, which suggests that there may be a place for exotic eucalypts in restoration.

Based on those observations, we started to consider planting eucalypts alongside native species characteristic of later successional stages. Eucalypts have a good market value, they grow quickly, and there are varieties adapted to a range of latitudes and well-established techniques to grow them. We thought maybe they could sequester large amounts of carbon, build a forest structure quickly to enhance natural regeneration of native species, shade exotic grasses, and provide economic returns from one or two timber rotations. Then eucalypts would be harvested which would enhance growth of slower-growing, intercropped native species and provide space for natural regeneration of native pioneers. Maybe eucalypts could help to upscale forest restoration.

Would it work? The idea certainly raised many questions and concerns from ecologists and foresters alike. Would eucalypts outcompete both planted and naturally regenerating native species for light and water? Would harvesting eucalypts in the middle of native trees be operationally viable and would it destroy planted and regenerating native trees? Would the profit obtained from eucalypt wood be sufficient to offset the high costs of restoration plantations? All of this needed to be tested on the ground. No passionate discussion between ecologists and foresters would resolve these heated debates.

We brought together the best silvicultural practices from the paper and pulp industry and the knowledge Brazilian scientists and practitioners have accumulated about tropical forest restoration over the past 30 years. We set up three large experiments to assess the impacts of incorporating exotic eucalypts as a transitional stage in tropical forest restoration on aboveground biomass accumulation, native woody species regeneration, harvesting, and financial viability, and found many interesting results. Biomass accumulation was nine times greater in mixed eucalypt-native species plantations than native only plantings due to fast eucalypt growth. Nonetheless, the growth of native trees was not affected or only slightly reduced by eucalypts prior to logging.

Contrary to common beliefs, eucalypts did not slow the natural regeneration of native woody species before or after eucalypt logging. In fact, a diverse community of tropical forest tree seedlings established in the understory. A year following logging, native tree cover was slightly lower from the damage caused by eucalypt harvesting, but tree cover regrew quickly. The number of tree species that regenerated and the growth of planted native trees were similar across treatments in the post-logging period. Importantly, the income generated by eucalypt wood production offset 44-75% of restoration implementation costs.

Overall, our results show that this approach should be included in the toolbox of potential management strategies to help achieve large-scale restoration of degraded tropical forests. Other combinations exotic and native trees should be tested in various regions worldwide to evaluate their potential to help pay the bill of global forest landscape restoration commitments. Of course, the selecting the most appropriate strategies to use will depend on specific project goals and site conditions.

Read the full article, Exotic eucalypts: From demonized trees to allies of tropical forest restoration? in Journal of Applied Ecology.