Synergies and trade-offs between tree cover expansion efforts within and outside forests to achieve climate, biodiversity and human well-being outcomes

Pooja Choksi , Jennifer S. Powers, Laura Toro, Smitha Krishnan, Forrest Fleischman

Increasing tree cover is hailed as a leading climate mitigation strategy, yet there is increasing evidence that there may be trade-offs between trees in different parts of the landscape. Existing science, policy, and practice on natural climate solutions (NCS) assumes that trees outside of forests, for example on farms, homesteads, or in urban areas, are synergistic with forest conservation: if people have more access to trees outside forests, the thinking goes, they will use fewer trees in the forest, leading to forest conservation. However, recent evidence shows that trees inside and outside forests often serve different purposes and produce different ecological, social or economic outcomes. As a result, in many contexts trees outside of forests do not necessarily substitute for trees inside of forests, and in some circumstances, an increase in trees outside of forests could even contribute to forest loss. In this perspective, we review the existing evidence on dynamics and trade-offs of trees inside and outside forests to describe pathways that lead to trade-offs and synergies and identify critical research gaps that, if addressed, can inform the implementation of ecosystem restoration and climate mitigation programs and policies. These diverse pathways are not widely recognized in science, policy, or practice, leading to potentially unproductive policy investments and leaving significant research gaps that we identify in this paper. Additionally, there is a risk that tree planting programs that are not cognizant of potential trade-offs may have negative repercussions for forest conservation, biodiversity, carbon storage, and rural livelihoods. Recognizing pathways that lead to negative outcomes, and identifying research gaps that could anticipate or prevent such negative outcomes will help scientists, policymakers and practitioners move forward with more effective NCS.