Why All Forest Carbon Projects Depend on Robust Counterfactual Baselines

With restoration and tree-planting projects back in the spotlight, partly driven by growing scrutiny of baselines in avoided-deforestation projects, we are revisiting a paper we published five years ago in Science: Active Restoration Accelerates the Carbon Recovery of Human-Modified Tropical Forests. The study combined two decades of permanent-plot measurements with high-resolution biomass maps, produced with airborne LiDAR, in Sabah, Malaysian Borneo. This allowed us to directly compare carbon recovery in areas that had been allowed to regenerate naturally with areas that had received active restoration, including enrichment planting of dipterocarps and routine cutting back of climbers and competing vegetation. Ultimately we found that in addition to the already impressive natural regeneration in this area, active restoration accelerated biomass accumulation substantially, around 50% faster than in the areas naturally regenerating. As funding shifts towards restoration projects, often viewed as more 'additional' than avoided-deforestation, this work is relevant because it established a robust empirical baseline on which to measure the efficacy of active restoration.

Natural regeneration performed well, but only because the forest had been protected from further disturbance. The surrounding forest, outside the study area, was logged repeatedly (Reynolds et al., 2011). Without sustained protection, the recovery rates we measured would not have been possible, and neither the impressive gains from natural regeneration nor the additional gains from active restoration would persist; both depend on the balance between growth and degradation. Growth and degradation are not separate silos; they result from interacting anthropogenic and natural processes that coexist and must be measured simultaneously.

The growing scrutiny of baselines in avoided-deforestation projects essentially questions the counterfactual, i.e. what would have occurred in the absence of the conservation intervention and, more specifically, how much deforestation and degradation would have occurred. However, the factor that is often missed is that a counterfactual is only an estimate as the true baseline is unknowable. This is inevitable because observing "what would have happened" would require leaving the area unprotected and observing the deforestation rate, which is obviously incompatible with implementing a conservation project. There are, however, a variety of methods that enable us to estimate this counterfactual and quantify the uncertainty empirically, although these methods are still in development and remain underutilised in forest carbon projects.

Just as with avoided deforestation projects, restoration and tree-planting projects also require a counterfactual to estimate their impact. In the restoration case, the primary questions are: if you hadn't planted a tree, what would have been there in its place? Bare ground, or perhaps some natural regeneration? And, if forest recovery is the most likely alternative scenario for the site, what would the rate of recovery have been? In both restoration and avoided-deforestation projects, the underlying question is the same: how does the carbon balance change over time compared with what would have happened without the intervention?

Although active restoration accelerated carbon recovery, the additional gains need to be considered alongside the restoration costs. Comparing the difference between natural regeneration and active restoration with global cost estimates makes it clear that, given the expense, restoration is more justified in the most degraded or disconnected areas where natural regeneration would be most inhibited. Recent global assessments also show that the majority of remaining forest carbon potential lies in existing forests, meaning that protection delivers much of the climate benefit, with restoration valuable where forests have been heavily degraded or removed (Mo et al., 2023). The baselines of avoided-deforestation projects have been criticised, but restoration projects also depend on a counterfactual to demonstrate impact. The issue is not whether baselines exist, but whether they are robust. Impact evaluation of both restoration and avoided-deforestation ultimately relies on the same principle - clearly estimating the difference in carbon balance between the intervention and a robust counterfactual baseline.