Environment

(1) Many national reforestation programmes measure their success by a single figure: the area of land on which tree cover has been restored. The figure has obvious appeal. It can be checked from satellite images, compared across countries and reported every year. Yet a growing number of ecologists argue that, used on its own, it rewards the wrong kind of forest.

(2) The difficulty is that satellites register canopy, not the forest beneath it. Within a decade, a plantation of a single fast-growing species and a regenerating native woodland can look much the same from orbit, yet they differ in almost every property that restoration is meant to recover. Single-species plantations typically support fewer species of birds and insects and store less carbon in their soils, and because their trees are usually harvested on short rotations, they may release much of the carbon they have absorbed within a few decades. Where plantations replace grassland or scrub that was itself valuable habitat, an area target can even record as a gain what is, for wildlife, a loss.

(3) Area targets also shape the choices of landowners. In one upland district studied by the ecologist Ines Varga, landholders were paid a fixed sum for each hectare that showed closed canopy five years after enrolling in a scheme. Natural regeneration, which relies on seeds carried from nearby woodland by wind and birds, is cheap but slow and uneven; after five years, much regenerating land still looks like scrub. Stands of a single imported pine, by contrast, reliably closed their canopy in time. Almost every participant planted pine, including owners whose land adjoined old native woodland and would have regenerated on its own at a fraction of the cost.

(4) None of this means that area should be abandoned as a measure. Varga herself argues that it remains indispensable for detecting large-scale clearance, which no other indicator reveals as quickly or as cheaply. Her objection is to using area as the sole basis for payment. She proposes that payments be divided: a smaller sum for canopy area and a larger one tied to indicators that are harder to satisfy with a quick planting, such as the number of native tree species present or the return of particular woodland birds, even though such indicators require costly field surveys.