Forest value: More than commercial

Abstract

In their Research Article “Positive biodiversity-productivity relationship predominant in global forests” (14 October, p. 196), J. Liang et al. establish a positive relation between forest productivity and commercial value to show the importance of tree species richness for supporting high productivity. They estimate that a drop in the current level of species richness in forests to one species would cost US66 to 90 billion annually. However, species richness and ecological productivity are poor indicators for commercial value. According to the richness-value assumption, megadiverse tropical forests would provide high commercial value. In fact, the opposite is usually true. Cubbage et al. (1) showed that species-rich subtropical or tropical native forests are much less profitable than planted monocultures. In these forest types, only 20% of tree species or fewer are commercially valuable, and the harvests may be as low as 0.7 cubic meters per hectare per year (2), less than 10% of global average productivity according to Liang et al. Planted forests provide a large part of the commercial value of the world's forests. The area of planted forests expanded from 167 to 278 million ha between 1990 and 2015, contributing 46% of the world's industrial roundwood in 2012 (3). The most profitable planted forests are not necessarily species rich (4), although forest types with two or more species may compete with monocultures in the temperate zone (5). For example, in South America, 88% of planted forests consist of introduced exotic species, usually grown as monocultures. Such forest types show very high economic return (6). There is no economic evidence for the loss in commercial value that Liang et al. claim.