Item

Abstract

Forests strongly aect climate through the exchange of large
amounts of atmospheric CO2 (ref. 1). The main drivers of
spatial variability in net ecosystem production (NEP) on
a global scale are, however, poorly known. As increasing
nutrient availability increases the production of biomass
per unit of photosynthesis2 and reduces heterotrophic3
respiration in forests, we expected nutrients to determine
carbon sequestration in forests. Our synthesis study of 92
forests in dierent climate zones revealed that nutrient
availability indeed plays a crucial role in determining NEP
and ecosystem carbon-use eciency (CUEe; that is, the ratio
of NEP to gross primary production (GPP)). Forests with
high GPP exhibited high NEP only in nutrient-rich forests
(CUEe D 33 4%; mean s.e.m.). In nutrient-poor forests,
a much larger proportion of GPP was released through
ecosystem respiration, resulting in lower CUEe (6 4%). Our
finding that nutrient availability exerts a stronger control on
NEP than on carbon input (GPP) conflicts with assumptions of
nearly all global coupled carbon cycle–climate models, which
assume that carbon inputs through photosynthesis drive
biomass production and carbon sequestration. An improved
global understanding of nutrient availability would therefore
greatly improve carbon cycle modelling and should become a
critical focus for future research.