Measuring net primary production in forests: Concepts and field methods

There are pressing reasons for developing a better understanding of net pri mary production (NPP) in the world's forests. These ecosystems play a large role in the world's carbon budget, and their dynamics, which are likely to be responding to global changes in climate and atmospheric composition, ha ve major economic implications and impacts on global biodiversity, Although there is a long history of forest NPP studies in the ecological literature , current understanding of ecosystem-level production remains limited, Fore st NPP cannot be directly measured; it must be approached by indirect metho ds. To date, field measurements have been largely restricted to a few aspec ts of NPP; methods are still lacking for field assessment of others, and pa st studies have involved confusion about the types of measurements needed. As a result, existing field-based estimates of forest NPP are likely to be significant underestimates. In this paper we provide a conceptual framework to guide efforts toward imp roved estimates of forest NPP. We define the quantity NPP* as the summed cl asses of organic material that should be measured or estimated in field stu dies for an estimate of total NPP. We discuss the above- and belowground co mponents of NPP* and the available methods for measuring them in the field, We then assess the implications of the limitations of past studies for cur rent understanding of NPP in forest ecosystems, discuss how field NPP* meas urements can be used to complement tower-based studies of forest carbon flu x, and recommend design criteria for future field studies of forest NPP.