Primary productivity of planet earth: biological determinants and physical constraints in terrestrial and aquatic habitats

The habitability of our planet depends on interlocking climate and biogeochemical systems. Living organisms have played key roles in the evolution of these systems. Now man is perturbing the climate/biogeochemical systems at an unprecedented pace. In particular, the global carbon cycle is being forced directly by changes in carbon fluxes (e.g. fossil fuel burning and deforestation/reforestation), and indirectly through changes in atmospheric chemistry (e.g. stratospheric ozone depletion and increases of green house gases). Nutrient cycles are also being perturbed, with implications for the carbon cycle. It is imperative that we learn how these changing conditions will influence terrestrial and oceanic photosynthesis and biogeochemistry. Understanding the controls on primary productivity of the biosphere is one of the fundamental aims of global change research. This forum addresses several key questions regarding the role of the biota in the carbon cycle. It begins with Ian Woodward's overview of the global carbon cycle and concludes with John Raven's historical perspective of the negative feedbacks that influenced the evolution of embryophytes in the Devonian. In between, the forum focuses on the process of net primary production (NPP). Despite differences in the structures of planktonic and terrestrial ecosystems, notably of response of biomass to environmental change, there are common problems affecting both terrestrial and oceanic studies of NPP. This has resulted in the parallel evolution of approaches to NPP research in very different milieux involving advances in the technology required to study interacting processes that cut across a range of space and time scales (Table 1). The problems include estimating NPP of whole plants and phytoplankton populations from gas exchange measurements on leaves or subpopulations, accounting for heterotrophic metabolism in gas exchange measurements, extrapolating from small scales to global NPP (Table 2), developing mechanistic models of NPP and biomass accumulation, and relating NPP to the cycling of other elements. Although small-scale measurements will continue to be a staple tool in investigations of NPP, use of open system measurements systems have necessarily come to the fore. These include free-air CO₂ exchange in terrestrial systems and water mass tracking in aquatic systems. Deliberate experimental manipulations will increasingly supplement correlative studies to derive insights into environmental regulation of NPP and the feedback between plant productivity and biogeochemical cycles