Abstract
AbstractTerrestrial ecosystems sequester 2.1 Pg of atmospheric carbon annually. A large amount of the terrestrial sink is realized by forests. However, considerable uncertainties remain regarding the fate of this carbon over both short and long timescales. Relevant data to address these uncertainties are being collected at many sites around the world, but syntheses of these data are still sparse. To facilitate future synthesis activities, we have assembled a comprehensive global database for forest ecosystems, which includes carbon budget variables (fluxes and stocks), ecosystem traits (e.g. leaf area index, age), as well as ancillary site information such as management regime, climate, and soil characteristics. This publicly available database can be used to quantify global, regional or biome‐specific carbon budgets; to re‐examine established relationships; to test emerging hypotheses about ecosystem functioning [e.g. a constant net ecosystem production (NEP) to gross primary production (GPP) ratio]; and as benchmarks for model evaluations. In this paper, we present the first analysis of this database. We discuss the climatic influences on GPP, net primary production (NPP) and NEP and present the CO2 balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates. Globally, GPP of forests benefited from higher temperatures and precipitation whereas NPP saturated above either a threshold of 1500 mm precipitation or a mean annual temperature of 10 °C. The global pattern in NEP was insensitive to climate and is hypothesized to be mainly determined by nonclimatic conditions such as successional stage, management, site history, and site disturbance. In all biomes, closing the CO2 balance required the introduction of substantial biome‐specific closure terms. Nonclosure was taken as an indication that respiratory processes, advection, and non‐CO2 carbon fluxes are not presently being adequately accounted for.
Highlights
By sequestering large amounts of atmospheric carbon, forests play an important role in the global carbon cycle and are thought to offer a mitigation strategy to reduce global warming (Schimel et al, 2001)
We discuss the climatic influences on gross primary production (GPP), net primary production (NPP) and net ecosystem production (NEP) and present the CO2 balances for boreal, temperate, and tropical forest biomes based on micrometeorological, ecophysiological, and biometric flux and inventory estimates
We expect that more GPP, NEP, and Re data will become available in the near future, especially for South America
Summary
By sequestering large amounts of atmospheric carbon, forests play an important role in the global carbon cycle and are thought to offer a mitigation strategy to reduce global warming (Schimel et al, 2001). Many more data on carbon cycling in forests have become available in recent decades. Additional insight in global NPP was gained by analyses of updated comprehensive data summaries (Scurlock & Olson, 2002; Ciais et al, 2005), as well as by modelling studies, such as the Potsdam NPP model intercomparison study (Cramer et al, 1999). Global patterns (both spatial and temporal) in gross primary production (GPP) and respiration (Re) are mainly based on modelling exercises Exceptions include analyses of NEP and GPP measurements from eddy covariance flux networks (Valentini et al, 2000; Janssens et al, 2001; Law et al, 2002; Reichstein et al, 2003) and a synthesis of the CO2 balance of a boreal, temperate and tropical forest site (Malhi et al, 1999)
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