Forest biomes are major reserves for terrestrial carbon, and major components of global primary productivity. The carbon balance of forests is determined by a number of component processes of carbon acquisition and carbon loss, and a small shift in the magnitude of these processes would have a large impact on the global carbon cycle. In this paper, we discuss the climatic influences on the carbon dynamics of boreal, temperate and tropical forests by presenting a new synthesis of micrometeorological, ecophysiological and forestry data, concentrating on three case‐study sites. Historical changes in the carbon balance of each biome are also reviewed, and the evidence for a carbon sink in each forest biome and its likely behaviour under future global change are discussed. We conclude that there have been significant advances in determining the carbon balance of forests, but there are still critical uncertainties remaining, particularly in the behaviour of soil carbon stocks.

Understanding terrestrial carbon metabolism is critical because terrestrial ecosystems play a major role in the global carbon cycle. Furthermore, humans have severely disrupted the carbon cycle in ways that will alter the climate system and directly affect terrestrial metabolism. Changes in terrestrial metabolism may well be as important an indicator of global change as the changing temperature signal. Improving our understanding of the carbon cycle at various spatial and temporal scales will require the integration of multiple, complementary and independent methods that are used by different research communities. Tools such as air sampling networks, inverse numerical methods, and satellite data (top-down approaches) allow us to study the strength and location of the global- and continental-scale carbon sources and sinks. Bottom-up studies provide estimates of carbon fluxes at finer spatial scales and examine the mechanisms that control fluxes at the ecosystem, landscape, and regional scales. Bottom-up approaches include comparative and process studies (for example, ecosystem manipulative experiments) that provide the necessary mechanistic information to develop and validate terrestrial biospheric models. An iteration and reiteration of top-down and bottom-up approaches will be necessary to help constrain measurements at various scales. We propose a major international effort to coordinate and lead research programs of global scope of the carbon cycle.

The California Mediterranean savanna has harsh summer conditions with minimal soil moisture, high temperature, high incoming solar radiation and little or no precipitation. Deciduous blue oaks, Quercus douglasii Hook. and Arn., are winter-deciduous obligate phreatophytes, transpiring mostly groundwater throughout the summer drought. The objective of this work is to fully characterize the seasonal trends of photosynthesis in blue oaks as well as the mechanistic relationships between leaf structure and function. We estimate radiative load of the leaves via the FLiES model and perform in situ measurements of leaf water potential, leaf nitrogen content, an index of chlorophyll content (SPAD readings), photosynthetic and electron transport capacity, and instantaneous rates of CO2 assimilation and electron transport. We measured multiple trees over 3 years providing data from a range of conditions. Our study included one individual that demonstrated strong drought stress as indicated by changes in SPAD readings, leaf nitrogen and all measures of leaf functioning. In the year following severe environmental stress, one individual altered foliation patterns on the crown but did not die. In all other individuals, we found that net carbon assimilation and photosynthetic capacity decreased during the summer drought. SPAD values, electron transport rate (ETR) and quantum yield of photosystem II (PSII) did not show a strong decrease during the summer drought. In most individuals, PSII activity and SPAD readings did not indicate leaf structural or functional damage throughout the season. While net carbon assimilation was tightly coupled to stomatal conductance, the coupling was not as tight with ETR possibly due to contributions from photorespiration or other protective processes. Our work demonstrates that the blue oaks avoid structural damage by maintaining the capacity to convert and dissipate incoming solar radiation during the hot summer drought and are effective at fixing carbon by maximizing rates during the mild spring conditions.

Map of PhenoCam sites.

Continuous eddy convariance measurements of carbon dioxide, water vapor and heat were measured continuously between an oak savanna and an annual grassland in California over a 4 year period. These systems serve as representative sites for biomes in Mediterranean climates and experience much seasonal and inter-annual variability in temperature and precipitation. These sites hence serve as natural laboratories for how whole ecosystem will respond to warmer and drier conditions. The savanna proved to be a moderate sink of carbon, taking up about 150 gC m-2y-1 compared to the annual grassland, which tended to be carbon neutral and often a source during drier years. But this carbon sink by the savanna came at a cost. This ecosystem used about 100 mm more water per year than the grassland. And because the savanna was darker and rougher its air temperature was about 0.5 C warmer. In addition to our flux measurements, we collected vast amounts of ancillary data to interpret the site and fluxes, making this site a key site for model validation and parameterization. Datasets consist of terrestrial and airborne lidar for determining canopy structure, ground penetrating radar data on root distribution, phenology cameras monitoring leaf area index and its seasonality, predawn water potential, soil moisture, stem diameter and physiological capacity of photosynthesis.