Uptake of carbon dioxide by forests is an important part of the global carbon budget. Although there is widespread agreement about what drives variability in uptake on short timescales—the process scale of hours and days—researchers are interested in what drives photosynthesis, respiration, and net carbon sequestration on seasonal and interannual scales.

FluxNet, a network of continental networks, now has more than 400 active sites and thousands of years of site data—enough to look into variabilities and to correlate them with environmental parameters. Many previous interannual studies found temperature as an important driver. Others found nitrogen availability or succession of species to drive temporal patterns.

Christoph Thomas, an atmospheric scientist who studies atmosphere–vegetation interaction and turbulence, observes exchanges of energy, water, carbon dioxide and methane in Oregon forests at three AmeriFlux sites, in collaboration with the Terra-PNW group in the College of Forestry. Working with continuous observations over seven years from the Metolius Flux site in Central Oregon, Thomas and colleagues show the importance of soil moisture as a driver of interannual carbon uptake in this summer drought-stressed forest. The decrease in uptake is particularly striking after several drought years in a row. The seven year record is the longest of its kind in the State of Oregon and one of the longest in North America.

Thomas found that water as a driver did not show up with monthly or calendar-season aggregates routinely used by researchers. However when researchers "thought like a tree," the importance of interannual variability of plant-available soil moisture became clear. Researchers defined a novel "hydro-ecological year" of four seasons, based on thresholds in the water cycle to which the trees respond:

• Winter (ground frozen, liquid water not available to plants, often snow).
• Growing season, non-limited (trees thrive; liquid water, nutrients, and sunlight abundant; soil and air warmed).
• Growing season, limited (soil moisture in the system starts to decline; older trees may use deeper roots to pump up deeper water at night).
• Growing season, drought (trees regulate photosynthesis and actively close stomates on leaves, through which trees exchange gas with the atmosphere).

Researchers wanted to be able to learn from the past to better predict the future. They correlated the 7-year record in the drought-stressed forest at the Metolius site with regional temperature and moisture patterns; these were very coherent across the region. The entire 7 years fell into a relatively dry and warm period in phase with the Pacific decadal oscillation. In a cooler, more humid cycle, researchers would expect the carbon sink of the system to increase.

Since carbon uptake is directly linked to water, any changes in the water cycle—either natural variation, climate change, or management actions—may strengthen or lessen the terrestrial carbon sink in these systems. This could apply to land development decisions or to optimizing the number of trees to plant for maximum carbon dioxide uptake.