Just to put the terrestrial biosphere in perspective, there is about ten times more carbon contained in all land plants (plus the soil they grow on) than all the “extra” anthropogenic carbon currently in the atmosphere. Most of the terrestrial carbon is contained in forests which have been significantly depleted by mismanagement. The question is whether Northwest forests are more likely to store or release carbon under a changing climate. 

The coupled processes of photosynthesis and respiration/decomposition mirror each other at a global scale to help regulate CO₂ levels and our climate¹. Photosynthesis captures water and CO₂ and liberates oxygen to create biomass, while respiration consumes biomass and oxygen to liberate CO₂ and water. Depending on temperature and moisture conditions, among other factors, photosynthesis sometimes dominates leading to net carbon uptake. At other times respiration/decomposition dominate leading to net carbon release². Whether our forests ultimately become net carbon sources or net carbon sinks under the future climate of the Northwest depends on factors that remain uncertain, such as the amount of summer precipitation vs. drought stress, the effects of future climate on fuels and fire hazard, the effects of CO₂ enrichment and climate change on plant physiology, whether forests geographically expand or contract, and whether forests are exploited or protected³. 

The good news is that slight to moderate warming may increase our forests’ ability to store carbon through increased growth and geographic expansion. Pacific Northwest forests might become significant carbon sinks and help mitigate climate change if growing conditions remain favorable and disturbances like fire do not significantly increase. Under warm-wet conditions growing seasons will lengthen, and forest or woodland communities could expand into current rangelands, thus raising the possibility that northwest forests could absorb CO₂ and become a significant net carbon sink⁴. 

The bad news is that there is likely a warming threshold above which our forests will likely decline due to drought stress and increased disturbances⁵. Drought stress limits the potential photosynthetic benefits of longer growing seasons and CO₂ enrichment. Increasing temperature also increases rates of respiration and decomposition, so under a future climate scenario like this, northwest forests could wither, recede geographically, and become a significant net carbon source. The IPCC tells us that some warming has already occurred and that existing levels of CO₂ already commit us to some additional warming. There is considerable uncertainty about when we may cross the sink to source threshold⁶. 

El Niño/Southern Oscillation (ENSO) is a prominent source of multi-year variability in weather and climate around the world. The main signature of ENSO is a periodic (~every 3-8 years) reduction in winds moving westward across the Pacific ocean. This allows warm water to move eastward across the tropical Pacific Ocean. ENSO has strong impacts on ocean nutrient cycling and associated fish populations and birds. ENSO has repercussions far beyond the Pacific ocean, including periodic wide-scale drought in many regions of the world. Scientists have found a correlation between periodic phenomena like ENSO and years with anomalous global increases in CO₂ which appear to be linked to CO₂ releases from plants, soil, and fire⁷.While there remains debate about this, some have predicted that ENSO may become more frequent and sustained under global warming which could cause a positive feedback favoring respiration over photosynthesis on a global scale⁸. 

The source/sink differences could also manifest themselves differently across geography and time periods. “In regions where drought stress is not important because of high levels of precipitation, or if increases in CO₂ concentration increase water use efficiency and thus reduce water stress, longer growing seasons could result in increased growth. Where drought stress is important, a longer growing season may mean only that plant respiration exceeds photosynthesis for a longer time, which would result in reduced growth^”9. So, it is conceivable that moist forests west of the Cascades might remain net carbon sinks, while the dryer forests east of the Cascades might become net sources. 

Another study looked at the effects of CO₂ enrichment and climate change on vegetation in the mid- and high-latitudes of the northern hemisphere and found opposing effects in spring and summer. CO₂ uptake was apparently enhanced during warm wet spring season, but looking over the entire growing season, including the dryer summer, CO₂ uptake did not increase¹⁰. Another paper estimated that western forests might increase in spatial extent while decreasing in their carbon density, i.e., more forested acres, but fewer trees per acre¹¹.

The bottom line is that if we carefully conserve our forests, they can play a substantial role in mitigating our current carbon predicament. Even if forests shift from becoming a carbon sink to a carbon source, continued forest conservation will help mitigate the consequences. To manage forests for resilience, they must be allowed time to grow and accumulate carbon while natural disturbance processes are allowed to self-regulate, thus ensuring that live vegetation is maintained below the water-limited carrying capacity and fuels will be maintained below the threshold for uncharacteristic fire.