The contribution of CO2 fertilization to the future global Carbon cycle has been uncertain, especially in forest ecosystems. This uncertainty may significantly affect climate change predictions. Researchers from the USA, under the leadership of Richard Norby from the Oak Ridge National Laboratory, examined the response of net primary productivity (NPP) to elevated Co2 in four free-air CO2 enrichment experiments in forest stands. Can their results change our trust towards climate modeling exercises? Read more, what they have found!

Scientists started with a hypothesis that net primary productivity (NPP) should increase as fossil-fuel combustion adds to the atmospheric (CO2). In other words, they assumed that increased carbon uptake into the biosphere in response in to rising CO2 (“CO2 fertilization”) could create a negative feedback that slows the rate of increase in atmospheric carbon dioxide. Such discovery could strongly affect predictions of future atmospheric carbon dioxide.

Researchers quoted studies and experiments, which unequivocally shown that plants can grow faster and larger in CO2-enriched atmosphere and the mechanisms of response are well understood. Nevertheless, they paid attention that relevance of such studies in prediction of global carbon budgets over decadal time frames, may be limited. They wrote:

“The problem is especially severe with forests because of the inherent difficulty in conducting relevant experiments at appropriate spatial and temporal scales with large and long-lived organisms and complex ecosystems.”

Researchers examined data from the four the Free-Air CO2 Enrichment (FACE) experiments operating in forest stands. The FACE experiments represent a broad range of productivity, climatic and soil conditions, stand developmental history, and life history characteristics of the dominant species, although all four selected spots are young stands in the temperate zone.

Two experiments were initiated in established monoculture plantations that had fully occupied the sites: an evergreen loblolly pine (Pinus taeda) stand at Duke-FACE and a deciduous sweetgum (Liquidambar styraciflua) at ORNL-FACE. The other two experiments, Apen-FACE and POP-EUROFACE, comprised simple multispecies assemblages dominated by Populus spp. that were initiated on previously bare ground.

Researchers found that the relationship between NPP in elevated CO2 was remarkably consistent across a broad range of productivity. The analysis indicated a 23% increase in forest NPP as atmospheric CO2 increases to 550 ppm over the next few decades.

In addition, it was pointed that although many important forest types were missing in this analysis (e.g. boreal and tropical forests, in which FACE experiments have not been conducted), the range in NPP comprised much of the range observed among forests globally.

Source: Norby R.J., E.H. DeLucia, B.Gielen et al. 2005.  Forest response to elevated carbon dioxide is conserved across a broad range of productivity. Proceedings of the National Academy of Sciences of the United States of America, vol. 102, no.50.