6.9. Körner

Keynote Presentations from the 3rd ALTER-Net Summer School, Peyresq 2 - 14 September 2008

ch.koerner -at- unibas.ch

Institute of Botany, University of Basel, Schönbeinstrasse 6, 4056 Basel, Switzerland

Title of the talk: Forest ecosystems in a CO₂-rich world (pdf: 10MB)

Summary of the talk by Luca Marazzi: Students´ summary (pdf)

Abstract

Forest ecosystems in a CO₂-rich world

Forests represent nearly 85 % of the global biomass carbon-pool. It is a central question in global change research whether plants, forests in particular, will accumulate more carbon when exposed to elevated CO₂ concentrations. I will discuss the likelihood of this possibility. Carbon is a rate limiting resource for plant growth to the extent other, non-carbon resources and environmental determinants permit. Some of these resources are often used more efficiently by plants when CO₂ availability is increased. However, if, for instance, less water or soil nutrients are consumed per unit of carbon incorporated in biomass, this can exert ecosystem feedback that may negatively affect biomass production. Nutrient depleted tissue is recycling slower, and reduced transpiration creates a drier and warmer atmosphere that stimulates evapotranspiration. The issue boils down to the question, whether forest growth is source (photosynthesis) or sink (meristems) limited. The classical view is that source activity drives sink activity. I will advocate that, on a global scale, it is the other way round, namely that source activity is driven by sink activity (except for growth in deep shade, when light is limiting). Tissue formation is far more sensitive to environmental stresses and the availability of resources other than carbon, than is photosynthesis. This is greatly diminishing the likelihood of a carbon fertilization effect on wild plants, including forests. The second major issue is whether a stimulation of tree growth ― should it occur in response to elevated CO₂ ― translates into greater landscape stores of carbon. One of the greatest confusions in the relevant literature is that tree growth is treated as "C-sequestration", although these are two rather different issues. Any stimulation of tree growth that affects all age classes of trees similarly has no effect on landscape-wide carbon stores, it rather accelerates the forest carbon cycle. For tree growth to affect the landscape C-capital (sustained C-sequestration), the age distribution (demography) of forests must be altered, for which we have no experimental evidence. While it is obvious that the expansion of forested land area ties up carbon in biomass, the only two ways to store more carbon in already existing forests is to change their age structure or to accrete carbon in soil humus. The latter removes mineral nutrients from the nutrient cycle and is not sustainable. A sustained change in tree age distribution can be achieved by forest management, but is unlikely to result from elevated atmospheric CO₂.

Recommended background literature on this presentation:

• Körner Ch (2003) Carbon limitation in trees. J Ecol 91:4-17. doi:10.1046/j.1365-2745.2003.00742.x
• Körner Ch, Asshoff R, Bignucolo O, Hättenschwiler S, Keel SG, Peláez-Riedl S, Pepin S, Siegwolf RTW, Zotz G (2005) Carbon flux and growth in mature deciduous forest trees exposed to elevated CO₂. Science 309:1360-1362, doi:10.1126/science.1113977.
• Körner Ch (2006) Plant CO₂ responses: an issue of definition, time and resource supply. New Phytol 172:393-411, doi:10.1111/j.1469-8137.2006.01886.x