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Improved understanding of terrestrial ecosystem dynamics through the development and application of a mechanistic model of plant growth.

Improved understanding of terrestrial ecosystem dynamics through the development and application of a mechanistic model of plant growth.

Supervisors: Andrew Friend, Ulf Büntgen

Future terrestrial biosphere carbon pool responses to climate and atmospheric CO2 change are modelled using Dynamic Global Vegetation Models (DGVMs). These models generate quantitative estimates on the feedbacks between global vegetation and the atmosphere. This makes an adequate representation of natural processes within these models crucial

DGVMs are highly simplified representations of the real terrestrial biosphere, and model plant physiological processes using numerous assumptions and mathematical formulas. These DGVMs are currently almost exclusively photosynthesis-driven and are therefore very carbon supply-centric. However, it is argued that plant growth as modelled in DGVMs is very different to our understanding drawn from experimental evidence (e.g. Fatichi et al. 2014). Photosynthesis, the carbon source for a tree, may not be the most limiting factor when it comes to carbon sequestration. Rather, there is good evidence for growth (or tree internal carbon sink-) processes to be affected more strongly by the environment than photosynthesis (e.g. temperature and water availability (Körner 2003) (Muller et al. 2011)). Growth may hence act as the major control on carbon sequestration in trees (and hence forest ecosystems) and so needs to be correctly incorporated into DGVMs.

Wood formation (cell division, cell expansion, and cell wall thickening) is a major component of tree growth and the predominant process of carbon storage in trees and hence an important process in the carbon cycle dynamics. Dr. Andrew Friend has developed a model (unpublished) that replicates these wood formation processes described above. Most of these processes are modelled to be controlled by both external environmental influences (water availability, temperature) and intrinsic influences (latent tree carbohydrate reserve and hormones). The current version of the model needs to be investigated, tested and possibly improved drawing from physiological and molecular evidence before it can be used for any further hypothesis testing.

The overall aim is to later use this mechanistic radial tree ring framework to apply a balanced source-sink approach to Dynamic Global Vegetation Models. Additionally, it can be used as a tool to help interpret inter-annual variability in tree ring widths. Also, integrated into global models, it has the potential to help improve projections on global vegetation responses to climate change.

References:

  • Fatichi, S., Leuzinger, S., & Körner, C. (2014). Moving beyond photosynthesis: From carbon source to sink-driven vegetation modeling. New Phytologist, 201(4), 1086–1095. doi:10.1111/nph.12614
  • Muller, B., Pantin, F., Génard, M., Turc, O., Freixes, S., Piques, M., & Gibon, Y. (2011). Water deficits uncouple growth from photosynthesis, increase C content, and modify the relationships between C and growth in sink organs. Journal of Experimental Botany, 62(6), 1715–1729. doi:10.1093/jxb/erq438
  • Körner, C. (2003). Carbon limitation in trees. Journal of Ecology, 91(1), 4–17. doi:10.1046/j.1365-2745.2003.00742.x