Modelling the components of plant respiration: Representation and realism

This paper outlines the different ways in which plant respiration is modelled, with reference to the principles set our in Cannell and Thornley (Annals of Botany 85: 55-67, 2000), first in whole-plant 'toy' models, then within ecosystem or crop models using the growth-maintenance paradigm, and finally representing many component processes within the Hurley Pasture (HPM) and Edinburgh Forest Models (EFM), both of which separate C and N substrates from structure. Whole-plant models can be formulated so that either maintenance or growth respiration take priority for assimilates, or so that growth respiration is the difference between total respiration and maintenance associated with the resynthesis of degraded tissues. All three schemes can be converted to dynamic models which give similar, reasonable predictions of plant growth and respiration, but all have limiting assumptions and scope. Ecosystem and crop models which use the growth-maintenance respiration paradigm without separating substrates from structure, implicitly assume that maintenance respiration is a fixed cost, uncoupled to assimilate supply, and use fixed rate coefficients chosen from a range of measured values. Separation of substrates in the HPM and EFM enables estimates to be made of respiration associated with local growth, phloem loading, ammonium and nitrate N uptake, nitrate reduction, N-2 fixation and other mineral ion uptake, leaving a 'residual maintenance' term. The latter can be explicitly related to C substrate supply. Simulated changes in grassland respiration over a season and forest respiration over a rotation show that the ratio of total respiration to gross canopy photosynthesis varies within the expected limited range, that residual maintenance accounts for 46-48 % of total respiration, growth 36-42 %, phloem loading 10-12 % and the other components for the small remainder, with the ratios between components varying during a season or forest rotation. It is concluded that the growth-maintenance approach to respiration, extended to represent many of the component processes, has considerable merit. It can be connected to reality at many points, it gives more information, it can be examined at the level of assumptions as well as at the level of predictions, and it is open to modification as more knowledge emerges. However, currently, there are still parameters that require adjustment so that the predictions of the model are acceptable. (C) 2000 Annals of Botany Company.