Aboveground respiration

It is convenient to seperate respiratory fluxes by the group of organisms respiring. We can thus separate ecosystem respiration into two components: autotrophic and heterotrophic respiration. Again, we can separate these processes further into their aboveground and belowground components for ease of investigation. We consider aboveground processes here.

Heterotrophic aboveground respiration

Animals, fungi, and heterotrophic microorganisms all exist both above- and belowground, and especially the latter two play important roles as decomposers. Whereas animals may contribute a non-trivial proportion of respiratory fluxes in an ecosystem, they are often ignored by modelers because they are too mobile in time and space. However, larger herbivore grazing can dominate plant community composition and plant canopy structure, which in turn plays a large role in all aspects of ecosystem respiration (Wilsey et al. 2002; Bremer et al. 1998; LeCain et al. 2000). A major component of aboveground respiration that is often overlooked is the harvesting of grasses for consumption and respiration of grazers elsewhere. All of this harvested and consumed carbon is returned to the atmosphere quickly. Ungulates and large herbivores also contribute significant methane fluxes to the atmosphere, which have important atmospheric chemistry and global change consequences, but are not considered here. Decomposers of standing biomass or aboveground plant material help release nutrients stored in wood and other plant materials and are central components of every ecosystem.

Autotrophic aboveground respiration

Plant respiration can be separated into two components: photorespiration, and dark (mitochondrial) respiration. Photorespiration may not be considered by ecosystem modelers (see Stand-level models), but it is worthwhile to understand the process. Photorespiration is the non-productive oxygenase reaction between oxygen (O₂) and the enzyme ribulose bisphosphate carboxylase oxygenase (Rubisco), and is more common in plants that use the C₃ photosynthetic pathway. The products of photorespiration are 3-phosphoglyceric acid which enters the Calvin cycle, and glycolate which is quickly broken down to release CO₂. Photorespiration represents a carbon loss to the plant, and many plants have developed the alternate C₄ and CAM photosynthetic pathways to minimize photorespiration. Details of C₄ and CAM photosynthesis are not discussed here.

Dark respiration is a misnomer; whereas it is best measured in the dark in the absence of photorespiration, it occurs all the time in living plants. Dark respiration is the functional equivalent of mitochondrial respiration in other organisms; it occurs in plant mitochondria itself. It is the three-phase process (glycolysis, Krebs cycle, and oxidative phosphorilzation/electron transport discussed before) that creates energy by oxidizing sugars. Dark respiration is often conceptually separated into two components: growth respiration, the biosynthesis of structural compounds, and maintentance respiration, the energy required by the normal activities of living cells. It is worthwhile to understand the meaning of growth and maintenance respiration, and the assumptions made by considering growth and metabolism separately.

The growth and maintenance respiration concepts stem from McCree's (1970) empirical relationship where R_P is plant respiration, P is gross photosynthesis (growth), W is living dry mass that needs to be maintained, k is dimensionless and c is a rate (time^-1) This simple relationship spawned intense research in basic plant biology and plant/ecosystem modeling, but may overly simplify respiration as 'growth' and 'maintenance' share biochemical reactions (e.g. lignin biosynthesis, Amthor 2000).

Understanding respiration is understanding plant function, and Amthor (2000) identifies the following processes most in need of energy from mitochondrial respiration:

1) biosynthesis of new structural biomass

2) translocation of photosynthate

3) uptake of ions from soil

4) Assimilation of N and S into organic compounds

5) Protein turnover

6) Cellular ion-gradient maintenance

Thus, the need for energy from dark respiration may be driven by a number of biotic and abiotic factors that are related to the status, life history, and environment of the plant. Therefore, we can expect a diversity of plant respiratory responses when the plants' abiotic environment changes, and hence, the impacts of global changes on plant respiration cannot be fully predicted (Amthor, 1991) across all species and ecosystems. General models are more attainable.

Taking a step back to McCree's (1970) conceptual model, elevated CO2-mediated increases in photosynthesis and possibly plant size increase the right-hand side of the growth/maintenance equation , which can be balanced by an increase in respiration on the left-hand side. This relationship may be oversimplify plant function and respiration dynamics, but does generate the testable hypothesis that a potential increase in growth due to elevated CO2 will be offset by increases in respiration, a hypothesis that is usually substantiated (Wang and Curtis, 2002). Whereas conceptual models are useful, rigorously incorporating the full range of variability in plant respiration responses to global changes should lead to more biological insight as well as more accurate ecological forecasts (Clark et al 2001).