Quantifying Carbon Sequestration by Terrestrial Vegetation

Photosynthetic carbon reduction is the basis of productivity, providing photosynthate for biomass accumulation from the level of an

┬ęCAB International 2007. Greenhouse Gas Sinks (eds D.S. Reay, C.N. Hewitt, K.A. Smith and J. Grace)

individual leaf up to an ecosystem and, further, the globe. However, because productivity is variously defined, a short summary of the derivations of the different definitions is provided as a foundation to this chapter (Fig. 2.1).

Rates of gross photosynthesis summed for individual plants, entire canopies, regions or the globe constitute gross primary productivity (GPP). However, direct measurements of gross photosynthesis are seldom made. The problem is that leaf dark respiration (flleaf) occurs simultaneously with photosynthesis. Consequently net rates of leaf photosynthesis (^net) are typically measured. Therefore,

These limitations aside, GPP provides a good starting point from which to develop a structural framework for assessment of productivity of terrestrial vegetation. When moving up in scale from leaves to an individual plant to a forest stand, productivity estimates must account for losses of CO2 from autotrophic respiration by all respiring components of the plant (_Ra), including the shoots CRsh00t) and

(a) Quercus ilex forest

Leaves 150

Roots 316

Leaves 150

Roots 316

TER = 1450

GEP = 2000

TER = 1450

(b) Mountain grassland

(b) Mountain grassland

Aboveground = 157

Roots = 310

Fig. 2.1. Representative measurements of annual ecosystem productivity are shown for two Italian ecosystems: (a) an evergreen Quercus ilex forest and (b) a mountain grassland. The two ecosystems display similar GEP/NEP ratios of 0.27 (forest) and 0.22 (grassland), respectively, evidence of similar respiratory costs per unit of carbon fixed. However, productivity in the forest ecosystem is double that of the grassland and from the perspective of long-term carbon sequestration in biomass, the accumulation of carbon in woody biomass is much more significant. (From unpublished data provided by Sabina Dore.)

Aboveground = 157

Roots = 310

Fig. 2.1. Representative measurements of annual ecosystem productivity are shown for two Italian ecosystems: (a) an evergreen Quercus ilex forest and (b) a mountain grassland. The two ecosystems display similar GEP/NEP ratios of 0.27 (forest) and 0.22 (grassland), respectively, evidence of similar respiratory costs per unit of carbon fixed. However, productivity in the forest ecosystem is double that of the grassland and from the perspective of long-term carbon sequestration in biomass, the accumulation of carbon in woody biomass is much more significant. (From unpublished data provided by Sabina Dore.)

^net + Rleaf roots (Rroot) in addition to Rleaf. The result is an estimate of net primary productivity (NPP):

Unlike GPP, NPP can in theory be directly measured. In practice, however, it is a time-consuming labour-intensive process, in which accumulation and turnover of biomass, not just in leaves, shoots and roots but in all carbon pools such as exudates and volatile organic compounds, should in theory be assessed over a given period of time. In reality they seldom are, and the accuracy of NPP assessments have long been questioned (Long et al., 1989; Geider etal., 2001). Of course, vegetation forms but one component of ecosystems and consequently assessments of whole ecosystem productivity need to account for further respiratory losses of CO2 from heterotrophic organisms within the soil (Rh), as well as those from vegetation. The result is an estimate of net ecosystem production (NEP):

Recent application of the eddy covariance technique to terrestrial ecosystems has provided a method to continually measure instantaneous net CO2 exchange between the biosphere and the atmosphere (NEE) and, when these measurements are integrated over time, NEP (recent review Baldocchi, 2003). Arguably the principle methodological uncertainty surrounding estimates of NEP made using eddy covariance centre on night time measurements, and the fact that the atmospheric conditions required for the technique to work are often not met (Baldocchi, 2003). However, eddy covariance measurements of just CO2 will not measure non-CO2 losses of carbon from an ecosystem such as other carbon trace gases (Crutzen et al., 1999; Walter et al., 2001) or dissolved organic or inorganic carbon (Ludwig et al., 1998), which in situations where they are substantially present would result in overestimation of NEP. The addition of total ecosystem respira tion (TER = Ra + Rh) during the photoperiod to NEP measured during the photoperiod only provides an estimate of gross ecosystem productivity (GEP). Although GPP and GEP are often used interchangeably, they do differ to a small extent, with GEP containing a photorespiratory component that GPP does not (Waring and Running, 1998).

Over longer periods of time (decades and longer) ecosystem productivity is often subject to large-scale disturbance that results in large carbon losses (Rd). Disturbance from fire, pest outbreaks or logging are examples. The subtraction of these terms from NEP results in an estimate of net biome productivity (NBP) (Schulze et al., 2000):

Changes in productivity provide the basis for changes in the size of the terrestrial carbon sink. However, as detailed above, productivity is defined in many ways and because there is no guarantee that photosynthesis and respiration are tightly coupled over a range of timescales from days to decades, these definitions can be contradictory. For example, GPP could in theory increase at the same time as NPP decreases, a consequence of a more than proportionate increase in Ra over GPP, as may happen during a period of active growth. Alternatively increases in GPP and NPP can mask decreases in NEP, as a result of large increases in respiratory losses from heterotrophic soil organisms, and may occur after a short-term rain event or a longer-term disturbance event. Determining the extent to which GPP/NPP or GEP/NEP ratios are conserved across ecosystems and timescales remains a major research priority. These complications need to be borne in mind as we make both general and specific references to productivity throughout this chapter.

Ultimately, partitioning of photosyn-thate into plant biomass components, all with different turnover rates and resistance to decomposition, will have as significant an impact on carbon sequestration as rates of productivity. While some plant compounds such as carbohydrates and proteins are important energy sources for soil organisms and respire quickly, others such as lipids, or lignin and cellulose are considered more resistant to breakdown (Gleixner, 2005). From the perspective of carbon sequestration in terrestrial vegetation, the accumulation of woody biomass and production of long-lived wood products are the keys to increasing carbon sequestration and are the focus of this chapter.

Was this article helpful?

0 0

Post a comment