512
C.B. Field et al.
lead to improved understanding of the ecological and physiological regulation
of CO2 exchange at the level of the leaf and the individual plant.
Fourth, increasing evidence that human activity is changing the earth
places a high priority on an extensive approach to ecophysiology. Photosynthesis determinations of global extent are critical both for assessing a key
quantity in the global carbon budget and for regional monitoring for change
detection. The role of CO2 as a climate-forcing factor (Lashof and Ahuja
1990) and the current uncertainty concerning the direction and quantity of
carbon flux from the terrestrial biosphere (Tans et al. 1990; Enting and
Mansbridge 1991; Quay et al. 1992) combine to make terrestrial photosynthesis and the sensitivity of terrestrial photosynthesis to future change
central issues in the global change agenda.
24.1.1 A Range of Approaches
Remote sensing products potentially useful for assessing photosynthesis fall
into three categories. One class of remote sensing products is related to
photosynthetic capacity or the potential for CO2 uptake. Examples of these
products include leaf area index (LAI) , total canopy nitrogen, and the
fraction of incoming solar radiation absorbed. A remote sensing-based assignment of ecosystem type also falls into this class. A second class of
products is related to instantaneous CO2 exchange. Methods that probe
fluorescence, the status of the xanthophyll cycle pigments, or that use
canopy temperature in a calculation of stomatal conductance are examples
of this class. A third class of products provides environmental parameters
needed in photosynthesis models. Remote sensing products can lead to estimates of surface temperature, incoming solar radiation, and soil moisture.
Products from these three classes can serve as inputs to different types of
photosynthesis models operating at a range of temporal scales. Indices
related to photosynthetic capacity need to track phenomena for which the
dominant dynamics tend to be seasonal. In contrast, instantaneous CO2
exchange has important dynamics on time scales from seasonal to brief
shading by passing clouds. The temporal resolution required for environmental parameters in photosynthesis models ranges from minutes for biochemically based models to a month or more for ecosystem production
models.
24.2 Models: from Radiance to CO2 Exchange
Every remote sensing-based estimate of photosynthesis involves a model
calculation - remote sensing, as we use the term here, measures radiances,
which are never sufficient in themselves for determining CO2 flux. Models
C.B. Field et al.
lead to improved understanding of the ecological and physiological regulation
of CO2 exchange at the level of the leaf and the individual plant.
Fourth, increasing evidence that human activity is changing the earth
places a high priority on an extensive approach to ecophysiology. Photosynthesis determinations of global extent are critical both for assessing a key
quantity in the global carbon budget and for regional monitoring for change
detection. The role of CO2 as a climate-forcing factor (Lashof and Ahuja
1990) and the current uncertainty concerning the direction and quantity of
carbon flux from the terrestrial biosphere (Tans et al. 1990; Enting and
Mansbridge 1991; Quay et al. 1992) combine to make terrestrial photosynthesis and the sensitivity of terrestrial photosynthesis to future change
central issues in the global change agenda.
24.1.1 A Range of Approaches
Remote sensing products potentially useful for assessing photosynthesis fall
into three categories. One class of remote sensing products is related to
photosynthetic capacity or the potential for CO2 uptake. Examples of these
products include leaf area index (LAI) , total canopy nitrogen, and the
fraction of incoming solar radiation absorbed. A remote sensing-based assignment of ecosystem type also falls into this class. A second class of
products is related to instantaneous CO2 exchange. Methods that probe
fluorescence, the status of the xanthophyll cycle pigments, or that use
canopy temperature in a calculation of stomatal conductance are examples
of this class. A third class of products provides environmental parameters
needed in photosynthesis models. Remote sensing products can lead to estimates of surface temperature, incoming solar radiation, and soil moisture.
Products from these three classes can serve as inputs to different types of
photosynthesis models operating at a range of temporal scales. Indices
related to photosynthetic capacity need to track phenomena for which the
dominant dynamics tend to be seasonal. In contrast, instantaneous CO2
exchange has important dynamics on time scales from seasonal to brief
shading by passing clouds. The temporal resolution required for environmental parameters in photosynthesis models ranges from minutes for biochemically based models to a month or more for ecosystem production
models.
24.2 Models: from Radiance to CO2 Exchange
Every remote sensing-based estimate of photosynthesis involves a model
calculation - remote sensing, as we use the term here, measures radiances,
which are never sufficient in themselves for determining CO2 flux. Models