steady increase throughout the day. In the morning, under conditions of low vapor
pressure deficit and high internal water content, the stomata open allowing for
photosynthesis to occur with minimal water loss. Under conditions of high net
radiation, the increase in the vapor pressure deficit leads to an increase in stomatal
resistance until midafternoon.
In temperate softwood forests, Gash and Stewart (1975) reported a pattern of
variation along the day, with increasing canopy resistance between 100 and
300 s/m. Lee and Black (1993b) as well as Stewart and de Bruin (1985), for
softwood stands indicate that canopy resistance increases throughout the afternoon,
due to an increase in vapor pressure deficit and/or a decrease in net radiation. In
coniferous stands, Oke (1992) also referred to daily increases of canopy resistance from 100 to 350 s/m, due to an increase in atmospheric vapor pressure deficit.
Factors such as vapor pressure deficit, net radiation, and air moisture influence
canopy resistance variations mainly in the short term (Baldocchi et al. 1997;
Kelliher et al. 1995). According to these authors, the short-term factors interact with
long-term factors intrinsic to biogeochemical processes and collectively influence
stomatal activity, photosynthesis, and evapotranspiration processes. In well-drained
soils, for example, conditions of modest rainfall and short growing season lead to
low rates of decomposition and mineralization of organic matter, low nutrient
availability, low growth rates, nutrition, and leaf development. This provides a
setting for low leaf area, and large stomatal resistance, thereby limiting photosynthesis and transpiration.
4.6 Carbon Sequestration in Forests
Emissions of carbon dioxide and other greenhouse gases, such as methane and
nitrous oxide, have increased since the nineteenth century. Indeed, atmospheric
CO 2 content has risen from 280 ppm in the early phase of the Industrial Revolution
to 370 ppm at present. According to the Intergovernmental Panel on Climate
Change (IPCC) in the absence of precautionary measures, including afforestation,
there will be an increase in atmospheric CO 2 concentration to 500 ppm by 2050,
which will increase further throughout the remainder of the twenty-first century
(e.g., Rodrigues and Oliveira 2006). The Kyoto Protocol defined a target of 5% for
reduction of CO 2 emissions by industrialized countries during the period 2008–
2012. A reasonable goal using measures proposed in the Kyoto Protocol would be
the stabilization of atmospheric CO 2 concentrations at 550 ppm throughout the
twenty-first century. This would correspond to an overall air temperature increase
of 2–3 °C above the current level.
Forests contribute greatly toward carbon sequestration or sinking. Strategies for
mitigating global warming should have long-term goals, bearing in mind that the
residence time of GHG in the atmosphere is 50 to 100 years. The theoretical
contribution of the long-term Portuguese forestry sector in reducing GHGs in terms
of carbon sequestration under the National Plan on Climate Change should be
124
4 Exchange of Energy and Mass Over Forest Canopies
pressure deficit and high internal water content, the stomata open allowing for
photosynthesis to occur with minimal water loss. Under conditions of high net
radiation, the increase in the vapor pressure deficit leads to an increase in stomatal
resistance until midafternoon.
In temperate softwood forests, Gash and Stewart (1975) reported a pattern of
variation along the day, with increasing canopy resistance between 100 and
300 s/m. Lee and Black (1993b) as well as Stewart and de Bruin (1985), for
softwood stands indicate that canopy resistance increases throughout the afternoon,
due to an increase in vapor pressure deficit and/or a decrease in net radiation. In
coniferous stands, Oke (1992) also referred to daily increases of canopy resistance from 100 to 350 s/m, due to an increase in atmospheric vapor pressure deficit.
Factors such as vapor pressure deficit, net radiation, and air moisture influence
canopy resistance variations mainly in the short term (Baldocchi et al. 1997;
Kelliher et al. 1995). According to these authors, the short-term factors interact with
long-term factors intrinsic to biogeochemical processes and collectively influence
stomatal activity, photosynthesis, and evapotranspiration processes. In well-drained
soils, for example, conditions of modest rainfall and short growing season lead to
low rates of decomposition and mineralization of organic matter, low nutrient
availability, low growth rates, nutrition, and leaf development. This provides a
setting for low leaf area, and large stomatal resistance, thereby limiting photosynthesis and transpiration.
4.6 Carbon Sequestration in Forests
Emissions of carbon dioxide and other greenhouse gases, such as methane and
nitrous oxide, have increased since the nineteenth century. Indeed, atmospheric
CO 2 content has risen from 280 ppm in the early phase of the Industrial Revolution
to 370 ppm at present. According to the Intergovernmental Panel on Climate
Change (IPCC) in the absence of precautionary measures, including afforestation,
there will be an increase in atmospheric CO 2 concentration to 500 ppm by 2050,
which will increase further throughout the remainder of the twenty-first century
(e.g., Rodrigues and Oliveira 2006). The Kyoto Protocol defined a target of 5% for
reduction of CO 2 emissions by industrialized countries during the period 2008–
2012. A reasonable goal using measures proposed in the Kyoto Protocol would be
the stabilization of atmospheric CO 2 concentrations at 550 ppm throughout the
twenty-first century. This would correspond to an overall air temperature increase
of 2–3 °C above the current level.
Forests contribute greatly toward carbon sequestration or sinking. Strategies for
mitigating global warming should have long-term goals, bearing in mind that the
residence time of GHG in the atmosphere is 50 to 100 years. The theoretical
contribution of the long-term Portuguese forestry sector in reducing GHGs in terms
of carbon sequestration under the National Plan on Climate Change should be
124
4 Exchange of Energy and Mass Over Forest Canopies