It decreases at a rate of 0.54–0.65 °C per 100 m of ascent, but significant variation is
introduced by meteorology, local topography and height above the ground (Barry and
Chorley 1987; Körner 2003; Rolland 2003). At cold temperatures, physical and
chemical reactions slow down, as do the assimilation of energy and metabolic activity
(Schmidt-Nielsen 1997). This process has the most remarkable consequences for
plants and ectotherms, and either leads to reduced activity or triggers costly homeostatic responses to offset the passive reaction to reduced temperature and freezing
(Sakai and Larker 1987; Gillooly et al. 2001). Endotherms may be affected in a similar
way when they are outside their thermoneutral zone (Angilletta et al. 2010), and lower
critical temperatures are reached more frequently at high elevations. Dormancy (including diapause and hibernation) is a mechanism to escape cold weather or resource
shortage over the winter and corresponds to a period when growth, development and
physical activity are temporarily arrested. Through its effect on metabolism, temperature limits the rates of production throughout ontogeny and reproduction, thus
directly influencing phenology, growth and reproductive patterns, and life-history
correlates such as body size (Atkinson 1996; Atkinson and Sibly 1996; Angilletta
et al. 2004). Temperature is also tightly associated with seasonality, and in turn, with
productivity, and thus also controls the above processes indirectly via these variables.
11.2.2 Atmospheric Pressure
Atmospheric pressure, the moisture content of air and the partial pressures of
biologically relevant gases, such as oxygen and carbon dioxide, decrease relatively
uniformly with increasing elevation, and impact gas exchanges in plants and respiration in animals. Pressure effects on photosynthesis are, however, smaller than
predicted from the decline in the ambient pCO 2 alone because the increased rate of
molecular diffusivity, induced by thinner air, is counteracted by the descent of
atmospheric temperature, decelerating diffusion. Moreover, up to 80% of the total
CO 2 transfer resistance between air and the chloroplasts is in the liquid phase,
which is not influenced by pressure (Körner 2007). An improvement in the rate of
oxygen intake with elevation has been observed in animals through ventilation or
changes in blood composition (Rourke 2000), as well as increased porosity of bird
eggshells to facilitate O 2 diffusion to the embryo (Körner 2007).
11.2.3 Precipitation
Unlike temperature and atmospheric pressure, precipitation exhibits non-linear
relationships with elevation, and regional, rather than global, patterns. Precipitation
tends to increase with altitude at low elevations, but exhibits no pattern at very high
elevations, or declines above the cloud zone in tropical areas (Nagy and Grabherr
2009). Orographic precipitation in the form of snow typically characterises high
11 Life-History Responses to the Altitudinal Gradient
255
introduced by meteorology, local topography and height above the ground (Barry and
Chorley 1987; Körner 2003; Rolland 2003). At cold temperatures, physical and
chemical reactions slow down, as do the assimilation of energy and metabolic activity
(Schmidt-Nielsen 1997). This process has the most remarkable consequences for
plants and ectotherms, and either leads to reduced activity or triggers costly homeostatic responses to offset the passive reaction to reduced temperature and freezing
(Sakai and Larker 1987; Gillooly et al. 2001). Endotherms may be affected in a similar
way when they are outside their thermoneutral zone (Angilletta et al. 2010), and lower
critical temperatures are reached more frequently at high elevations. Dormancy (including diapause and hibernation) is a mechanism to escape cold weather or resource
shortage over the winter and corresponds to a period when growth, development and
physical activity are temporarily arrested. Through its effect on metabolism, temperature limits the rates of production throughout ontogeny and reproduction, thus
directly influencing phenology, growth and reproductive patterns, and life-history
correlates such as body size (Atkinson 1996; Atkinson and Sibly 1996; Angilletta
et al. 2004). Temperature is also tightly associated with seasonality, and in turn, with
productivity, and thus also controls the above processes indirectly via these variables.
11.2.2 Atmospheric Pressure
Atmospheric pressure, the moisture content of air and the partial pressures of
biologically relevant gases, such as oxygen and carbon dioxide, decrease relatively
uniformly with increasing elevation, and impact gas exchanges in plants and respiration in animals. Pressure effects on photosynthesis are, however, smaller than
predicted from the decline in the ambient pCO 2 alone because the increased rate of
molecular diffusivity, induced by thinner air, is counteracted by the descent of
atmospheric temperature, decelerating diffusion. Moreover, up to 80% of the total
CO 2 transfer resistance between air and the chloroplasts is in the liquid phase,
which is not influenced by pressure (Körner 2007). An improvement in the rate of
oxygen intake with elevation has been observed in animals through ventilation or
changes in blood composition (Rourke 2000), as well as increased porosity of bird
eggshells to facilitate O 2 diffusion to the embryo (Körner 2007).
11.2.3 Precipitation
Unlike temperature and atmospheric pressure, precipitation exhibits non-linear
relationships with elevation, and regional, rather than global, patterns. Precipitation
tends to increase with altitude at low elevations, but exhibits no pattern at very high
elevations, or declines above the cloud zone in tropical areas (Nagy and Grabherr
2009). Orographic precipitation in the form of snow typically characterises high
11 Life-History Responses to the Altitudinal Gradient
255