312
D .L. Spittlehouse
demand. However, stomata tend to close as the vapor pressure deficit
increases , thereby offsetting effects of temperature increases. Increasing
the atmospheric carbon dioxide concentration could result in a reduction
in stomatal conductance and increases in leaf area (Eamus & Jarvis ,
1989). Rosenberg, McKenney , and Martin (1989) and de Bruin and
Jacobs (1993) show that under a realistic range of conditions, transpiration
rates will change by less than ±10%. Thus, it is probable that changes in
rainfall will be the dominant factor in changing soil water status .
Droughts during July and August can also affect reforestation success.
Spittlehouse and Childs (1990) found a summer drying pattern similar to
that shown in Figure 15.1. Dry summers following dry springs can result
in severe moisture stress even in established trees. This situation occurs
10% of the time at present, 16% with a 20% decrease in rainfall, and 6%
a 20% increase. An increase in air temperature could result in heat stress
since maximum temperatures at seedling height are already in the high
30s (0e) during the summer. The effect of changes in winter air temperature on chilling and budburst is important for growth in subsequent years
and is considered later.
This analysis is species and site specific. However, the simulations
could also be used to assess effects on natural regeneration of Douglas-fir,
other plant species, and of soil texture. Fire risk could also be considered.
Precipitation patterns and soils vary across the area where Douglas-fir
occurs, but conditions described here are not atypical. Analyses could
readily be done for other sites.
Management Implications and Response
Figure 15.1 and Table 15.1 indicate that these stony , sandy sites are
extremely sensitive to variations in water availability. Any climate change
that results in decreased rainfall in late spring and early summer is a
serious threat to the establishment of Douglas-fir. This may be made
worse if a warmer climate causes snow melt to occur earlier than at
present. Table 15.1 indicates a relatively high chance of failure under
present conditions. Consequently, strategies to deal with future changes
are of use now.
A number of adaptation and mitigation options are available. Planting
time could be change, though any move to earlier planting to avoid the
droughts or to take advantage of warmer conditions needs to consider the
potential for increased frost risk (McCreary, Lavender, & Hermann ,
1990). A similar analysis would show whether there is less risk to fall
planting. Second, the large size of the marginal class indicate s that it is
necessary to ensure good control of any vegetation that might compete
for soil moisture. A third option is to consider alternates to clearcut
harvesting, such as shelterwoods or partial cuts, to reduce the evaporative
demand on the seedling and reduce soil surface temperature (Spittlehouse
D .L. Spittlehouse
demand. However, stomata tend to close as the vapor pressure deficit
increases , thereby offsetting effects of temperature increases. Increasing
the atmospheric carbon dioxide concentration could result in a reduction
in stomatal conductance and increases in leaf area (Eamus & Jarvis ,
1989). Rosenberg, McKenney , and Martin (1989) and de Bruin and
Jacobs (1993) show that under a realistic range of conditions, transpiration
rates will change by less than ±10%. Thus, it is probable that changes in
rainfall will be the dominant factor in changing soil water status .
Droughts during July and August can also affect reforestation success.
Spittlehouse and Childs (1990) found a summer drying pattern similar to
that shown in Figure 15.1. Dry summers following dry springs can result
in severe moisture stress even in established trees. This situation occurs
10% of the time at present, 16% with a 20% decrease in rainfall, and 6%
a 20% increase. An increase in air temperature could result in heat stress
since maximum temperatures at seedling height are already in the high
30s (0e) during the summer. The effect of changes in winter air temperature on chilling and budburst is important for growth in subsequent years
and is considered later.
This analysis is species and site specific. However, the simulations
could also be used to assess effects on natural regeneration of Douglas-fir,
other plant species, and of soil texture. Fire risk could also be considered.
Precipitation patterns and soils vary across the area where Douglas-fir
occurs, but conditions described here are not atypical. Analyses could
readily be done for other sites.
Management Implications and Response
Figure 15.1 and Table 15.1 indicate that these stony , sandy sites are
extremely sensitive to variations in water availability. Any climate change
that results in decreased rainfall in late spring and early summer is a
serious threat to the establishment of Douglas-fir. This may be made
worse if a warmer climate causes snow melt to occur earlier than at
present. Table 15.1 indicates a relatively high chance of failure under
present conditions. Consequently, strategies to deal with future changes
are of use now.
A number of adaptation and mitigation options are available. Planting
time could be change, though any move to earlier planting to avoid the
droughts or to take advantage of warmer conditions needs to consider the
potential for increased frost risk (McCreary, Lavender, & Hermann ,
1990). A similar analysis would show whether there is less risk to fall
planting. Second, the large size of the marginal class indicate s that it is
necessary to ensure good control of any vegetation that might compete
for soil moisture. A third option is to consider alternates to clearcut
harvesting, such as shelterwoods or partial cuts, to reduce the evaporative
demand on the seedling and reduce soil surface temperature (Spittlehouse