15. Effects of Climate Change on Forest Ecosystems
315
occurrence of consecutive dry years can increase the susceptibility of the
trees to insects and disease . A mortality model (e.g ., LeBlanc & Foster,
1992) could be incorporated to determine if tree death is induced at
these sites. As was the case in the seedling analysis, the calculation of
transpiration has the physiology implicit rather than explicit. As noted
earlier, the changes in stomatal conductance and leaf area in response to
increased carbon dioxide concentrations are likely to effect transpiration
by less than 10%. Increasing atmospheric concentrations of carbon
dioxide could act as a " fertilizer," because the photosynthetic rate increases with an increase in the carbon dioxide concentration. This could
offset some of the above predicted decrease in growth, though not
mortality induced by drought. Increased insect grazing of foliage, changes
in partitioning of photosynthate, long-term acclimation, and nutrient
deficiencies may limit gains that can be made (Eamus & Jarvis, 1989;
Running & Nemani, 1991; Pastor & Post, 1993). Many Douglas-fir sites
in British Columbia are nitrogen limited.
Changes in precipitation may be accompanied by changes in temperature. This may increase the length of the growing season and the
need for water, and increase respiration. Photoperiod response will not
be changed , thus trees may not be able to take full advantage of earlier
warming. Temperature increases in the winter and spring may reduce the
ability of the trees to meet their chilling requirement. McCreary et al.
(1990) found over a range of ecotypes that there is sufficient chilling for
Douglas-fir if the mean December to February temperature is below 9°C.
The mean December to February temperature in the warmest part of the
Douglas-fir zone in British Columbia is 4°C, though over the last 94 years,
14 years exceeded soc. However, most of area in British Columbia
occupied by Douglas -fir is 1°C or more cooler than the warmest area and
it would require a warming of over 4°C in any year to exceed the
threshold. This is greater than is predicted by climate models (Boer et al.,
1992).
Management Implications and Response
Changes in the rainfall during the summer will affect Douglas-fir growth,
and thus other components of the ecosystem. An approximation of
potential response is that each site moves down one site class. Management
options include longer rotations, reduced harvest rates , or smaller volumes
or piece sizes of wood . Other species such as lodgepole pine may be
preferable on the drier sites.
Monitoring
As before , assessing how the trees and understory are responding over
time is required. Inventories are part of forestr y activities, but there is an
315
occurrence of consecutive dry years can increase the susceptibility of the
trees to insects and disease . A mortality model (e.g ., LeBlanc & Foster,
1992) could be incorporated to determine if tree death is induced at
these sites. As was the case in the seedling analysis, the calculation of
transpiration has the physiology implicit rather than explicit. As noted
earlier, the changes in stomatal conductance and leaf area in response to
increased carbon dioxide concentrations are likely to effect transpiration
by less than 10%. Increasing atmospheric concentrations of carbon
dioxide could act as a " fertilizer," because the photosynthetic rate increases with an increase in the carbon dioxide concentration. This could
offset some of the above predicted decrease in growth, though not
mortality induced by drought. Increased insect grazing of foliage, changes
in partitioning of photosynthate, long-term acclimation, and nutrient
deficiencies may limit gains that can be made (Eamus & Jarvis, 1989;
Running & Nemani, 1991; Pastor & Post, 1993). Many Douglas-fir sites
in British Columbia are nitrogen limited.
Changes in precipitation may be accompanied by changes in temperature. This may increase the length of the growing season and the
need for water, and increase respiration. Photoperiod response will not
be changed , thus trees may not be able to take full advantage of earlier
warming. Temperature increases in the winter and spring may reduce the
ability of the trees to meet their chilling requirement. McCreary et al.
(1990) found over a range of ecotypes that there is sufficient chilling for
Douglas-fir if the mean December to February temperature is below 9°C.
The mean December to February temperature in the warmest part of the
Douglas-fir zone in British Columbia is 4°C, though over the last 94 years,
14 years exceeded soc. However, most of area in British Columbia
occupied by Douglas -fir is 1°C or more cooler than the warmest area and
it would require a warming of over 4°C in any year to exceed the
threshold. This is greater than is predicted by climate models (Boer et al.,
1992).
Management Implications and Response
Changes in the rainfall during the summer will affect Douglas-fir growth,
and thus other components of the ecosystem. An approximation of
potential response is that each site moves down one site class. Management
options include longer rotations, reduced harvest rates , or smaller volumes
or piece sizes of wood . Other species such as lodgepole pine may be
preferable on the drier sites.
Monitoring
As before , assessing how the trees and understory are responding over
time is required. Inventories are part of forestr y activities, but there is an