310
D .L. Spittlehouse
enhancing Douglas-fir within any "new" range, because there are many
factors besides climate that control tree growth.
Reforestation
Issue of Concern
Changes in climate, particularly rainfall, could affect the establishment of
Douglas-fir seedlings after a disturbance such as harvesting. In situations
with low summer rainfall, significant numbers of recently planted seedlings
die through dessication. Seedlings are susceptible to variations in weather
conditions since they exploit only the top 0.2m or less of the soil profile.
They require 30 to 60 days of adequate soil water (wetter than -0.1 MPa)
after planting to develop their root system to allow them to withstand
summer droughts (Spittlehouse & Childs, 1990).
Scenarios
Soil water availability is regulated by rainfall, evaporative demand,
competing vegetation, and the ability of the soil to store water. Seedlings
are planted in April, and snow melt and April rainfall result in a wet soil
at planting time. Rainfall in May and June is critical for maintaining
adequate soil water for establishment. The site considered is at an elevation
of 350 m, on a 15° SE-facing slope with a stony loamy sand (50°28' N,
123°00' W) . A physically based, layered, soil-water balance model and
daily solar radiation, air temperature, and rainfall data were used to
determine how soil water availability was affected by variations in May
and June rainfall (Spittlehouse & Childs, 1990). These data were used to
define three categories of suitbility for seedling establishment (soil water
status) in terms of May and June rainfall. They are: adequate (no soil
drying-May plus June rainfall >60mm), marginal (some soil dryingMay plus June rainfall between 30 and 60mm) , and too dry (severe soil
drying-May plus June rainfall <30mm). The relative variation in
evaporative demand is much smaller than the changes in rainfall and is
not considered. Each year of a monthly rainfall record for 1908 to 1989
was then categorized using the above criteria. Rainfall in May averaged
33 ± 20 mm (ranged from 2 to 93 mm), and 32 ± 21 mm (range from 0 to
103 mm) in June. Figure 15.1 shows the soil water status for each year,
and the percentage of years in each category is presented in Table 15.1.
About 50% of the years in the marginal category are at the drier end of
the range. In this case, the timing of the rainfall and control of the
competing vegetation is critical for successful establishment.
The sensitivity to changes in rainfall was determined by reevaluating
the rainfall record after increasing or decreasing rainfall by 20%. The
decrease in the mean rainfall is similar to changes predicted by global
climate models (e.g., Boer et al. , 1992). Average May rainfall was 40mm
D .L. Spittlehouse
enhancing Douglas-fir within any "new" range, because there are many
factors besides climate that control tree growth.
Reforestation
Issue of Concern
Changes in climate, particularly rainfall, could affect the establishment of
Douglas-fir seedlings after a disturbance such as harvesting. In situations
with low summer rainfall, significant numbers of recently planted seedlings
die through dessication. Seedlings are susceptible to variations in weather
conditions since they exploit only the top 0.2m or less of the soil profile.
They require 30 to 60 days of adequate soil water (wetter than -0.1 MPa)
after planting to develop their root system to allow them to withstand
summer droughts (Spittlehouse & Childs, 1990).
Scenarios
Soil water availability is regulated by rainfall, evaporative demand,
competing vegetation, and the ability of the soil to store water. Seedlings
are planted in April, and snow melt and April rainfall result in a wet soil
at planting time. Rainfall in May and June is critical for maintaining
adequate soil water for establishment. The site considered is at an elevation
of 350 m, on a 15° SE-facing slope with a stony loamy sand (50°28' N,
123°00' W) . A physically based, layered, soil-water balance model and
daily solar radiation, air temperature, and rainfall data were used to
determine how soil water availability was affected by variations in May
and June rainfall (Spittlehouse & Childs, 1990). These data were used to
define three categories of suitbility for seedling establishment (soil water
status) in terms of May and June rainfall. They are: adequate (no soil
drying-May plus June rainfall >60mm), marginal (some soil dryingMay plus June rainfall between 30 and 60mm) , and too dry (severe soil
drying-May plus June rainfall <30mm). The relative variation in
evaporative demand is much smaller than the changes in rainfall and is
not considered. Each year of a monthly rainfall record for 1908 to 1989
was then categorized using the above criteria. Rainfall in May averaged
33 ± 20 mm (ranged from 2 to 93 mm), and 32 ± 21 mm (range from 0 to
103 mm) in June. Figure 15.1 shows the soil water status for each year,
and the percentage of years in each category is presented in Table 15.1.
About 50% of the years in the marginal category are at the drier end of
the range. In this case, the timing of the rainfall and control of the
competing vegetation is critical for successful establishment.
The sensitivity to changes in rainfall was determined by reevaluating
the rainfall record after increasing or decreasing rainfall by 20%. The
decrease in the mean rainfall is similar to changes predicted by global
climate models (e.g., Boer et al. , 1992). Average May rainfall was 40mm