314
D .L. Spittlehouse
35% slope, gravelly sandy loam 0.4 to 1.1 m deep , 190 to 300 m elevation
(48°50'N, 124°08'W) . Simulations were done using daily solar radiation,
air temperature and rainfall data for the April to October period of 1951
to 1983, and were then repeated for a 20% reduction in mean rainfall.
Transpiration deficit was correlated with the site index (the projected
height of the trees at 100 years). Transpiration deficit is the difference
between what the forest would transpire to meet the atmospheric demand
if the soil had adequate water , and the actual transpiration. For the
unadjusted rainfall, the regression between mean transpiration deficit
(TD mm) and site index (SI m) was SI = 54.9 - (0.5 ± 0.07 * TD) , se =
±4.2 m, R
2 = 0.910. The average tran spiration deficit for 1951 to 1983 for
the reduced rainfall case was then used in this equation to predict the
expected site index for the seven sites. The relationships are presented in
Figure 15.2 in terms of site moisture class, which vary from very xeric
(dry, rocky site at the top of the slope) to hygric (a water table present
for much of the year) (Meidinger & Pojar, 1991). All sites are predicted
to have reduced growth , but the dry sites are the most sensitive to the
reduction in rainfall. Smith et al. (1992) noted different responses across
site types.
Care must be taken to ensure that the model is not applied outside of
conditions for which it was calibrated. Some of the years at the xeric and
very xeric site are now drier than any experienced in the calibration range
of the model. The frequency of extremely dry years increased , and the
60 r - - - - - - - - - - - - - - - - - - - - - ,
Site Index
•
Subxeric
Mesic
Hygric
Submesic
Subhygric
•
;0
/
.
/
/
/4
/
Present Climate / / /
/
/
/
/ .
/
/
/
/
/
.
/
/
/
/
/
/
•
10 V
X '
ery eric
Xeric
20
50
40
(m)
30
Figure 15.2. Relationship between expected height of the trees at 100 years (site
index) and current site moisture class for Dougla s fir stands. The dashed line and
points show the current situation. The solid line shows the predicted site index for
the present site moisture class after a 20% reduction in rainfall.
D .L. Spittlehouse
35% slope, gravelly sandy loam 0.4 to 1.1 m deep , 190 to 300 m elevation
(48°50'N, 124°08'W) . Simulations were done using daily solar radiation,
air temperature and rainfall data for the April to October period of 1951
to 1983, and were then repeated for a 20% reduction in mean rainfall.
Transpiration deficit was correlated with the site index (the projected
height of the trees at 100 years). Transpiration deficit is the difference
between what the forest would transpire to meet the atmospheric demand
if the soil had adequate water , and the actual transpiration. For the
unadjusted rainfall, the regression between mean transpiration deficit
(TD mm) and site index (SI m) was SI = 54.9 - (0.5 ± 0.07 * TD) , se =
±4.2 m, R
2 = 0.910. The average tran spiration deficit for 1951 to 1983 for
the reduced rainfall case was then used in this equation to predict the
expected site index for the seven sites. The relationships are presented in
Figure 15.2 in terms of site moisture class, which vary from very xeric
(dry, rocky site at the top of the slope) to hygric (a water table present
for much of the year) (Meidinger & Pojar, 1991). All sites are predicted
to have reduced growth , but the dry sites are the most sensitive to the
reduction in rainfall. Smith et al. (1992) noted different responses across
site types.
Care must be taken to ensure that the model is not applied outside of
conditions for which it was calibrated. Some of the years at the xeric and
very xeric site are now drier than any experienced in the calibration range
of the model. The frequency of extremely dry years increased , and the
60 r - - - - - - - - - - - - - - - - - - - - - ,
Site Index
•
Subxeric
Mesic
Hygric
Submesic
Subhygric
•
;0
/
.
/
/
/4
/
Present Climate / / /
/
/
/
/ .
/
/
/
/
/
.
/
/
/
/
/
/
•
10 V
X '
ery eric
Xeric
20
50
40
(m)
30
Figure 15.2. Relationship between expected height of the trees at 100 years (site
index) and current site moisture class for Dougla s fir stands. The dashed line and
points show the current situation. The solid line shows the predicted site index for
the present site moisture class after a 20% reduction in rainfall.