WSP00036

• File Number: 8200.750
• Description: San Juan River General
• State: CO
• Basin: San Juan/Dolores
• Water Division: 7
• Date: 3/1/1975
• Author: BOR
• Title: San Juan Ecology Project - Interim Progress Report - March 1975
• Water Supply Pro - Doc Type: Report/Study

of ,growth was recorded on May 28, 12 days earlier than
the other two years.

The date of final snow melt influences the timing of
growth initiation in oak by the effect of melt water
on soil temperatures which is probably one of the more
important factors affecting growth initiation. Data
for the springs of 1972 and 1973 indicate that soil
temperatures at -50 cm and -90 cm must be at or above
5 C for two to three weeks prior to the onset of
growth (Fig. 7). The initiation of growth in 1972 did
not occur earlier in that season possibly due to
insufficient:day length. Temperature sums, or the
number of degree-days above 0 C also can influence the
initiation of growth by its affect on snow melt, wa~
ing of the soil, and on bud temperatures. Monthly
temperature sums recorded at the Durango Station were
higher in 1972 than in 1971 and 1973, and for the
last two years temperature sums of at least 343
degree-days were recorded at the study site prior to
initiation of growth. This sum was reached about 25
days earlier,in 1972 than in 1973.

Browse Prod~tion-

The amount of browse produced in a given year may be
affected by the length of the growing season and by
the rate of growth during that season. The length of
the growing season, however, appears to be stable in
oak brush. Even though the date of growth initiation
may have been influenced by snow melt and spring
temperatures, the length of the growing season for
the last three years has remained relatively constant
(Table 3).

Table 3. Growing season of Gambe1 oak at its upper
elevational limits on Missionary Ridge.
Final Growing
Snow Growth End of Season
Melt Initiation Growth (days)
1971 5-13 6-9 7-3 24
1972 3.4 5-28 6-24 27
1973 5-5 6-9 7-3 24

Temperature sums during the growing seasons for the
last three years are not significantly different, but
there has been about a two-fold increase in the amount
of precipita~ion each year with 1.96 cm, 4.32 cm, and
8.20 cm of water being recorded during the 1971, 1972
and 1973 seasons respectively. This increase in
moisture, however, did not appear to enhance the grow
growth of oak because the amount of browse produced
for the three years is not significantly (95% level)
different (Table 4). The lack of response in growth
to increased-precipitation is probably due to the
fact that Gambel oak has low moisture requirements and
therefore is most often found on soils with low soil
water retention characteristics. Any additional
water simply drains through the 80il. This is the
case on the study site. The almost two-fold increase
in precipitation in 1973 over 1972 did not increase
soil moistures. The water potential of the soil at
-50 cm and -90 em-depth was the same for both years
throughout each season (between 0 and -5 bars) and
was not considered to be limiting to growth of oak
(Fig. 8).

Successional Status-

Gambel oak has low moisture requirements and there-
fore, is found on soils with low soil water retention
characteristics. In areas where it is found on soils
of higher soil water availability, oak brush will most

often be replaced by the mo I competitive" speciea
of .spen or ponderos. Pine.jenata for determining the
s~ccessional status of oak lUSh on Missionary Ridge
has been collected, but the analysis is not complete
enough to make any conclusi s at this time.

4.6.4 Summary
Winter Distribution-

Tbtal precipitation, freque
and temperature all influen
of baring of the winter ran
slope migration patterns of
with severity of the winter
elk occur during winter is
a certain critical depth.
penetrable snow will cause
e~osed winter range if ava
o~ more of penetrable snow
added winter stress and are
and/or oak habitats on sout
higher degree than other ba
during the winter period.

C~ll ving-

cy of storms, cloud cover,
e the frequency and rate
e. As a result, down
elk in the winter vary
The elevation at which
elated to a snow line of
depth of 35 - 40 em of
lk to move to more
l~ble. Areas with 70 cm
over probably represent
usually avoided. Fescue
aspects are used to a
itat types or aspects

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Spring movements of elk bac into the high summering
a~eas are associated more s rongly with the greening
up of the open fescue parks than directly with a
receding snow line. Snow p obably does not limit the
actual location of calving n Missionary Ridge. For
the three calving seasons s udied, there appeared to
be ample habitat available or calving below the snow
line, with the prime calvin areas open each spring.

Oak-

The timing of final snow me
growth in oak by as much as
cooling effect of melt wate
A delay in growth initiatio
decrease the length of the
appears relatively constant
browse by oak is not enhanc
tation because of the low w
i~tics of the soils on whic
As a result, the production
significantly different for
study. In areas where an i
results in an increase in s
oak may be-replaced by the
such as aspen or ponderosa
the overall production of 0

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t may delay the onset of
two weeks through the
on soil temperatures.
, however, does not
rowing season which
The production of
d by increased precipi-
ter holding character-
it is commonly found.
of browse has not been
the three years of
crease in moisture
i1 water availability,
ore competitive species
ine and thereby decrease
k browse.

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Table 4. Production of Gam el oak browse for the
1971, 1972 and 19 3 growing seasons.

Production of Biomass

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Year 1 Yea 2 Year 3
. Stand 1 2.4 6 4 4.S
Stand 2 6.2 6 7 11.1
~:
Year 1 Yea 2 Year 3
Stand 1 24.S 43 0 21.2
Stand 2 40.9 65 5 68.2

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