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

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001580

4.4. TREE BIOMASS (Tom Landis and Dr. E. W. Mogren)

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4.4.1. Introduction and Ob1ectives

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Changes in snowfall patterns in the San Juan Mountains
as a result of weather modification could potentially
affect the wood production of forest communities in
the subalpine zone of the target area. These possible
effects of snow on forest production can be divided
into two categories; direct effects where snow could
alter the length of the short growing season in the
subalpine, and indirect effects where snow could
influence the temperature and moisture regimes of the
environment which influence the production patterns
of the forest. Both direct and indirect influences
of snow should also be considered in a temporal
context; snow effects may be cumulative in nature and
may not be obvious in short-term studies.

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There are two main forest types in the subalpine zone
of the target area. The upper elevational forests
are composed of stands of spruce-fir and in the lower
elevational areas, relatively pure stands of quaking
aspen are found. The spruce-fir association is
composed of only two tree species, Engelmann spruce
(Pieea en~elmannii Parry) and subalpine fir (Abies
lasiocarpa (Hook.)Nutt.). These forests are mature
and uneven-aged and considered climax for this region.
Engelmann spruce is the dominant species, usually
comprising more than 85 percent of the standing crop.
Quaking aspen (Populus tremuloides Michx.) forests
are varied in age and structure, but most stands are
wholly aspen, with a small percentage of coniferous
species in the understory. These forests represent a
substantial timber resource and large areas of mature
trees are suitable for commercial harvest. Therefore,
the possible effects of weather modification on the
wood production of these forests should be investi-
gated.

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The research objectives of the Tree Biomass Project
may be best stated in terms of two null hypotheses.
The first null hypothesis involves the production
performance of individual spruce and aspen trees and
proposes that the annual wood produ~tion of these
trees is adversely affected by the direct or indirect
influences of snow. The second hypothesis states that
bolewood production of aspen and spruce-fir stands is
affected by these same factors.

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4.4.2. Results and Discussion

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The experimental procedures of the project have been
reported in detail in reports of previous years.
They will not be discussed in this report; rather,
the results and tentative conclusions of this project
will now be presented.

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~iomass and production of the spruce-fir association

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The production relationships of this forest type were
studied in more detail than the aspen type because
of the extensive acreage of the spruce-fir type in the
subalpine zone and therefore, this type was sampled
more intensively. The following discussion deals
exclusively with wood production of individual trees
and stands of the spruce-fir type. Aspen production
will be treated later.

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Individual tree production was measured at the four
intensive study sites of the San Juan Ecology Project
over the past 20 years using stem analysis techniques
(Figure 1).

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. . . . x
. .
x~3.12
s~0.08
MR 3200m
l-
I-
. . . . . . . . . . . . . . ,
I-
x~ 2.65
s~ 0.03
MR3500m
. . . x
x ~ 2.70
s~0.06
we 3100 m
. . . x
. .
l-
I-
I- x~3.42
I- s~ 0.07
I- we 3300m

0>
~ 3
o
o
no 2
D-
o
>
<{

0>
"
3" 3
o
no 2
D-
o
>
<{

0>
~3
d
~ 2
D-
o 1
~

Ol
"
3"3
o
no 2
D-
01
>
<{

73 71 69 67 65 63 61 59 57 55
TIME (years)

Figure 1 - Bolewood production per tree.

Annual bolewood production of five "individual
production trees" at each site was averaged to provide
a typical production response of individual trees to
sno~related variables. The average production
pattern over the 20 year study eeriod was remarkably
stable around the period mean (x), as indicated by
the low standard deviation values (s). These data
indicate that spruce trees are quite consistent in
their annual wood production although they do exhibit
a 3-4 year cycle in their production during the 20
year period. This cycle could be ascribed to the seed
production characteristics of the species. Heavy
seed crops are produced every 2-6 years (Alexander,
1958), and these heavy seed crops could outcompete
the vascular cambium for current and stored photosyn-
thate. During these heavy seed years, the wood
production would be slightly reduced and during light
seed years, it would be slightly greater. Accepting
this hypothesis, the production pattern of the
individual spruce trees becomes even more stable.

Stand bolewood production during the 20 year study
period is also shown to remain relatively constant
at the four intensive study sites (Figure 2). This
pattern would be anticipated because stand production
reflects the cumulative response of all individual
trees in the plot. Also, such constant production is
an attribute of the climax seen in uneven-aged stands
(Whittaker, 1970).

The bole biomass or standing crop of the four study
sites was monitored since 1971 on permanent plots and
these data exhibit two types of perturbations during
this period (Figure 3). The first type can be seen
at the 3300m location on Wolf Creek Pass, where a

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