WMOD00593

• Contract/Permit #: 14-06-D-7052
• Title: Ecological Impacts of Snowpack Augmentation in the San Juan Mountains, Colorado
• Date: 3/1/1976
• State: CO
• Weather Modification - Doc Type: Report

DENDROCHRONOLOGY AND DENDROECOLOGY 1/

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ABSTRACT

Paula V. Krebs and Lesley F. Tarleton

Topography: level to gentle (6 percent) sloping ENE-
facing clearing at bottom of a very low wooded
ridge; no drainage into immediate core tree area;
drainage is into a ravine directly at base of
ridge; stand trees are at least 50 m (155 ft.) out
away from the ravine; soil good, clayish, no surface
rocks or rock outcroppings.

W., and Jack D. Ives (Eds.) 1976. Ecological impacts of snowpack augmentation in the
Colorado. Final Report, San Juan Ecology Project, Colorado State University Publ.,

Dendrochronologic data from three ponderosa pine
sites within the San Juan study area, and historic
climatologic data have been used in the development
of regression models for the prediction of tree
growth increments. The " general approach of the study
involved the application of two such models, stepwise
regression analysis and bivariate analysis. These
were based on climatologic data from sites outside
the target area to predict values of tree growth
increments for the years after the initiation of
the snow augmentation project. The coring sites are
within an area which theoretically is being sub-
jected to a change in moisture regime due to cloud
seeding effects. A significant difference between
predicted and actual tree growth increments would be
interpreted as evidence of this change in climate.
A major handicap in the application of this method
to the San Juan Ecology Project is the small sample
of post seeding cases.

Four main climatic variables account for 71 per-
cent of the total growth response variance. These
variables are precipitation for the preceding Decem-
ber, June mean maximum temperature, August mean
maximum temperature, and April mean temperature.
Precipitation for the December prior to the year of
growth layer formation is consistently a principle
variable in all regressions.

A dendroclimatic evaluation of historical climate
trends was supported when compared to the actual
climatic record. The thicknesses of tree growth
layers were projected using prediction formulae de-
rived from the regression analysis. No significant
departure in actual growth response from the pre-
dicted growth response has been noted. Changes due
to weather modification have not been detected.
This indicates the general climatic trend of the
coring sites is paralleling the climatic trend of
the recording station.

This method of predicting tree growth response may
lead to a means of evaluating the effect of weather
modification, given the opportunity for astatis-
tically valid sample.

INTRODUCTION

Dendrochronologic analysis of ponderosa pine (Pinus
ponderosa) offers potential for monitoring the in-
fluence of weather modification on tree growth. The
possibility of retrieving climatic information from
growth layers depends largely on the conditions which
serve as limiting factors to tree growth in an area
(Fritts 1971). The effect of limiting climatic fac-
tors on tree growth is best observed in years of
stress." Trees form thin growth layers when pre-
cipitation is low if precipitation is the limiting
factor, and when temperature is low if temperature
is the limiting factor. Under extremely adverse
growing conditions, an annual growth layer may be
microscopic or even absent.

l!

In Steinhoff, Harold
San Juan Mountains,
Fort Collins
Present Address:

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On dry sites, where moisture supply serves as the
limiting factor, the growth layer characteristics
of ~. ponderosa should reflect available soil
moisture, and thus, precipitation. Changes in
precipitation, such as those due to winter oro-
graphic snow augmentation, should induce the same
response in tree growth as natural climatic fluc-
tuations would. The approach taken in this study
was to develop a correlation/regression model for
the tree growth-climate relationship in the years
prior to the start of the snow augmentation program,
and to use this in evaluating the growth pattern of
the following years.

Dendrochronologic analysis depends on the recog-
nition of growth layers and the identification of
annual increments of tree growth. This involves the
averaging of information provided by separate
cores from single trees into tree sequences and the
subsequent merging of a set of tree sequences into
a master chronology for a particular site. The
master chronology supplies info~tion concerning
diagnostic features characteristic of an area, such
as thin or absent growth layers for certain years,
and provides a generalized pattern of the growth
response of that area. The growth pattern can then
be correlated with various climatic parameters.

DATA COLLECTION

Data utilized in the dendrochronology study con-
sisted of master chronologies compiled for three
sites within the study area and climatological
records of temperature and precipitation for seven
nearby stations.

Dendrochronologic Data

The growth of trees in the San Juan area is assumed
to be a response to the amount of available soil
moisture. Study sites were selected to represent
three expressions of a soil moisture gradient based
on seasonal aspects; Station SJS-l has near-
optimum moisture conditions; SJS-2, on a broad
ridge crest, is a dry site; and SJS-3 is a moist
site throughout the year. The site locations are
shown in Figure 1. Site characteristics for the
three areas analyzed are summarized as follows:

SJS-l

Geographic location: 0.4 miles west on Trail Creek
Road (F.S. #2639) from junction with Piedra Road
(F.S. #2631) near Bridge Campground, approximately
23 miles NW of Pagosa Springs, San Juan National
Forest, Hinsdale County, Colorado; ele. 2700 m
(8,175 ft.).

INSTAAR, University of Colorado, Boulder, Colorado

80309.

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