July 1965 Harold C. Fritts 421
TREE-RINGEVIDENCEFORCLIMATICCHANGES IN WESTERNNORTHAMERICA
HAROLD C. FRITTS
Laboratory of Tree-Ring Research, The University of Arizona, Tucson, Ariz.
ABSTRACT
The relationships between climatic factors and fluctuations in dated tree-ring widths are statistically evaluated'
A wide ring indicates that the year's climate was moist and cool, and a narrow ring dry and warm. In general, ring
width relates to a 14-month period from June through July but most tree-ring chronologies exhibit a closer relationship
with autumn, winter, and spring moisture than with summer moisture. The climatic relationships for evergreen
trees are attributed largely to the influence of environmental factors on photosynthesis and the accumulation of
food reserves. Under abnormally dry and warm conditions, especially during the autumn, winter, and spring, little
food is accumulated, new cells are formed more slowly during the growing period, and the resulting ring is narrow.
Relative 10-yr. departures are calculated for the entire length of 26 tree-ring chronologies from western North
America. Those portions after 1500 are used to map areas of high and low moisture. Periods of widespread drought
are noted in 1576-1590,1626-1635,1776-1785,1841-1850,1871-1880,1931-1940. Periods of widespread and
above average moisture occurred during 1611-1625,1641-1650,1741-1755,1826-1840,1906-1920. The moist
periods of 1611-1625, and 1906-1920 were most widespread and markedly above average.
1. INTRODUCTION
The details of the climatic history of the United States
during recent centuries are not known. In this period, as
in more ancient times, there is much indirect evidence of
significant changes of climate. Dendroclimatic analysis
represents an especially promising source of information
on the chronology and character of such climatic changes,
especially those in the semiarid regions of western North
America.
It is the purpose of this report first to present some
recent statistical analyses of the climatic factors influenc-
ing ring growth. This is followed by a brief discussion of
the current theory concerning the model of tree growth
and climate. In the last part, relative 10-yr. departures
for ring growth are calculated from 26 North American
tree-ring chronologies, and maps of these departures are
constructed for the interval from A.D. 1501 to 1940.
2. ANALYSES OF TREE-RINGANDCLIMATIC
RELATIONSHIPS
Three study areas were chosen to center around three
weather stations within forest stands where relatively
homogeneous and long weather records are available.
Fort Valley, Ariz., 35'16' N., 111'44' W., has an elevation
of 7,345 ft. It is located near the San Francisco Peaks
in a ponderosa pine (Pinus ponderosa Laws.) forest north
of Flagstaff, Ariz. Pinyon pine (Pinus edulis Engelm.)
occurs at slightly lower elevations and at greater distances
from the main mountain mass. Idaho Springs, Colo.,
39"45' N., 105'33' W., has an elevation of 7,556 ft. It is
in a narrow valley with scattered stands of second growth
ponderosa pine and Douglas-fir (Pseudotsuga menziesii
(Mirb.) Franco) occurring on the adjacent mountain
slopes. Mesa Verde National Park, Colo., 37'12' N.,
108'30' W., has an elevation of 6.960 ft. It is located on
a southward sloping plateau which is forested with pinyon
pine, occasional ponderosa pine, and Utah juniper
(Juniperus osteosperma (Torr.) Little). Douglas-fir stands
are restricted to deep canyon slopes and to the higher
elevations.
Climatic summaries for the three stations are presented
in table 1. Fort Valley records the highest annual
precipitation. However, the annual precipitation of this
region has been shown to decrease markedly with in-
creasing distance from the San Francisco Mountain
mass [l], and this is accompanied by vegetational changes
to pinyon pine-juniper and then to grassland commu-
nities [7]. The seasonal precipitation regime exhibits
two maxima associated with high frequencies of winter
and summer storms. The Fort Valley and Idaho Springs
stations exhibit comparable temperature regimes though
the latter is 210 ft. higher. The annual precipitation a t
Idaho Springs is lower than at the other stations, but
the monthly distribution exhibits one maximum centering
around the tree growth season of May through August.
The amount of annual precipitation a t Mesa Verde
National Park is intermediate between the other stations,
but temperatures often average 7' F. higher, so that
evapotranspiration probably reduces its effectiveness.
Precipitation maxima occur during the winter and sum-
mer, but the relative amounts are more uniformly dis-
tributed throughout the entire year than a t t h e other
two stations.
422 MONTHLY WEATHER REVIEW Vol. 93, No. 7
TABLE 1.-Climatic summary for the three weather stations used in screening analysis. (From U.S. Weather Bureau, Climatography of the United States, Climatic Summary of the United States-Supplement .for 1931 Through 1952, No. 11-2, Arizona, and No. 11-5, Colorado)
STATION MONTHLY PRECIPITATION (Inches)
J a n . F e b . Mar. Apr. M a y J u n e July A u g . S e p t . O c t . N o v . D e e .
AMual total
FortValley ______._______._______
18.41 15.27 1.86 2.14 1.97 1.22 1.02 .73 1.47 1.85 1.78 1.59 1.00 1.78 MesaVerde _________.______.___ __
22.84 .3 7 .5 6 .9 7 2.07 2.19 1.56 2.29 2.20 1.16 .85 .68 .37 Idaho Springs ___.__.._.______.__
2.35 2.43 2.02 1.66 .64 .59 2.50 3.76 1.81 1.51 1.11 2.46
"
MEAN MONTHLY TEMPERATURE (OF.) Annual mean J a n . F e b . Mar. Apr. May J u n e J u l y Aug. S e p t . O c t . N o v . D e c .
"
IdahoSprings __..______.._______
50.5 29.4 33.1 38.7 4R.4 57.2 67.1 73.3 71.1 64.0 52.6 39.6 32.0 MesaVerde ________.____.__._____
24.5 27.2 32.7 40.7 47.6 55.5 62.9 61.5 55.4 44.9 34.1 27.9 42.9 Fort Valley ______._.___..._______
25.2 27.5 32.3 40.8 48.1 57.1 62.7 61.6 55.0 45.3 34.2 28.6
I
43.2
During the summers of 1961 through 1963, several forest
stands on semiarid sites were sampled near each weather
station. Where possible, several samples were taken from
a variety of sites and species so that differences in the
tree-growth responses might be evaluated. Five or more
dominant trees of comparable age were selected from each
stand. Two or four radii were sampled from each tree
using a Swedish increment corer. At first, a sample of
four cores from five trees was used but later a sample of two
cores from a larger number of trees was thought to be more
representative of the regional tree-growth patterns. All
rings on the increment cores were dated. Approximately
the last 100 rings were analyzed but the length of record
varied from 93 to 106 yr. because of the limited age of a
few stands and differences in the year when the samples
were taken. Ring widths on each core were measured,
standardized, and analyzed by computer techniques [4].
The location, elevation, size, and several statistical
characteristics of each sample are presented in table 2.
These characteristics include: the first order serial correla-
tion, average mean sensitivity [12], and standard deviation
of the average chronology for the sampled group. The
mean standard error of the group chronology and the
percentages for three variance components are listed on the
TABLE 2.-Sifes, sample sizes, and statistical characteristics for chronologies used in screening analyses. Serial correlation, mean sensitivity, standard deviation, and standard error are calculated for the mean chronology of the sampled groups. The percent variance provides a measure of the relative variability in chronology of the whole group as compared l o di3erence.s in chronologies among trees and among radii within trees
SPECIES AND LOCATION SITE ELEVA- TION
(It.)
DOUGLAS-FIR
Fort Valley __.__.
~
._...__._... -
N-Facing Slope ___.._.....__ Idaho Springs. ....__..._____._
NE-Facing Slope ...___ ...___
Old-Tree Canyon 1 ___._.....
Navajo Canyon 1 __.___..__._
Bobcat Canyon _...._....___
Park Point ._._ ......__..____ Mesa Verde- - -. - ..__..____.._
~
NW-Facing Slope." _..___..
PONDEROSA.PINE
Fort Valley _____..____..____._ Mountain Slope ....._______.
Forest Interior. _..._
~
_..__..
Near Forest Border _________ Forest Border __...___.._..._
Idaho Springs ___..____________ N-Facing Slope .....__.___..
Mesa Verde .______._..__._____
S-Facing Slope _....__.....__
Moccasin Mesa" - ...._ __ .._.
Bobcat Canyon _...._...__..
PINYON PINE
Fort Valley _...__...__..._....
Lower Forest Border __..___.
Upper Forest Border ____..__
Mesa Verde." - _._. .____..____
Chapin Mesa Mid _..._..__..
Chapin Mesa Crest __.___....
Weth. Mesa Mid. 1 _____..__..
Bobcat Canyon ._.._....._.. Chapin Mesa Low .____...._.
Navajo C . E-Facing __.__...
Navajo C. W-Facing ______..
BRISTLECONE PINE
Idaho Springs __.._._.._______. Mount Evans ___..._....___.
8.500
7,700 8, OM)
8, 100 6,900
6,500 6,8M)
8,500 7,400
7,300 7,300
7,700 7,900
7,000 6,700
7,300 6,300
;G
$E
7,000 6,700
6,500
10,250
Years
102
103 103
104 104 104 106
102 102 103 103
103 103
93
104
103 103
104 106
104 104 103 106 106
103
Trees Cores
5 M
9
1
18
9 18
5 20
5
:I ;
5 5
M
5
20
5
20
20
7
20 20 5 5
34 17
14
5 20 5
20
5 20
5 20
5 M
5
5 20
20 5 20
5 20
5 20
Chronology Characteristics
Serial Corr.
0.66
.10
.23
.58 .29 .30 .23
.54 .65
.70
.63
.14 .28
.51
.03
.53 .59
. 20. .32
.33 .37
.52 .31 .60
.37
Sensl- Stand.
f:iz
I
Dev.
.4 7 .4 2
.36 .61 .44
.70 .53 .68 .66 .53
.2 0
.49
.2 8 .22 .27
.58 .53 .68
.44 .37 .47 .43
.35 .23 .26 .31
.31 .48 .44 .34
.32 .30 .49 .45 .49
.59 .58
.50 .56
.42 .35 .51 .59 .46
"
Stand. Error
"
0.079
f110
,086
,095 ,086
,115 ,091
. OW ,096
,133 ,103
,117 ,076
.OW ,123
,093
. 102
,092
,142
. 107 ,101
. 125 .119 ,122
,071
% Variance
Radii
22
17 14
12
11
13 10
31
27 11
12
21 21
24 29
20 15
23 22
13 17 23 ?O
24 25
-
Trees
7
34 19
14 8
8 9
22 18
9 12
28
17
34 24
16 13
24 23
1;
21
11
14 9
15
"
Group
"
71
49 67
74 81
81 79
51 51 80
76
51 62
47 42
64 72
60
-
1 Originally described and presented by Fritts, Smith, and Stokes [SI.
July 1965 Harold C. Fritts 423
right in table 2. The variance components measure the
percentage of the variability due to differences in chronolo-
gies along the several radii, differences in chronologies
among the several trees, as compared to the remaining
variability in the chronology for the group. All computa-
tions are made with standardized tree-ring indices [4].
The consistently high variability and low error in tree-
ring chronologies from the three areas demonstrate the
high similarity in growth response for all trees within a
group. Only the two samples of ponderosa pine from
Mesa Verde exhibit more variability in chronologies
between different radii and different trees than in the
chronology of the group. Although catastrophic events
such as fire, cutting, or insect infestation could affect
the growth of the entire stand and thus alter the group
chronology, the high variability from year to year and
the consistency among widely scattered stands and dif-
ferent species that were sampled is evidence that climatic
variation from year to year is responsible for a large part
of the variation in widths of annual rings.
The three Douglas-fir samples from elevations under
7,000 ft. a t Mesa Verde (table 2) exhibit high standard
deviations and a high percent of variance in the group
chronology. The same pattern appears in the lowerele-
vation samples of ponderosa and pinyon pine, but these
chronologies exhibit more site variability. Serial cor-
rdlation is more pronounced in forest border ponderosa
and pinyon pine than in forest border Douglas-fir.
Several of the samples described in table 2 were merged
or augmented, and climatic relationships exhibited by
each chronology were analyzed using the weather records
from the neighboring station in a stepwise multiple
regression analysis [3]. The monthly weather data were
grouped into eight intervals prior to and concurrent
with the season of growth. The total precipitation
and average temperature were calculated for each inter-
val. The intervals are: (1) previous June, (2) previous
July, (3) previous August-September, (4) previous Oc-
tober-November, (5 ) previous December-February, (6)
current March-May, (7) current June, and (8) current
July. The climatic parameters of each interval were
correlated with the corresponding ring-index values, and
a multiple regression equation for predicting ring-indices
was built in a stepwise fashion by entering variablesin
order of their correlation with the residual variance.
The result is a multiple regression equation including
only those variables which are found to be significantly
related to the ring-width variability [3].
First, analyses were made on each chronology using only
precipitation or only temperature during the eight inter-
vals as possible predictors of growth. Where available,
all three measures of temperature (maximum, mean, and
minimum) were tried in separate analyses. Then in order
to allow for serial correlation in the chronologies, the
index values for the three preceding rings were included
as possible variables, and all the above analyses were
rerun. The inclusion of the three preceding indices
776-1,72 0-65-4
generally improved prediction. The ranges of multiple
correlations obtained in these analyses are:
1. 0.636-0.906 using precipitation and prior growth.
2. 0.000-0.839 using maximum temperature and prior
growth.
3. 0.212-0.850 using mean temperature and prior
growth.
4. 0.226-0.653 using minimum temperature and
prior growth.
These results show that precipitation is generally more
highly related to ring-width variation than is temperature.
Further analyses were made by including both precipita-
tion and temperature during the eight intervals as in-
dependent variables. The products of temperature and
precipitation were also included to allow for interaction.
In 5 single analysis the total precipitation for each interval
represents variables 1 through 8, the average maximum
temperature for each interval represents variables 9
through 16, and the product of precipitation and average
maximum temperature represents variables 17 through 24.
Each analysis was run a second time with the ring indices
for the three preceding years included as variables 25
through 27. Similar analyses were also run using mean
temperature and average minimum temperature in place
of average maximum temperature. As was observed in
the earlier analyses, the inclusion of the three preceding
growth indices generally improved prediction. The
ranges of multiple correlations obtained in these analyses
are :
1. 0.744-0.937 using precipitation, maximum tem-
peratures, and prior growth.
2. 0.721-0.924 using precipitation, mean tempera-
tures, and prior growth.
3. 0.741-0.947 using precipitation, minimum tern-
peratures, and prior growth.
These results show a marked increase in prediction
when both precipitation and temperature are used as predic-
tors of growth. Temperature was entered as a variable and
improved prediction over the use of precipitation alone
in all analyses but two. These two exceptions are the
forest interior ponderosa pine and pinyon pine samples
near Fort Valley, Ariz., where only precipitation and
previous growth were selected as the significant factors.
In many cases the products of precipitation and tem-
perature were entered so that the multiple regression
equation was complex and difficult to evaluate by visual
inspection. Therefore, to aid interpretation, selected
values for precipitation and temperature were substituted
into the final regression equations and the predicted ring
width was calculated [6]. Using these computations it is
possible to ascertain which climatic conditions for each
interval relate to high and low growth. The variance
ratios for the individual regression coefficientswereused
t o test for significance and to arrange the climatic-interval
effects in order of. their importance. The regression
coefficients for prior growth conditions were handled in a
similar fashion.
424 MONTHLY WEATHER REVIEW Vol. 9 3 , No. 7
TABLE 3.-Stand locations, sample descriptions, and generalized results of screening analysis of tree-ring chronologies from 7 replicated samples of Douglas-fir. Climatic and growth conditions are those correlated with narrow annual rings and numbers in front of conditions indicate order of importance as determined from variance ratios. All variables exceed a 2.5 variance ratio
Location ........................... F O R T V A L L E Y , I D A H O S P R I N G S , C O L O R A D O MESA V E R D E , C O L O R A D O
I ARIZONA 1 I I I -
N-Facing Slope 7,700
1
Old-Tree Can.1
6,800
Navajo Can.1
6,500
................................
Elevation (ft.)
2o Sample size .........................
49 Years analyzed. ....................
2 Miles from weather station ..........
NE-Facing Slope Site
8,500 ......................
f 5 ' Z e s
NW-Facing Slope
57
Bobcat Canyon
6,900 8,100 8,000
Park Point
2 41
5 1 40 9 trees 18 cores 5 trees 6 trees 20 cores 24 cores
57 9 trees 18 cores
il 6 trces 30 cores precip. max. temp. precip. mean temp precip. m a x . t e m p . I E F 7 e m p . I Ef:%mp, precip. max. temp. Variables used ......................
SEASON INTERVAL 2 -
CLIMATIC CONDITIONS ASSOCIATED W
"
Prev. June .........................
Prev. July- ........................
3 dry 3 dry, warm 5 warm, dry
6 warm, wet 5 warm Current July .......................
1 dry 4 dry, warm 4 dry, warm Current June .......................
4 warm, dry 1 dry 1 warm, dry 4 dry, cool Current Mar.-May .................
2 warm 2 warm, dry Prev. Dec.-Feb ....................
2 dry 9 dry 6 dry, warm Prev. 0ct.-Nov ....................
5 dry 8 dry 8 dry, warm Prev. Aug.-Sept.. .................
7 dry, warm
ITH NARROW RINGS
6 dry, warm 5 cool 7 dry or warm
7 dry, warm 4 dry, warm
1 dry, cool 3 dry 6 dry
kg
2 dry 5 dry, warm
7 dry or warm
4 dry
1 dry
2 dry
5 dry
6 dry
GROWTH INTERVAL
PRIOR GROWTH CONDITIONS ASSOCIATED WITH NARROW RINGS
I ___- I I I I I _____ "-
Prev. Year ......................... 1 low 2 Years Prev .......................
3 Years Prev-.. .................... 3 low I 7 1 0 ~ 3 low I 210w
5 low
1 llow 3 low 1 3 low
"I I I I- I "I I
...............
yo Unexplained Variance M u l t i p l e 0 .8 7 9 0 .8 7 2 0.875 0.785 0.892 0.913 0.916
1 20.3 I 15.6 I 20.4 I 29.0 1 17.6 I 13.1 I 13.2 ...........
1 Originally described and presented by Fritts, Smith, and Stokes [6].
TABLE 4.--Stand locations, sample descriptions, and generalized results of screening analysis of tree-ring chronologies f r o m 6 replicated samples of ponderosa pine. Climatic and growth conditions are those correlated with narrow annual rings and numbers in front of conditions indicate order of importance as determined from variance ratios. All variables exceed a 2.5 variance ratio
MESA VERDE, COLORADO
I
IDAHO SPRINGS, COLORADO Location .......................
~
........................ FORT VALLEY. ARIZONA
7,950
Forest Interior
1 49 10 trees
7,300
Forest Border
17
Bobcat Canyon Moccasin Mesa
2 5
40
20 cores 5 trees
41
7,000 6,700
5 trees 20 cores
Site .....................................................
Miles from Weather Station Elevation (ft.)
Years Analyzed .........................................
Sample Size. ...........................................
Variables used". .......................................
..........................................
.............................
N-Facing
7,700 1
57 7 trees 17 trees 49 10 trees 40 cores preclp. mean temp.
40 cores
mean temp. precip.
14 cores 34 cores
preclp. preclp. max. temp. min. temp. precip. m i n . t e m p . I gncliLmp.
CLIMATIC CONDITIONS ASSOCIATED WITH NARROW RINGS
4 wet 7 wet 3 dry, warm 5 warm
6 dry 7 dry, warm
2 dry, cool 4 dry or warm
1 dry, cool
5 dry 7 dry
6 dry 5 warm
1 warm
5 dry 6 warm
5 dry
1 dry 1 warm, dry 2 dry
6 wsrm, dry 4 dry 5 dry, warm
3dry
8 dry
3 dry
1 dry
2 dry, warm 3 dry, warm 3 dry or warm
PRIOR GROWTH CONDITIONS ASSOCIATED WITH NARROW RINGS
SEASON INTERVAL
..............................................
Prev. July"
Prev. June
............................................
Prev. Aug.-Sept ........................................
Prev. 0ct.-Nov .........................................
Prev. Dec.-Feb .........................................
Current Mar.-May .....................................
Current June ...........................................
Current July ............................................
GROWTH INTERVAL
2 low 1 low- 4 low
6 low
I
410w
1
710w
...........................................
2 Years Previous Previous Year
3 Years Previous .......................................
........................................
8 low
2 low 3 low
1
2 low 4 low
0.854 1 0.821
I
0.822 1 2 .5 2 7 .0 2 1 .4 0.741
1 41.6
0.905 0.824
8.6
1 18.2
....................................
% Unexplained Variance- Multiple Correlation
...............................
July 1965 Harold C. Fritts 425
TABLE 5."Stand characteristics, sample descriptions, and generalized results of screening analysis of tree-ring chronologies f r o m 7 replicated samples of pinyon pine at Mesa Verde National Park. Climatic and growth conditions are those correlated with narrow annual rings and numbers in front of conditions indicate order of importance as determined from variance ratios. All variables exceed a 8.5 variance
ratio
Location- - - ____. __ __ ____ ____ ____ __ - CHAPIN MESA I W E T H E R I L L M E S A 1 N A V A J O C A N Y O N
Middle 1
7,000 2
41 5 trees
20 cqres preclp. max. temp.
6,500 W-Facing
1 38 5 trees 20 wres precip. min. temp.
Site ................................
Elevation (It.) ......................
Crest 8 400 Miles from weather station __._.____ 5'
Years analyzed ..................... 41
Sample size ......................... {i:Fes
Variables used ...................... { preclp' max. temp.
Middle
7,150 6,700
Low
1 6 38 41
5 trees 5 trees 20 cores
precip.
20 wFes
max. temp. max. temp. preclp.
6,900 2 40
5 trees 5 trees
6,600
20 cores 20 cores
precip. precip. min. temp. min. temp.
CLIMATIC CONDITIONS ASSOCIATED WITH NARROW RINQS SEASON INTERVAL
Prev.June------------------------- Prev. July __________.__.____________
Prev. Aug.-Sept .................... 1 dry Prev. 0ct.-Nov ....................
4 warm
6 warm Current July .......................
Current June .......................
2 dry, cool Prev. Dec.-Feb 3 warm Current Mar.-May _______________..
....................
3 dry
6 warm
1 &Y
2 dry wol 7 dry: warm
4 dry, cool
5 dry, warm
8 dry
2 warm, dry
4 dry
4 warm wet 1 dry, ;arm 1 dry
: g, cool
4 warm
2 *Y
4 warm, dry
1 dry
2 dry
4 warm 5 warm 3 warm
; :;, cool 1 b Y 6 dry 2 dry
1 5dry
3 dry
PRIOR QROWTH CONDITIONS ASSOCIATED WITH NARROW RINQS QROWTH INTERVAL I
5 low
I
610w
6 low 3 low
I
51oW 7 low
1
610w 8 low ...................... 2 Years Previous 3 Years Previous ...................
Previous Year
5 low ...................
Multiple Correlation ...............
% Unexplained Variance.. 0.937 0.891 16.5 I
.........
11.8 I 5.2 1.1 1
2.1
I
0.901 0.897 0.883
I
0.910 0.947 8.9
I 1.3
1 Originally described and presented by Fritts, Smith, and Stokes (61.
The best predicting relationships for each analyzed
chronology are presented in tables 3-6 by expressing the
relationship as climate and prior growth conditions associ-
ated with the formation of narrow rings. Only those
factors which proved significant are entered in the tables.
Numbers indicate order of importance for the interval
and the most significant climatic factor within the interval
is entered.first. Additional information provided in the
tables is stand locations, sample descriptions, climatic
variables used to obtain the equation, multiple correlation,
and percent of unexplained variance. The last term is
calculated by subtracting from the chronology variance
its error variance and the variance explained by regression,
and then dividing by the chronology variance to obtain a
percent. The resulting value provides a measure of the
relative failure of the analysis to obtain a perfect growth-
climatic relationship.
Analyses for seven chronologies of Douglas-fir are
shown in table 3. The samples are arranged from left
to right in approximate order of increased aridity of site.
The percentage of unexplained variance shows that the
closest relationship between ring widths in Douglas-
fir and climate occurs in the most arid sites. Moisture
conditions from August through May appear to most
closely relate to ring-width variation. The width of
the third previous ring is highly significant, while the
two previous ring widths nppear unrelated.
With higher elevation nnd less aridity of site, the
moisture during the current and previous June and July
assumes more importance and the growth during the
TABLE 6.-Stand locations, sample descriptions, and generalized re- sults of screening analysis of tree-ring chronologies from replicated samples of pinyon pine north of Fort Valley, Ariz., and bristlecone p i n e o n M t . E v a n s , Colo., near Idaho Springs. Climatic and growth conditions are those correlated with narrow annual rings and numbers in front of conditions indicate order of importance as de- termined from variance ratios. All variables exceed a 8.5 variance ratio
Location .............................. Fort Valley, Arizona Idaho Springs, Colorado
~~
Bristlecone pine 10.250 6
57
20 wres 10 trees
precip. max. temp.
"
Species ................................
{ Variables used
Sample size ............................
49 Years analyzed ........................
6,800 Elevation (ft.) 21 Miles from Weather Station
Pinyon pine .........................
............
{io cqres 10 trees
preclp. mean temp. .........................
I
CLIMATIC CONDITIONS ASSOCIATED WITH NARROW RINGS
- -
Season interval I I I I
1 2warm Previous June .........................
Previous July .........................
Prev. Aug.-Sept ......................
Prev. 0ct.-Nov .......................
Prev. Dee.-Feb .......................
3 dry Current Mar.-May ....................
1 dry
Current July ..........................
Current June ..........................
9 dry 3 dry
7 dry, warm 4 wet, warm
PRIOR GROWTH CONDITIONS ASSOCIATED WITH NARROW RINQS
~~ ~
Growth interval
I
5 low 8 low
"- --I
Previous Year ......................... 2 low
2 Years Previous ......................
3 Years Previous ...................... 4 low
Multiple correlation ...................
Percent unexplained variance- ._______
I '2;: 1 36.4
0.778
426 MONTHLY Vol. 93, No. 7
two previous years may be more significant. The high
significance of the width of the third previous ring in
all but the Park Point sample indicates that climatic
conditions which are favorable or unfavorable for ring
growth may also influence other processes in the tree
which become manifest three years later in the formation
of a wide or narrow ring.
A generalized growth-climatic model for Douglas-fir
may be obtained by averaging the results of the seven
analyses in table 3. Conditions of low moisture and
high temperature during March through May relate
most significantly to narrow rings. Moisture and temper-
ature during the current June, previous October through
November, and previous December through February,
are next in importance. Moisture and temperature
conditions during the previous June and August through
September appear to follow. in importance, while cli-
matic factors during the previous and current July
exhibit no clear relationship to the annual ring increment.
Analyses for six samples of ponderosa pine are present'ed
in table 4. The multiple correlation and percent unex-
plained variance exhibit no clear relationship with site.
The Moccasin Mesa and forest border samples werecol-
lected 5 and 17 mi. from the weather stations, so it might
be expected that these series would not exhibit a high re-
lationship with the weather records especially during t,he
summer months. It was noted that the south-facing
slope chronology from Idaho Springs contained some non-
climatic variability probably related to site disturbance.
This is a possible explanation for the low correlation and
high unexplained variance obtained for that sample.
The variability in the intervals which relate to growth
is sudiciently high to obscure any single pattern that can
be discerned in the growth relationships for the different
sites. An average picture for all six analyses indicates
that low moisture and high temperature during December
through May is most significantly related to narrow rings
in ponderosa pine. Moisture and temperature during the
previous October through November, and current June
through July are next in importance while the climate dnr-
ing the previous June through September appears least
important. A high relationship between growth and the
widths of the three preceding rings is evident in the
samples of ponderosa pine.
Analyses for seven pinyon pine samples at Mesa Verde
National Park are presented in table 5, and a sample from
near Fort Valley is presented in table 6 . Ring widths in
all pinyon pine samples t i t Mesa Verde are generally
directly related to precipitation and inversely related to
temperature. The relationship of growth to the three
previous ring widths is less significant than for Douglas-fir
and ponderosa pine. Considerable consistency is ex-
hibited among the screening results for all eight samples.
The climate for the interval December through February
is either first or second in importance in all analyses.
The previous October through November climate is al-
most as consistently significant, followed by the climate
of March through May. Temperature and precipitation
relationships during the current June and July also appear
significant for the Mesa Verde samples.
The pinyon pine analyses differ from those of the other
species in that the autumn, winter, and spring climate from
October through May consistently account for a very large
portion of the ring-chronology variance. The climate of
the current June and July exerts a smaller influence. The
absence of a relationship with the climate during October-
November and June and July at Fort Valley is conceiv-
ably a result of the distance between the weather station
and the sampled sites. This would introduce error espe-
cially in the correlation with summer and early autumn
climate.
The results of an analysis of a single sample of bristle-
cone pine (Pinus aristata Engelm.) growing at its extreme
lower limits on Mount Evans, Colo., are presented in
table 6. A low multiple Correlation ancl high percent
unexplained variance in this analysis may be attributed
both to possible site disturbance ancl to the distance and
difference in elevation between the sample and the Iclaho
Springs weather station. However, the mdysis suggests
that the tree-ring chronology of this high elevation species
relates most closely to the climate of the previous June,
previous August through September, current March
through May, and current July. The winter climate is
significant but assumes a low order of importance. Since
growth of this species appears to relate primarily to the
highly variable summer climate, i t is not surprising that
correlations with the records of the relatively distant Idaho
Springs weather station are low.
The above analyses show that variations in tree-ring
widths from four southwestern conifer species clearly
relate to variations in climatic parameters. There is a
consistent, direct relationship of ring width with pre-
cipitation and an inverse relationship with temperature.
The latter is less important, and in many cases, tempera-
ture appears to influence growth only if moisture is
present in the soil. Opposite relationships occasionally
appear in regression analyses, but coefficients exhibiting
these opposite relationships are infrequent and not highly
significant. Narrow rings in Douglas-fir and ponderosa
pine imply low precipitation and high temperatures
throughout the entire year, with a somewhat greater
weightplacedon the climate of autumn, winter, and
spring. A narrow ring in pinyon pine implies a dry,
warm previous autumn, winter, and spring and a hot
June or July. A narrow ring in bristlecone pine implies
a dry, warm climate during the year with greatest weight
placed on the spring, summer, and autumn periods.
3. A SUGGESTED MODELFORTHEPHYSIOLOGICAL
RELATIONSHIPSCAUSINGRING-WIDTHGROWTH
TOCORRELATEWITHVARIATIONSINCLIMATE
There are several recent studies which support and
complement the above results. In two studies [7, 81
changes in tree-ring characteristics along a gradient
July 1965 Harold C. Fritts 421
FIGURE 1 .-A
V
D e c r e a s e d C e l l E n l a r g e m e n t
f r o m M e r i s t e m a t i c T i s s u e
a n d D i f f e r e n t i a t i o n
Decreosed
T e r m i n a l
Growth
Respiration
N e e d l e a n d S t e m ->Less Photosynthetic
A r e a P r o d u c e d *P h o t o s y n t h e s i s
D e c r e o s e d C o n c e n t r o t i o n s
o f Growth Hormones -_____>L e s s A s s i m i l a t i o n
Shorter
Growing
o f c e l l p a r t s 6 Fbz:s ,",::E:',",,<- Consumption
lncreosed
o f F o o d s
X y l e m C e l l s
Narrower
Differentiated
R e d u c e d C a m b i a l
Activity
A N A R R O W T R E E R I N G I S F O R M E D
. schematic.diagram of the model hypothesized for the relationships between low precipitation and high
production of a narrow tree ring.
temperatures and the
from the forest interior to the forest border are analyzed.
A decrease in the forest density, which indicates increased
site aridity, is associated with greater year-to-year vari-
ability in the tree-ring chronologies, higher correlations
among trees, and higher variances in common among
trees within sites. The authors suggest that on semiarid
sites, climate is most frequently limiting to plant processes,
and therefore the variations in ring widths exhibit, the
highest relationship to variations in climate. Schulman
[12] had previously recognized this phenomenon and
applied it to dendroclimatic investigations by selecting
and studying only trees from semiarid sites.
A study by Fritts, Smith, and Stokes [6] represents
a detailed analysis of the biological and statistical rela-
tionships dealt with in this report. Among other things
they analyze radial growth and needle length in pinyon
pine and Douglas-fir, and relate them to site factors and
the climatic regime. They analyze statistical samples
from Mesa Verde (three of which appear in this report)
and describe the procedures and resulks of screening
analyses. They apply their findings to the reconstruc-
tion of a drought sequence at' Mesa Verde during pre-
historic times, using a Douglas-fir chronology which
extends from A.D. 435 to A.D. 1963.
These authors propose that the formation of a wide or
narrow tree ring is largely a function of the available food
supplies which are initially manufactured by photo-
synthesis and accumulated throughout the previous year.
Climate influences ring growth primarily through its
control of photosynthesis and other processes affecting
the accumulation of stored foods. They suggest that
cambial activity and growth in the spring and early
summer usually continue until the stored food is expended.
However, they recognize that climatic conditions can
sometimes directly induce the cessation of growth. The
proposed model is schematically diagramed in figure 1.
Precipitation is the primary climatic control but tem-
perature may modify its influence. Both factors affect
the water relations o f ' the tree, which in turn, govern
photosynthesis and the accumulation of food. Tempera-
ture may also directly influence food accumulation
through its control of photosynthesis, respiration, and
the assimilation of foods in the plant. Fritts [5] presents
direct evidence that during hot and dry periods food
consumption frequently exceeds food production in
conifers on lower forest border sites, but during cooler
and more moist periods, photosynthesis rates are higher
and respiration rates lower, water relations less critical
and the net accumulation of photosynthate may be
greater. High rates of net photosynthesis have been
measured during winter days, even though the trees may
be frozen a t night [5].
The left portion of the diagram in figure 1 shows how
water relations during the time of growth may also
influence ring width. Water stress may reduce cell
enlargement in the meristems of the growing buds. This
428 MONTHLYWEATHERREVIEW Vol. 93, No. 7
may reduce the concentration of hormones that are pro-
duced by the meristems, and these low hormone concen-
trations may reduce cambial activity or cell enlargement
and cause the ring to be narrow. High water stress
may also decrease the number and size of new stems and
leaves, and thus affect the amount of new photosynthetic
area that is produced. With less photosynthetic area,
the efficiency with which the tree responds to climate is
reduced, so that less food can be made, and the rings
during subsequent years will be proportionately narrower.
This relationship could produce the serial correlation
which is evident in the above statistical analyses.
It may be concluded from the results of the individual
screening analyses that there is a close relationship between
the widths of annual rings from trees on semiarid sites and
variations in the aridity of the yearly climates. Growth
analyses and preliminary results from physiological studies
indicate that these relationships may be largely attributed
to the accumulation of stored foods in the tree during
periods when neither moisture nor temperature are limit-
ing to the food-accumulating processes. Therefore, the
ring width represents an integration of the favorableness of
the environment of approximately a year's duration,
though direct effects of environment on growth and rela-
tionships producing serial correlation are complicating
factors. In general it may be inferred that the wider the
ring, the more moist and cool was the climate.
Chronologies from different species and sites may
measure somewhat different seasonal periods. For ex-
ample, the ring widths of extremely low elevation trees,
such as lower forest border pinyon pine, may be con-
trolled largely by moisture during the autumn, winter, and
spring because high summer temperatures rapidly evapo-
rate soil moisture and are unfavorable for accumulation of
foods. Likewise, ring widths of high elevation arid-site
trees, such as bristlecone,pine, may be more influenced by
precipitation during spring, summer, and autumn because
extremely low temperatures during the winter period
prevent photosynthesis and food accumulation from oc-
curring at that time. Since most dendroclimatic work has
dealt with sites a t intermediate elevations, it may be
inferred that most chronologies provide relatively reliable
estimates of the occurrence of drought for the entire year
beginning in June and ending in the following June or July.
Schulman ' [12] came to the same general conclusion,
though he did not consider temperature as a factor and his
analyses accounted for less variance.
4. DENDROCLIMATIC ANALYSIS OF WESTERNNORTH
AMERICA
Tree-ring chronologies from 26 stations through western
North America were selected for their location, length,
and reliability (table 7). Douglas-fir chronologies were
generally selected, but in some areas, the best chronologies
available were for other species. All chronologies were
converted to standardized ring-width indices, so they
are relatively free of changes arising from growth and
maturation of individual trees.
Whenever possible, chronologies based upon only a few
trees were avoided for they exhibit the most error. How-
ever, chronologies l , 3, 10, 15, and 24 (table 7), which are
based upon the fewest number of trees, are strategically
located and are used only for lack of better data. Recent
analyses indicate that the error in a small sample, such
as five trees, may not be excessive if care is taken to select
comparable sites, but any further reduction in sample size
grestly inflates the error component. Because the earliest
portions of most chronologies, especially the small samples,
may be derived from a very limited number of trees, these
early chronologies undoubtedly contain the most error.
This error is further inflated by the use of archaeological
materials to extend chronologies into still earlier times.
All 26 chronologies include the years 1651 through 1920
TABLE 7.-Tree-ring chronologiespresented i n table 8 andplotted on maps i n figures 2-10. BCP=bristlecone pine, BCS=big conespruce (Pseudotsuga macrocarpa (Vasey) Mayr), DF=Douglas-jir, JP=Jeffrey pine (Pinus jeffreyi Grev. and Balf.), LP=limber pine (P. flexilis James), Pnn= pinyon pine, PP= ponderosa pine __ __
No.
"_
2 1
3 4 5 6 7 8
10 9
11 12
13
15 14
16 17 18
20 19
21
23 22
24 25 26
.. -.
I
N A M E
I NORTH
Durango, Mexico .................................................. 23 48 Big Bend, Tex .................................................... 2913 Casas Grandes, Mexico ............................................ 30 07 Southern Arizona ..................................................
33 59 S. Calif., Big Cone Spruce
32 14
37 12 Mesa Verde Natl. Park, Colo
37 24 White Mts.. Calif
35 12 Flagstaff, Ariz
36 41 Navajo Natl. Monument. Ariz
35 43 Upper Rio Grande, N . Mex
34 10 San Bernardino Mts.. Calif
32 56 Cloudcroft, N . Mex ...............................................
.........................................
.......................................
........................................
.....................................................
....................................
......................................
.................................................
Arkansas River, Colo .............................................. 38 South Platte River, Colo .......................................... 39 N.E. California .................................................... 40 S.E. Oregon .......................................................
45 Upper Missouri DF, Mont ........................................
44 Snake River, Idaho ................................................
42
Banff, Canada 51
Columbia River, Wash 48
Fraser River. Canada-. ........................................... 51
............................................
.....................................................
~~~~ ~
E . Central California
52 Jasper, Canada ....................................................
45 Upper Missouri LP, Mont ........................................ .40 North Platte River, Colo .......................................... 39 Nine Mile Canyon, Utah ..........................................
37
~~
..............................................
29 21 25
03
12
39
58
09
55 45 46 55 19 54
LONGITUDE WEST
105 36 103 19
110 17 108 22
105 45
106 06 117 25
117 00 111 40
108 28 11032
118 10
105 57 105 35 120 38
112 51 120 20
119 29 111 33
11529 121 52 119 02 11027 106 19 111 12 118 04
SPECIES
D F D F
DF, PP, Pnn D F D F BCS DF, PP, Pnn P P P P
D F D F
BCP
D F D F P P P P DF D F D F , P P D F
D F , P P JP D F D F LP D F
of TREES MAX. NO
15 5
27 6
60 8
11 12 23 5 12 10
16 15 5 15 20 52 29 33 41 14 18 6 21 8
PERIOD USED
1641-1940 1646-1945 1526-1960 1416-1950 1516-1960 1386-1950
1351-1930 911-1930
576-1960 701-1955
801-1950 73-1945
1431-1950 1426-1940 1486-1930 1456-1930 12%-1950
1651-1940 1306-1950
1421-1940 1461-1950
1356-1940 1201-1945 1336-1945
1541-1945 981-1950
SOURCE (Ref. No.)
ScbUlman (Un- [el
pubhsbed data).
12-, Table 45
.12'
Table 40 12'
Table 36 12'. , Table 75 12'
Table 76 .lZ', , Table 44
[l2 : Table 31 12: ' Table 33
[12', Table 28
12:, Table 77
12' Table 41
12:: Table 30
k,
11
Table 43
July 1965
il
2
E
a w E
430
h
m
0
July 1965 Harold C. Fritts 431
7761172 0-65"5
432 MONTHLY WE
A.D., so this 270-yr. period was chosen as a standard
interval. The mean and the standard deviation for the
indices in each chronology were calculated for this
standard interval. Then, for the entire length of each
chronology, mean indices were calculated for 10-yr. periods
starting with the year 1 and 6 of each decade. Because of
tbe lag in the response of growth to climate, it is thought
that the tree growth interval starting with the year 1
corresponds most closely to the climate of the usual decades
which begin with the year ending in a zero.
Each 10-yr. mean is converted to a relative departure
by subtracting the mean and dividing by the standard
deviation of the chronology during the 270-yr. standard
interval. This subtraction and division largely correct
for differences in the variability and means of the 26
series, but do not eliminate the influence of serial correla-
tion. As a result, the 10-yr. relative departures some-
times appear overestimated in those series which exhibit
the highest serial correlations.
The relative departures for the 26 chronologies are
presented in table 8. The data for the interval 1501
through 1940 were plotted on maps, and lines of equal
values were drawn through positive and negative depar-
tures of 0.2, 0.6, 1.0, 1.4, and 1.8. Care was taken to
draw lines based exclusively on the data and smoothing
was avoided except in cases where wide discrepancies
occur among chronologies in close proximity (numbers 6
and 8, 12 and 22, and 18 and 25). In such cases, lines
were drawn according to the mean for the two departures.
The series of maps showing chronology location and the
lines of equal departure are presented in figures 2-10.
The areas with a positive growth departure exceeding
0.6 are shaded with vertical lines. The areas with a
negative growth departure exceeding 0.6 are shaded with
dots. I n t h e following discussion, conditions of high
precipitation and low temperatures, are inferred from
positive growth departures and are referred to as moist
climates. Conditions of low precipitation and high
temperatures are inferred from negative growth depar-
tures and are referred to as dry climates.
Random error or statistical noise is unavoidable in
such an analysis, especially in the earliest portions of
certain chronologies. This error creates discrepancies in
individual chronologies and should not be considered as
climatically significant. For example, a period such as
1786-1795, which exhibits numerous small high and low
growth areas, provides little information on the regional
climate. However, most of the maps show large areas
of similar departure and gradual changes over great
distances. These major regional similarities and changes
are most probably a result of similarities and changes in
climate.
Figure 2 contains maps for the first half of the 16th
century, a period characterized by the persistence of dry
conditions centering in the Colorado River Basin but by
alternating wet and dry conditions to the north.
1501-1515: Moisture is below average in the Inter-
ATHER REVIEW Vol. 93, No. 7
mountain area but above average to the
south.
1511-1525: Dry conditions occur farther east, centering
in the north-central Rockies and the Colorado
River Basin.
1521-1530: Dry conditions intensify in the Southwest, and
the Northwest becomes moist.
1526-1535 : The Lower Colorado continues to be extremely
dry, and the Northwest and Intermountain
areas become dry.
1531-1550 : The central Pacific and northern Rockies
become moist; the Colorado River Basin
remains dry.
1541-1555: The prolonged dry climate in the Colorado
River Basin is finally broken by moist condi-
tions developing in areas along the Mexican
border and the central Rocky Mountains.
The Northwest becomes dry.
The maps in figure 3 show the last half of the 16th
century, which was a period with extensive areas of wet
and dry extremes.
1551-1565: Moist conditions intensify in the north-central
Rockies while the climate of the Rio Grande
Basin and Canada become dryer.
1561-1570: The area from the Colorado River to. the
Canadian border and the eastern slope of the
southern Rocky Mountains becomes more
moist, while the Upper Rio Grande Basin
becomes dry.
1566-1585: Dry conditions intensify in the Southwest and
northern Rockies; a major drought develops
until it finallyextends throughout theentire West.
1581-1605: Dry conditions become more restricted to the
Rocky Mountain areas as moist conditions
develop in the Rio Grande and Gila River
Basins and in the Northwest.
Figure 4 contains maps for the first half of the 17th
century, which was also a period with extensive areas of
moist and dry extremes.
1601-1625: An extensive, extremely moist climate expands
throughout the central portions of the West.
1616-1635: A dry climate develops in Canada and the
Pacific Northwest and gradually spreads
southward as the area of moist climate moves
northeastward.
1631-1645: Moist conditions which persist in the eastern
and southern Rocky Mountain areas begin to
intensify and expand as dry conditions to the
west diminish.
1641-1650: Moist conditions intensify in the Rio Grande
and Upper Colorado River Basins, expand
into the southern and north-central portions
of the West, but do not reach their previous
maximum extent.
1646-1655: The extreme conditions which prevailed over
western North America for a century become
more localized and confined to the Southwest.
July 1965 Harold C. Fritts
434 MONTHLY WEATHER Vol. 93, No. 7
4 8 8
July 1965 Harold C. Fritts 43 5
s P e
4 36 MONTHLYWEATHERREVIEW Vol. 9 3 , No. 7
July 1965 Harold C. Fritts
438 MONTHLYWEATHERREVIEW Vol. 93, No. 7
I I
SI
I
b 9
July 1965 Harold C. Fritts 439
3 P
440 MONTHLYWEATHERREVIEW Vol. 9 3 , No. 7
July 1965 Harold C. Fritts 441
442 MONTHLY WE
Figure 5 covers the last half of the 17th century, a
period exhibiting little climatic change.
1651-1660: Moisture is above average in the Rio Grande
Basin and dry conditions prevail over the
northern Rockies.
1656-1690: Northern California, sou thern Oregon, and
southern Idaho receive above average mois-
ture but the Upper Colorado River Basin is
generally dry. In other areas only local
changes of short duration occur.
1686-1705: The climate of the Northwest is first dry but
becomes wet while in the extreme Southwest
the climate is first wet, then becomes dry.
Below average moisture conditions persist in
the Upper Colorado River Basin and central
Rocky Mountains.
Figure 6 includes maps for the first half of the 18th
century, a period initially exhibiting little climatic change.
Later, more extreme conditions developed.
1701-1715: Above average moisture continues in the
Northwest. The Southwest is somewhat dry.
1711-1730: A moist climate in the Upper Rio Grande
Basin expands into the central Rocky Moun-
tains, while the Northwest becomes dry.
1726-1740: Moist conditions move farther north into the
Rocky Mountains as dry conditions intensify
in the Southwest.
1736-1755: Dry conditions occur along the Canadian
border and in the central Rockies, but are
replaced by an extensive moist climate which
moves from south to north.
The maps in figure 7 cover the last half of the 18th
century, which was a period exhibiting complex patterns
of moist and dry conditions.
1751-1760: Dry conditions develop in southern California
and the northern Rockies.
1756-1765: The entire-Pacific Coast becomes dry, while
Mexico is moist.
1761-1775: The climate of the West becomes more moist,
especially in areas along the Mexican border
and in the northern Rockies.
1771-1790: Dry conditions developing in northern Cal-
ifornia and the Great Basin cover the area
west of the Rocky Mountains.
1786-1805: Dry conditions move to the Pacific Northwest
and extend into the Canadian and northern
Rocky Mountains. The climate of California
and the Colorado River Basin becomes more
moist.
Figure 8 covers the fist half of the 19th century, a
period of more persistent and extensive climatic change.
1801-1820: The Pacific Northwest and northern Mexico
become moist; the northern Rockies and the
Colorado River Basin become dry.
1816-1830 : Dry conditions persist in California and the
Southwest, the climate of the Pacific North-
west becomes more average, and areas of the
ATHER REVIEW Vol. 93, No. 7
Rocky and Sierra Madre Mountains become
moist.
1826-1841: The Rocky Mountains, the Southwest, and
Mexico become moist.
1836-1850: Dry conditions develop over the Pacific Coast
and expand throughout the Far West. Moist
conditions persist in the Upper Colorado
River Basin and in the central Rockies.
Figure 9 includes maps for the last half of the 19th
century, which was a period of fluctuating but below average
moisture.
1851-1865: Dry conditions intensify in California and
throughout the Rocky Mountain chain.
1861-1875: California becomes moist but the Canadian
and Mexican border regions remain dry.
1871-1880: Dry conditions prevail east of the Sierra
Nevada and the Cascade Mountains and
throughout the Rocky Mountain Range, but
central California and the Canadian Rockies
are moist.
1876-1890: The Colorado River and central Rio Grande
Basins are dry. The Canadian Rockies
remain moist.
1886-1900: Southern California and the Lower Colorado
River Basin become moist, and dry condi-
tions intensify in the Rio Grande Basin and
extend northward from the Upper Colorado
to the Pacific Northwest.
1896-1905: Dry conditions return to the Colorado River
Basin; the Southwest and Rocky Mountain
areas are generally dry.
The maps in figure 10 span the first 40 years of the
20th century, a period starting with abundant moisture
and ending with a widespread drought.
1901-1910: The Pacific Coast and northern Rockies be-
come moist.
1906-1915: Extremely moist conditions prevail through-
out western North America.
1911-1935: Drier conditions develop in the Pacific
Northwest, intensify, and move southwest-
ward as the area of extreme moist climate
diminishes.
1931-1940: Extremely dry conditions extend throughout
the West except for the area of southern
New Mexico and Chihuahua.
These maps represent a tentative reconstruction of the
patterns in climate during the past four and a half cen-
turies. As new and better data are added the maps will
undoubtedly be revised. The author has outlined his
methods and results which employ only dendroclimatic and
botanical techniques. Verification from other lines of
evidence should also be considered. A preliminary search
of these sources indicates more agreement than disagree-
ment. However, many of these data are qualitative
and difficult to evaluate in terms of the precisely dated
chronologies derived from tree rings.
The maps represent only a mode@ beginning of dendro-
July 1965 Harold C. Fritts 443
climatic investigation. More stations must be sampled
and incorporated in the present study, and earlier time
periods should be analyzed. However, there are fewer
chronologies during these early periods and errors may be
greater, for fewer specimens are available and these
specimens are likely to be relatively short archaeological
pieces. Because of these difficulties, mapping of the
climate prior to the 16th century was not attempted.
However, plots and comparisons of the early data show
climatic variations comparable to those mapped in figures
2-10.
It is hoped that this kind of investigation will provide a
basis for relating the short periods of recorded weather to
climatic patterns of the past as well as provide clues to
the nature and causes of climatic change. It is reasonable
tqinfer that this type of analysis may be applied to other
arid or cold areas of the world. A series of such analyses
on a global scale would provide highly valuable regional
information on long-term fluctuations in atmospheric
circulation and on possible factors influencing the heat
balance and water economy of marginal lands.
ACKNOWLEDGMENTS
This work was supported by the U.S. Weather Bureau, contract
Cwv-10798. The analyses of the tree-growth relationships were also
sponsored by the National Geographic Society and the U.S. Depart-
ment of Interior, National Park Service, through the Wetherill Mesa
Archaeological Project. The author acknowledges the able assist-
ance of David G. Smith and Richard L. Holmes. He is also
indebted to past and present staff at the Laboratory of Tree-Ring
Research for the development of the regional tree-ring chronologies,
and to the Numerical Analysis Laboratory, The University of
Arizona, for free computing time and services. He wishes to express
his appreciation for the help of James A. Erdman, Maurice E.
Cooley, and Julie McMahan, who contributed to various phases of
the project.
REFERENCES
1. H. S. Colton, Precipitation about the San Francisco Peaks,
Arizona, Museum of Northern Arizona Technical Series, No. 2,
1958, 18 pp.
2. A. E. Douglass, Photographic Tree-Ring Chronologies and the
Flagstaff Sequence, Tree-Ring Bulletin, vol. 14, No. 2,1947,
3. H. C. Fritts, An Approach to Dendroclimatology: Screening
by Means of Multiple Regression Techniques, Journal of
Geophysical Research, vol. 67, No. 4, Apr., 1962, pp. 1413-1420.
4. H. C. Fritts, Computer Programs for Tree-Ring Research,
TreeRing Bulletin, vol. 25, Nos. 3-4, 1963, pp. 2-7.
5. H. C. Fritts, The Continuation of A Study of the Physiological
Relationships Underlying Correlations of Tree-Ring Width and
Climate, Report and Proposal to the National Science
Foundation, GB-1025, Jan. 1965, 22 pp.
6. H. C. Fritts, D.G. Smith, and M. A. Stokes, The Biological
Model for Paleoclimatic Interpretation of Mesa Verde Tree-
Ring Series, in Contributions of the Wetherill Mesa Ar-
chaeological Project, Assembled by Douglas Osborne, Memoir
of the Society for American Archaeology, vol. 2, Part 2, 0th
1965 (in press).
7. H. C. Fritts, D. G. Smith, J. W. Cardis, and C. A. Budelsky,
Tree-Ring Characteristics Along a Vegetational Gradient in
Northern Arizona, Ecology, vol. 46, 1965 (in press).
8. H. C. Fritta, D. G. Smith, C. A. Budelsky, and J. W. Cardis,
The Variability of Ring Characteristics Within Trees aa Shown
by a Reanalysis of Four Ponderosa Pine, Tree-Ring Bulletin,
vol. 27,1965, (in press).
9. E. Schulman, An 800-Year Douglas-Fir at Mesa Verde,
Tree-Ring Bulletin, vol. 14, NO. 1, 1947, pp. 2-8.
10. E. Schulman, Dendrochronology at Navajo National Monu-
ment, Pree-Ring Bulletin, vol. 14, No. 3,1948, pp. 18-24.
11. E. Schulman, Dendrochronology in Northeastern Utah,
Tree-Ring Bulletin, vol. 15, Nos. 1-2, 1948, pp. 2-14.
12. E. Schulman, Dendroclimatic Changes in Semiarid America,
University of Arizona Press, Tucson, 1956,142 pp.
13. S. D. Scott, Tree-Ring Dating in Mexico, Ph. D. dissertation,
University of Arizona, Tucson, 1963, (pp. 137-138).
14. T. L. Smiley, S. A. Stubbs, and B. Bannister, A Foundation for
the Dating of Some Late Archaeological Sit.es in the Rio Grande
Area, New Mexico, University of Arizona Bulletin, vol. 24,
No. 3, 1953, pp. 54-55.
15. M. A. Stokes, Dendroclimatology of the Sacramento Moun-
tains, New Mexico, U.S. Air Force Report, Contract
pp. 10-16.
AFlg(604)-7474, 1961,7 pp.
[Received Februury 6, 1966; revised May 17, 17661
New Weather Bureau Publication
ResearchProgressand Phns of the U.S. WeatherBureau,Fiscal Year 1$64,
Washington, D.C., Jan. 1965, 150 pp. For Sale by Superintendent of Docu-
ments, U.S. Government Printing Office, Washington, D.C., 20402. Price
$1 .oo.
Annual report of progress made on the research program of the Weather
Bureau in the fiscal year ending July 1964, and plans for continued research
in fiscal year 1965.