United States 

Department of 

Agriculture 

Forest Service 

Intermountain 

Research Station 

Research Paper 

INT-373 

December 1986 

Adaptive Variation in 

Pinus ponderosa 

from Intermountain 

Regions. II. 

Middle Columbia 

River System 

G. E. Rehfeldt 

This file was created by scanning the printed publication.

Errors identified by the software have been corrected;

however, some errors may remain.
THE AUTHOR 

GERALD E. REHFELDT is a plant geneticist at the Intermountain Research Station's Forestry Sciences Laboratory 

in Moscow, 10. 

RESEARCH SUMMARY 

Seedlings representing 138 populations of ponderosa 

pine from drainages of the middle Columbia River system 

were grown and compared in common environments. Field 

studies of 3-year-old trees on mesic and xeric test sites 

detected population differentiation for traits reflecting 

growth and development. Populations that expressed the 

greatest growth potential under mesic culture suffered the 

greatest reduction in height under xeric culture. Conse- I 

quently, a different set of populations was performing the 

best on the two sites when 3-year height was expressed as 

the growth from a common height at age 2. 

Greenhouse studies of the periodicity of shoot elongation 

revealed that populations of high growth potential achieved 

a large stature by means of a long duration, late cessation, 

rapid rate, and large amount of elongation. 

Multiple regression models accounted for as much as 

60 percent of the variance between populations and 

described adaptive landscapes in which populations from 

low elevation have a high growth potential while those from 

high elevations have a low growth potential. Geographic 

variation at a common elevation indicated that populations 

of high growth potential come from areas where the frostfree period is long and precipitation is relatively high. Thus, 

growth potential is directly related to the length of the 

growing season, which can be truncated by either frost or 

moisture stress. 

Adaptive landscapes are used to construct guidelines for 

limiting seed transfer in artificial reforestation. In general, 

seed from a single source should not be transferred more 

than ±200 m in elevation. 

Intermountain Research Station 

324 25th Street 

Ogden, UT 84401 
Adaptive Variation in Pinus 

ponderosa fr0!'1lntermoun~ain 

Regions. II. Middle Columbia 

River System 

G. E. Rehfeldt 

INTRODUCTION 

Populations that occupy contrasting environments tend 

to differ genetically for numerous traits that convey environmental adaptation. In accordance, adaptive differentiation in the widespread ponderosa pine (Pinus ponderosa) 

is well documented (Wang 1977) and is related to the 

elevation and geographic origin of the seed. For just the 

Inland Northwest, populations from mild environments 

tend to have a high growth potential while populations 

from areas where growing seasons are short tend to have 

a low growth potential (Wang 1977; Madsen and Blake 

1977; Rehfeldt 1979b, 1980, 1986). 

Forest managers rely on geographic patterns of adaptive 

variation to estimate the distance that seeds can be transferred from their origin before maladaptation begins to 

limit the productivity of artificial regeneration. However, 

previous studies were of insufficient power and scope for 

providing managers with this information. Consequently, a 

series of studies was begun to determine the appropriate 

limits to seed transfer for the Intermountain Region. 

While the first study of this series concerned populations 

from central Idaho (results published in Forest Science, 

Rehfeldt 1986), the present involves populations from the 

middle Columbia River system of northeastern Washington, northern Idaho, and western Montana. 

MATERIALS AND METHODS 

Population differentiation was studied in seedlings from 

138 populations that sampled the ecologic, geographic, and 

elevational distribution within which the species is of commercial importance in the Inland Northwest (fig. 1). 

Genetic diversity of each population was sampled in a 

manner to assure that seedling populations represented a 

large number of parental trees. Most populations were 

represented by an equal volume of wind-pollinated cones 

from 10 trees. Some, however, were represented by 

samples of cones collected from several squirrel caches. 

Genetic diversity was assured by selecting cones with a 

variety of morphologies, sizes, and colors. Seedlings were 

used in separate studies of (1) growth and development in 

the field and (2) the periodicity of shoot elongation in a 

greenhouse. The results from both were used to assess 

patterns of genetic variation. 

Growth and Development 

Seedlings were grown for 6 months in plastic containers 

(65 cm3) in a shadehouse at Moscow, ID (lat. 48.5° N., 

long. 116.7° W.). In the fall, seedlings were planted at two 

sites at about 670 m elevation at the Priest River Experimental Forest, 190 km north of Moscow. Although both 

sites averaged 90 frost-free days, one was on sandy loam 

soil and was maintained under a mesic cultural regime. 

This site was tilled before planting, competing vegetation 

was controlled, and the planting was irrigated twice each 

summer during the course of the study. The second site, 

on glacial till, was maintained under xeric cultural conditions. Irrigation was provided early in the second growing 

season to ensure seedling establishment, but water supplemental to natural precipitation, which averages 81 cm, 

was withheld thereafter. This site was also tilled before 

planting, and competing vegetation, gophers, and white 

grubs were controlled. Nine seedlings, planted in row 

plots, represented each population in each of three blocks 

at each site. At the mesic site, 0.5 m separated rows and 

0.3 m separated trees within rows. To allow conversion of 

the test to a demonstration plot, the corresponding 

spacings at the xeric site were wider than at the mesic 

site: 1 m and 0.5 m. 

Population performance was described by five variables: 

1. Height, the 3-year height (centimeters) of individual 

trees. 

2. Leaf length, the length (centimeters) of a leaf from 

near the center of the 3-year shoot of an individual tree. 

3. Adjusted height, 3-year height (centimeters) of individual trees adjusted by regression on 2-year height, a 

value that depicts the 3-year height expected if all trees 

had been the same height at age 2. Consequently, the 

value is relatively independent of early environmental 

effects (such as seed weight or transplanting shock) and 

genetic effects and is capable of reflecting the adaptive 

response of populations to a particular environment in a 

relatively short time. 

4. Xeric height reduction, the amount (centimeters) by 

which the height of trees cultured under xeric conditions 

failed to reach the potential expressed under mesic conditions. The value was calculated as the difference between 

the xeric height of an individual tree and the mean mesic 

height of the population from which the individual 

originated. 
BRITISH COLUMBIA 

-- o 

I I 

o 100 MI 

+ 

Figure 1-Geographic distribution of ponderosa pine (shading) 

within the major drainages of the middle Columbia River system 

and location of sampled populations. Letters A to G locate the 

elevational clines presented in figure 2. 

5. Drought mortality, the proportion of trees within 

each plot that died from drought under xeric conditions. 

Analyses. of variance were performed on each variable 

according to a model in which the effects of populations, 

planting sites, and blocks within sites were considered as 

random variates. A harmonic mean of 7.31 reflected the 

number of seedlings in each block. 

Periodicity of Shoot Elongation 

Seedlings from 98 of the populations were grown for 

2 years in plastic containers (740 cm3) in a shadehouse at 

Moscow. Each population was represented by nine seedlings in each of three blocks. In mid-March of the second 

growing season, seedlings were moved into a greenhouse 

before shoot elongation had begun. Greenhouses were 

maintained under natural photoperiods and light intensities. Temperatures were held at about 24 ± 3 °C during 

the day and were allowed to cool to a minimum 13°C at 

night. All seedlings were measured three times each week 

until elongation was well under way. Thereafter, measurements were made twice each week until elongation of the 

preformed shoot had ceased. 

2 

As described by Rehfeldt and Wykoff (1981),' shoot 

elongation of individual trees was expressed by a modified 

logistic function with a hyperbolic time term: 

Y = [1 + be<-rX+clX)]-l 

where Y is the proportion of total increment attained by 

day X; b, r, and c are regression coefficients; and e is the 

base for natural logarithms. 

Regression statistics allowed calculation of the following 

variables for describing the periodicity of shoot elongation 

of individual seedlings: (1) initiation of elongation: the day 

on which 2 mm of growth had occurred; (2) cessation of 

elongation: the day on which all but 2 mm of growth had 

occurred; (3) duration of elongation: the number of days 

between initiation and cessation; (4) rate of elongation: 

elongation (millimeters) per day during the period of maximum elongation; and (5) total elongation (millimeters). 

Population differentiation was assessed according to an 

analysis of variance in which the effects of populations and 

blocks were considered to be random. A harmonic mean of 

8.83 reflected the number of seedlings from each population in each block. 
Patterns of Genetic Variation 

Multiple regression models were used to relate genetic 

variation between populations to the ~levation and geographic location of the seed source. Independent variables 

included elevation, latitude, longitude, northwest departure, southwest departure, and their squares. Squared 

values were used to accommodate the possibility .of nonlinear patterns of variation. Northwest and southwest 

departures were derived by rotating the grid of latitude 

and longitude by 45°. Elevation was considered without 

geographic interaction because preliminary analyses suggested that the relationship between performance and 

elevation was similar throughout the region of study. 

Thus, 10 independent variables were included in a stepwise 

regression for maximizing R2 (SAS 1982) according to the 

general model: 

Y i = (30 + ~ (31 Xij + ~ (32X~j J J 

where Y i is the performance of population i; Xij is independent variable J' for population i; and (30, (31, and (32 are 

regression coefficients, j = 1 ... 5. 

Adequacy of a model was judged according to the goodness of fit (R2), residual variance (Sy.x), patterns displayed 

by residuals (Draper and Smith 1981), and the degree by 

which results were biologically plausible. 

RESULTS 

The analyses detected differences between populations 

for numerous traits involving (1) growth and development 

and (2) the periodicity of shoot elongation. Genetic variation in these traits was related to the elevation and 

geographic origin of the seed. 

Growth and Development 

Planting sites had a tremendous effect on the growth 

and development of seedlings. Under mesic cultural conditions, the average 3-year-old tree was 46 cm tall with 

leaves 14 cm long, but under xeric conditions, the average 

tree was only 23 cm tall with leaves 7 cm long. Thus, xeric 

culture reduced the growth potential expressed under optimal growing conditions by an average of 23 cm. Values 

of adjusted height show that if trees on both sites had 

been the same height at age 2, those growing under the 

mesic regime would still have been 11 em taller at age 3 

than trees on the dry site even though 3-year height accounted for 69 percent of the variance in 2-year height. 

Because of these differences associated with cultural 

conditions, main effects of test sites accounted for most 

(68 to 85 percent) of the variance for those variables 

measured on both sites and, therefore, dominate analyses 

of variance (table 1). Nevertheless, differences between 

populations were detected for all variables measured on 

both sites even though the main effects of populations 

accounted for less than 5 percent of the total variance 

(table 1). 

The significant interactions (table 1) for height and adjusted height indicate that the differences between populations on one site were not the same as the differences 

between the same populations on the other site. Because 

these interactions accounted for nearly a third as much 

variance as the main effects of populations, subsequent 

analyses involving population means will consider height 

and adjusted height from each site as separate variables. 

By contrast, the lack of a significant interaction for leaf 

length shows that the relative performance of populations 

was similar at each site even though leaves were much 

shorter under xeric conditions. 

Populations also differed significantly in the reduction in 

height attributable to xeric culture. The absolute amount 

of reduction ranged from 28 cm (56 percent) for one _ 

population to 14 cm (43 percent) for another. But populations did not differ in mortality under xeric conditions 

because drought mortality reached only 16 percent and 

was confined to essentially one block. 

Periodicity of Shoot Elongation 

Shoot elongation of individual seedlings was completed 

between 31 and 53 days after being placed in the greenhouse. This means that 11 to 17 observations were 

Table 1-Results of analyses of variance of growth and development in field environments presented 

as intraclass correlations, the ratio of a variance component to the sum of all components 

Variable 

Source of 3-year Adjusted Leaf Xeric height 

variance height height1 length reduction2 

Sites 0.850* * 0.689* * 0.850* * 

Blocks in site .000 .002* * .008* * 0.006* * 

Populations .028* * .021 * * .004* * .317* * 

Sites x populations .010* * .009* * .002 

Experimental error .007* * .047* * .020* * .103* * 

Within plots .106 .232 .116 .574 

Error mean square3 107.433 72.969 5.824 3.119 

• 'Statistical significance of the F-value at the 1 percent level of probability. 

13-year height adjusted by regression on 2-year height. 

20ifference between xeric height and the growth potential expressed by mesic height. 

3Absolute value of the interaction mean square that was used to test for differences between populations. 

3 

Drought 

mortality 

0.977* * 

.012 

.010 

.027 
Table 2-Results of analyses of variance for the periodicity of shoot elongation presented as intraclass correlations, the ratio of a variance component to the sum of all components 

Variable 

Source of 

variance Elongation Initiation Cessation Duration Rate 

Replications 0.04* * 0.03* * 0.01 * * 0.01 * * 0.07* * 

Populations .20** .02* * .34* * .17* * .15* * 

Interaction .05** .09* * .16* * .03* * .06* * 

Within plots .71 .86 .49 .79 .71 

Error mean square 1 766.931 4.811 18.109 22.553 1.428 

*. Significance of the F-value at the 1 percent level of probability. 

1Absolute value of the interaction mean square that was used to test for differences between populations. 

I 

Table 3-Results of stepwise regressions for relating genetic variation to the elevation and geographic origin of the seed 

Number of 

independent Residual 

Variable variables R2 mean square 

Growth and development 

3-year mesic height 

3-year xeric height 

Adjusted mesic height 

Adjusted xeric height 

Leaf length 

Xeric height reduction 

Periodicity of shoot elongation 

Elongation 

Initiation 

Cessation 

Duration 

Rate 

•• Statistically significant at the 1 percent level. 

available for the logistic regression that described shoot 

elongation of individual trees nearly perfectly. Values of 

R2 ranged from 0.90 to essentially 1.0, averaging 0.99. 

Analyses of variance not only detected significant differences between populations for all variables (table 2) but 

also attributed substantial variance to the effects of populations for all variables except the initiation of shoot elongation. These effects were reflected in mean differences 

between populations as large as 9 days in cessation, 

10 days in duration, 2.8 mm per day in rate, 73 mm total 

elongation, but only 2.3 days in initiation. Although these 

differences seem small, culturing plants under optimal conditions allows shoot development to proceed rapidly, and 

developmental events are condensed into a short time 

interval. Under natural conditions development occurs 

over a long period and differences in the periodicity of 

shoot elongation become relatively large. 

Patterns of Genetic Variation 

By accounting for as much as 60 percent of the variance 

between popUlations, regression models were statistically 

significant for all variables except the initiation of shoot 

4 

5 

6 

6 

6 

5 

8 

5 

5 

6 

5 

6 

0.52* * 10.09 

.31 * * 4.55 

.17* * 2.14 

.13* * 1.91 

.10* * .20 

.40* * 6.20 

.49* * 86.98 

.09 .24 

.59** 1.74 

.58* * 1.89 

.43* * .14 

elongation (table 3) and drought mortality, two variables 

for which popUlation differentiation was not pronounced 

(tables 1 and 2). Thus, genetic differences between populations follow systematic patterns that can be related to the 

elevation and geographic origin of the seed. These patterns can be presented as (1) elevational clines for several 

geographic localities (fig. 2), (2) geographic clines at a constant elevation (fig. 3), or (3) geographic clines at the base 

elevation, the lowest elevation at which the species occurs 

in a given locality (fig. 4). 

These results, however, were based on five to eight independent variables and, therefore, are susceptible to 

overfitting, a condition that occurs when models are fit to 

single samples rather than to general trends described by 

all samples (Draper and Smith 1981). Consequently, in this 

paper, population differentiation is interpreted according 

to the least significant difference (Steel and Torrie 1960) 

at the 80 percent level of probability-lsd(0.2). A relatively 

low level of probability is used to guard against accepting 

no differences between populations when differences actually exist. Thus, in figure 2, populations separated by an 

elevational interval sufficient to subtend a mean difference 

equal to lsd(0.2) are judged to be statistically different at 
50 

40 

30 

27 

23 

19 

MESIC HEIGHT (em) 

I • • 

ADJUSTED XERIC HEIGHT (em) 

• • • 

• 

•• 

• -G F 

0 

, 

400 1200 2000 

40 

36 

32 

5 

4 

3 

400 

CESSATION (day) ..... 

•• # .. 

••• 1 

... t·.· ~ 

I : ~~ .. .~G 

• RATE (mm/day) 

I • 

• 

1200 2000 

ELEVATION OF SEED SOURCE (m) 

Figure 2-Elevational clines for four variables at seven geographic 

locations keyed to figure 1. Brackets quantify Isd (0.2). 

DURATION MESIC HEIGHT 

LEAF LENGTH ADJUSTED XERIC HEIGHT 

Figure 3-Geographic patterns of variation for four variables 

described by isopleths (contour lines) of equal performance at the 

mean elevation (1,075 my. The inteNal between isopleths equals 

1/2 [ Isd (0.2)]. Isopleths represent positive or negative deviations 

from the mean value (x) of aI/ populations. 

5 
Figure 4-Geographic patterns of variation for 

mesic height presented as isopleths of equal 

performance for base elevations, the lowest 

elevation at which ponderosa pine occurs in 

a given area. The interval between isopleths 

equals V2[lsd(O.2»). /sopleths represent 

positive or negative deviations from the mean 

(x) of all base populations. 

about the 80 percent level of probability. In figures 3 and 

4, the distance between isopleths (contour lines) is scaled 

to lh[lsd(0.2)]. This means that populations separated by a 

distance equivalent to two isopleths are assumed to differ 

with a probability of about 0.2. 

The relationship between the elevation of the seed 

source and population performance is presented in figure 2 

for four variables that span the range of responses for all 

variables. Not shown are the elevational clines for adjusted mesic height and xeric height reduction, which 

nearly duplicated that for mesic height. Clines for the 

cessation and amount of shoot elongation were similar to 

that of the duration of elongation, and clines for adjusted 

xeric height, leaf length, and the initiation of shoot elongation failed to depict differentiation in excess of lsd(0.2). 

These elevational clines indicate that growth potential 

decreases as elevation of the seed source increases. In contrast to populations from high elevations, those from low 

elevations tend to be tall, they exhibit a late cessation, 

long duration, and rapid rate of elongation, and under 

xeric culture, their growth is reduced the most. 

The elevational interval associated with a mean difference between populations that equals lsd(0.2) is approximately 440 m for the linear cline involving cessation of 

shoot elongation (fig. 2). Although the interval for the 

nonlinear clines would change as elevation changes (fig. 2), 

population differentiation at the mean elevation (1,075 m) 

can be detected across about 400 m for both mesic height 

and adjusted xeric height, but about 450 m for rate of 

elongation. 

Geographic patterns of variation at the mean elevation 

are illustrated in figure 3 for the four variables that represent the range of patterns. Patterns for all variables not 

6 

presented follow those for the duration of shoot elongation 

and mesic height. Thus, when comparing populations from 

the same elevation, populations from west central areas 

display the largest values of traits reflecting growth 

potential. From this area, growth potential declines in all 

directions. 

Patterns for adjusted xeric height and leaf length deviate from the general. Figure 3 shows that if all populations had been the same height at age 2, those from the 

dry lands to the east would have been tallest (largest adjusted xeric height) at age 3 on the xeric site. Geographic 

clines for leaf length indicate that southern populations 

have the longest leaves while those from the northwest 

have the shortest. 

Because geography ~nd elevation are not independent of 

each other, genetic variation between populations at the 

base elevation (fig. 4), the lowest elevation at which 

ponderosa pine occurs in a given locality, is arranged 

along much steeper clines than variation at a constant 

elevation (fig. 3). As judged by the number of and distance 

between isopleths, geographic clines at the base elevation 

are nearly twice as 'steep as those for a constant elevation. 

Figure 4 shows that populations of the highest growth 

potential should come from among base populations from 

the west central region. From this area, growth potential 

of base popUlations decreases regardless of whether elevation is decreasing toward the northwest or increasing 

toward the south or east. The average distance between 

differentiated populations occurs on a relatively small scale 

(54 km) across the landscape. 

Similarities in patterns of variation between the variables (figs. 2 and 3) result from intercorrelations, some of 

which are extremely strong (table 4). PopUlations that 

were tall under mesic conditions exhibited other characters 

associated with a high growth potential: a late cessation, 

long duration, fast rate, and large amount of shoot elongation. Although these same populations tended to be the 

tallest under xeric culture (table 4), they suffered the 

greatest xeric height reduction in that the growth potential expressed under mesic culture was reduced the most 

by xeric culture. 

DISCUSSION 

Results have demonstrated genetic differentiation between populations for a variety of loosely intercorrelated 

traits. These intercorrelations, such as those in Pseudotsuga menziesii (Rehfeldt 1983b), Pinus contort a (Rehfeldt 

1983a), and P. ponderosa from central Idaho (Rehfeldt 

1986), involve traits that are part of an annual sequence of 

developmental events. This sequence begins with dehardening in the spring, includes shoot elongation, leaf expansion, bud development, and lignification, and concludes 

with cold acclimation. The components are inter correlated 

because the entire sequence must fit into a growing 

season of finite length. Thus, populations that inhabit environments where growing seasons are short cease developmental events early and are, therefore, short, even 

when grown under optimal conditions. 

For ponderosa pine, a species that commonly occurs in 

habitats that border the prairie, the growing season may 
Table 4-Simple correlations among population means. Underlined coefficients failed to reach statistical significance at the 5 percent 

level. 

Variable Code, H3-X AH-M AH-X 

Growth and development 

3-year mesic height H3-M 0.74 0.56 0.31 

3-year xeric height H3-X .28 .69 

Adjusted mesic height AH-M .19 

Adjusted xeric height AH-X 

Leaf length LL 

Xeric height reduction RED 

Drought mortality DO 

Periodicity of shoot elongation 

Elongation EL 

Initiation IN 

Cessation CES 

Duration OUR 

Rate RATE 

be limited by drought or frost or both. The species inhabits environments that differ by over 60 days in frostfree period and by 60 cm in precipitation (fig. 5). That 

populations have become differentially adapted to environmental differences of this magnitude is an expected 

result of natural selection. 

Adaptive differentiation can be expressed by significant 

interactions involving populations and environments. However, the interactions that were evident for 3-year height 

and adjusted height arose for different reasons. On the 

one hand, the interaction for 3-year height represents a 

scale effect rather than an adaptive response. As evidenced by strong simple correlations (table 4), populations 

FROST-FREE PERIOD (days) 

LL RED DO EL IN CES OUR RATE 

0.30 0.84 0.02 0.76 -0.14 0.68 0.68 0.74 

.35 .25 - .18 .64 - .14 .48 .50 .64 

-.02 .57 .04 .31 -.02 .30 .29 .29 

.22 - .11 -.29 .24 -.01 .17 .16 .24 

.15 - .18 .21 - .18 .08 .12 .23 

.18 .55 -.08 .56 .56 .53 

-.07 .07 .03 .01 -.06 

-.37 .82 .87 .98 

-.06 -.29 -.37 

.97 .72 

.78 

retained similar rankings in each environment, but the absolute difference in height of any two populations was 

greater under mesic than under xeric conditions. 

On the other hand, the interaction for adjusted height 

reflects differential adaptation. Under mesic culture, 

populations of greatest growth potential would still have 

been tallest at age 3 even if all trees had beEm the same 

height at age 2. Thus, the superiority of these populations 

has increased between ages 2 and 3. But under xeric conditions, populations from dry eastern areas grew the most 

from a common height at age 2. This suggests that if 

xeric culture were to be maintained, these eastern populations would eventually become tallest on this particular site. 

PRECIPITATION (em) 

Figure 5-Regional environmental patterns of variation (enlarged from U. S. Department 

of Commerce 1968). 

7 
Regression models described population differentiation 

according to the elevation and geographic location of the 

seed source. As elevation increases, the length of the 

frost-free period rapidly decreases. Baker (1944) has 

shown that the frost-free period in the Inland Northwest 

varies by about 80 days across an elevational interval of 

1,000 m. Consequently, populations from low elevation 

generally exhibit a high growth potential associated with a 

long duration and late cessation of elongation. Populations 

from high elevation express a low growth potential largely 

because developmental events must be completed within a 

short frost-free period. These general trends are remarkably similar to those described by the height of 2-year-old 

trees growing in nursery beds (Madsen and Blake 1977) 

and also are similar to the patterns exhibited by populations from central Idaho (Rehfeldt 1986). For both central 

Idaho and the Inland Northwest, an elevational interval of 

about 400 m is sufficient for detecting (80 percent level of 

probability) population differentiation. 

Geographic patterns of genetic variation at either a 

constant or the base of elevation (figs. 3 and 4) were pronounced, consistent, and uniform. Moreover, these patterns roughly correspond to geographic patterns in 

precipitation and in the frost-free period (fig. 5). Populations of highest growth potential tend to occur in the 

west-central region where frost-free periods are long and 

precipitation is relatively high. From this area, growth 

potential of populations decreases in all directions in a pattern that is similar to that of the frost-free period. Toward 

the northwest, however, the rapid decline in growth potential is associated with only a small decline in the frost-free 

period but with a huge reduction in precipitation. Thus, 

the northwestern populations, which may originate from 

the lowest elevations (400 m), display a low growth potential. In addition, the eastern populations that exhibited the 

lowest growth potentials come from areas characterized by 

both low precipitation and a short frost-free period. 

The elevational cline is much steeper than the geographic. In figure 3, the distance between any two isopleths 

represents half the distance required to detect (80 percent 

level of probability) genetic differentiation at a constant 

elevation. Because differentiation can be detected between 

populations separated by about 400 m in elevation, the 

distance between geographic isopleths represents the 

amount of genetic variation that occurs across 200 m of 

elevation at a single locality. Thus, the total amount of 

geographic variation that occurs within the region at a 

constant elevation can also occur across 1,000 m at a 

single locality. 

Regardless, adaptive differentiation occurs along two 

interdependent clines. This means that populations of 

similar adaptive norms recur at different elevations across 

the landscape. As shown in figure 2, populations with a 

relatively long duration of shoot elongation, 38 days, occur 

at about 700 m at locality A (fig. 1), 750 m at E, 1,000 m 

at D, and 1,300 m at both A and B. Thus, similar genotypes are distributed across the landscape in a pattern 

that is oblique to elevation. 

8 

Because patterns of genetic variation are systematically 

related to environmental variation, one logically accepts 

the patterns as reflecting adaptive differentiation. As 

such, the patterns have direct application to artificial reforestation. To maximize productivity, planted trees must 

be adapted to the planting site. Adaptation is secured by 

limiting the distance that seeds are transferred from their 

origin. Consequently, limits to seed transfer must reflect 

geographic and elevational patterns of variation. 

One estimate of an appropriate limit to seed transfer is 

the smallest geographic or elevational interval across 

which differentiation is detected (Rehfeldt 1979a). Differentiation along the steepest elevational clines (fig. 2) 

suggests that a seed zone for ponderosa pine should encompass not more than 400 m of elevation. This means 

that seed from a single source should not be transferred 

more than ± 200 m. In addition, the geographic clines of 

figure 3 describe seed zones that should not encompass 

more than two geographic bands between isopleths. 

Approximately three geographic zones would be suitable 

for the region, and seeds from a single source should not 

be transferred a distance equivalent to more than ± 1 

band. 

Although seed zoning is an administratively simple procedure for limiting seed transfer, the procedure tends to 

be inflexible, inefficient, and uneconomical. This is because 

seed zones compartmentalize continuous genetic variation 

and fail to take advantage of the recurrence of similar 

genotypes at different elevations in geographically 

separated localities. 

By contrast, floating transfer guidelines are based on 

the recurrence of similar environments in a pattern that is 

oblique to elevation. According to floating guidelines, seed 

can be transferred across isopleths (fig. 3), but each time 

seed is transferred across a geographic interval equaling 

the interval between isopleths, the elevations at which the 

seed is to be used should be adjusted. When transferring 

across isopleths of high to low value, the interval should 

be adjusted downward by 200 m. When transferring from 

lower to higher value, the interval should be adjusted upward by 200 m. 

For example, assume that seed originates from 1,000 m 

on the isopleth representing the mean of all popUlations 

(fig. 3). This seed should be used between 800 and 1,200 m 

in lands adjacent to the isopleth. In transferring the seed 

across one isopleth of larger value, the seed should be 

used between 1,000 and 1,400 m. Transfers of two contours should be used between 1,200 and 1,600 m. And in 

transferring the seed across an isopleth of lesser value, 

the seed should be used between 600 and 1,000 m. In this 

way, seed from a single source or seed orchard can serve 

a much broader geographic area than under the concept of 

discrete seed zones. 

These recommendations for limiting seed transfer 

evolved from statistical models based on the performance 

of young trees under controlled conditions. The models 

need practical verification. On the one hand, the environmental events responsible for the systematic patterns may 
occur so infrequently that managers might risk transferring fast-growing populations into severe environments in 

an attempt to increase productivity. Indeed, genetic differentiation may have been detected at levels associated 

with productive differences so small as to be immaterial. 

On the other hand, small adaptive differences observed at 

young ages may portend large differences in the future. 

Regardless, verification can come only from planting programs that not only incorporate these guidelines but also 

maintain precise records on the exact location from which 

planted trees originated. Productivity of such plantings 

will test the applicability of these guidelines. 

REFERENCES 

Baker, F. S. Mountain climates of the western United 

States. Ecological Monographs. 14: 223-254; 1944. 

Draper, N. R.; Smith, H. Applied regression analyses. 

New York: Wiley and Sons; 1981. 407 p. 

Madsen, J. L.; Blake, G. M. Ecological genetics of 

ponderosa pine in the northern Rocky Mountains. Silvae 

Genetica. 26: 1-8; 1977. 

Rehfeldt, G. E. Ecological adaptations in Douglas-fir 

(Pseudotsuga menziesii var. glauca) populations. 1. North 

Idaho and northeast Washington. Heredity. 43: 383-397; 

1979a. 

Rehfeldt, G. E. Genetic variation in southern Idaho 

progeny tests after 11 years. General Technical Report 

INT-75. Ogden, UT: U.S. Department of Agriculture, 

~u.s. GOVERNMENT PRINTING OFFICE:1986-773-039/41030 9 

Forest Service, Intermountain Forest and Range Experiment Station; 1979b. 12 p. 

Rehfeldt, G. E. Genetic gains from tree improvement of 

ponderosa pine in southern Idaho. Research Paper 

INT-236. Ogden, UT: U.S. Department of Agriculture, 

Forest Service, Intermountain Forest and Range Experiment Station; 1980. 9 p. 

Rehfeldt, G. E. Adaptation of Pinus contorta populations 

to heterogeneous environments in north Idaho. Canadian 

Journal of Forest Research. 13: 405-411; 1983a. 

Rehfeldt, G. E. Ecological adaptations in Douglas-fir 

(Pseudotsuga menziesii var. glauca) populations. III. 

Central Idaho. Canadian Journal of Forest Research. 13: 

626-632; 1983b. 

Rehfeldt, G. E. A~aptive variation in Pinus ponderosa 

from Intermountain regions. 1. Snake and Salmon River 

basins. Forest Science. 32: 79-92; 1986. 

Rehfeldt, G. E.; Wykoff, W. R. Periodicity of shoot 

elongation among populations of Pinus contort a from 

the northern Rocky Mountains. Annals of Botany. 48: 

371-377; 1981. 

SAS Institute. SAS user's guide: statistics. Cary, NC: 

SAS Institute, Inc.; 1982. 584 p. 

Steel, R. G. D.; Torrie, J. H. Principles and procedures of 

statistics. New York: McGraw-Hill; 1960. 481 p. 

Wang, C. Genetics of ponderosa pine. Research Paper 

WO-34. Washington, DC: U.S. Department of 

Agriculture, Forest Service; 1977. 24 p. 

U.S. Department of Commerce. Climatic atlas of the 

United States. Washington, DC: U.S. Department of 

Commerce, Environmental Data Service; 1968. 80 p. 
Rehfeldt, G. E. Adaptive variation in Pinus ponderosa from Intermountain regions. II. 

Middle Columbia River system. Research Paper INT-373. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station; 1986. 9 p. 

Seedling populations were grown and compared in common environments. 

Statistical analyses detected genetic differences between populations for numerous 

traits reflecting growth potential and periodicity of shoot elongation. Multiple regression models described an adaptive landscape in which populations from low elevations have a high growth potential while those from high elevations have a low growth 

potential. Patterns of genetic variation reflect adaptation to a growing season of 

variable length. Adaptive landscapes are used to construct guidelines for limiting seed 

transfer in artificial reforestation. In general, seed from a single source should not be 

transferred more than ± 200 m in elevation. 

KEYWORDS: microevolution, genetic differentiation, seed zones, seed transfer 

INTERMOUNTAIN RESEARCH STATION 

The Intermountain Research Station provides scientific knowledge and technology 

to improve management, protection, and use of the forests and rangelands of the Intermountain West. Research is designed to meet the needs of National Forest 

managers, Federal and State agencies, industry, academic institutions, public and 

private organizations, and individuals. Results of research are made available 

through publications, symposia, workshops, training sessions, and personal contacts. 

The Intermountain Research Station territory includes Montana, Idaho. Utah. 

Nevada, and western Wyoming. Eighty-five percent of the lands in the Station area, 

about 231 million acres, are classified as forest or rangeland. They include 

grasslands, deserts, shrublands, alpine areas, and forests. They provide fiber for 

forest industries, minerals and fossil fuels for energy and industrial development, 

water for domestic and industrial consumption, forage for livestock and wildlife. and 

recreation opportunities for millions of visitors. 

Several Station units conduct research in additional western States, or have missions that are national or international in scope. 

Station laboratories are located in: 

Boise, Idaho 

Bozeman, Montana (in cooperation with Montana State University) 

Logan, Utah (in cooperation with Utah State University) 

Missoula. Montana (in cooperation with the University of Montana) 

Moscow, Idaho (in cooperation with the University of Idaho) 

Ogden, Utah 

Provo. Utah (in cooperation with Brigham Young University) 

Reno. Nevada (in cooperation with the University of Nevada)