沙漠研究 24-1, 223-230 (2014)
Ἃ₍◊✲ 24-1, 223 - 230 (2014)
Journal of Arid Land Studies
Journal of Arid Land Studies－ DT11 Refereed Paper －㸫DT11 Refereed Paper㸫

Evaluation of Drought Tolerance of Selected Provenances of Taxodium
Geoffrey C. DENNY*1) and Michael A. ARNOLD2)
Abstract: Screening studies of open-pollinated families in a greenhouse suggest a geographic component to variation in droughttolerance of Taxodium distichum. The open-pollinated families from mesic eastern localities were less tolerant of drought thanopen-pollinated families from more xeric western populations. Further drought screenings suggest that T. distichum seems to avoid
drought by limiting water loss from the shoots and tolerate drought tolerance by osmotic adjustment. The field screening supported
the conclusions of the greenhouse-based studies that western populations of Taxodium distichum are generally more drought tolerant
than eastern populations. Field performance under xeric conditions improved as populations were sampled from east to west in the
U.S. and then south into Mexico, following a general environmental gradient of decreasing precipitation. The implication is thatwhen choosing Taxodium for use in more xeric conditions, care should be taken to select western genotypes.
Key Words: Baldcypress, Montezuma cypress, Pondcypress, Taxodium distichum, Water relations

1. Introduction
Taxodium distichum (L.) Rich. is a widely adaptable tree
species for landscape use, tolerating both wet and dry soils, andair pollution (Cox and Leslie, 1988; Wasowski and Wasowski,1997). Watson (1983) reports tolerance to varying nutrient
availability conditions, a wide range of soil aeration levels, andsomewhat extreme pH levels. It is fast growing, has reliablefeathery foliage, and a nice form (Arnold, 2002; Cox andLeslie, 1988). Two varieties, var. distichum (baldcypress) andvar. imbricarium (Nutt.) Croom (pondcypress), have fairly
good fall color some years, while var. mexicana Gordon(Montezuma cypress) remains semi-evergreen (Arnold, 2002).
It is an extremely long-lived tree, with a life span of up to 700
years possible (Cox and Leslie, 1988). All of these factors
allow T. distichum to tolerate many environmental stresses,
making it a promising choice for urban landscapes. However,there are a few limitations to this species. While it is tolerantof substantial soil salts, it tends to defoliate when leaves comeinto contact with salty irrigation water, tends to develop
chlorosis on sites with high pH, and has a tendency to “brownout” in periods of extended or severe drought (Arnold, 2002).
Urban surfaces and compacted soils frequently decrease
the amount of water that infiltrates into the root zone of trees;
moreover, trees must compete with turf and other vegetation
for the available water (Zwack and Graves, 1998). Therefore,water deficit situations can be common in urban areas.
Zwack and Graves (1998) also point out a need for “tree taxa
that maintain landscape function during episodes of variable
and adverse soil moisture”. St. Hillaire and Graves (2001)* Corresponding Author: gcd42@msstate.edu

suggested that a strategy for selecting ornamentals with“superior resistance to drought stress” was to select from
populations native to relatively xeric habitats. The purpose ofthis study was to determine if there is a geographic basis fordrought tolerance in Taxodium and to evaluate selected
provenances in an effort to select those which yield individuals
that are most adaptable/tolerant to this environmental stress.

2. Materials and Methods
Open-pollinated family identity was coded with fouralphanumeric characters. The first two letters signify thegeneral geographic origin of the mother tree. ‘MX’ signifies
south Texas and Mexico, ‘TX’ signifies central Texas, and ‘EP’
denotes the southeastern U.S. The numeral is unique to an
open-pollinated family from a given geographic area. The
final letter indicates the taxonomic variety. ‘M’ indicates thatthe open-pollinated family belongs to the variety mexicanum,‘D’ indicates var. distichum, and ‘I’ indicates var. imbricarium.
2.1. Greenhouse screening 1
Thirteen open-pollinated families of Taxodium distichum
were collected in the late summer and fall of 2003. Seeds
were collected off a single mother tree at several locations (Fig.
1) representing the ecophysiographic variation throughout the
species’ range. After collection, seeds were stratified for 90 d
at 2ºC. Localities representing ‘normal’ seed sources (mesic
eastern U.S. sites), as well as sites representing more xeric
environmental conditions (western U.S. and Mexican sites)
were sampled. Seeds were planted in 36 cm × 51 cm × 10
cm deep flats (Kadon Corp., Dayton, Ohio) filled with medium
㸦Received, September 15th, 2013; Accepted, November 12th, 2013㸧

Box 9555, Mississippi State, MS 39762
1) Plant & Soil Sciences Dept., Mississippi State University2)Dept. of Horticultural Sciences, Texas A&M University

224

hourly using dielectric soil moisture probes (Decagon Devices,
Inc., Pullman, Wash.).
Growth and morphology (height, trunk diameter, dry mass,root:shoot ratio and time to mortality), as well as xylem water
potential data were analyzed using univariate analysis in the
GLM procedure of SPSS (version 12.0.2 for Windows, SPSS
Inc., Chicago, Ill.). Hierarchical cluster analysis usingsquared euclidean distance as a measure and thenearest-neighbor method in SPSS utilizing pre-dawn water
potentials and mortality was used to generate dendrograms.

Fig. 1. Locations of mother trees providing seeds for open-pollinatedfamilies of Taxodium distichum used in drought tolerance
screenings.
Symbols represent collection sites andopen-pollinated family identity is indicated beside symbol.

vermiculite (Sun Gro Horticulture, Bellevue, Wash.) on 12 Mar.
2004, and germinated before the end of Apr. 2004. On 8-10
Apr. 2004, 200 seedlings of each open-pollinated family weretransplanted into 9.6 L containers (Nursery Supplies, Inc.,
Kissimmee, Fla.) filled with 3 pine bark : 1 coarse perlite (byvolume) substrate amended with 6.53 kg m-3 15N-3.9P-9.9K
controlled-release fertilizer (Osmocote® Plus, Scotts Co.,
Marysville, Ohio), 0.89 kg m-3 micronutrient fertilizer
0N-0P-0K-6Ca-3Mg-12S-17Fe (Micromax®, Scotts Co.,
Marysville, Ohio), 1.78 kg m-3 CaSO4 (United States Gypsum
Co., Chicago, Ill.) and 4.15 kg m-3 CaMgCO3 (Oldcastle Stone
Products, Thomasville, Pa.). Plants were grown outdoors
under 55% light exclusion in a nursery area and irrigated byhand as needed.
Initial drought tolerance evaluations were conducted in agreenhouse beginning 6 June 2005. Containers werearranged in a completely randomized design. Plants were
subjected to a regime of decreasing irrigation frequency,
beginning with a daily watering, followed by a 2 d interval
between irrigations, then a 3 d period, etc. The study wasterminated 8 Aug. 2005, after the 10 d interval between
irrigations. Plant height and trunk diameter, as well as shootand root dry mass were taken on the last day of the experimentto evaluate plant growth and biomass partitioning. Heightand diameter measurements were taken at the initiation of theexperiment and at its end. Pre-dawn xylem water potential
was measured just before irrigation at the end of each dry
down cycle using a pressure chamber (Model 610, PMS
Instrument Company, Albany, Ore.). Volumetric watercontent of five containers chosen at random was measured

2.2. Greenhouse screening 2
Four open-pollinated families of Taxodium distichum wereselected for screening in the spring of 2006 (Fig. 1). Families
were selected to represent the ecophysiographic variation
between the “Mexican” type populations (extreme south Texas
and Mexico, Family MX5M) and those from central Texas(Families TX1D, TX2D, TX5D) because of the superior
performance of genotypes from these regions in the initial
screening. Cuttings off multiple trees per family from a stock
block maintained in the field in College Station, Texas wererooted on 20 March 2006. Cuttings were treated with a 8000
mg L-1 IBA and 4000 mg L-1 NAA dip (Dip ‘n Grow, Inc.,
Clackamas, Ore.) and were placed in 36 cm × 51 cm × 10 cm
deep flats (Kadon Corp., Dayton, Ohio) filled with coarse
perlite (Sun Gro Horticulture, Bellevue, Wash.). Rooted
cuttings were planted on 12 May 2006 into 2.5 L containers(Nursery Supplies, Inc., Kissimmee, Fla.) filled with calcined
clay (Oil-Dri Corp. of America, Alpharetta, Ga.) amended with
6.53 kg m-3 of 15N-3.9P-9.9K controlled release fertilizer(Osmocote® Plus, Scotts Co., Marysville, Ohio), 0.89 kg m-3
micronutrient
fertilizer
0N-0P-0K-6Ca-3Mg-12S-17Fe(Micromax®, Scotts Co., Marysville, Ohio), 1.78 kg m-3 CaSO4(United States Gypsum Co., Chicago, Ill.), 4.15 kg m-3
CaMgCO3 (Oldcastle Stone Products, Thomasville, Pa.).
Plants were grown in a greenhouse with 26.7ºC / 23.9ºC
day/night temperature set points. Typical light levels as
measured in mid-afternoon on 30 Aug. 2006 were 702 ȝmolm-2 s-2 PAR.
Drought tolerance evaluations were conducted in thegreenhouse beginning on 4 September 2006. Containers
were arranged in a completely randomized design. Plants
were subjected to an acute drought stress by withholding water.
Plants within a family were harvested when at least half of thetreated plants in that family showed foliar death. Plant height
and trunk diameter, as well as shoot and root fresh and dry
mass, were measured at the end of the experiment to evaluate
plant growth and biomass partitioning. Pre-dawn xylem
water potential was measured at harvest using a pressurechamber. Volumetric water content at harvest of all the225containers was calculated from fresh and dry masses andvolume measurements of the substrate. The mass of thewater present in the containers was calculated as the difference
between the fresh and dry mass of the substrate. The densityof water was assumed to be 1.0, allowing the easy conversion
from mass to volume. The volumetric water content of thesubstrate was then calculated with the following formula:
V% = (volumewater / volumesubsrate) × 100. Growth andmorphology (height, trunk diameters, dry masses, root:shootratios, shoot and substrate water contents, and days tomortality), as well as xylem water potential data, wereanalyzed using univariate analysis in the GLM procedure of
SPSS (version 12.0.2 for Windows, SPSS Inc., Chicago, Ill.).
2.3. Pressure-volume curves
In spring 2006, three open-pollinated families of Taxodium
distichum were selected which represent the ecophysiographic
variation throughout the tested species’ range (Fig. 1). The
genotypes used represented seed sources from the southeastern
U.S. (family EP8D, Vidalia, La.), central Texas (family TX6D,
Atascosa River, Texas), and the Rio Grande Valley of Texas(family MX5M, Progresso, Texas). Plants were grown as
described in section 2.2. Containers were arranged in acompletely randomized design on a single bench during
growth and were irrigated as needed.
On 18 Sept. 2006, three rooted cuttings from each of thethree selected families were used to perform a pressure-volume
analysis as described by Turner (1988). Care was taken to
ensure that shoots had comparable amounts of foliage pershoot, as is suggested by Neufeld and Teskey (1986). Shoots
were cut and allowed to rehydrate to full turgor in distilledwater for 18 h in the dark at 5.5ºC. Fresh mass (FW) of eachcutting was measured followed immediately by its xylem
water potential beginning at the end of the rehydration period
and then every 30 min thereafter until xylem water potential
reached -4.0 MPa. The initial fresh mass is referred to fromhere on as TW. Dry mass (DW) of each cutting was alsomeasured. Relative water content (RWC) of the cuttings wascalculated using the following formula:
RWC=[(FW-DW)÷(TW-DW)]×100
Water contents, fresh mass to dry mass ratios, and xylem
water potential data were analyzed and parameter estimates
generated using univariate analysis in the GLM procedure in
SPSS (version 12.0.2 for Windows, SPSS Inc., Chicago, Ill.).
2.4. Field screening
Open-pollinated families of Taxodium distichum werecollected in the late summer and fall of 2003. Seeds from asingle mother tree at all the locations on Figure 1, representing
the ecophysiographic variation throughout the species’ range

were collected and stratified (90 d at 2ºC). Localities
representing “normal” seed sources (mesic, acidic eastern U.S.
sites), as well as sites representing more extreme
environmental conditions (more xeric, alkaline western U.S.
and Mexican sites) were sampled. Plants were grown as
described in section 2.1.
The field site was located at the Texas A&M Research andExtension Center at Overton (USDA hardiness zone 8a). The
soil at the site is a Bowie very fine sandy loam, 1% to 4%
slopes and has a pH of approximately 6.5. The trees were
irrigated as needed the first year only. Seedlings were planted
on 29 June 2004. Plants were arranged in a randomized
complete block design with 13 families in 20 blocks containing
2 replications of each family per block. Tree heights andtrunk diameters were measured at the time of planting andagain in the next three Decembers. Growth indices for bothheight and trunk diameter were calculated as follows: growthindex = (new measure – previous measure)/previous measure.
This is analogous to relative growth rate calculations, except it
is based on non-destructive measures rather than dry masses(Arnold et al., 2007). Growth data were analyzed using
univariate analysis in the GLM procedure and hierarchicalcluster analysis in SPSS (version 12.0.2 for Windows, SPSS
Inc., Chicago, IL.). Survival data were analyzed with the
Chi-square procedure in SPSS.

3. Results and Discussion
3.1. Greenhouse screening 1
There were significant differences in pre-dawn water
potentials among families after 5, 6, 7, 8 and 10 d of imposed
drought (P  0.05) (Fig. 2). Additionally, there was asignificant difference in the mean survivable water deficit
among families (P  0.05) (Fig. 3). A hierarchical cluster
analysis of the families based on pre-dawn xylem water
potentials from the 5, 6 and 7 d drought periods (Fig. 2) and thesurvivable drought period of each family (Fig. 3) generated adendrogram showing the relationship among families based on
their performance in this screening (Fig. 4). The parameters
utilized in this analysis were selected because theopen-pollinated families showed the most separation during
the 5, 6 and 7 d drought periods. After the 7 d drought period,
some of the individuals exhibited canopy death, leading to aless negative pre-dawn xylem water potential measurement.
Time to canopy death was selected as a parameter in aneffort to account for this phenomenon. The dendrogram
divides the families into two main groups. The eastern
populations all fall into one group (Families EP2D, EP4D,
EP5I, EP6D and EP8D) and the Texas (Familes MX5M,
TX2D and TX8D) and Mexican (Families MX3M and226

Fig. 4. Dendrogram generated by a hierarchical cluster analysis
based on pre-dawn xylem water potentials from the 5, 6 and 7
d drought periods and the maximum survivable drought
period showing the relationship among thirteen
open-pollinated families of Taxodium distichum.

Fig. 2. Pre-dawn xylem water potentials of 13 open-pollinatedfamilies of Taxodium distichum after 5 (A), 6 (B) and 7 (C) ddrought periods. Values represent means of three observations± standard errors.

Fig. 3. Maximum survivable drought period of thirteen
open-pollinated families of Taxodium distichum. Values
represent means of three observations ± standard errors.

MX4M) populations fall into the other. The exception is the
open-pollinated family (TX6D) from Poteet, TX, whichclusters with the eastern populations. Within the cluster ofeastern populations there are two groups. The first includes
populations that all belong to the variety distichum, and thesecond includes families from both variety distichum (TX6D,
Poteet, TX) and var. imbricarium (EP5I, Fowl River, AL andEP6D, Biloxi, MS). No significant differences in root toshoot ratios were found (P = 0.372).
The clustering of the families suggests that there is ageographic component to variation in drought tolerance of
Taxodium distichum. The observed geographic pattern iswhat might be expected. The open-pollinated families fromeastern localities were less tolerant of drought thanopen-pollinated families from western populations. This is
likely due to a general trend in decreasing rainfall as we movefrom east to west in the southern U.S.
The implication is that when selecting genotypes for more
xeric situations, an effort should be made to obtain genotypes
from central Texas or Mexico. Additionally, open-pollinatedfamilies from south Texas and Mexico appeared less stressed attimes of xylem water potential measurement, although no data
were taken on general appearance because of its subjective
nature.
3.2. Greenhouse screening 2
There were significant (P  0.05) treatment effects related
to water deficits among the families in all the parameters
measured. Well-irrigated control plants did not differ
significantly among open-pollinated families. Treatment
plants from family MX5M had 50% survival after 11 d without
irrigation, while families TX1D, TX2D and TX7D had 50%
survival after only 8 d. Volumetric water content of the227

Fig. 5. Volumetric water content of the substrate at the time of
harvest for four open-pollinated families of Taxodium
distichum. Values represent means for eight observations ±standard errors.

Fig. 6. Water content of shoots based on dry weight at the time of
harvest for four open-pollinated families of Taxodium
distichum. Values represent means for eight observations ±standard errors. Dark bars represent control plants, while light bars
represent drought treatment plants.

substrate at the time of harvest was significantly lower (P 
0.01) for family MX5M than the other families, which did not
differ significantly from each other (Fig. 5).
Shoot water content at harvest was significantly lower (P 
0.01) for family MX5M than the other families (Fig. 6).
Shoot water content of family TX1D was significantly higher(P  0.05) than all other families (Fig. 6). Families TX2D
and TX5D did not differ from each other, but did differ fromfamilies MX5M and TX1D.
An estimated substrate water depletion rate was calculatedby dividing the difference between the mean substrate
volumetric water content for the well watered control plants nd
the observed substrate volumetric water content for thetreatment plants by the number of days to harvest. Family

Fig. 7. Substrate volumetric water content loss rate for fouropen-pollinated families of Taxodium distichum. Values
represent means for eight observations ± standard errors.

a MX5M showed a lower estimated water depletion rate thanthe other families (P  0.01) (Fig. 7). No significant
difference was found between the root to shoot ratios of thefamilies (P = 0.11) (data not shown).
These results support the observation in the initial
screenings that the Mexican families appeared less water
stressed compared to the central Texas families. After similardrought periods, the Mexican genotypes had higher watercontents per unit dry mass. They were able to withstandlonger droughts than central Texas families because they wereable to survive at lower substrate volumetric water contents.
They also removed water from the substrate at a lower rate
(Fig. 7), implying that they are better at controlling water loss
from their shoots. The Mexican genotype was also able toextract more water from the substrate (Fig. 5). This suggeststhat Taxodium may utilize both drought tolerance and drought
avoidance as mechanisms for resisting drought stress.
3.3. Pressure-volume curves
Pressure-volume analysis allows many plant-waterparameters to be derived including: total water content,
turgid/dry mass ratio, relative water content, apoplastic andsymplastic water contents, relative symplastic water contents,
osmotic pressure at full and zero turgor, relative water content
at zero turgor, bulk moduli of elasticity, and tissue moisturerelease curves (Turner, 1988). It also provides the neededparameters to create a Höfler diagram (Turner, 1988).
Pressure-volume curves have been used extensively to
examine many aspects of plant-water relations by numerous
authors (Fan et al., 1994; Roberts et al., 1981; White et al.,2001). Li (1998) utilized pressure-volume analysis tocompare leaf water relations of Eucalyptus microtheca F.

228

Table 1. Analysis of covariance table for the rates of shoot relative
water content decrease, xylem water potential decrease, andchange in the fresh to dry mass ratio in threeopen-pollinated families of Taxodium distichum.
Measured22
Characteristic r
Relative watercontent

Xylem water
potential

0.94

0.83

Fresh : Dry
M ass Ratio

0.72

Xylem water
potential

0.84

Table 2. Parameter estimates for the rates of shoot relative watercontent decrease, xylem water potential decrease, andchange in the fresh to dry mass ratio in threeopen-pollinated families of Taxodium distichum. The
values in column b are the parameter estimates generated byanalysis of covariance.

S ource

S ignificanceM odelIntercept<0.001<0.001

MeasuredCharacteristic

F amily
Family
Time
T
ime

0.958
0.958
<0.001<0.001

Family X TimeM odelIntercept<0.001<0.001<0.001

FamilyRWC
Family X RWC
M odel<0.001<0.001<0.001<0.001

Intercept
Family<0.001<0.001TimeT
ime
Family X TimeM odel<0.001<0.001<0.001<0.001

Intercept
FamilyF
amily<0.00100.063
.063

Time
T
ime
Family X Time

<0.001
<
0.001
0.039

Family(M X5M )Family(TX6D)
Relative water
Family(EP8D)
content
Family(M X5M ) X Time
Family(TX6D) X Time
Family(EP8D) X Time
Family(M X5M )Family(TX6D)
Xylem water
Family(EP8D)
potential
Family(M X5M ) X RWCFamily(TX6D) X RWCFamily(EP8D) X RWCFamily(M X5M )Family(TX6D)
Fresh : Dry M ass Family(EP8D)
Ratio
Family(M X5M ) X Time
Family(TX6D) X Time
Family(EP8D) X Time
Family(M X5M )Family(TX6D)
Family(EP8D)
Xylem water
potential
Family(M X5M ) X Time
Family(TX6D) X Time
Family(EP8D) X Time

Muell. provenances.
Relative water content of each of the samples wascalculated for each measurement point. The rate of relative
water content (RWC) loss was significantly different among
the eastern family (EP8D) and both the central Texas (TX6D)
and south Texas (MX5M) families (P  0.001) (Table 1).
The families from Texas did not significantly (P  0.05) differ
in RWC loss rates. The difference in rate of RWC loss
among the three families in the study supports the findings ofthe previous screenings of the open-pollinated families.
Eastern families tended to desiccate (decrease in RWC) more
rapidly than south Texas families, while families from central
Texas tended to be intermediate. The relationship between theplant water potential and RWC is referred to as the water
potential isotherm or the moisture release curve for a tissue(Turner, 1988). It has been used to determine the drought
resistance characteristics of various species (Jones et al., 1981).
Xylem water potential also differed significantly inresponse to decreasing RWC among all three families (Table 1).
The south Texas family (MX5M) showed the largest decreasein xylem water potential per unit change in RWC, followed by
the central Texas family (TX6D) and then the eastern family(EP8D). The south Texas family has the steepest moisturerelease curve (Table 2), implying that its tissues retain water
more tightly than the other families. The eastern family

Parameter

95% Confidence Interval
Lower
Upper
Bound
Bound
99.61
97.63
101.5999.62
97.64
101.61
99.99
97.87
102.11
-4.18
-4.85
-3.51
-5.79
-6.47
-5.12
-11.25-12.1-10.39-14-15.62-12.38
-7.94
-9
-6.88
-4.9
-5.55
-4.26
0.13
0.11
0.15
0.06
0.05
0.08
0.03
0.03
0.04
3.33
3.18
3.47
3.36
3.22
3.51
4.11
3.96
4.26
-0.09
-0.14
-0.04
-0.14
-0.18
-0.09
-0.33
-0.39
-0.28
-0.69
-0.91
-0.48
-0.98
-1.19
-0.76
-1.04
-1.27
-0.81
-0.56
-0.63
-0.48
-0.45
-0.52
-0.37
-0.42
-0.51
-0.33

b

has the shallowest moisture release curve implying that thewater in these plants is held the least tightly. The moisturerelease curve of the central Texas family was intermediate.
Xylem water potential of family MX5M decreases slightly
faster than those of either of the other two families (Table 2).
The fresh mass to dry mass ratio decreases more rapidly in theeastern family (EP8D) compared to the Texas families (Table2). The turgid to dry mass ratio has been shown to correlate
well with osmotic adjustment in some species (Turner, 1988).
This is logical because the higher the ratio is the more water
per unit dry mass the plant contains. However, this may be
due to higher osmotic potentials or to more elastic cells.
Differences in the rate of change in the ratio between freshmass and dry mass over time can indicate the concentration ofosmolytes in the leaf tissue. This may be the case here. As
the tissues began to desiccate, the south Texas (MX5M) familyshows only a slight decrease in the fresh to dry mass ratio andthe slope of the line does not differ significantly from 0 (Table2). The central Texas family (TX6D) also shows only a slightdecrease in fresh to dry mass ratio during desiccation.
Although the slope of the regression line for this family does
not differ significantly from that of the south Texas family(MX5M), it is less than zero (Table 2). The slope of the linear
regression for the eastern family (EP8D) differs from those ofboth Texas families and is significantly less than zero (Table 2).

229

Fig. 10.
Fig. 8. Percent cumulative survival of 13 open-pollinated families of
Taxodium distichum for 2005 (A) and 2006 (B) in Overton,
Texas.

Fig. 9. Height growth index for 13 open-pollinated families of
Taxodium distichum in 2005 at Overton, Texas. Symbols
represent means ± standard error of 40 observations.

This means that as the tissue desiccates it holds relatively lesswater than it did while it was wetter. This would not be
expected if the plant was utilizing osmotic adjustment as a
strategy to resist drought, which would likely give the opposite
result.
3.4. Field screening
There was significant variation in tree survival in thesecond and third growing season at the Overton field site (Fig.
8). The Chi-square test for survival in both seasons foropen-pollinated family was highly significant (P  0.0001).
In both years, the western genotypes generally had higher
survival percentages than genotypes from more mesic, eastern

Trunk diameter growth index for 13 open-pollinatedfamilies of Taxodium distichum in 2005 (A) and 2006 (B) at
Overton, Texas. Symbols represent means ± standard errorof 40 observations.

sources (Fig. 8). This pattern is especially striking in the third
season (2006) cumulative survival where none of the easternfamilies had above 25% cumulative survival (Fig 8 b).
There was significant variation in the height growth index(Fig. 9) and the trunk diameter growth index for 2005 amongfamilies (P  0.0001) (Fig. 10). In 2006, only the variation intrunk diameter growth indices was significant (P  0.0001).
A similar pattern to that observed in the survival percentages
was evident. The western families grew faster in height (Fig.
9) and trunk diameter during 2005 and in trunk diameter
during 2006, when compared to eastern genotypes (Fig. 10).
This pattern is similar to that observed in thegreenhouse-based drought screenings. These results support
the conclusions of the greenhouse-based studies that western
populations of Taxodium distichum are generally more droughttolerant than eastern populations. Field performance under
xeric conditions improved as populations were sampled fromeast to west in the U.S. and then south into Mexico, following ageneral environmental gradient of decreasing precipitation.

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