Transcript Module 3: Local Environmental Gradients

Module 3: Local Environmental Gradients
SEE-U 2001, Biosphere 2 Center, AZ
Professor Tim Kittel, TA Erika Geiger
Yuko Chitani
Mei Ying Lai
Lily Liew
Asma Madad
Adam Nix
Eli Pristoop
J.C. Sylvan
Introduction
Individuals of populations are adapting to survive. How are
ecological gradients defined? An ecological gradient can be defined
as a change in an environmental factor, such as water availability, soil
pH, salinity, temperature, over a specified distance. In our sampling
we explored the water gradient of Velvet Mesquite. We sampled two
different extremes with the water gradient based on water availability;
one was in a topographically low area that has the most available
moisture for the Velvet Mesquite. The other area that we sampled was
on a ridge, which is a high topographic area. The comparison of the
two areas illustrates the affects of the two extreme gradients on the
Velvet Mesquite.
How do local populations respond to ecological gradients? Not all
differences between populations are ecotypes since some differences
are simply the result of plasticity, the phenotypic as opposed to
genetic response to different environmental conditions. If the
differences between or among populations are not genetically based,
then are they influenced by environmental factors? Furthermore, what
could different traits be between the higher topography Velvet
Mesquite and the lower topography Velvet Mesquite. What possible
differences could the two environments have, if the Velvet Mesquite
has different traits?
Site A = Arroyo
Site B = Ridgeline
Natural History of Mesquite
•
•
•
•
The flowers are pollinated mainly by bees, and the resulting
pods are produced in huge quantities in good years. Fallen
pods are quickly infested with bruchid beetle larvae, and
eaten by a variety of larger animals. Germination is enhanced
by passage through the guts of large animals; otherwise a few
years of weathering is needed to release the seeds from the
endocarp. The foliage is also an important browse for
numerous animals. Mesquite is long-lived, probably a couple
of centuries in favorable sites.
Honey mesquite
(Prosopis glandulosa var. torreyana)
The mesquite’s root system is the deepest documented; a live
root was discovered in a copper mine over 160 feet (50 m)
below the surface. Like all known trees, however, 90% of
mesquite roots are in the upper 3 feet of soil. This is where
most of the water and oxygen are. The deep roots presumably
enable a mesquite to survive severe droughts, but they are
not its main life support.
Dense mesquite stands are called bosques (pronounced
BOSE case). Once abundant on floodplains in the southwest
U.S., most have been cut down or killed by rapid lowering of
water tables. Only scattered remnants still exist. In the low
desert, velvet mesquite is restricted to flood plains and large
washes. At higher elevations it also occurs on dry hillsides.
On the rocky slopes of Arizona Upland it is sparse and
dwarfed to shrub size.
•
(Phillips 2000)
Site A = Arroyo
Site B = Ridgeline
Methods
Prosopis velutina, commonly known as the Velvet Mesquite, was chosen
as the focus species for a study of a lowland wash area and an elevated
ridge area. Two study sites of differential moisture levels were chosen
upon the Biosphere 2 Campus. Study Site A was a wash region located
behind the Arroyo apartments and Study Site B was an elevated ridge
located near the Biosphere 2 Hotel. A total of 10 Prosopis trees were
sampled at each site through utilization of the linear transecting method.
A measured distance was marked parallel to a geological feature at each
site, and the trees that transected this distance were marked and sampled.
Samples were not collected from shaded and/or juvenile trees.
Ten leaf samples were collected from the side of the tree with the highest
sun exposure, which in the case of Site A trees, was the southern side.
Site B trees had maximal sun exposure on all sides, but only the southern
side was sampled for consistency in analysis. The leaves were harvested
between 110 and 138 for all the trees at both sites in order to ensure
consistent sampling. After the leaf samples were collected from each
specimen, the GPS was collected for each individual Prosopis studied
within both sites.
Four characteristics were analyzed from the collected leaf samples using
the Independent Samples T-test once repeat measures had been averaged
for each tree: number of pinnae per leaf, length of each pinnae, length of
mid-leaflet per pinnae, and width of mid-leaflet per pinnae. Tables
(“Comparative Analysis of the the Morphological Characteristics of
Prosopis Velutina” and “Statistical Data” below) were generated for both
sites utilizing both the Excel and SPSS program.
Site A = Arroyo
Site B = Ridgeline
SAMPLING AREA
Ridgeline
Arroyo
Site A = Arroyo
Site B = Ridgeline
Results
In order to determine whether there is a significant
environmental water gradient with respect to the Velvet Mesquite
in the local biome, we analyzed pinnate and leaflet characteristics
from our samples and discovered the following patterns:
1.
On average, samples taken from ridgeline trees have more pinnae per leaf (2.67
+/- 0.4246mm) than samples taken from velvet mesquite in the arroyo. (2.34 +/0.2959mm) Though, statistically speaking, we can only be 94% sure of this result
(Sig. P< 0.06).
2.
There is no significant difference (LP < 0.695), according to our data, between
the average length of individual pinnae sampled from both these groups (57.78
+/- 11.67mm vs 59.42 +/- 5.62mm).
3.
There is no appreciable difference between the lengths of the leaflets (LS <
0.867). On average, leaflets from ridgeline samples were 11.40 +/- 0.66 mm,
where arroyo samples were 11.55 +/- 0.54mm.
4.
However, the value for the average width of the leaflet is larger (3.51mm) from
the arroyo in comparison to the width of the leaflets from the ridgeline trees
(2.85mm). Leaflets tend to be 19% wider on trees near the arroyo (WS < 0.05).
5.
The compound values we calculated for this experiment also pointed to no
significant difference between populations. These values were, for B and A
populations respectively: average area of pinnate (1086.57 +/- 348.80mm)
(1002.04 +/- 381.52mm) (Sig. AP> 0.611); area of leaflet (27.45 +/- 9.02mm)
and (31.92 +/- 6.54mm) (Sig, AS > 0.223); area of leaf (2873.76 +/- 1111.59mm)
and (2494.96 +/- 604.43mm) (Sig. AL> 0.360).
Please view “Comparative Analysis of the the
Morphological Characteristics of Prosopis Velutina” and
“Statistical Data” below.
Site A = Arroyo
Site B = Ridgeline
Comparative Analysis of the Morphological Characteristics of
Prosopis Velutina from Two Different Environmental Gradients
Group Statistics
AVG # of Pinnea
AVG L. of Pinnea
AVG. L of Subleaf
AVG. W of Subleaf
Average Area of Pinnae
Average Area of
Sub-Leaflet
Total Area of Leaf
SITE
B
A
B
A
B
A
B
A
B
A
B
A
B
A
N
10
10
10
10
10
10
10
10
10
10
10
10
10
10
Mean
2.6710
2.3404
57.7766
59.4213
11.3975
11.5535
2.8493
3.5145
1086.5656
1002.0419
27.4472
31.9171
2873.7572
2494.9662
Std. Deviation
.4246
.2959
11.6677
5.6232
2.6792
1.0563
.6662
.5447
348.8029
381.5221
9.0196
6.5444
1111.5906
604.4344
Std. Error
Mean
.1343
9.358E-02
3.6897
1.7782
.8472
.3340
.2107
.1722
110.3012
120.6479
2.8522
2.0695
351.5158
191.1389
Site A = Arroyo
Site B = Ridgeline
Statistical Data
Independent Samples Test
Levene's Test for
Equality of Variances
F
AVG # of Pinnea
AVG L. of Pinnea
AVG. L of Subleaf
AVG. W of Subleaf
Average Area of Pinnae
Average Area of
Sub-Leaflet
Total Area of Leaf
Equal variances
assumed
Equal variances
not assumed
Equal variances
assumed
Equal variances
not assumed
Equal variances
assumed
Equal variances
not assumed
Equal variances
assumed
Equal variances
not assumed
Equal variances
assumed
Equal variances
not assumed
Equal variances
assumed
Equal variances
not assumed
Equal variances
assumed
Equal variances
not assumed
Sig .
1.124
2.145
1.390
.237
.000
.760
9.566
.303
.160
.254
.633
.999
.395
.006
t-test for Eq uality of Means
t
df
Sig . (2-tailed)
Mean
Difference
Std. Error
Difference
95% Confidence
Interval of the
Difference
Lower
Upper
2.020
18
.059
.3306
.1637
-1.33E-02
.6744
2.020
16.074
.060
.3306
.1637
-1.63E-02
.6774
-.402
18
.693
-1.6447
4.0958
-10.2496
6.9603
-.402
12.967
.695
-1.6447
4.0958
-10.4954
7.2061
-.171
18
.866
-.1560
.9107
-2.0693
1.7574
-.171
11.732
.867
-.1560
.9107
-2.1453
1.8334
-2.445
18
.025
-.6652
.2721
-1.2369
-9.35E-02
-2.445
17.316
.025
-.6652
.2721
-1.2386
-9.19E-02
.517
18
.611
84.5237
163.4694
-258.9129
427.9603
.517
17.857
.611
84.5237
163.4694
-259.1098
428.1572
-1.268
18
.221
-4.4699
3.5239
-11.8734
2.9336
-1.268
16.420
.222
-4.4699
3.5239
-11.9248
2.9850
.947
18
.356
378.7910
400.1218
-461.8337
1219.4157
.947
13.894
.360
378.7910
400.1218
-479.9983
1237.5803
Site A = Arroyo
Site B = Ridgeline
Discussion:
How Does the Environmental Water Gradient Affect the Leaf
Morphology of Velvet Mesquite Trees in the Sonoran Desert
Biome?
We sampled two groups of Velvet Mesquite trees, one in a wash area and one along a
ridgeline. What we were trying to establish was some kind of morphological difference between
these groups, so that we could formulate some hypotheses about the way that a water gradient
influences the leaf morphology of this species. What we discovered was that establishing these
differences is no simple task. Based on our sample size the only significant (and statistically viable)
conclusion we could draw is that trees that grow at higher, drier elevations tend to produce narrower
leaflets. Whether this is an effect the water table remains to be determined. The only other potential
difference we discovered was in average number of pinnate per leaf. Trees in the arroyo tended to
have less pinnae. We measure pinnae length, leaflet length, average pinnae area, average sub-leaf
area, and total leaf area, but we found that there was no meaningful statistical differences between the
two populations. However, we did notice some differences just through general observation, and
have made some hypotheses thereon. For instance:
In the lower elevations the Velvet Mesquite had flowers, and the Velvet Mesquite on the
ridgeline had no flowers. This could be due to the fact that at lower the elevation more moisture is
available, and at the higher the elevation less moisture is available. Many insects surrounding the
Velvet Mesquite depend upon the pollen in the flowers of the velvet Mesquite. In the lower elevation
we observed many insects present, however, in the higher elevations the presence of insects was not
obvious. We also observed that the pinnae from site A tended to be greener and the pinnae from site
B tended to be curvier. The influence that moisture has on the characteristics of Velvet Mesquite is
apparent, and this must have an effect on all of the surrounding species.
Site A = Arroyo
Site B = Ridgeline
Conclusion
Statistically, the morphology of Velvet Mesquite did not vary greatly
along a water gradient. The only statistically meaningful difference
we observed was that leaflets from a Velvet Mesquite in an
environment in which water was more readily available were wider
than leaflets from a Velvet Mesquite in a drier environment. Based on
the lack of morphological differences and the lack of genetic isolation,
we must conclude that the Mesquite trees that we sampled are
members of the same population. The different characteristics the
Mesquite had were a plasticity response rather then different genetic
coding.
In this exercise, we learned various new methods to collect and
analyze data:
• Linear sampling with line-intercept
• Stratified sample - taking samples from the same species across
different environments.
•Installing sampling control factors such as moisture, sunlight,
elevation, and sampling consistently within the 110° and 138° range.
• Anatomy of the Velvet Mesquite leaf.
•Labeling sample bags with GPS to avoid confusion.
•SPSS Statistical Analyses software, mean, standard deviation, and
significant error.
•Treating both the pinnae and the sub-leaf as ellipses:
A = pi(r1r2)
Site A = Arroyo
Site B = Ridgeline
References
Bowers, Janice Emily and Brian Wignall. 1993. Shrubs and Trees of the Southwest Deserts. Southwest
Parks and Monuments Association, Tucson.
Phillips, Steven J. and Patricia Wentworth Comus. 2000. A Natural History of the Sonoran Desert.
Arizona-Sonoran Desert Museum Press, Tucson.