Module 3: Local Environmental Gradients

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

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

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

MethodsProsopis 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

Arroyo

Ridgeline

Site A = Arroyo

Site B = Ridgeline

ResultsIn 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

10 2.6710 .4246 .1343

10 2.3404 .2959 9.358E-02

10 57.7766 11.6677 3.6897

10 59.4213 5.6232 1.7782

10 11.3975 2.6792 .8472

10 11.5535 1.0563 .3340

10 2.8493 .6662 .2107

10 3.5145 .5447 .1722

10 1086.5656 348.8029 110.3012

10 1002.0419 381.5221 120.6479

10 27.4472 9.0196 2.8522

10 31.9171 6.5444 2.0695

10 2873.7572 1111.5906 351.5158

10 2494.9662 604.4344 191.1389

SITEB

A

B

A

B

A

B

A

B

A

B

A

B

A

AVG # of Pinnea

AVG L. of Pinnea

AVG. L of Subleaf

AVG. W of Subleaf

Average Area of Pinnae

Average Area ofSub-Leaflet

Total Area of Leaf

N Mean Std. DeviationStd. Error

Mean

Site A = Arroyo

Site B = Ridgeline

Statistical DataIndependent Samples Test

1.124 .303 2.020 18 .059 .3306 .1637 -1.33E-02 .6744

2.020 16.074 .060 .3306 .1637 -1.63E-02 .6774

2.145 .160 -.402 18 .693 -1.6447 4.0958 -10.2496 6.9603

-.402 12.967 .695 -1.6447 4.0958 -10.4954 7.2061

1.390 .254 -.171 18 .866 -.1560 .9107 -2.0693 1.7574

-.171 11.732 .867 -.1560 .9107 -2.1453 1.8334

.237 .633 -2.445 18 .025 -.6652 .2721 -1.2369 -9.35E-02

-2.445 17.316 .025 -.6652 .2721 -1.2386 -9.19E-02

.000 .999 .517 18 .611 84.5237 163.4694 -258.9129 427.9603

.517 17.857 .611 84.5237 163.4694 -259.1098 428.1572

.760 .395 -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

9.566 .006 .947 18 .356 378.7910 400.1218 -461.8337 1219.4157

.947 13.894 .360 378.7910 400.1218 -479.9983 1237.5803

Equal variancesassumed

Equal variancesnot assumed

Equal variancesassumed

Equal variancesnot assumed

Equal variancesassumed

Equal variancesnot assumed

Equal variancesassumed

Equal variancesnot assumed

Equal variancesassumed

Equal variancesnot assumed

Equal variancesassumed

Equal variancesnot assumed

Equal variancesassumed

Equal variancesnot assumed

AVG # of Pinnea

AVG L. of Pinnea

AVG. L of Subleaf

AVG. W of Subleaf

Average Area of Pinnae

Average Area ofSub-Leaflet

Total Area of Leaf

F Sig.

Levene's Test forEquality of Variances

t df Sig. (2-tailed)Mean

DifferenceStd. ErrorDifference Lower Upper

95% ConfidenceInterval of the

Difference

t-test for Equality of Means

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

ConclusionStatistically, 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.