Plant density on the north-facing and south-facing slope of Tumamoc Hill in Tucson, Arizona

Brad Fiero

RESEARCH QUESTION

How does plant density differ between the north-facing and south-facing slope of Tumamoc Hill in Tucson, AZ?

BIOLOGICAL INTEREST

South-facing slopes receive more direct sunlight than north-facing slopes, thus will experience higher solar radiation, higher temperatures, higher rates of evaporation, and lower soil moisture. Plants need sunlight and a certain amount of heat to photosynthesize and grow (which they get more of on the south-facing slope), but they also need water (which they get less of on the south-facing slope). If plant density is higher on the south-facing slope, then this suggests that greater sunlight/heat was more important than the lesser amount of moisture in affecting how many plants germinated and grew on a particular slope. If plant density is higher on the north-facing slope, then this suggests that greater moisture was more important than the lesser sunlight/heat in affecting how many plants germinated and grew on a particular slope.

HYPOTHESES/DEDUCTIONS

H0: plant density will be similar on the north-facing and south-facing slopes of of Tumamoc Hill in Tucson, AZ.
D0: The p value for the chi square test will be 0.05 or greater.

H1: plant density will be greater on the north-facing than the south-facing slope of Tumamoc Hill in Tucson, AZ.
D1: the p value for the chi square test will be less than 0.05 and the density will be greater on north-facing slope.

H2: plant density will be greater on the south-facing than the north-facing slope of Tumamoc Hill in Tucson, AZ.
D2: The p value for the chi square test will be less than 0.05 and the density will be greater on south-facing slope.

METHODS

Dates and Times: On February 21, 2009 from 9 am until 1 pm I sampled the north-facing slope, and on February 22, 2009 from 8:30 am until 12:30 pm I sampled the south-facing slope.

Locations: North and south sides of Tumamoc Hill located on the West side of Tucson, Arizona. Tumamoc Hill is a protected site reaching 947 m (3107 feet) high (~200 m above surrounding), and contains natural sonoran desert vegetation.

Sample: On the north and south sides of Tumamoc Hill (determined by compass), I started at an arbitrary point 100 meters up from the bottom of the hill, and I laid out five 50-meter transects (10 transects total), 20 meters apart, and running along the contour of the slope (not up and down the slope). The area sampled was everything within 5 meters of each transect line (500 square meters per transect line). Total area sampled on each side of the hill was 2500 square meters.

Data Collection: I recorded the number of individuals for each tree, shrub, and succulent plant species that are at least 10 cm (4") tall within 5 meters of each transect line. [I will confirm that this would take about 8 hours to have completed].

Data Analysis: I summed the data from the five transects on each side of the hill to calculate density per 2500 square meters. I analyzed values for all plants combined and grouped by life form (trees, shrubs, succulents). I compared numbers using the chi square test to determine if there were significant differences in plant density between the two sides of Tumamoc Hill.

RESULTS

Shrub density was significantly greater on the north-facing slope (95/2500m2) versus the south-facing slope (39/2500m2), but tree and succulent densities did not differ significantly between slopes (see Table 1). This resulted in overall plant density being significantly greater on the north-facing slope (110/2500m2) compared to the south-facing slope (55/2500m2) of Tumamoc Hill.

DISCUSSION

Tentative Conclusion 1: Shrub density was significantly greater on the north-facing slope (95/2500m2) versus the south-facing slope (39/2500m2).

Causal Question 1: Why was shrub density significanly greater on the north-facing slope compared to the south-facing slope?

Multiple Hypotheses for Causal Question 1:

• Hypothesis 1: This may be because [explanation].
  • Support from literature and critical thinking (cite sources)
  • Support from literature and critical thinking (cite sources)
• Hypothesis 2: This may be because [explanation].
  • Support from literature and critical thinking (cite sources)
  • Support from literature and critical thinking (cite sources)
• Hypothesis 3: This may be because [explanation]
  • Support from literature and critical thinking (cite sources)
  • Support from literature and critical thinking (cite sources)

Tentative Conclusion 2 (note: in this case there are multiple tentative conclusions, but most studies have only one tentative conclusion. Indeed, if your study has multiple tentative conclusions, you may want to talk to me about narrowing them down): Tree density (5 versus 4 trees per 2500m2) and succulent density (10 versus 12 succulents per 2500m2) was not significantly different between the north-facing slope and south-facing slope (39/2500m2) of Tumamoc Hill (Table 1).

Causal Question 2: Why was tree and succulent density not significanly different on the north- versus south-facing slope?

Multiple Hypotheses for Causal Question 2:

• Hypothesis 1: This may be because [explanation].
  • Support from literature and critical thinking (cite sources)
  • Support from literature and critical thinking (cite sources)
• Hypothesis 2: This may be because [explanation].
  • Support from literature and critical thinking (cite sources)
  • Support from literature and critical thinking (cite sources)
• Hypothesis 3: This may be because [explanation]
  • Support from literature and critical thinking (cite sources)
  • Support from literature and critical thinking (cite sources

Ways to Improve/Extend Study: If I were to do it again, I would make the following changes for the following reasons. [Note: state the changes and why, but you don't actually write this sentence].

Next Question: The results of this study suggest another interesting question which is [question]. [Note: state the question, but you don't actually write this sentence].

ANNOTATED BIBLIOGRAPHY

Annotation 1: MacMahon, J. A. 1985. Deserts. Alfred A. Knopf, New York.
This source presents information on the differences in light intensity, temperature, and soil moisture on north-facing slopes versus south-facing slopes which was critical in many of my explanations above. This source also presents information on ....

Annotation 2: Full citation of second source goes here.
Annotation for second source goes here.

Table 1. Tree, shrub, and succulent density (per 2500 m2) on the north- versus south-facing slope of Tumamoc Hill in Tucson, AZ as observed February 21 and 22, 2009. Significance (p<0.05) determined by chi square analysis.