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Plant density on the north-facing versus 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. Perhaps through more literature review and data collection, I will come up with other factors that may affect plant density on the two slopes.

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.

CONSIDERATIONS

Adequate Sample Size: Based on my visit to the site, I am certain there will be plenty of trees to give me a large sample size and plenty of space to take more than 8 hours to survey. I estimate the north slope of Tumamoc Hill to have at least 600 meters x 800 meters (480,000 square meters) to sample and the south slope to have at least 500 meters by 500 meters (250,000 square meters) to sample and that there are plenty of trees, shrubs, and succulent plants. Based on my pilot study of one 50x5 meter transect strip on each slope of Tumamoc Hill, I estimate that I will be able to sample 2500-3000 square meters in 8 hours which is sufficient for my study.

Controlled Variables: I will sample areas with similar steepness on both slopes and will not sample within washes or areas significantly impacted by humans on either slope because all three of these variables have been found to impact plant density.

METHODS

Dates and Times: On February 21, 2009 from 9 am until 1 pm I will sample the north-facing slope, and on February 22, 2009 from 9:00 am until 1:00 pm I will sample 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) elevation (~200 m above surrounding), and contains natural sonoran desert vegetation.

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

Data Collection: I will record 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.

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

RESULTS

     See Table 1.

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 28, 2009. Significance (p<0.05) determined by chi square analysis.

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