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Module 9:
The Effects of Elevation on Forest
Microclimate,Flora and Insect Life
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
In addition, when the aspect of a slope changes, the amount of solar radiation also changes. The difference in amount of light influences the plant species found at each site.
Insects depend on certain plants to survive, and if those plants are able to adapt to a particular elevation, species of insects which depend on these plants will similarly be found at these elevations.
By analyzing plant and insect samples collected at four varying gradients on Mount Lemmon, we can attempt to answer these questions:
What is the general climatic trend as we move along an elevation gradient?
How do plant and insect species respond to changing light and temperature found at increasing elevations?
At which elevation is the greatest species richness?
Introduction:
In this exercise, we studied how climatic factors and biodiversity change with increasing elevation gradients at Mount Lemmon. We measured climatic factors such as temperature, relative humidity, wind speed, and pressure. We also obtained local weather records to evaluate precipitation, We also studied changes in biodiversity by sampling the plant and insect species found along the various gradients.
As elevation increases, abiotic factors such as temperature and light also change. Temperatures tend to decrease as elevation increases, and plants must adapt to both changing elevation and temperatures. According to Humboldt’s Rule, plant communities change with altitude and temperature (Danoff-Burg, Module 9). Some plants will adapt better to the decreasing temperatures found at higher elevations than other plants.
Figure 1: Relationship Between Latitude and Altitude
Relationship between latitude and altitude, copyright 1990, Wadsworth Inc.
8500 ft (4)
7600 ft (3)
7840 ft (2)
4380 ft (1)
Figure from Reading the Landscae of America
Mary Theilgaard Watts
Methods
Since our main objective was to test the factors that change along an elevation gradient, such as moisture, precipitation, solar radiation, temperature, soil, etc., we controlled the elevation by laying out an elevation transect. We tested four elevations in total:
Site One: Molino Basin – 4380 ft. / 1335 m.
Site Two: Bear Wallows – 7840 ft. / 2280 m.
Site Three: Marshall Gulch – 7600 ft. / 2316 m.
Site Four: Mt. Lemmon – 8500 ft. / 2591 m.
We laid out a 25-meter transect line in all the sites, except for Site One. In Site One, we laid out two 25-meter lines. We laid the first one to test the chaparral community and the second one to test the oak woodland community.
Before laying out these lines, however, there were few factors that we decided to uniformly test in all four sites. These factors were:
General site description – As a group, we decided on distinct features of each site.
Elevation (in ft.) – We utilized both the topographic map and the GPS. In both Site One and Three, the two readings did not correspond. We chose the topographic map reading over the GPS because we knew precisely where we were on the map and the GPS probably encountered difficulties in tracking enough satellites to provide accurate readings for elevation.
Latitude/Longitude – We recorded the readings from the GPS. The GPS appears more accurate in determining horizontal location than vertical location.
Maximum/Minimum temperatures (C) – Our tireless Professor Tim and TA Erika strapped thermometers along all four site locations 3 days before our experiments in order to map the change in temperature across these sites.
Relative humidity (%) – We measured the moisture content in the air by using a sling psychrometer.
Wind speed (km/hr) – Using an instrument called the anemometer, which operates on Bernoulli’s principle, we measured the wind speed across all four sites.
Atmospheric Pressure (in. Hg) – We used a barometer to measure this and also to estimate the altitude.
Slope angle – We estimated the slope angles by using trigonometric functions.
Aspect – We used a compass and measured a down-slope line perpendicular to the transect line. Unfortunately, we were not able to measure the slope angle and the aspect for Site One.
After these factors were determined in each site, we laid out the transect lines. We then determined species richness, not percent cover, along the lines. We recorded species richness for both plants and arthropods.
We included an additional site in our data. A pine forest site, known as Mile 16, was included in our analysis. The data from this additional site was collected in the year 2000. We also compared our data with year 2000’s data done at Molino Basin, Marshall Gulch, and Mt. Lemmon. Lastly, we included climate patterns from Tucson airport (with an elevation of 2579ft. / 786 m.) Although we did not measure the species richness in this area and the measurements in this site were based on a daily average, we included this low elevation in order to show a general climatic trend across different elevations.
Methods Cont’d
TEAMWORK
Results
Relative humidity and wind speed are directly proportional to elevation (Figure 1, 2).
Barometric pressure and temperature are inversely proportional to elevation (Figure 3, 4).
Plant species richness also seems to be inversely proportional to elevation; though based on our observations, there is greater biotic productivity at the higher (and moister) elevations (Figure 5).
Arthropod morphospecies seem to be most abundant at intermediate elevations (Figure 5).
Soil types varied between sites—the % of organic material seems to increase; this may not have been directly dependent on elevation.
Considering the possibility that vegetation might be a function of climate--as in the equation provided by Jack Major (1951) V= f (cl, p, r, o, t)--, we took stock species richness at four sites along a microclimatic gradient in the Santa Catalina mountains. We measured several climatic indicators at each site including temperature, barometric pressure, wind speed, relative humidity, and elevation. These are our results:
Table of Data Results
Figure 1
Elevation vs. Relative Humidity
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Tucson Airport Molino Basin Marshall Gulch Bear Wallows Mt. Lemmon Summit
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Figure 2
Elevation vs. Wind Speed
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Tucson Airport Molino Basin Marshall Gulch Bear Wallows Mt. Lemmon Summit
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Figure 3
Elevation vs Atmospheric Pressure
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Tucson Airport Molino Basin Marshall Gulch Bear Wallows Mt. Lemmon Summit
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Elevation (m) ATM press. (in. of Hg)
Figure 4
Elevation vs. Temperature
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28.027.5
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2119
1411 12
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Max temp (degrees C) Min temp (degrees C)
Figure 5Elevation vs. Species Richness
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76 5
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9 97
1335 1335
21242390 2316
2591
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Molino BasinScrub
Molino BasinOak Wood
Area 51 Bear Wallows Marshall Gulch Mt. Lemmon
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Figure 6
Plant and Insect Diversity vs Elevation
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Elevation (meters)
Plant BiodiversityArthropod BiodiversityPoly. (Arthropod Biodiversity)Poly. (Plant Biodiversity)
Results Cont’dRegionally, these sites all have the same climate. In Major’s application of Jenny’s formula , regional climate is represented by the value r. We also created climate diagrams based on data collected at weather stations in the vicinity of this gradient. These diagrams illustrate the relation between average monthly precipitation, average monthly temperature and elevation in the Santa Catalinas. Since we collected our data over the course of one day these diagrams give us the best picture we have of the relation between elevation and precipitation. These four sites (and by corollary our four sites) lie within the same regional climate, thus we were able to eliminated r from this experiment. Based on these diagrams we recognized some of the following trends:
They exhibit the same pattern in preciptation (two rainy seasons interspersed with long dry seasons, and similar seasonal temperature fluctutations (even if they are in different ranges.
Precipitation is directly proportional to elevation.
Temperature is inversely proportional to elevation.
mm
USA
SABINO CANYON, ARIZONA32.18°N / 110.49°W / 805m
[34-34] +20.5°C 332mm
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-10Data compiled from
1948-1982
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U.S.A.
ORACLE 2 MILES SOUTHEAST32.60°N / 110.70°W / 1375m
[ ] +16.7°C 576mm
°C+0.0+0.0
+0.0+0.0
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1950-2000
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U.S.A.
ORACLE, ARIZONA32.60°N / 110.78°W / 1402m
[ ] +16.8°C 492mm
°C+0.0+0.0
+0.0+0.0
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1893-1949
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U.S.A.
PALISADE RESEARCH STATION32.42°N / 110.72°W / 2426m
[ ] +9.3°C 790mm
°C+0.0+0.0
+0.0+0.0
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1965-1981
DiscussionAs our group rode up to the first site, we were observing the
vegetation along the Catalina Highway. At about 2,400 feet {731.52 meters} there was Velvet Mesquite, fairyduster, Arizona walnut, saguaro cactus, Arizona sycamore, creosote bush, Fremont cottonwood, ocotillo, barrel cacti, yellow paloverde, jumping cholla, and Arizona alder. The first site was not that windy but it was warm and increasing in temperature by the minute.
On the way up to the second site we observed the following vegetation along the highway from about 3,500 feet – 5,000 feet {1066.8-1524 meters}; Saguaro Cactus, Mesquite, yellow paloverde, creosote bush, jumping cholla, ocotillo, Arizona walnut, Arizona sycamore, Emory oak, Mexican blue oak, Arizona white oak, gambel oak, silver leaf oak, and alligator juniper. It was evident that with increasing elevation, changes occurred with vegetation. From 5,000 to 6,000 feet {1524-1828.8 meters} we observed Arizona sycamore, mesquite, gambel oak, Arizona white oak, emory oak, ocotillo, alligator juniper, Mexican pitch pine, madrone, ponderosa pine, and Douglas fir. From about 6,000 feet to 7,840 {1828.8-2390 meters} the vegetation followed the previous patterns of changing with elevation. We observed Silver leaf Oak, Ponderosa pine, madrone, alligator juniper, Fremont cottonwood, emory oak, Mexican blue oak, gambel oak, Arizona walnut, white fir, aspen grove, and Douglas fir.
The third site was an area that had similar vegetation as the second site. In fact, the elevation was 7,600{2316 meter}, and 240 feet{74 meter} below the second site. Both sites had steep slopes (that required skill to maintain balance), and they were located close to the bottom of the slope near the arroyos. The third site was on a northeast slope, and the second site was on a southwestern slope. The significance in this was the amount of solar radiation loading on a northwest slope is greater then a southwestern slope, which affects soil temperature and evaporation rate. With this in mind, the only major vegetation difference was that site Three had some grass, and site Two had no grass on the forest floor.
The fourth site was at an elevation of 8,500 feet {2591 meters}. The area had a lot of grass, and was very cool and windy. We sampled an area just below the top of the slope; the slope was not that steep that people had a hard time maintaining their balance like previous slopes.
Discussion, Cont’dAtmospheric pressure decreases with increase in
elevation. A decrease in the number of molecules in the air above occurs with an increase in elevation. When comparing the minimum temperature to the maximum temperature for all the sites, a pattern is apparent. For site One, the temperature at its maximum was 39.5, and minimum was at 18.5 degrees Celsius. At about 3,460 feet higher, the maximum temperature drops down to 28 and the minimum is 13.5 degrees Celsius. At the top of the mountain the maximum was about 24, and the minimum was 12 degrees Celsius. Why does temperature decrease in higher elevations? The Ideal gas law: pV=nRT, basically states that pressure times volume is equal to the number of molecules in the air which create the temperature. The decrease in pressure that occurs with increased elevation means that fewer molecules are present. The decrease in molecules in the air means that the molecules are traveling farther to collide and this means a lower temperature. The higher the elevation the decrease in temperature only applies to the troposphere.
When looking at the wind speed relative to elevation, there seems to be an increase in speed with elevation. The first site had a wind speed of 5.5 km/hr, the second site 8 km/hr, third site 10 km/hr, and finally the forth site 11 km/hr. Why does wind speed increase with elevation? The lower topographical areas have mountains, trees, and hills that slow the flow of wind. However in higher topographical regions the mountains and hills are not blocking the wind, allowing for higher topographical areas to have more and faster winds.
The relative humidity on site one was 45% relative humidity, site two was 40%, site three 32%, and site four was 35% relative humidity. Site four was higher in elevation than site three, yet site four has a higher level of relative humidity. Topography influences precipitation as far as how much precipitation that some areas receive. As the relative humidity increases so do the levels of precipitation. Could the time of day affect the accuracy of these tests? If the relative humidity were taken at all the sites, at the same time would another pattern develop? Because we measure the first site early in the morning there was the highest percent of relative humidity. If we had measured that site again at the end of the day there would have been a very low percent of relative humidity. This is because as the day goes on the climates of certain places change, with the influential factors such as solar radiation levels, precipitation, wind, and temperature. As the day went on that first site got hotter, with this change it lowers the relative humidity level. If we had tested all four sites at the same time of day, the results with climate would be more exact to illustrate a climate pattern.
When analyzing the insect and plant data from all of the sites some acute patterns appear. The first site (4380 feet Oak Woodland and Chapparal) line A, had twenty-three different species of vegetation. On the other hand, line B had fourteen different species of vegetation. This site was important because the two different lines represented two different vegetation areas. Site one was a transition zone from oak woodland vegetation to chaparral vegetation. There was six different species of arthropod morphotaxa from line A. From line B there where five different species of arthropod morphotaxa. Even though these lines were not that far apart they reflect the slight changes in vegetation and arthropod morphotaxa that occur in a transition zone.
Site two(7840 feet Ponderosa Pine/Oak Pine Woodland) had seventeen different species of vegetation. Site two also had nine different species of arthropod morphotaxa, which is an increase of 3-4 species of arthropod morphotaxa. Site three(7600 feet Oak Pine Woodland) which was about 240 feet {74 meters} lower then site two had 19 different species of vegetation. There where nine different species of arthropod morphotaxa.
On site four (8500 feet Mixed Conifer Forest) the vegetation decreased to only six different species of vegetation, and seven species of arthropod morphotaxa. On the highest elevation that we counted the number of vegetation species and arthropod morphotaxa, there was a decrease in both. This can be attributed to the topography and the climate factors that go along with higher and lower topography levels.
Some possible sources of error could have been with the accuracy of the GPS readings. The time of day differed from site to site and this could affect the accuracy of the data, regarding climate patterns. For instance the relative humidity and temperature changed from site to site as the day moved on.
Discussion, Cont’d
Plant and Insect Diversity vs Elevation
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1300 1350 1400 1450 1500 1550 1600 1650 1700 1750 1800 1850 1900 1950 2000 2050 2100 2150 2200 2250 2300 2350 2400 2450 2500 2550 2600
Elevation (meters)
Plant BiodiversityArthropod BiodiversityPoly. (Arthropod Biodiversity)Poly. (Plant Biodiversity)
ConclusionWhat is the general climatic trend as elevation gradient increases? How do changing climatic factorsinfluence biodiversity at increasing elevations? At which elevation is the greatest species diversity? In this exercise, we learned to use several new instruments and techniques to gather climatic data:--Sling psychrometer (measures relative humidity)--Anemometer (measures windspeed)--Barometer (measures atmospheric pressure and altitude)--Trigonometric functions (measure slope angle)--Compass to measure down a slope line perpendiular to transect (measure aspect)
From the data gathered, we learned that elevation influences climate as follows:
• Temperature decreases at increasing elevations; according to the Ideal Gas Law, less atmospheric pressure is exerted at higher elevations so air molecules are farther apart and release less energy (i.e. temperature) as they collide with each other.
• As elevation increases, relative humidity increases as it is a function of temperature; at cooler temperatures, relative humidity is higher as there is more moisture in the air.
• At higher elevations, wind speed increases due to less surface roughness such as hills, mountains, and trees to disrupt the flow of wind.
• Precipitation increases as relative humidity increases, but this is not necessarily true because at increased elevations in tropical areas, there is a cap of subsiding air at higher elevations.
• As elevation increases, atmospheric pressure decreases due to the decreasing number of molecules in the air column above..
- Plant species richness is inversely proportional to elevation; thus the climatic factors which are directly proportional to elevation seem to have a limiting potential on species richness. Arthropod morphotaxa diversity did not change much with elevation changes.
From this exercise, we learned that as elevation changes, species diversity also differs. At higher elevations, there were more evergreens such as Douglas Fir and white fir forests, whereas at lower elevations, there is more vegetation that grew closer to the ground, for example, cacti, grass, and forbs. However, species abundance was greater at site one, the lowest elevation, than at the other sites.
References and AcknowledgementsArnberger, Leslie P. Flowers of the Southwest Mountians. Tucson: Southwest Parks and Management Association, 1982.
Bowers, Janice Emily. Shrubs and Trees of the Southwest Deserts. Tucson: Southwest Parks and Management Association, 1993.
Danoff-Burg, James A. “Module 9: Light and Temperature” CERC, Columbia University
Dodge, Nat N. Flowers of the Southwest Deserts. Tucson: Southwest Parks and Management Association, 1985.
Elmore, Francis H. Shrubs and Trees of the Southwest Uplands. Tucson:Southwest Parks and Management Association, 1976.
Epple, Anne Orth. A Field Guide to the Plants of Arizona. Lew Ann Publishing Company, Mesa,Arizona. 1995.
USDA Natural Resources Conservation Service National Plant Database <http://plants.usda.gov/plants>
Gould, Frank W. Grasses of the Southwestern United States. University of Arizona Press, Tucson. 1993.
Grassesof Southeastern Arizona. Coronado RC&D and Area Inc., And Conservation Districts of Southeastern Arizona.
Kearney, Thomas H. and Peebles, Robert H. Arizona Flora. University of California Press, Berkeley. 1960.
Major, J. 1951. A functional factorial approach to plant ecology. Ecology, 32 (3): 392-412.
Niehaus, Theodore F. A Field Guide to Southwestern and Texas Wildflowers. Houghton MifflinCo., New York. 1984.
Phillips, Steven J. and Patricia Wentworth Comus. 2000. A Natural History of the Sonoran Desert. Arizona-Sonoran Desert Museum Press, Tucson.
Summer Forbs of Southeastern Arizona. . Coronado RC&D and Area Inc., And Conservation Districts of Southeastern Arizona.
Theilgard Watts, Mary. Reading the Landscape of America. New York: Collier Books, 1975. P. 354
