Download plant adaptations

TUCSON’S BIOGEOGRAPHY
INTRODUCTION
Biogeographers seek to answer such basic questions as why are there so many living creatures?
Why are they distributed in the way that they are? Have they always occupied their current distribution
patterns? Is the present activity of human beings affecting these patterns and, if so, what are the
prospects for the future?
Biogeography’s scope is very wide, thus we cannot cover all aspects in a single lab. In this lab, you
will be introduced to the biogeographic analysis of plant species and communities. We will be
examining some common species that grow in and around Tucson. To do this, we will be using data
collected from the Santa Catalina Mountains, located just north of Tucson.
The data in this lab are based on those presented in:
Niering, W.A. and Lowe, C.H. 1984. Vegetation of the Santa Catalina Mountains: Community
types and dynamics. Vegetation 58: 3-28.
PLANT ADAPTATIONS
Because each species has a unique evolutionary history, they are uniquely adapted to a given range
of conditions. In some cases, species are well suited to tolerate the shady conditions in a forest under
story or to out compete species with which they often co-occur. In others, the adaptations allow the
species to tolerate harsh environmental conditions. For example, there are a number of adaptations
that allow plants to live in the dry conditions of the Sonoran Desert. Such adaptations include:
Succulence: plants have a specialized fleshy tissue in roots, stems, or leaves for the conservation of
water. Most succulents are xerophytes, plants preferring dry climates, such as cactus or aloe.
Waxy coatings: the surfaces of leaves or trunks are resinous to prevent water loss.
Specially adapted root systems: some species have very shallow but extensive root systems to
catch rain quickly when it does fall; others have very deep roots to reach the groundwater.
Specially adapted "leaves" (spines): leaves contain openings called stomates that allow CO2 and
O2 to enter and leave the plant. Some species have spines that perform the same function as leaves.
Because of their smaller surface area, spines reduce the amount of H2O lost to the environment and,
in some cases, help to protect the plant (including adapted bark in the form of thorns for added
protection). Most photosynthesis is carried out in organs called chloroplasts that contain the pigment
chlorophyll (which gives the "green" to green leaves); spiny species usually have green trunks that are
photosynthetically-active to make up for the absence of leaves.
Deciduousness: species drop leaves at certain times (e.g., during times of drought) and regrow their
leaves during the wet season.
Seed dormancy: in ephemeral species, individuals live for only a few weeks or months. Their seeds
survive dry periods and sprout following rainfall. These species sprout, grow rapidly, reproduce and
die in a short time span.
SPECIES DISTRIBUTION
The distribution of species is determined by their adaptations to environmental conditions (abiotic
factors such as air temperature, precipitation, soil nutrients, etc.) and by their biotic relationships with
other species. This is called the ecological niche of a species. It refers to the functional role played by
an organism as well as the physical space it inhabits, including how and where it obtains its energy,
and how it influences other species and the environment around it.
How do scientists measure the size of plant populations in a study area? They could count every
individual plant, but you could imagine how long this would take for small plants in a large area - just
imagine trying to count all of the individual blades of grass in a football field! When conducting plant
ecology research on invasive or other species, scientists often select several smaller sample plots
inside a larger study area. The scientists thoroughly study the smaller areas. The vegetation within
these sample plots is identified by species, and specimens are counted and measured for size.
This information is then used to make generalizations about the larger study area.
A plant community can be defined as an assemblage of plant species, which interact among
themselves and with their environment within a time-space boundary. The spatial boundaries of plant
communities, called ecotones, can be gradual or abrupt.
Plant communities are often named for the dominant species of the community. A species may be
dominant either by its physical size, such as the Monterey cypress forest, or by its numerical
dominance. Not all communities have one species that is dominant; these are named for physical or
generic characteristics, such as grasslands or scrub, or by habitat, such as coastal bluff communities.
ARIZONA’S “SKY ISLANDS”
In the southeastern corner of Arizona, several impressive mountain ranges rise above the hot
lowlands of the Sonoran and Chihuahuan Deserts. These "sky islands" include the Santa Catalina,
Rincon, Santa Rita, Galuiro, Huachuca, Pinaleno (Mt. Graham), and Chiricahua Mountains (figure 1).
They extend into New Mexico and Mexico.
Figure 1. Sky island mountain ranges of Arizona, New Mexico, and adjacent Sonora and
Chihuahua (Marshall 1957)
The biological diversity of these mountain ranges is amazing. One can hike from desert scrub up to
cool forests of pine, fir, and spruce in a single day, passing through a host of other plant communities
along the way. Some mountains of the Sky Islands region are characterized by the presence of
perennial streams. These are fed by occasional deep winter snows in the higher elevations combined
with heavy summer rains.
Since the last glaciation, the sky islands have become relatively isolated from each other. Expanding
desert grasslands and desert scrub in the valleys ("the sea" between the sky islands) have limited
genetic interchange between populations and created environments with high evolutionary potential.
The resulting sky islands ecosystems show a high number of endemic species, and harbor most
threatened and endangered species in the Southwest.
The amounts of precipitation and solar radiation received by specific locations on a mountain
influence the communities found at different elevations. They also influence the communities on the
various exposures of its slopes: east and west, north and south. These bands of communities that
vary along gradients of precipitation, solar radiation and temperature, are referred to as life zones
(figure 2). In the Santa Catalina Mountains (located east of Tucson), these changes in the life zones
are experienced as elevation increases.
In this lab, we will study the distribution of plant communities on Mount Lemmon. At the base of Mount
Lemmon, warm temperatures and low precipitation prevail. The Lower Sonoran Zone is characterized
by a desert-scrub community of sparse vegetation, including prickly pear cactus, saguaro, ocotillo,
palo verde, and cholla in the Sonoran Desert near Tucson.
Figure 2. Cross-sections of sky islands showing the life zones
Moving up in elevation, the communities of the Upper Sonoran Zone vary with elevation, exposure of
the slope, and amount precipitation. On the drier, warmer, south-facing slope of Mount Lemmon the
plant community is desert grassland.
On the north-facing slope, at the same elevation, is an oak woodland characteristic of a cooler,
moister environment. In some areas, the oaks are mixed with piñon, juniper, and manzanita, with
grasses covering the ground between the larger woody plants.
At Inspiration Rock on Mount Lemmon, the mixed conifer forest of the Canadian Zone is found. There,
the air temperature is lower and precipitation is higher. In this forest, Douglas-fir and white fir become
more numerous and pines become less abundant.
LAB QUESTIONS
SECTION A: Plant adaptations
1. Your teaching assistant will take you on a short walk around Campus. As your teaching assistant
introduces you to various plant species and their specific adaptations, take notes about these species
so you can fill out table 1.
Table 1. Adaptations of Species Discussed During the Campus Tour
Species
Succulent?
(yes / no)
Photosynthetic
trunk/stems?
(yes / no)
Spines
or
thorns? (yes
/ no)
Deciduous?
(yes / no)
Saguaro
Creosote bush
Ocotillo
Prickly pear
Mesquite
Palo verde
Ponderosa pine
SECTION B: Species distribution in the Santa Catalinas Mountains
The data in table 2 show the mean density of seven species at nine elevations along a gradient from
Tucson to the top of Mount Lemmon. Density refers to the number of individual plants per acre – thus
at 2950 ft, there was an average of 15 Carnegiea per acre, but no Juniperus or Quercus.
Elev
(ft)
3000
3600
4250
4920
5550
6550
7200
7870
8900
Table 2. Mean species density from Tucson to Mount Lemmon summit
Saguaro
Ponderosa Douglas-fir
Mesquite Mexican
Gambel
cactus
pine
Oak
Pseudotsuga Prosopis Pinyon
Juniperus Carnegiea Pinus
pine
meziesii
velutina
Quercus
deppeana gigantea
ponderosa
Pinus
gambelii
cembroides
0
15
0
0
12
0
0
0
100
0
0
121
0
0
0
3
0
0
40
0
0
25
0
0
0
0
20
0
89
0
0
0
0
88
2
30
0
0
0
0
15
32
0
0
120
15
0
0
74
0
0
295
42
0
0
10
0
0
15
149
0
0
0
Alligator
Juniper
1. Using the data in table 2, graph the distribution of species vs. elevation on figure 3. Juniperus
deppeana is already graphed as an example. Use different colors or line types to graph each species.
Do not forget to provide a legend to identify which line type or color represent which species.
300
Density (Indiv/ha)
270
240
210
180
150
120
90
60
30
0
2500
3750
5000
6250
7500
8750
10000
Elevation (ft)
Figure 3. Distribution of species as a function of elevation
2. Which two species are best adapted to hot, dry conditions?
3. Which species is best adapted to cool, moist conditions?
North-facing
South-facing
11000
10000
Spruce-fir
Elevation (ft)
9000
Mixed conifer
8000
7000
Pine-oak
6000
5000
Pinon-juniper
4000
Desertscrub
3000
2000
1000
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Figure 4. Altitudinal association of major plant communities in southern Arizona
Because the elevation change from Tucson to the top of the Santa Catalina Mountains (Mt. Lemmon)
occurs over such a short distance, it is possible to see a rapid turnover of plant communities that are
composed of species with relatively similar niches and traits.
4. Figure 4 shows the altitudinal zonation of different plant communities (desert scrub, piñon-juniper,
pine-oak, mixed conifer, spruce-fir) in Southern Arizona. Using the data in table 2 and figure 3,
determine the community to which each of the seven species listed below is most closely associated.
Alligator Juniper Juniperus deppeana:
_________________________________________________________________________________
Saguaro cactus Carnegiea gigantea:
_________________________________________________________________________________
Ponderosa pine Pinus ponderosa:
_________________________________________________________________________________
Douglas-fir Pseudotsuga meziesii:
_________________________________________________________________________________
Mesquite Prosopis velutina:
_________________________________________________________________________________
Mexican Pinyon pine Pinus cembroides:
_________________________________________________________________________________
Gambel Oak Quercus gambelii:
_________________________________________________________________________________
5. Note that the communities zones on figure 4 occur at slightly different elevations depending on the
side of the mountain you are looking at. The lines slant downward to the right. For example, desert
scrub occurs all the way up to around 4500 ft on south-facing slopes but is limited to around 3900 ft
on north-facing slopes. Explain why this occurs.
6. Listed below are the heights of five southern Arizona sky islands. Describe the type of forest
community you expect to find at or near the summit for each of these sky islands.
a) Santa Catalina Mountains (Mt. Lemmon): 9,157 ft
b) Tucson Mountains (Wasson Peak): 4,686 ft
c) Picacho Peak: 3,370 ft
d) Kitt Peak (Quinlan Mountains): 6,880 ft
e) Pinaleno Mountains (Mt. Graham): 10,720 ft