Bryan Hummel
March
16, 1999.
Introduction:
To look at the effects of succession and to determine which plants
come into an area and at what time after a disturbance, our Ecology class made
a trip to the Mission Trail area in south San Antonio. We visited four different areas that were
disturbed at different times in the past and we did a visual survey of the
woody plants, wildflowers and other herbs.
A radiometer was used to determine the average amount of radiation that
reaches the ground.
Methods:
The area of south San Antonio
supported a large number of American Indians and was later settled by Spaniards
because of the availability of water.
There have been large areas of human disturbances since the Indians
arrived and the disturbances did not stop with the arrival of the Spanish. Large areas are still being cleared
today. Four different sites were
selected for several reasons. The sites
were relatively close together, the dates of the disturbances were known and
the times of disturbance were approximately evenly distributed over the last
eighty years. At these sites we looked for the common woody (tree), herb and
wildflower species. We looked for
nitrogen fixing species as a signal to the amount of available nitrogen added
to the soils. The common leaf shapes,
the layers of vegetation, and amount of radiation reaching the ground were also
examined. The total number of species
was estimated for each site.
Results:
|
8 Years |
Woody Plants |
Wildflowers |
Herbs |
Radiation
Data |
|
|
|
|
|
|
|
|
Huiasche * |
Day Flower |
Coastal
Brumuda |
0.9 |
|
|
Mesquite * |
Evening
Primrose |
Speargrass |
|
|
|
Bumelia |
Fleabain
Daisy |
? |
|
|
|
Condalia |
Indian
Blanket |
? |
|
|
|
Hackberry |
Lantana |
? |
|
|
|
Unknown |
Queen's
Anne Lace |
? |
|
|
|
Yucca |
Skull Cap |
? |
|
|
|
|
Texas
Thistle |
? |
|
|
|
|
Texas
Verbena |
? |
|
|
|
|
Turks Cap |
? |
|
|
|
|
|
|
|
|
Total # of
species |
7 |
10 |
10 |
0.9 |
Table 1.
Shows the woody species, wild flower
species, herbs and radiation data for the four different sites. The trees with the “*” are the nitrogen
fixers. There is no standard deviation
for the Radiation Data because there was only one measurement. The units for the radiation results are
(1000*micromoles/meter/second*(.01 watt/meter)*10 lux)
Site one was a grassland type habitat that had been disturbed eight years prior to our study. There was no canopy and the small nitrogen fixers were just coming into place. The dominant plants were grasses and wildflowers and because of the lack of a canopy, the ground cover was very dense. Many of the small trees that were taking hold were nitrogen-fixers. All of the woody species had many small leaves, mainly pinnately shaped, sometimes all the way down to ground level. There were seven species of woody plant, but they were small and so spread out that they did not block a significant portion of light from hitting ground level.
|
25 Years |
Woody
Plants |
Wildflowers |
Herbs |
Radiation
Data |
|
|
|
|
|
|
|
|
Huiasche * |
Dayflower |
Dew Berry |
0.1 |
|
|
Retama * |
Evening
Primrose |
Prickly
pear |
0.06 |
|
|
Mesquite * |
Horsemint |
Ragweed |
0.08 |
|
|
Bumelia |
Kisses |
? |
0.06 |
|
|
Hackberry |
Purple
Flower |
? |
0.07 |
|
|
|
Queens
Anne Lace |
|
|
|
|
|
Wine cup |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Total # of
species |
5 |
7 |
15 |
0.37 |
|
|
|
|
Average |
0.074 |
|
|
|
|
Standard
Deviation |
0.016733201 |
Table 2.
Shows the woody species, wild flower
species, herbs and radiation data for the twenty-five-year-old site. The trees with the “*” are the nitrogen
fixers. The units for the radiation results are the same as in Table 1 for all
Tables 1 to 4.
Site two had not been disturbed in
25 years. There was one dense canopy at
about twenty feet composed of 90% nitrogen-fixers. Three out of Five of the woody species were nitrogen-fixers. Under this canopy there was less light than at
any of the other sites. The low
standard deviation reflects the fact that the canopy cover was more consistent
than at the other test areas. The leaf structure of the canopy was composed of
small leaves on several levels to catch more light. Even with the dense cover,
there was enough light to support an array of grasses and wildflowers. The plants on the ground level had much
broader leaves, for example the ragweed that grew to about five feet in the
shady but protected haven.
|
50 Years |
Woody
Plants |
Wildflowers |
Herbs |
Radiation
Data |
|
|
|
|
|
|
|
|
Bumelia |
Turks Cap |
|
0.16 |
|
|
Pecan |
|
|
0.09 |
|
|
Persimmon |
|
|
0.16 |
|
|
Shunkbush |
|
|
0.16 |
|
|
Spiny
Hackberry |
|
|
0.05 |
|
|
Sugar
Hackberry |
|
|
0.07 |
|
|
Unknown |
|
|
0.03 |
|
|
|
|
|
|
|
Total # of
Species |
7 |
10 |
15 |
0.72 |
|
|
|
|
Average |
0.102857143 |
|
|
|
|
Standard
Deviation |
0.0564843 |
Table 3.
Shows the woody species, wild flower
species, herbs and radiation data for the fifty-year-old site.
Site
three has been relatively undisturbed since it was cleared fifty years
ago. The nitrogen fixing trees have
been out competed and trees like Pecan and Bumelia have replaced them. There is now a middle layer of vegetation
above the ground level, but still well below the upper canopy. The leaf structure of the dominant plants
tends to be larger and closer to the ends of the branches to collect the
available light before it is filtered through a taller layer of leaves. Even with the low light conditions that
persisted under the canopy, plant life was vigorous. This site had the most
number of species overall. The large
pecan trees that towered over the rest of the vegetation were not cleared fifty
years ago. Since they “survived” the
disturbance they were not counted, but the younger trees from their larger
parents’ pecans were counted.
|
80 Years |
Woody
Plants |
Wildflowers |
Herbs |
Radiation
Data |
|
|
|
|
|
|
|
|
Anaqua |
Bed Straw |
Dew Berry |
0.01 |
|
|
Cedar Elm |
Day Flower |
Virginia
Creeper |
0.2 |
|
|
Ligustrum |
Four
O'clock |
|
0.25 |
|
|
Persimmon |
Indian
Paintbrush |
|
0.09 |
|
|
Mulberry |
Queen's
Anne Lace |
|
0.06 |
|
|
Spiny
Hackberry |
|
|
0.12 |
|
|
Sugar
Hackberry |
|
|
0.05 |
|
|
Texas
Pastiche |
|
|
0.07 |
|
|
|
|
|
|
|
Total # of
species |
8 |
5 |
10 |
0.85 |
|
|
|
|
Average |
0.10625 |
|
|
|
|
Standard
Deviation |
0.080876891 |
Table 4.
Shows the woody species, wild flower
species, herbs and radiation data for the oldest (eighty-year-old) site. Notice that there are no nitrogen fixers.
The eighty-year-old site was also
devoid of large nitrogen fixers that were present at the earlier sites. There were eight species of woody plants,
which topped the list for the four sites.
There was again a middle canopy composed of small trees, shrubs and
vines such as Virginia creeper. The
lower level of vegetation was less rich than the previous sites. The Radiation data also reveals some
information on which plants are found in that particular place. The radiation data shows that this site gets
more light to the ground on average than any of the sites with a canopy. Along with a high rate of radiation reaching
it was sporadic. This site had the
highest standard deviation of all the younger sites, representing the holes
that sometimes cast their ray of light into an othrewise shaded forest floor. The plants growing in the more shaded areas
tended to be taller and more lush than the plants in the brightest spots. Again the leaf shape tended to be towards
larger leaves at the top of each layer of vegetation which reached for all the
available light. In the bright spots,
there were also species that were more typical in a sunnier area such as Indian
Paintbrush.
Discussion:
It
became fairly easy to visualize the progression of the different habitats as
plants go through succession. It took a
site a short time to recover, but several years to get to their climax
community. Site one was beginning to
get some nitrogen fixing trees after eight years. Site two was composed of mainly nitrogen fixing trees that
improved the soils to a point that facilitated the non-fixing trees to grow
better. These later species eventually
out competed the early nitrogen fixers for space and thus at the fifty year old
site there were no more nitrogen fixers.
The stages of succession were apparent in the San Antonio floodplain
once you knew what to look for.
Bryan Hummel
Ecology 3434
Ribble; 8:30 TR
April 20, 1999.