Bryan Hummel

                                                                                                            March 16, 1999.

 

Succession Along the San Antonio River Floodplain.

 

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.

                                    

Succession Along the San Antonio River Floodplain.

 

Bryan Hummel

Ecology 3434

Ribble; 8:30 TR

April 20, 1999.

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