Download Effects of light availability on Streptanthus bracteatus, a rare annual

Copyright
by
Elizabeth Ann Ramsey
2008
Effects of light availability on Streptanthus bracteatus, a rare annual
plant, and implications for reintroduction
BY
Elizabeth Ann Ramsey, B.S.; A.A.
Report
Presented to the Faculty of the Graduate School of
The University of Texas at Austin
in Partial Fulfillment
of the Requirements
for the Degree of
Master of Arts
The University of Texas at Austin
December 2008
Effects of light availability on Streptanthus bracteatus, a rare annual
plant, and implications for reintroduction
Approved by
Supervising Committee:
Norma Fowler
C. Randy Linder
ABSTRACT
Effects of light availability on Streptanthus bracteatus, a rare annual
plant, and implications for reintroduction
Elizabeth Ann Ramsey, MA
The University of Texas at Austin, 2008
Supervisor: Norma Fowler
To determine the light requirements of the endangered annual Streptanthus
bracteatus (bracted twist-flower, Brassicaceae), I conducted two experiments. In the
first, adult plants were grown outside in full sun, 42% shade, and full sun for 58% of the
day. In the second, seedlings were grown indoors at four light levels. Light intensity did
not affect the survival of either adults or seedlings. However, high degrees of full
sunlight allowed better growth of adult plants and enabled reproduction. Increasing
amounts of available light increased the growth of seedlings. Reintroduction into habitats
with high light availability and no overlying canopy is recommended for the success of
this species.
iv
TABLE OF CONTENTS
List of Figures ........................................................................................................ vi
Effects of light availability on Streptanthus bracteatus, a rare annual plant, and
implications for reintroduction .......................................................................1
Abstract ...........................................................................................................1
Introduction .....................................................................................................1
Methods...........................................................................................................5
Study species ..........................................................................................5
Seed germination ...................................................................................5
Greenhouse care and field preparation ..................................................6
Outdoor experimental procedures and measurements ...........................7
Experimental conditions and measurements for seedlings ....................9
Analyses ...............................................................................................10
Results ...........................................................................................................10
Adult vegetative characters ..................................................................10
Reproductive characters .......................................................................11
Seedling experiment results .................................................................12
Discussion .....................................................................................................14
Effects of light availability on adult plants ..........................................15
Effects of light availability on seedlings ..............................................20
Implications for reintroduction ............................................................20
Bibliography ..........................................................................................................31
Vita ......................................................................................................................35
v
LIST OF FIGURES
Figure 1:
Mean plant diameter (in) and mean number of offshoots produced at
harvest ...............................................................................................23
Figure 2:
Mean total biomass at harvest, composed of vegetative and reproductive
biomass (mg) .....................................................................................24
Figure 3:
Mean stem diameter at harvest (mm)................................................25
Figure 4:
Mean maximum inflorescence height (in) of reproductive plants ....26
Figure 5:
Mean number of apparently viable seeds produced at harvest by
reproductive plants ............................................................................27
Figure 6:
Reproductive allocation ....................................................................28
Figure 7:
Mean stem length and mean number of leaves at harvest (cm) ........29
Figure 8:
Growth rate: age (days) versus log biomass at harvest (mg) ............30
vi
EFFECTS OF LIGHT AVAILABILITY ON STREPTANTHUS BRACTEATUS,
A RARE ANNUAL PLANT, AND IMPLICATIONS FOR
REINTRODUCTION
ABSTRACT
In this study, I investigate the effects of light availability on a rare annual plant,
Streptanthus bracteatus (bracted twist-flower; Brassicaceae), in order to provide
information guiding future reintroduction plans. I hypothesize that plants exposed to
higher light availability grow and reproduce better than those under lower light
conditions. To test this hypothesis, I conducted two experiments: one outdoors on potted
adults, and one indoors on seedlings. I found that light availability did not affect the
survival of either adults or seedlings. However, higher light availability allowed better
growth of both adults and seedlings, and enabled reproduction for adults. Higher light
conditions with low to minimal shade (less than 35%) are recommended for the success
of this species. Current growing conditions beneath dense woodland canopy are probably
inappropriate, and reintroductions should be positioned in more open, sunny habitats.
INTRODUCTION
Streptanthus bracteatus is a rare annual forb endemic to only four counties in
central Texas. It occurs on the eastern and southern edges of the Edwards Plateau in
juniper-oak woodland understory; however, plants exist both under dense canopy and in
more open, shrubby clearings (Zippin 1997). Several populations have been extirpated
by urban housing developments, particularly in the city of Austin. Only 15 populations
1
remain in Bexar, Medina, Travis, and Uvalde Counties, and many are in decline. Five of
these populations occur on public land (Bee Creek Preserve, Mount Bonnell City Park,
and Barton Creek Greenbelt in Travis County; Garner State Park in Uvalde County;
Eisenhower State Park in Bexar County) and are therefore subject to some degree of
protection from becoming extinct. Those on private land, however, are unfortunately at
risk of being lost. Though S. bracteatus is considered rare, it is not listed under the U.S.
Endangered Species Act; it has a G2S2 NatureServe (“imperiled”) rank. It is clear that
remaining populations must be protected to ensure survival and persistence of the
species, and to avoid federal listing.
The rarity of S. bracteatus is evident by the existence of very few populations and
the small size of those remaining, features which often indicate genetic concerns due to
the effects of genetic drift and inbreeding (Gilpin and Soulé 1986; Barrett and Kohn
1991; Ellstrand and Elam 1993; Newman and Pilson 1997; Lofflin and Kephart 2005;
Menges 2006). Zippin’s study on the population biology of this plant identified a few
possible factors contributing to its rarity (1997). First, plants found in areas with thinned
or removed overstory were larger compared to those under more dense canopy. Because
woody cover and canopy density in the area has steadily increased since at least the mid19th century (Foster 1914; Buechner 1944; Johnston 1963; Van Auken 1988; Fuhlendorf
and Smeins 1997), it is conceivable that the more open, favorable habitat type for S.
bracteatus has been reduced. Second, this species may be negatively affected by whitetailed deer (Odocoileus virginianus), whose population densities are very high in central
Texas (Buechner 1944; Correll and Johnston 1979; Young and Traweek 1999; Armstrong
and Young 2002). Deer tend to prefer annual forbs in spring and early summer (Kohn
2
and Mooty 1971; Bryant et al. 1981; Armstrong et al. 1991; Waller and Alverson 1997;
Russell et al. 2001), which is a dangerous time for S. bracteatus plants completing their
life cycle.
Furthermore, studies have identified substantial deer herbivory on S.
bracteatus plants, lowering biomass, seed set, and survival (Dieringer 1991; Zippin
1997). Other factors to which rarity may be owed include recent evolutionary origin of
several species in the genus and possible specialization upon a rare central Texas habitat
type (Zippin 1997).
An informal group devoted to recovery of this species was founded by several
acquainted citizens and individuals working for local universities, preserves, and
government agencies. The group organized a reintroduction plan and made at least ten
attempts to reintroduce the species and form persistent populations in what was thought
to be appropriate habitat. However, all of these efforts failed; it became apparent that
more information about habitat requirements would be necessary before another
reintroduction would be attempted. Life history information and growth requirements are
of great importance in determining causes of rarity and appropriate habitat for
reintroduction, but this data is often lacking (Holsinger and Gottlieb 1991; Smith et al.
1993; Schemske et al. 1994; Fiedler and Laven 1996; Falkner et al. 1997; Helenurm
1998; Pegtel 1998; Holl and Haynes 2006). The only reliable information guiding site
selection for S. bracteatus reintroduction at present is protection from deer.
A few additional habitat preference factors have been identified as particularly
important by the recovery group. There is a possibility that soil magnesium content may
play an important role due to the similarity of magnesium-rich substrate underlying the
remaining populations; this factor is being investigated at Texas A & M University by
3
Alan Pepper, a member of the recovery group. Another factor of interest and importance
due to its direct effects on plant growth, survival, and reproduction is light (Boardman
1997; Aleric and Kirkman 2005; Vandenbussche et al. 2005; Lambers et al. 2006).
Though S. bracteatus is found primarily beneath woodland canopy, researchers who have
grown the plant note its apparent preference for higher light than a plant suited for shady
conditions might have (N. Fowler, pers. comm.). In Medina County, Zippin found that
plants growing in less shaded areas were larger, which may also indicate that the species
is better adapted to more open habitat (1997). His studies also showed that S. bracteatus
plants are subject to stong deer herbivory, which may be a major factor in population
declines. Several other studies have shown that deer herbivory can have negative effects
upon plant growth, reproduction, and survival, which can be particularly important when
subjected plants are rare (Waller and Alverson 1997; Augustine et al. 1998; Fletcher et al.
2001; Russell et al. 2001). A study on Ashe juniper seedlings (Juniperus ashei) showed
that those growing beneath juniper trees, which have low branches and prickly, scale-like
leaves, are protected from deer herbivory (Russell and Fowler 2005). Therefore, S.
bracteatus plants, which are often found in this location, may be growing there due to
deer herbivory rather than shade adaptation.
The goal of this study is to identify the light preferences of S. bracteatus, both in
the adult and seedling stage. I address the hypothesis that S. bracteatus plants prefer high
light conditions over more shaded conditions, as evidenced by increased plant growth and
reproduction. To test this hypothesis, I conducted two experiments. In the first, plants
were grown in a greenhouse until they reached adulthood and placed outdoors under the
following light treatments: control (full sun), 42% shadecloth, and sun wall (a vertical
4
wall set behind the plant). The second experiment was conducted indoors upon seedlings
and included the following treatments: control (no shade), 30% shadecloth (low shade),
53% shadecloth (medium shade), and 75% shadecloth (high shade).
METHODS
Study species
Streptanthus bracteatus (bracted twist-flower; Brassicaceae) is a winter annual
that produces a basal rosette following germination in October or November (Zippin
1997). Plants then overwinter and produce a single, but relatively tall inflorescence in
early spring, typically in April. Flowering appears to peak in May but may continue
through early summer, and seeds are produced in dehiscent, elongate fruits called siliques
(Zippin 1997; Enquist 1987; Correll and Johnston 1979). Greenhouse-grown individuals
produce offshoots, or branches from the main stem that become small rosettes; as more
offshoots are produced, the original rosette is seen to diminish in size.
Seed germination
Due to low availability of wild seed, seed for experimentation was derived from
greenhouse-grown offspring of one plant from the now largely destroyed Valburn Drive
population in Travis County. In January 2007, seeds to be used for the outdoor light
experiment were chosen based upon large size and proper embryo shape. Seeds were
surface-sterilized by soaking in distilled water for 30 minutes, 95% ethanol for 5 minutes,
and then in 10% bleach for 5 minutes before being thoroughly rinsed with distilled water.
5
I used the following soil mixture for seeding, as recommended by another
researcher familiar with growing the species: 2 Metro-Mix 200: 2 Metro-Mix 702: 1
perlite. One seed was dropped onto the damp soil surface of small individual pots and
then lightly dusted with sifted soil. Seeds were misted and re-covered with sifted soil as
needed to remain moist. Fluorescent lights elevated ~3 ft above the bench provided an
additional 12 hours of light during the day.
Greenhouse care and field preparation
As the seedlings grew, powdery mildew and insect infestations were treated with
Bonide® Liquid Copper fungicide, Safer® Garden Fungicide, Green Light® Neem
Concentrate insecticide/fungicide, and Garden Tech Sevin® insect killer as necessary. In
order to prevent fungus gnats, Gnatrol® Bacillus thuringiensis (ssp. israelensis) was
applied. All plants were re-potted twice to accommodate increasing size. By June, 29
plants reached adulthood and were considered healthy enough to go outdoors. To reduce
heat absorption and the associated loss of soil moisture, the outside of each pot was
covered with aluminum foil duct tape. Finally, plants were randomly assigned to
treatments.
The plants were then allowed to “harden-off,” or acclimate to outdoor conditions,
for two weeks within a large, open, and deer-fenced area of The University of Texas at
Austin Brackenridge Field Laboratory. A 15 X 10 m area was mowed and covered with
black plastic to prevent vegetation encroachment. The plants sat upon small wooden
pallets during this time and for the duration of the experiment to prevent them from
sitting directly upon the hot black plastic or in any standing water. The sun walls were
6
1.5’ squares of foamboard (an insulation material used in construction) that I nailed to a
wooden pallet and supported additionally by attaching a wooden frame-like backing and
side-arm. Sun walls were used to simulate the more natural condition of a plant growing
beneath a tree within woodland fringes; several hours of full sunlight would probably
reach such a plant for part of the day. Shade tents were constructed using black
shadecloth (50%), pvc pipes, and zip ties; these extended 1.5’ above the pallet surface
and shaded each plant in all directions. The position of each plant within a four-column
and ten-row arrangement was determined randomly. I aligned the sun walls facing
roughly eastward and toward the rising sun. Plants in the control treatment sat in full
sunlight.
Outdoor experimental procedures and measurements
The experiment began on July 18, 2007 and continued for 9 weeks. All plants
were watered as needed, fertilized using granular Garden Safe™ All Purpose Plant Food
5-3-3 (with micronutrients), and treated for insects and powdery mildew using the same
treatments used in the greenhouse, with the addition of St. Gabriel Laboratories® Insect
Dust (diatomaceous earth), Safer® Insect Killing Soap, and Green Light® Bacillus
thuringiensis (ssp. kurstaki) Worm Killer. The perimeter of the experimental area was
treated with Ortho Bug-Geta® snail and slug killer as needed. As fire ants became a
problem in some pots, these and the perimeter of the experimental area were treated with
Spectracide® Fire Ant Mount Destroyer.
Light availability of each treatment was
determined with the LI-190SA quantum sensor (Li-Cor, Inc., Lincoln, Nebraska USA).
The average of three readings was recorded at plant height on a clear day in August at
7
1500 hours for each treatment, while wall plants were shaded. Photosynthetically active
radiation (PAR) ± 1 standard error for the three treatments was: no shade, 1324.33 ±
11.06; 42% shadecloth, 550.67 ± 9.44; and sun wall, 160.67 ± 5.66 μmol m-2 s-1. Though
50% shadecloth was used in constructing the shadecloth tents, only 42% of full sunlight
actually reached the plants inside. Hours of the day’s sunlight and shade received by wall
plants on a clear day were: 5 hours of full sunlight (~58%), 2 hours of partial shade
(~23%), and 1.67 hours of full shade (~19%). During the experiment, 13.57” of rain fell,
which was abnormal compared to an average amount of 5.92”. Because the plants were
exposed to several days of heavy rainfall early in the experiment, many plants became
severely wilted and were taken into a nearby greenhouse for two days to recover. As a
result, three plants died, likely because the soil was too saturated for roots to receive
adequate oxygen. Six plants died later in the experiment due to aphid infestations that
were not alleviated by several insecticides, and one plant died of unknown causes.
The following measurements were taken every two weeks from the start of the
experiment: plant diameter, measured as the greater of two visually maximal extendedleaf plant diameters, stem height, measured from the soil surface to the shoot apical
meristem, plant height, measured from the soil surface to the tallest leaf, inflorescence
height, measured from soil surface to inflorescence apex, and number of offshoots
produced. The above-ground portion of each plant was harvested, dried, and weighed at
the end of the experiment. Below-ground biomass could not be reliably separated from
the soil. Stem diameter at the clear stem/root interface was measured before the plants
were dried.
Any phenological events and herbivore issues were recorded as they
occurred, and all siliques were collected before dehiscence.
8
After the experiment, the
number of apparently viable seeds was evaluated by counting those with properly shaped
embryos derived from siliques that were no longer chlorophyllous.
Reproductive
allocation was calculated as dry reproductive biomass, including the entire above-ground
stem and bracts of the inflorescence, divided by total above-ground dry biomass.
Experimental conditions and measurements for seedlings
The seedling light experiment was conducted indoors under grow lights and in a
window to ensure good germination and survival. It began on November 16, 2007 and
continued for five weeks. The seed source, selection process, and surface-sterilization
procedure were identical to those for the outdoor experiment. 40 seeds were divided
evenly into 4 light treatments: control (no shade), 30% shadecloth (low shade), 50%
shadecloth (medium shade), and 70% shadecloth (high shade). Each seedling was grown
in a small peat pot with Metro-Mix® 700 soil and seeds were sown and in the same
manner as for the outdoor experiment. I applied shade treatments by cutting a small
square of shadecloth about the same size as the top of each pot and elevating it above the
soil surface using toothpicks. Treatment assignment was determined randomly and pots
were placed in trays upon shelving in a window; plants were regularly rotated within
trays to accommodate differences in exposure to light from the window. A bank of grow
lights was suspended about 6 inches above the tops of the peat pots to allow maximum
light exposure. Light availability of each treatment was determined using the same light
sensor as for the outdoor experiment. The average of three readings was recorded at plant
height on a clear day in December at 1100 hours for each treatment. Photosynthetically
active radiation (PAR) ± 1 standard error for the four treatments was: no shade, 106.33 ±
9
4.28; 30% shadecloth, 80 ± 3.30; 53% shadecloth, 56.33 ± 2.60; and 75% shadecloth, 32
± 1.89 μmol m-2 s-1.
Actual light transmission through the two higher shadecloth
percentages was slightly higher than the advertised amount (50% and 70%). Plants were
bottom-watered as needed in order for the soil to remain moist at all times. No treatment
for insects or fungus was necessary for the duration of the experiment.
Dates of
germination were recorded, and stem length from the soil surface to the shoot apical
meristem and number of leaves were measured every three days. At the end of the
experiment, I harvested, dried, and weighed the above-ground portions of all plants.
Below-ground biomass could not be reliably separated from the soil. Age at harvest and
above-ground biomass were used to calculate growth rate.
Analyses
Data from both experiments were analyzed using SAS (version 9.00). Variables
were analyzed for significance using simple one-way ANOVAs where possible or the
Kruskal-Wallis nonparametric tests where the assumptions of ANOVA could not be met
(even after log transformations were applied).
performed when results were significant.
Scheffé pairwise contrast tests were
I analyzed probabilities of survival and
reproduction using Fisher’s exact test.
RESULTS
Adult vegetative characters
After nine weeks of treatment, 29 plants of the 39 that entered the experiment
were still alive. 66.67% of control plants, 76.92% of shadecloth plants, and 78.57% of
10
wall plants survived treatment. Differences among treatments in survival rate were not
significant (Fisher’s exact test, d.f. = 2, Χ22 = 0.5475, p = 0.8109).
Plant diameter differed significantly among treatments (ANOVA, F2, 26 = 4.21, p
= 0.0271). The control and wall plants had significantly smaller plant diameters than the
shadecloth plants (Scheffé contrasts, p = 0.0127 and p = 0.0381, respectively; Figure 1).
Although the number of offshoots per plant did not differ significantly among treatments
(ANOVA, F2, 28 = 0.43, p = 0.6560), it tended to be negatively related to plant diameter
(Figure 1). As plants grew larger, they produced more offshoots. Because offshoots
generally had shorter leaves than the original rosette, which diminished as more offshoots
were produced, overall plant diameter decreased as the number of offshoots increased.
Although the differences among treatments did not reach significance (ANOVA,
F2,
28
= 2.48, p = 0.1035), wall plants tended to have the greatest total above-ground
biomass and shadecloth plants the least (Figure 2). Similarly, shadecloth plants tended to
have smaller stem diameters (ANOVA, F2, 28 = 1.84, p = 0.1783, Figure 3). The other
measures of vegetative size did not differ among treatments (stem height: ANOVA, F2, 22
= 0.37, p = 0.6956; maximum rosette height: ANOVA, F2,
vegetative stem biomass: ANOVA, F2,
28
27
= 0.04, p = 0.9632;
= 0.96, p = 0.3962; vegetative leaf biomass:
ANOVA, F2, 28 = 0.34, p = 0.7154; total vegetative biomass: ANOVA, F2, 28 = 0.56, p =
0.5796).
Reproductive characters
Only seven of the 29 plants that survived flowered, and only six produced seed.
No shadecloth plants reproduced, 25% of control plants reproduced, and 45.45% of wall
11
plants reproduced; this difference was significant (Fisher’s exact test, Χ22 = 5.9147, p =
0.0492). Once the shadecloth treatment was dropped from the analysis, however, the
difference between control and wall plants was not significant (Fisher’s exact test, Χ21 =
0.8328, p = 0.6332).
Among plants that reproduced, maximum inflorescence height and the number of
apparently viable seeds per plant did not significantly differ between the two treatments,
though mean height was slightly higher in the control treatment (Kruskal-Wallis test, Χ21
= 0.0382, p = 0.8451; Figure 4) and seed number in the wall treatment (Kruskal-Wallis
test, Χ21 = 0.8571, p = 0.3545; Figure 5). Many of the seeds produced were immature,
but numerous non-chlorophyllous siliques contained seeds that appeared diseased or
malformed, possibly due to pest damage or poor genetics. One wall plant produced
~2800 apparently viable seeds; no other plant produced more than 250.
Wall plants had the greatest reproductive biomass; shadecloth plants had none
(Kruskal-Wallis test, Χ22 = 5.4312, p = 0.0662; Figure 2). When only reproductive plants
were considered, differences between control and wall plants were not significant
(Kruskal-Wallis test, Χ21 = 0.1500, p = 0.6985). Wall plants tended to allocate more
biomass to reproduction than control and shadecloth plants (Kruskal-Wallis test, Χ22 =
5.5710, p = 0.0617; Figure 6), although this difference disappears if only plants that
reproduced are considered (Kruskal-Wallis test, Χ21 = 0, p = 1).
Seedling experiment results
Stem length of seedlings increased with increasing shade and differed
significantly among treatments (Kruskal-Wallis test, Χ23 = 27.7097, p < 0.0001; Figure
12
7). Pairwise contrasts indicated that stem length also significantly differed in each
treatment pairing (Scheffé contrasts, control v. low/medium/high shade, low v. high
shade: p = 0.0005; low v. medium shade: p = 0.039; medium v. high shade: p = 0.0059).
Differences in leaf number among treatments were significant (Kruskal-Wallis test, Χ23 =
10.4239, p = 0.0153), and number of leaves decreased with increasing shade (Figure 7).
In pairwise contrasts, high shade seedlings had significantly lower leaf number than
control and low shade seedlings (Scheffé contrasts, p = 0.0023; p = 0.0225). Differences
in leaf number were marginally significant between control and medium shade treatments
(Scheffé contrast, p = 0.0805).
Total biomass and age at harvest were used to calculate growth rate, which
decreased as shade increased (Figure 8); differences among treatments were statistically
significant (ANOVA, F4,
33
= 17.67, p <0.0001). Pairwise comparisons indicated that
growth rates among treatments also significantly differed with the exception of the low
and medium shade comparison (Scheffé contrasts, control v. low shade: p = 0.0139;
control v. medium shade: p = 0.0002, control v. high shade: p < 0.0001; low v. medium
shade: p = 0.1077; low v. high shade: p = 0.0009; medium v. high shade: p = 0.0374).
13
DISCUSSION
Identification of appropriate habitat conditions is an essential requirement in
selecting sites for reintroduction of rare plants, particularly when extant populations
occur in habitats of questionable suitability. Due to the general importance of the light
environment for plant growth and reproduction (Boardman 1977; Vandenbussche 2003;
Lambers et al. 2006) and an apparent unexpected preference of S. bracteatus for high
light environments, this factor was chosen for investigation. In its current habitat, S.
bracteatus appears as a shade plant, representing either a shade-adapted genotype or an
acclimated phenotype, which all plants are capable of forming (Larcher 1995; Lambers
2006). Typically, high photosynthetic efficiency is an adaptation to one extreme of light
which prohibits equivalent efficiency in the opposite light conditions (Boardman 1977).
Low light intensities tend to induce stresses on plants by limiting photosynthesis, leading
to limited carbon gain and growth (Lambers 2006).
For shade-adapted plants,
repercussions of growth in high light environments may include damage to the
photosynthetic apparatus and increased water loss (Lambers 2006).
It is possible that the rosette growth form of S. bracteatus may be detrimental
beneath plant canopies. Bonser and Geber (2005) found a reduction in the fitness of
Arabidobsis thaliana and Brassica rapa rosette genotypes compared to upright genotypes
when grown in low light; notably, these species are also in the Brassicaceae.
The
threatened, rosette-bearing forb Boltonia decurrens was also found to be sensitive to low
light levels, which helps explain its disappearance after significant habitat alteration
(Smith et al. 1993).
14
Effects of light availability on adult plants
S. bracteatus adults responded quite differently to the two shade treatments I
applied. While the wall treatment was intended to be more natural, the walls should have
been constructed differently to provide more realistic shade in the intended amount.
Perhaps the wall could have been supplemented with suspended shadecloth directly
above the plant, though this would have made it difficult to determine exactly how much
shade was provided. Full shade for 42% of the day’s available sunlight is probably too
little compared to natural conditions beneath a tree. Thus, rather than an intermediate
amount of shade, this treatment represents an alternative application of shade (opaque
shade instead of filtered light through shadecloth). Perhaps a natural comparison to this
treatment would be the margin of a woodland clearing. Generally, adult plants tolerate
42% shadecloth poorly compared to full shade for 42% of the day afforded by the walls
and full sun in the control group.
The amount of shade applied did not affect survival. Deaths were largely due to
factors not directly related to light availability—aphid attack and chronically saturated
soils. Though aphid attack might seem random, the treatment implements themselves
may have allowed some protection. Walls may have prevented attack from aphids
coming from the surrounding vegetation on the western side of the experimental area;
two wall plants died from apparent aphid infestation.
Shadecloth tents completely
enclosed the plants within except for the lowest few inches of the pot, and the mesh
material probably provided protection since no shadecloth plants were attacked. Control
plants were wholly vulnerable to aphid attack, and three plants in this group died as a
result. All plants were subject to ambient rainfall, but the only plants that died of
15
apparent lack of oxygen were shadecloth plants.
Perhaps cooler and more humid
conditions within the shadecloth tents prevented water from evaporating from the soil as
quickly as in the other treatments. These causes of death may be indirectly related to the
treatments applied, but are probably not related to light availability itself. Furthermore, it
is unlikely that treatment affected survivorship in this case because the heavy rainfall
event happened early in the experiment, just five days after the initiation date.
Many of the vegetative measures I analyzed cannot be interpreted without
consideration of reproductive effort due to associated trade-offs in resource use. One of
the few measures that is less related is average stem diameter, which should increase with
plant vigor and overall size. Results of this analysis, though not significant, follow the
consistent trend that wall plants are most robust, followed by control and shadecloth
plants. Stem height and maximum plant height were vegetative measures that produced
no pronounced trends or significance and thus may not be biologically meaningful.
Measures of vegetative height may be less informative in determining S. bracteatus
health due to its rosette growth form. Because internodes do not elongate until bolting
occurs, and never elongate within the rosette itself, height is unlikely to confer any
significant advantage to growth.
The measures of plant diameter and offshoot number are difficult to interpret due
to differences in vegetative and reproductive allocation among treatments. Superior plant
health would likely be evidenced by larger rosette size, and thus higher plant diameter
values, but only before reproduction occurs. As plants become reproductive, the rosette
diminishes as resources are allocated to inflorescence production rather than vegetative
growth. Furthermore, it appears that as S. bracteatus grows, it begins to branch and
16
produce offshoots on the periphery of the central rosette.
Cues for this type of
reproduction are unknown, though plants grown in the greenhouse and in this experiment
seem to increase offshoot production with either increasing age or size.
As more
offshoots are produced, the overall plant diameter decreases as offshoots become larger
and the original central rosette diminishes. Each offshoot produced is much smaller than
the original central rosette and never reaches a similar size, so no matter how many
offshoots are produced, the overall plant diameter decreases.
By the end of the
experiment, almost all plants appeared as a collection of quite separate offshoots,
regardless of treatment. This separation seems to happen earlier if a plant becomes
reproductive, probably due to more advanced diminishment of all vegetative growth.
However, results for plant diameter in particular are clear: the mean diameter for
shadecloth plants was far higher than control or wall treatments means. A higher value
here is not a measure of greater health, however, as no shadecloth plants reproduced, and
the fewest number of offshoots were found in this treatment. The lower mean values in
control and wall groups are more likely evidence of advanced growth, as more offshoots
and inflorescences were produced in these treatments. An important conclusion from
these results is that there were no significant differences between control and wall
groups; each group was able to allocate resources to reproduction, which is a factor
essential to population persistence upon successful reintroduction, as these are annual
plants. Some low amount of full shading is apparently not a serious disadvantage, and
though the characters discussed so far indicate more success in the wall group,
differences compared to the control group have not been considerable or significant. It is
equally important to note here that a moderate amount of shade has prevented
17
reproduction from occurring, so care must be taken in reintroductions to prevent full
shading from exceeding 42% of sunlight hours in order for adult plants to be successful.
Probability of reproduction analysis further confirms these conclusions, as there
were no significant differences between control and wall groups, and both were
significantly different from the shadecloth group. Clearly, plants exposed to higher light
conditions were better able to reproduce than those under moderate shade. Inflorescence
heights were not significantly different and means were similar among reproducing wall
and control plants, so it is unlikely that this percentage of full shade negatively affected
this character.
Somewhat troubling, however, are results from the seed count.
Unfortunately, few plants produced particularly apparently viable seeds, and only one
plant produced a great number of seeds. The control plants that produced seeds both
made around 200 seeds that were visually assumed to be viable. The number produced
by wall plants, however, was highly variable: the four plants produced 5, 13, 146, and
2825 seeds, respectively. The plant that produced copious seeds began flowering quite
early, shortly after treatment began, and may be seen as a particularly vigorous plant
compared to others. Most other plants began reproducing a few weeks later and were still
producing flowers as the experiment was ending. The two wall plants that produced very
few seeds appeared to be experiencing a high degree of reproductive failure. Many pods
that were brown and visually ready to dehisce contained seeds of poor quality; many
were darkly colored, misshapen, and/or appeared to be deteriorating. I was unable to
diagnose these failures, but suspect either fungal damage or genetic issues. Because the
seeds used for my experiment are all the progeny of the progeny of one individual plant
from a small population, the existence of deleterious mutations is certainly possible. The
18
lack of significance and high variance of these results make it impossible to judge the
apparent viability of seed produced under full sun compared to low shade conditions.
The total biomass values, though only marginally significant, indicate that again,
wall plants were largest, followed closely by control plants. Shadecloth plants produced
far less biomass on average than either of the other groups. Vegetative biomass results
were again complicated by whether or not plants reproduced, possibly causing the lack of
significance. Control plants appeared to grow largest vegetatively, likely because fewer
control plants reproduced than wall plants. Even with the conversion of some vegetative
biomass to reproductive biomass in the control group, the mean vegetative biomass value
was still higher than that of the shadecloth group. This offers some support to the idea
that control plants were vegetatively more productive than shadecloth plants without
considering reproductive biomass at all. Reproductive biomass results were marginally
significant, though the same trend of better allocation in wall and control plants compared
to shadecloth plants exists.
A similar, but also marginally significant trend emerges
through analysis of reproductive allocation results.
Results of these analyses consistently support that control and wall plants grew
and reproduced better than shadecloth plants, indicating that higher light conditions are
more favorable for S. bracteatus adults. Although some results were not significant,
directional trends were consistent. The lack of significant differences between control
and wall groups may indicate that the plants tolerate some percentage of full shade well,
but it is apparent that under constant reduced light through shadecloth, growth and
reproduction are hindered.
19
Effects of light availability on seedlings
Compared to results from the outdoor experiment on S. bracteatus adults, the
seedling experiment results were clear. Stem length increased as the amount of shade
increased; seedlings elongated their stems to reach higher light conditions.
Stem
elongation under shaded conditions is considered a shade avoidance response, which may
increase the risk of prostration and mechanical damage and reduce competitive ability
(Franklin and Whitelam 2005; Vandenbussche et al. 2005). As seedlings exposed to
higher shade elongated their stems, fewer resources were available for leaf production, as
evidenced by the leaf number results. Similarly, the analysis of growth rate supports the
same conclusion: seedlings grow better as light availability increases. It is fortunate that
even 75% shade allowed seedling survival and growth, though these conditions should
not be seen as appropriate for reintroduction.
Implications for Reintroduction
Because seedlings grew larger and faster with increasing light availability and
adult plants were able to become larger and reproductive when allowed direct sunlight as
opposed to consistent shade through shadecloth, it is probable that the highly shaded
environments that S. bracteatus plants are found in are not ideal for its recovery and
success. Because increased shade did not affect survival, S. bracteatus is likely a shade
tolerant plant, though due to its increased growth under more direct sunlight, is probably
a facultative sun plant rather than a facultative shade plant (Lambers 2006).
Reintroduction sites should be chosen that allow direct sunlight for at least 60% of the
time sunlight is available.
20
Possible vegetation types that might provide this light environment are grassland
areas of the savannah matrix or possibly woodland fringes, though not interior portions of
wooded areas.
Savannahs with lesser degrees of woody encroachment and canopy
thickening would also be appropriate choices, both of which would eventually lead to
highly shaded habitat (Van Auken 1988). Aside from the repercussions of growth in the
shade due to light availability, higher shade may also intensify another health-limiting
issue—higher incidence of powdery mildew infection, caused by ascomycete fungi
(family Erysiphaceae).
S. bracteatus is highly susceptible to the fungus both in
greenhouse and field conditions, and the disease can become fatal if uncontrolled.
Powdery mildew was less problematic during my experiment, likely because it occurred
in mid-summer, when temperatures are typically too high for this fungus to survive (>85°
F). Wild S. bracteatus plants would normally have completed their life cycle by this
time, so are normally susceptible to powdery mildew due to lower spring temperatures.
Conditions that favor growth of the fungus are moderate temperatures, high humidity,
and reduced airflow, which can all be exacerbated by shading.
Further considerations for habitat choice will necessarily include protection from
white-tailed deer to ensure survival (Zippin 1997). Unfortunately, little can be done to
prevent the causes of death experienced by plants during my experiment. Aphid attack
was fatal if it occurred and could be a serious factor affecting S. bracteatus survival, but
prevention of attack in the field would be impossible. Zippin’s study on herbivory found
negative effects of insects, but aphids were not among the insect herbivores in question,
so their impact on natural populations is yet unknown (1997). The fatalities that occurred
due to heavy rainfall may be more of an issue for potted plants than those growing
21
naturally, since drainage would typically be much better in nature. Furthermore, the rain
events that occurred in summer 2007 are atypical in central Texas, where water scarcity
is usually of great concern. Therefore, these causes of death should not be given serious
consideration in choosing future reintroduction locales.
Another cause of concern is the production of poor seeds, which occurred both in
my experiment and in the greenhouse where seeds were produced for experimentation.
Some plants seem to produce poor seeds due to unknown causes, but there is suspicion of
serious genetic concerns due to inbreeding of siblings from one individual that was from
a small population itself (Gilpin and Soulé 1986; Barrett and Kohn 1991; Ellstrand and
Elam 1993; Newman and Pilson 1997; Lofflin and Kephart 2005; Menges 2006). I
cannot conclude that the seed sources remaining to be used in reintroductions are
genetically poor, as these are from natural, but small populations, but I suspect that this
may be an important issue, especially if production of healthy seeds is affected.
These experimental results support the idea that deer herbivory may have
sequestered remaining populations in more protected areas with inappropriate light
conditions.
The fact that populations in these habitats are waning yearly further
establishes the urgent need for better growing conditions. More light may not guarantee
population persistence, but will hopefully help in combination with protection from deer
and more information about nutrient requirements soon to be identified.
22
Figure 1:
Mean plant diameter (in) and mean number of offshoots produced at harvest
14
12
B
10
A
Means
A
8
6
4
2
0
Control
Shadecloth
Wall
Treatment
Plant diameter
Number of offshoots
Sample sizes for plant diameter were 8, 10, and 9, respectively; two plants in the wall
treatment were not measured because all offshoots were separate and did not form a
measurable diameter altogether. Sample sizes for number of offshoots were 8, 10, and
11. Error bars represent ±1 standard error. Duplicate letters above plant diameter
treatment means indicate a lack of significance between them (p > 0.05).
23
Figure 2:
Mean total biomass at harvest, composed of vegetative and reproductive
biomass (mg)
18
16
14
Means
12
10
8
6
4
2
0
Control
Shadecloth
Wall
Treatment
Total biomass
Vegetative biomass
Reproductive biomass
Sample sizes for each measure were 8, 10, and 11, respectively. Error bars represent ±1
standard error.
24
Figure 3:
Mean stem diameter at harvest (mm)
35
Mean stem diameter
30
25
20
15
10
5
0
Control
Shadecloth
Wall
Treatment
Sample sizes were 8, 10, and 11, respectively. Error bars represent ±1 standard error.
25
Figure 4:
Mean maximum inflorescence height (in) of reproductive plants
Mean maximum inflorescence height
40
30
20
10
0
Control
Wall
Treatment
Sample sizes were 2 and 5, respectively. Error bars represent ±1 standard error.
26
Figure 5:
Mean number of apparently viable seeds produced at harvest by
reproductive plants
Mean number of apparently viable seeds
1600
1400
1200
1000
800
600
400
200
0
Control
Wall
Treatment
Sample sizes are 2 and 4, respectively; all seeds produced by one wall plant were
immature and were therefore not counted. Error bars represent ±1 standard error.
27
Figure 6:
Reproductive allocation
0.5
Reproductive allocation
0.4
0.3
0.2
0.1
0.0
Control
Shadecloth
Wall
Treatment
Reproductive allocation is calculated as reproductive biomass divided by total biomass at
harvest. Sample sizes are 8, 10, and 11, respectively. Error bars represent ±1 standard
error.
28
Figure 7:
Mean stem length and mean number of leaves at harvest (cm)
D
2.6
2.4
2.2
A
2.0
AB
C
Means
1.8
ABC
1.6
1.4
B
C
1.2
1.0
A
0.8
0.6
Control
Low shade
Medium shade
High shade
Treatment
Stem length
Number of leaves
Sample sizes are 9, 9, 8, and 8, respectively. Error bars represent ±1 standard error.
Duplicate letters above treatments indicate a lack of significance between them (p >
0.05).
29
Figure 8:
Growth rate: age (days) versus log biomass at harvest (mg)
A
4
Log biomass at harvest (mg)
B
B
3
C
2
1
0
16
18
20
22
24
26
28
Age at harvest (days)
Control (no shade)
Low shade
Medium shade
High shade
Sample sizes are 9, 9, 8, and 8, respectively. Here, slopes are different, and all treatments
have the same y-intercept (before log transformation), which represents identical initial
starting size among all plants before light treatment affected growth. Error bars represent
±1 standard error. Duplicate letters to the right of the fitted lines for treatments indicate a
lack of significance between them (p > 0.05).
30
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34
VITA
Elizabeth Ann Ramsey was born in Dallas, Texas on November 7, 1980, the daughter of
Carol Ann Ramsey and Kenton Blake Ramsey. After receiving her diploma at
Duncanville High School, Duncanville, Texas, in 1998, she entered Tarrant County
College in Fort Worth, Texas and received an Associate of Arts with Cornerstone Honors
in 2000. Soon afterward she began at The University of North Texas, Denton, Texas,
where she became undecided about her English major and opted to take a break from her
studies. The following year of 2001 was spent in Ithaca, New York, where she lived with
her sister, a Cornell graduate student, and worked as a supervisor at Jo-Ann Fabrics and
Crafts. In the summer of 2002, she grew anxious to resume her undergraduate education
and returned to Arlington, Texas to attend The University of Texas at Arlington. There
she developed a love of botany as she worked as a research assistant in a plant ecology
laboratory. Her work in this setting and accomplishment of associated independent
research projects in Alaska as a McNair Scholar led to a publication in Oikos. She
received the degree of Bachelor of Science in Biology in August, 2005. Immediately
after, she entered the Graduate School at the University of Texas at Austin.
Permanent address:
1018 Dollins St, Katy, TX 77493
This report was typed by Elizabeth Ann Ramsey.
35