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LUDWIGIA REPENS
INTERFERENCE PLANT COMPETITION STUDY
PREPARED BY
Center for Reservoir and Aquatic
Systems Research
Baylor University
Waco, Texas 76798
BIO-WEST, Inc.
1812 Central Commerce Court
Round Rock, Texas 78664
March 2015
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STUDY PLAN
Objective
The overall objective of our proposal is to evaluate the competitive interactions between the
desirable native plant Ludwigia repens and two widespread and aggressive non-native species
Hygrophila polysperma and Hydrilla verticillata. Additional objectives include evaluating
competitive ability of Hygrophila polysperma under two light regimes in Landa Lake to better
understand the competitive outcome under shaded conditions common due to riparian shading of
the rivers. Evaluation of growth and competitive ability will be assessed between different
locations within each river so we best understand the likely competitive outcomes along the
length of these rivers.
Study Sites
Sites for the Hygrophila competition study will consist of field study sites located on the Comal
River in Comal County, Texas in Landa Lake, Upper Spring Run and the Old Channel. The
Hydrilla competition study will be conducted on the San Marcos River in Hays County Texas
with two sites, one site upstream and one site downstream of Rio Vista dam.
Introduction
The San Marcos and Comal Rivers have unique aquatic plant communities that support a wide
variety of native and endemic wildlife including several listed species. Non-native aquatic plant
species such as Hydrilla verticillata and Hygrophila polysperma are becoming increasingly
abundant in these systems (Lemke, 1989; Bowles and Bowles, 2001) and pose a threat to efforts
in reestablishing beneficial native aquatic vegetation (Bormann, 2012). The ability to predict and
anticipate changing dynamics and community structure in these plant communities is essential
for understanding how listed species could be impacted and to help guide the development of the
submerged aquatic vegetation module of the HCP Ecological model.
Data and Literature Review
Success of a plant species in competition with another plant species is driven by species
characteristics. These include growth rate, plant architecture, reproductive vigor and
susceptibility to herbivory (Spencer and Bowes, 1985) and in certain cases chemical defenses
(Gross, 2003). Non-native species may excel at one or several of these characteristics which
allow them full advantage over native species. Invasive aquatic plant species are well known for
their ability to spread rapidly via fragmentation of stems, basal rooting structures, such as
stolons, tubers or corms, or specialized structures, such as turions, which can detach and move
downstream or float on currents into new locations colonizing in rapid fashion (Sculthorpe,
1967; Langeland and Sutton, 1980). Typically aquatic plants reproduce asexually (Arbor, 1920;
Haynes, 1988) and vegetative structures are primed for growth upon settling into new habitat
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with root structures or leaves still attached (Sutton, 1996). As a consequence, in many cases,
invasion of an aquatic species into new areas can take very little time (Santamaria, 2002). For
example, Eurasion watermilfoil, Myriophyllum spicatum, a widespread problematic submersed
aquatic plant has been documented to establish and dominate littoral zones of lakes within two to
three years after introduction (Aiken et al., 1979; Newroth, 1985). A North American native
Elodea nuttaalii has spread rapidly in Japan’s largest lake covering the lake bottom within a few
years after introduction there (Kadono, 2004). Closer to home in the San Marcos system the
exotic plant Cryptocoryne becketti was documented to quickly establish and spread within 2
years after initial discovery with a recorded expansion rate of 80% a year (Doyle, 2001) and
annual mapping by BIO-WEST has shown the dramatic expansion of Hygrophila polysperma in
the Old Channel Study Reach of the Comal River (BIO-WEST, 2010)
With recent documented expansions of invasive aquatic plants within the San Marcos and Comal
systems data is needed to suggest how native plants may respond. Few studies regarding native
versus non native aquatic plant competition have been conducted with regard to either of these
systems. In one particular study Doyle et al. (2003) conducted a static container (35 gallon
barrels) within an outdoor raceway experiment to study the competitive ability of Ludwigia
repens against Hygrophila polysperma. Our experiment will expand upon that of Doyle et al
(2003) to help further understand the competitive outcome under more realistic environmental
flow and ambient light conditions.
In their experiment Doyle et al. (2003) found that Hygrophila polysperma had significant
interference with the growth of Ludwigia repens across all trials. Hygrophila polysperma plant
stems were longer and more highly branched and the relative growth rate for Hygrophila
polysperma was not significantly affected by the presence of Ludwigia repens while the relative
growth rate of Ludwigia repens was significantly impacted by Hygrophila polysperma. While
the 2003 study indicated the species’ ability to dominate Ludwigia repens it was conducted under
static conditions, as a container experiment. For our competition study an in situ experiment is
proposed. An in situ experiment provides for different conditions and exposure to real ambient
factors such as ambient light levels, consistent CO 2 and consistent water flow, which may be
more favorable towards Ludwigia repens growth. Also an in situ experiment minimizes transport
and handling of plants which can cause stress to plant propagules. For this study plant
propagules will only be handled when collecting and repotting otherwise they will grow much
like wild plants. For our study three plant species will be utilized. One native plant Ludwigia
repens and two non native species Hygrophila polysperma and Hydrilla verticillata.
Ludwigia repens J.R. Forst. (Red Ludwigia) is an obligate amphibious aquatic macrophyte native
to the San Marcos and Comal rivers and is common in many other water ways across Texas
(BONAP, 2014). In the past Red Ludwigia has been common to the San Marcos and Comal
Rivers (Doyle pers comm.) but recent mapping has shown its decline in aerial cover. Red
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Ludwigia is considered important habitat for the endangered fountain darter and is currently
being propagated and planted as part of a large scale restoration effort in both systems.
Hygrophila polysperma (Roxb) T. Anders (Hygrophila) is an exotic invasive plant introduced to
the San Marcos and Comal Rivers (Angerstein and Lemke, 1994). While morphologically
similar to Red Ludwigia (Doyle et al., 2003) it is not closely related. Hygrophila is an obligate
amphibious aquatic plant native to Eastern Asia. Popular in the aquarium trade it has been
introduced into several Florida waterways and is currently present in at least three waterways in
Texas and found in several locations in Mexico (Moro-Olivo et al., 2008). Hygrophila has
increased in abundance in both systems over the years. Recent system wide mapping of both
rivers by the Meadows Center (Williams 2010) and BIO-WEST (2013) has shown that
Hygrophila is wide spread in the San Marcos River found from Spring Lake to the confluence of
the Blanco River as well as in the Comal system where it can be found from the Upper Spring
Run to the confluence with the Guadalupe River (BIO-WEST, 2013). Hygrophila is commonly
found as a submersed perennial although it can grow as an emergent or terrestrially in wet
locations (Sutton and Dingler, 2000). It commonly reproduces and spreads by fragmentation or
nodal rooting. It is most often found in areas with slow to medium velocities and is easily
scoured in high flow events (Fast et al., 2008). It has a shallow lightly developed fibrous root
system.
Hydrilla verticillata (L.F.) Royle (Hydrilla) is an exotic invasive plant commonly problematic in
many waterways across Texas including the San Marcos River. Only one small colony of
Hydrilla exists in an off-channel pool of the Comal River and it has not expanded in over a
decade (Doyle pers. com.). This species is much more abundant in the San Marcos, and was
until recently common throughout the system. Now Hydrilla is mostly limited to below Spring
Lake dam as efforts to remove the plant from Spring Lake over the past 12 years have been
mostly successful (Williams et al., 2011). Hydrilla is an obligate submersed aquatic plant native
to Central Africa and Asia. Perhaps one of the most notorious aquatic invasive plants in North
America it can be found in many lakes and reservoirs across the United States. Hydrilla
commonly reproduces via fragmentation. It can also produce specialized structures called turions
which allow for dispersed vegetative reproduction (Madsen and Smith, 1999). Hydrilla also
produces tubers (subterranean turions) within the sediment which can stay viable for extremely
long periods of time (Owens et al., 2012) allowing for the plant to survive harsh conditions such
as drawdown, and scouring (Van and Steward, 1990). Hydrilla shoots emerge from a root crown
and are attached to well-developed rooting structures. It produces a thick canopy structure at the
water surface and is known to “bolt” in response to low light conditions giving it a quick
advantage over other native aquatic plants (McFarland and Barko, 1987).
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Materials and Methods
Two separate studies will be conducted to compare the competitive interactions of Red Ludwigia
with Hygrophila and Hydrilla. The Hygrophila competition study will be within the Comal
System which provides ideal conditions since recreation disturbances are limited in Landa Lake,
Upper Spring Run and the Old Channel. Since Hydrilla does not occur in the Comal system this
study will be conducted separately in the San Marcos River, even though higher recreation and
public use may impact that experiment.
Basic Competition Study Design.
The proposed experiments document initial establishment and growth of colonizing fragments in
three competitive environments (empty pots, pots with fragments of a competitor species, pots
with established plants of the competitor species). In addition, it allows for evaluation of the
continued growth of established plant communities with and without invasion by fragments of a
competing species.
For the Red Ludwigia versus Hygrophila or Hydrilla experiments seven treatments are proposed
(Table 1). This basic experimental design will be used at each location where an experiment is
conducted. Our treatment nomenclature utilizes lower case letters to designate fragments of a
species and capital letters to designate established plants. The first three treatments utilize only
plant fragments planted in to previously empty pots of sediment. These include freshly collected
Red Ludwigia fragments planted in monoculture into empty pots (ll), Hygrophila (or Hydrilla)
fragments planted in monoculture into empty pots (hh), a 50/50 mix of Red Ludwigia fragments
and Hygrophila (or Hydrilla) fragments (llhh). The use of newly sprigged fragments in empty
pots will provide information on the colonization potential of both species when free of
competitive pressures (ll and hh). The 50/50 fragment mixture (llhh) will provide information on
the competitive outcome of “equal start” low-competition environments.
Four treatments are proposed that utilize established plants of both species (Figure 1).
Fragments of the competitor species will be planted into the pots containing established plants
(llHH, hhLL) while other pots will allow the continued growth of the established plants without
any competitive pressure from invading fragments of the other species (HH, LL). The use of
these established plants in the experiment will allow us to understand the ability of an established
plant colony to resist invasion by a competitor (arriving as a viable fragment) and vice versa the
ability of a plant fragment to invade an established colony of the other species (high-competition
environment).
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Table 1. Treatments proposed for Ludwigia vs. Hygrophila (or Hydrilla) competition
experiment.
Symbol
Treatment
Count
ll
Ludwigia fragments into empty pot
8
hh
Hygrophila (or Hydrilla) fragments into empty pot
8
ll hh
50 / 50 mix Ludwigia and Hygrophila (or Hydrilla) fragments
into empty pots
8
ll HH
Ludwigia fragments planted into pots of established Hygrophila
(or Hydrilla)
8
hh LL
Hygrophila (or Hydrilla ) fragments planted into pots of
established Ludwigia
8
HH
Continued growth of established Hygrophila (or Hydrilla) plants
4
LL
Continued growth of established Ludwigia plants
4
Figure 1. Red Ludwigia growing in MUPPTs establish in only 3-4 weeks.
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Our experimental design will utilize Mobile Underwater Plant Propagation Trays (MUPPTs)
designed by BIO-WEST to allow culture of aquatic plants in situ for restoration work.
Treatments will be replicated 8 times except for the established monospecific treatments (HH
and LL) which will be replicated only four times. Space limitations within the MUPPTs limit
the total number of pots to 48 (see below). Since we anticipate less variability in the continued
growth of the established plots over time we have elected to lower the number of replicates for
those treatments.
The MUPPTs each hold four plastic nursery trays containing 12 nursery pots each for a total of
48 quart pots per MUPPT. However, to allow room for growth over the experimental growth
period without undue crowding, we will spread out the pots into alternate cells within the
MUPPTs. Two MUPPTs will be placed side by side at each location (Figure 2). The pots will be
assigned a location within the MUPPTs based on a stratified random design such that each
plastic tray receives one pot from each of the first 5 treatments (hh, ll, llhh, llHH, hhLL) and one
pot of either HH or LL. The pots used in the MUPPTs are 10.16 cm diameter X 10.16 cm tall
pots, referred to as “quart pots” in the plant nursery trade, with a maximum volume of 900 mL of
soil.
Figure 2. Alternating placement of plants within two MUPPTs. Each MUPPT holds four plastic trays of
12 quart pots. Pots with plants (green circles) will be distributed such that each plastic tray
holds six pots.
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Hygrophila Competition Study Design (Comal River)
The Red Ludwigia vs. Hygrophila competition study will be conducted at four sites on the
Comal River (Table 2). We propose to conduct replicates of the competition study at three main
locations in the River (Landa Lake, Upper Spring Run and Old Channel). At these locations we
will select sites with as much sun as available (=full sun). In Landa Lake we will select both a
full sun site as well as a restricted light site. Using the three locations in the river will allow us to
understand the likely competitive outcomes under differing environmental conditions within the
river while having both a full-sun site and a restricted light site in Landa Lake will allow us to
understand if the competitive outcome will differ in more light-restricted areas such as near tall
riparian trees that currently shade significant portions of the river. Hygrophila’s shade tolerance
is suspected but not widely investigated (Spencer and Bowes, 1985) and the possibility for the
species to be more shade-tolerant than native aquatic plant species means that a competitive
outcome may differ in light-restricted environments. Sites with similar depth will be selected.
However water flow, riparian canopy shade and perhaps other environmental factors will vary.
Table 2. Experimental design diagram for the Hygrophila vs. Red Ludwigia interference study.
Comal River
Landa lake
Full Sun
MUPPT MUPPT
I
II
4 ll
4 ll
4 hh
4 hh
4 llhh
4 llhh
4 llHH
4 llHH
4 LLhh
4 LLhh
2LL
2 LL
2 HH
2 HH
Upper Spring Run
Restricted Light
MUPPT MUPPT
I
II
4 ll
4 ll
4 hh
4 hh
4 llhh
4 llhh
4 llHH
4 llHH
4 LLhh
4 LLhh
2 LL
2 LL
2 HH
2 HH
Full Sun
MUPPT MUPPT
I
II
4 ll
4 ll
4 hh
4 hh
4 llhh
4 llhh
4 llHH
4 llHH
4 LLhh
4 LLhh
2 LL
2 LL
2 HH
2 HH
8
Old Channel
Full Sun
MUPPT MUPPT
II
4 ll
4 ll
4 hh
4 hh
4 llhh
4 llhh
4 llHH
4 llHH
4 LLhh
4 LLhh
2 LL
2 LL
2 HH
2 HH
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Hydrilla Competition Study Design (San Marcos River)
Two locations will be selected in the San Marcos System with one location upstream of Rio
Vista dam one location downstream (Table 3). Full-sun locations with similar depth will be
selected at these sites.
Table 3. Experimental design diagram for Hydrilla versus Red Ludwigia
San Marcos River
above Rio Vista
MUPPT I
MUPPT II
4ll
4ll
4hh
4hh
4llhh
4llhh
4llHH
4llHH
4LLhh
4LLhh
2LL
2LL
2HH
2HH
below Rio Vista
MUPPT I
MUPPT II
4ll
4ll
4hh
4hh
4llhh
4llhh
4llHH
4llHH
4LLhh
4LLhh
2LL
2LL
2HH
2HH
Experiment Implementation.
Native sediment from either the Comal or the San Marcos Rivers will be used for these
experiments. For established plants (LL or HH) healthy 20 cm apical stem fragments will be
planted and allowed a three to four week long pre-emptive grow out period. This growth period
has been shown to allow robust plant establishment and growth for Red Ludwigia (Figure 1) and
a similar establishment period is expected to suffice for the rapidly growing exotics as well. Ten
pots of established plants will be randomly selected at the beginning of the experimental growth
periods to quantify the initial biomass of these established pots as described below.
Two MUPPT frames will be secured side by side at a selected site at each location (Figure 2).
Sites with as much sunlight as available at the location will be selected. Treatments will be
assigned to the MUPPTs using the stratified random design explained above. Viable 20 cm long
freshly-collected apical fragments of the appropriate species will be collected and utilized to
initiate the experiments. Ten randomly selected apical fragments will also be harvested to
quantify the initial biomass of the fragments used in the experiments after drying to constant
weight at 60 °C (approx. 72 hours).
Monitoring
Experiments are expected to run for 10-12 weeks. This should allow sufficient time for the
plants to fill the pots and for the outcome of initial competitive interactions to be evident.
During this time, photosynthetically active radiation (PAR) will be measured continuously at
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each site at 5 minute intervals throughout the day using deployable data recorders. Each site will
also be visited at least twice per week and each pot will be visually assessed for evidence of
disturbance or death/loss of the plants. At least once per week velocity will be measured with a
Marsh McBirney water velocity meter and pH, conductivity and temperature will be measured
with an YSI multiparameter sonde.
Harvesting and Processing of Plants
After 10-12 weeks of growth the plants will be harvested and processed for biomass and
morphology data. The plants in each pot will be separated by species and the total number of
stems and length of each stem will be measured. Dry weight biomass data will be generated by
separating the samples of each species into above and below ground tissues and drying to a
constant weight at 60 °C for 72 hrs.
Data Analysis
Data analysis will follow that used by Doyle et al. (2003). Data will be tested for normality
using Cochran’s C test and transformed as needed to meet normality requirements. The
interference competition biomass and morphology data for each species will be analyzed
separately. A two-factor ANOVA (location X competition level) will be used to analyze the
results of the three-level interference competition for Ludwigia, Hygrophila or Hydrilla
fragments growing under varying levels of competitors (ll, llhh, llHH or hh, hhll, hhLL).
A separate two-factor ANOVA (location X invasion status) will be used to analyze the results
continued growth of the established plants with and without invasion by fragments of the
competitor species (LL, hhLL or HH, llHH).
Conclusion
In reality, as an in situ experiment conducted at differing locals within two different systems
each study location will be subjected to certain “location factors” such as differences in depth,
water velocity and PAR that cannot be controlled. These factors may actually provide insightful
data on how these plants grow and interact with each other in different locations of the river. If
for example Red Ludwigia can hold its own against Hygrophila under flowing conditions in the
Old Channel but can be easily outcompeted in more static conditions such as the Upper Spring
Run this will further improve our ability to predict the makeup of the plant community in those
locations as well as provide managers better data on the restorative ability of the system.
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