Download The impact of willow encroachment on water and carbon exchange

THE IMPACT OF WILLOW ENCROACHMENT ON WATER AND CARBON
EXCHANGE IN THE VEGETATION OF A SUBTROPICAL WETLAND
by
Michelle Louise Budny
A Thesis Submitted to the Faculty of
The Charles E. Schmidt College of Science
In Partial Fulfillment of the Requirements for the Degree of
Master of Science
Florida Atlantic University
Boca Raton, Florida
May 2015
Copyright 2015 by Michelle Louise Budny
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ACKNOWLEDGEMENTS
I would like to extend a world of gratitude to my advisor, Brian Benscoter, for his
guidance in my quest to become a better scientist and his patience towards my stumbles
along the way. I would also like to thank my committee members, Xavier Comas, Scott
Markwith, and Kimberli Ponzio, for their comments and suggestions throughout my
project.
Funding was provided by the U.S. Department of Energy as part of a larger
project with collaborators Ross Hinkle, Scott Graham, and Barclay Shoemaker. Florida
Atlantic University provided additional funding through the Provost Fellowship program.
St. Johns River Water Management District granted access to our research site at Blue
Cypress Management Conservation Area.
My fieldwork was possible thanks to assistance from Jake Dombroski, Liz Dutra,
Tristan Froud, Daniel Hagood, Diane Harshbarger, Lisa Reger, and Jaci Roberto. Lisa
and Liz along with Cara Abbott, Becca Dougherty, Cole Reintsma, and Kevin Nekblad
also provided invaluable encouragement and advice during the writing process for which
I am eternally thankful. Cynthia Berman-Gruen was instrumental in helping me navigate
the administrative path from start to graduation. Erika “K” Satterthwaite offered endless
emotional support and has been the best, most understanding roommate that anyone
could ever have.
Most of all, I would like to thank my parents, Eric and Mary Lou Budny, for their
unwavering support, not only throughout my graduate career, but in every decision and
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cross-country move I have made. They are my champions, and I am so fortunate for all
the opportunities they have given me.
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ABSTRACT
Author:
Michelle Louise Budny
Title:
Impact of Willow Encroachment on Water and Carbon Exchange
in the Vegetation of a Subtropical Wetland
Institution:
Florida Atlantic University
Thesis Advisor:
Dr. Brian Benscoter
Degree:
Master of Science
Year:
2015
Shortened periods of inundation due to water management have led to the
encroachment and expansion of Carolina willow (Salix caroliniana) in sawgrass
(Cladium jamaicense) marsh communities. Morphologic and physiologic differences
between sawgrass and willow have potential consequences for microhabitat conditions
and ecosystem function such as a reduction in temperatures and light availability and
changes in primary productivity. Since it is a woody shrub, willow is often assumed to
exhibit higher rates of transpiration than non woody plants, which in turn can affect
photosynthesis and carbon exchange and ultimately wetland water management. In this
study willow was found to have higher rates of stomatal conductance (gs) and
photosynthesis (Anet) than sawgrass. However, sawgrass had greater intrinsic water use
efficiency (WUE) than willow. This suggests that willow is capable of greater gas
exchange and carbon assimilation than sawgrass but requires more water. Understanding
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the implications of willow expansion will improve landscape models of wetland water
and carbon exchange and inform water management decisions.
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DEDICATION
To Ms. Valerie Frizzle, the earliest influence in my scientific career, who always
encouraged me to take chances, make mistakes, and get messy.
THE IMPACT OF WILLOW ENCROACHMENT ON WATER AND CARBON
EXCHANGE IN THE VEGETATION OF A SUBTROPICAL WETLAND
LIST OF TABLES ............................................................................................................. xi
LIST OF FIGURES .......................................................................................................... xii
LIST OF EQUATIONS ................................................................................................... xiii
CHAPTER 1 – INTRODUCTION: SHRUB ENCROACHMENT IN WETLANDS ....... 1
CHAPTER 2 – DIFFERENCES IN LEAF GAS EXCHANGE BETWEEN SAWGRASS
AND WILLOW .................................................................................................................. 6
Introduction ..................................................................................................................... 6
Subtropical wetland landscapes .................................................................................. 6
Ecophysiology of sawgrass and willow .................................................................... 10
Hydrologic management of south Florida wetlands ................................................. 11
Methods......................................................................................................................... 15
Field site description ................................................................................................. 15
Leaf gas exchange measurements ............................................................................. 15
Results ........................................................................................................................... 20
Discussion ..................................................................................................................... 24
ix
CHAPTER 3 – LANDSCAPE IMPLICATIONS OF WETLAND SHRUB EXPANSION
........................................................................................................................................... 28
REFERENCES ................................................................................................................. 41
x
LIST OF TABLES
Table 2.1: Comparison of physiological characteristics of sawgrass and willow ............ 14
Table 2.2: Parameters fit to Equation 1 derived from light response data ........................ 21
Table 3.1: Cover class community names and descriptions found in BCMCA with
spatial cover (ha) measured in 2001 and 2008............................................................ 35
Table 3.2: Parameters used to estimate landscape-level gas flux of sawgrass and
willow communities .................................................................................................... 37
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LIST OF FIGURES
Figure 2.1: Map of survey points within BCMCA ........................................................... 19
Figure 2.2 Light response curves for sawgrass and willow .............................................. 20
Figure 2.3 Mean Anet and gs for boardwalk and survey populations of sawgrass and
willow at PAR 2000 μmol m-2 sec-1. ......................................................................... 23
Figure 3.1: Vegetation surveys of Blue Cypress Management Conservation Area in
2001 and 2008 ........................................................................................................... 34
Figure 3.2 Spatial cover adjusted for species-specific estimates from plant community
data. ........................................................................................................................... 38
Figure 3.3: Total population carbon assimilation from willow and sawgrass .................. 39
Figure 3.4: Total population stomatal conductance from willow and sawgrass ............... 40
xii
LIST OF EQUATIONS
Equation 2.1: Rectangular hyperbola ................................................................................ 17
xiii
CHAPTER 1 – INTRODUCTION: SHRUB ENCROACHMENT IN HERBACEOUS
ECOSYSTEMS
Encroachment of woody plants into graminoid-dominated ecosystems is a
growing concern. Due to the morphologic and physiologic differences from herbaceous
plants and grasses, woody plants can alter the composition of the understory and change
environmental conditions. These changes may have a cascade of effects on ecosystem
functions. Shrub and tree encroachment has most commonly been reported in terrestrial
grasslands (Sankaran et al. 2005, Knapp et al. 2008, Barger et al. 2011, Eldridge et al.
2011). The extent of woody plant encroachment is largely driven by mean annual
precipitation (Sankaran et al. 2005, Knapp et al. 2008) with shrub cover increasing from
xeric sites to more mesic sites (Sankaran et al. 2005, Eldridge et al. 2011). In these
landscapes woody plants with a wide branching structure can shade many or all
understory plants while exhibiting leaf patterns and angles that minimize self shading to
maximize photosynthesis (Knapp et al. 2008, Barger et al. 2011). Furthermore, the
replacement of grasses by woody plant cover increases net ecosystem primary
productivity (Knapp et al. 2008) resulting in greater carbon storage in aboveground
biomass (Barger et al. 2011).
Similar to terrestrial grasslands, woody plant encroachment is also a growing
concern in wetlands (Cook et al. 1996, Berg et al. 2009, Marko 1999, Boyce et al. 2012).
However, drivers of shrub encroachment in wetlands are unique. In wetlands, woody
plant cover increases as these ecosystems become drier. Standing water maintains
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wetland plant communities by preventing shrub and tree seedlings from surviving to
maturity since most woody plant seedlings do not tolerate saturated or inundated soils
(Quintana-Ascencio et al. 2013, Castro-Morales et al. 2014). When water levels recede,
soil conditions become more favorable for woody shrubs and trees to establish and persist
(Rundel et al. 2014).
Established woody plants enable further woody seedling recruitment in wetlands,
especially under drier conditions (Knickerbocker et al. 2009). Once seedlings survive to
maturity, shrubs and trees with abundant aerenchyma (tissues for air transport) can
tolerate a broader range of hydrologic fluctuations within wetlands including prolonged
flooding and inundation (Zedler and Kercher 2004). As population establishment occurs,
woody plants have the potential to further modify wetland hydrology and
microtopography, facilitating greater woody encroachment. Elevated water loss due to
transpiration from broadleaf shrubs can draw down water tables (Gordon 1998, Berg et
al. 2009, Boyce et al. 2012). Additionally, greater accumulation of organic matter can
increase substrate elevation. Leaf litter from wetland shrubs in subtropical Florida has
slower decomposition rates than other marsh plants (Duffy 2014). Furthermore, willows
(Salix spp.) that have invaded riparian and wetland areas in Australia form dense root
mats that can trap sediment (Doody et al. 2011). Both of these factors allow drier
conditions to persist and promote or accelerate the spread of woody plant populations.
Thus, a positive feedback of shrub and tree establishment and habitat modification
perpetuates the cycle of woody plant encroachment into wetlands.
Shrub and tree encroachment can also change wetland community structure
(Zedler and Kercher 2004). Morphology of woody plants can exclude non-woody
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wetland species and change succession patterns (Lee et al. 2005a, b, Ponzio et al. 2006).
Tall branching shrubs with broad leaves can shade understory plants which can alter
primary production and change water surface temperatures (Koch and Rawlik 1993,
Doody et al. 2011). Some instances result in an understory of more shade tolerant
vegetation and replacement of light limited plants (Ponzio et al. in prep). Increasing
woody plant cover of floating marshes in the Mississippi River Delta has promoted
establishment of plant species commonly found in forest understories while the
occurrence of light-demanding marsh species declined (Battaglia et al. 2007). Often, the
understory is completely shaded leaving monoculture stands of woody plants (Zedler and
Kercher 2004, Rundel et al. 2014). In contrast, greater plant diversity has been observed
post shrub encroachment, but these increases in diversity are attributed to additional
weedy shrub species (Knickerbocker et al. 2009). In addition to the replacement of
herbaceous and graminoid species, shrub encroachment can reduce biodiversity of
wildlife. Woody plant encroachment can degrade the habitats for native lizards (Marko
1999), birds (Marko 1999, Grant et al. 2004), and spiders (Boyce et al. 2012) leading to
declines in these populations and reduced faunal richness.
Shrub encroachment can additionally impact wetland ecohydrology. Many
woody plants have deeper roots than herbaceous plants and graminoids. These
phreatophytic roots can access shallow aquifers and groundwater not readily available to
plants with shallower root systems (Zedler and Kercher 2004, Doody et al. 2011,
McLaughlin et al. 2012, Rundel et al. 2014). Increased water use can lead to water drawdowns which reduce the amount of water stored in aquifers and wetlands and the
availability of water for ecosystem processes. In addition, the morphology of woody
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plants can influence the rate at which water is lost to the atmosphere through
evapotranspiration (Rundel et al. 2014). Evapotranspiration (ET) describes the combined
effect of two mechanisms of water loss: transpiration, the transfer of water to the
atmosphere through the stomata of vascular plants, and evaporation, the vaporization of
water from the soil or water surface (Mitsch and Gosselink 2000). Broadleaf plants have
greater leaf surface area with more potential for transpirational water loss than
graminoids (Boyce et al. 2012). Combining higher transpiration rates with greater water
demand can exploit water storage and perpetuate drier marsh conditions. For example,
saltcedar (Tamarix spp.) along riparian areas in the southwest United States accesses
groundwater via deep tap roots and is thought to lose much of this water through
transpiration. However, local leaf area index can influence the degree of water loss
(Doody et al. 2011). Moreover, shrubs shading the understory can decrease temperatures
and reduce the potential for evaporation from the water surface (Koch and Rawlik 1993).
Ecophysiological characteristics of woody plants can change carbon cycling in
wetlands. Broad leaves that have higher transpiration rates may also have a higher
capacity to assimilate carbon through photosynthesis. If more shrubs with greater
photosynthesis rates spread throughout a marsh, the net primary production of the
ecosystem could increase. Woody plant tissues have greater carbon storage (Berg et al.
2009). For example, areas of the Florida Everglades where Melaleuca (Melaleuca
quinquenervia) trees have been introduced exhibited higher allocation of both above
ground and below ground biomass than areas without Melaleuca (Bodel et al. 1994,
Martin et al. 2009). The standing biomass changes environmental carbon allocation and
allows greater carbon sequestration within the vegetation of the marsh (Rundel et al.
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2014). However, as these plants have higher leaf turnover, the decomposition of biomass
can release sequestered carbon back to the atmosphere. Furthermore, shrub-induced
wetland drying can release carbon through peat oxidation (Berg et al. 2009). Greater
primary productivity and carbon assimilation by shrubs combined with carbon released
from soil drying could accelerate wetland carbon cycling over all.
In this thesis, I quantified the differences in leaf gas exchange between a native
graminoid, sawgrass (Cladium jamaicense) and a native but encroaching shrub, Carolina
willow (Salix caroliniana) to identify potential ecophysiological changes from shrub
encroachment in a subtropical wetland. In Chapter 2, I describe leaf-level net
photosynthesis and stomatal conductance rates for sawgrass and willow in Blue Cypress
Management Conservation Area (BCMCA). Additionally, these rates were used to
estimate physiological parameters associated with photosynthesis and intrinsic water use
efficiency of both species. In Chapter 3, I estimated ecosystem gas exchange rates in
BCMCA in 2001 and 2008 by combining leaf gas exchange measurements and landscape
vegetation cover data. The results of this study will provide a better understanding of
how shrub encroachment into graminoid wetlands can affect carbon and water cycles.
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CHAPTER 2 – DIFFERENCES IN LEAF GAS EXCHANGE BETWEEN SAWGRASS
AND WILLOW
Introduction
Subtropical wetland landscapes
Globally, shrub encroachment into herbaceous wetlands is a growing concern.
Sawgrass-dominated marshes of subtropical Florida are particularly vulnerable to
expansion of shrub populations due to anthropogenic activity such as the construction of
levees and dams for hydrologic diversion and landscape manipulation for agriculture.
Examples of ecosystems affected by growing populations of woody plants include the
Florida Everglades and marshes within the Upper St. John’s River Basin.
Sawgrass is one of the most dominant vascular plants in subtropical freshwater
wetlands of Florida (Loveless 1959, Craighead 1971), found as homogenous sawgrassdominated plains or on raised ridges characteristic of the patterned ridge-slough
landscape. Sawgrass is a sedge with a waxy cuticle that can grow clonally through
underground rhizomes. It is best adapted to grow in deep organic peat soils with long
hydroperiods typically inundating the marsh year round (Wade et al. 1980, Schedlbauer
et al. 2012). This species has adapted to thrive in low nutrient and oxygen-poor
environments (Steward and Ornes 1975, Wade et al. 1980). Plants can reach 3 m tall and
grow densely creating monoculture stands (Hofstetter and Parsons 1975, Hofstetter
1976). However, in many wetlands, spikerush (Eleocharis spp.), beak rush
(Rhynchospora spp.), arrowhead (Sagittaria latifolia), maidencane (Panicum
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hemitomon), spider lily (Crinum americanum), and pickerelweed (Pontedaria cordata)
are often found where sawgrass grows less densely (Loveless 1958, Craighead 1971,
Wade et al. 1980, Wood and Tanner 1990). As sawgrass leaves senesce, detrital organic
matter accumulates on the surface providing additional fuel for fires (Wade et al. 1980)
and leading to peat formation through slow decomposition (Craighead 1971). Plants can
survive frequent fires (every 3-5 years) utilizing energy storage in underground rhizomes
to recover rapidly after disturbance (Wade et al. 1980), facilitating the persistence of
sawgrass-dominated plant communities and the maintenance of peat accumulation over
time.
Sawgrass is adapted to withstand disturbance from frequent fire and fluctuating
water levels. However, anthropogenic manipulation of Florida wetlands has altered the
pattern of natural disturbance events (Kinser et al. 1997, Watts et al. 2010) and
hydrologic regimes, compromising the internal drivers of sawgrass community
maintenance. Changes in hydrology and nutrient enrichment due to human activity have
also promoted the spread and proliferation of native plants that were not as common in
sawgrass marshes. For example cattail (Typha spp.) invasion in the Everglades on
disturbed soils and following the addition of phosphorous has been widely documented
(Davis 1994, Newman et al. 1998, Weisner and Miao 2004). Such transitions in
community composition can have substantial implications for ecosystem function.
Shrubby vegetation encroachment has become a concern for wetland management
in the United States (Johnston 2003, Barger et al. 2011, Doody et al. 2011), Europe
(Medori et al. 2012), and Australia (Marko 1999, Doody et al. 2011). Shrub
encroachment in streams and wetlands can shade the understory potentially leading to
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decreased temperatures and light availability and changes in primary productivity (Doody
et al. 2011). Growth of shrubs can change ecosystem evapotranspiration rates and affect
the water table by accessing water not readily available to other plants with shallower
roots systems (Doody et al. 2011, Rundel et al. 2014). This is of particular concern for
water management.
Several species of woody plants are also found in subtropical wetlands. Carolina
willow (Salix caroliniana) is a shrub native to Florida which is commonly found on tree
islands and other elevated areas. Trees can grow up to 10 m tall, and trunks can be 35 cm
in diameter (Brockman 1979 in Kinser et al. 1997). Individuals may live up to 50 years,
but old plants are typically found in poor condition (Kinser et al. 1997). For a population
to become established, willows take advantage of low, partially flooded land and soils
that have been exposed following drought or disturbance events. They are shade
intolerant and grow readily after fires, in abandoned agricultural fields, around alligator
holes, and in areas of erosion (Craighead 1971, Kinser et al. 1997). Willows can form
dense stands that often exclude other vegetation by shading the understory (Ponzio et al.
2006). However, some herbaceous plants including pickerelweed, arrowhead, green
arum (Peltandra virginica), spilanthes (Acmella oleracea), great bulrush (Scirpus
californicus), figwort (Scrophularia spp.), pennywort (Hydrocotyle sp.), smartweed
(Polygonum sp.), false bishop’s weed (Ptilimnium capillaceum), and chain fern
(Woodwardia verginica) are found growing sparsely near willows in peat formed from
willow leaves and twigs (Craighead 1971, NPS 2006, Ponzio et al. 2006). The edge of
willow stands can also provide habitat for other woody species such as sweet bay
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(Magnolia virginiana), cocoplum (Chrysobalanus icaco), and buttonbush (Cephalanthus
occidentalis) to establish (NPS 2006, Ponzio et al. 2006).
The expansion of willow from upland to wetland areas has been attributed to
shorter hydroperiods following the construction of levees and canals (Quintana-Ascencio
et al. 2013), fire suppression, and landscape fragmentation (Lee et al. 2005a, 2005b).
Quintana-Ascencio et al. (2013) found that willow seedlings and cuttings are sensitive to
both drought and flooding conditions, but they can persist once they survive throughout
the wet season. They can also tolerate growing in hypoxic conditions and compacted
soils (Quintana-Ascencio et al. 2013). Willow expansion into herbaceous wetlands is
undesirable as it can reduce landscape heterogeneity, biodiversity, and ecological and
economic values (Kinser et al. 1997, Ponzio et al. 2006, Quintana-Ascencio et al. 2013).
Encroachment of willow has had some noticeable impact on sawgrass communities as
willow cover greater than 50% replaces sawgrass and reduces fuels needed for prescribed
fire (Kinser et al. 1997).
In addition to changes in community composition and plant diversity, willow
abundance may affect ecosystem function and microhabitat characteristics. Vegetation
structure and density changes the way air circulates around plants, influencing
evapotranspiration (ET) and microclimate gradients (Koch and Rawlik 1993). Dense
vegetation can slow airflow, decreasing the rate at which leaf gas exchange of carbon
dioxide (CO2) and water vapor occurs. Canopy conditions such as leaf density and angle
can also alter ET by influencing the amount of light that is transmitted through or
reflected away from the canopy (Koch and Rawlik 1993). Changes within the canopy
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structure that affect leaf gas exchange and microclimate conditions can influence net
primary productivity and hydrologic conditions of the ecosystem.
Ecophysiology of sawgrass and willow
Physiologically, leaf gas exchange depends on stomatal conductance, the capacity
for CO2 or water vapor to enter and exit the leaf through stomatal pores. Stomatal
conductance is influenced by the difference between leaf intercellular vapor pressure and
air vapor pressure as well as the abundance and diameter of stomata. Water stress can
cause stomata to close, which reduces water loss but also allows less CO2 to enter the leaf
(Lambers et al. 2006). The trade-off between photosynthetic gain (i.e., CO2 fixation) and
leaf water loss during CO2 uptake (i.e., transpiration) can be expressed as the leaf water
use efficiency (WUE: Lambers et al. 2006). While willow and sawgrass both utilize the
C3 photosynthetic pathway, woody C3 plants have a greater range of WUE among species
than that of other non-woody C3 plants (Table 2.1) due to morphological variability and
specialization of water movement through vascular tissue (Lambers et al. 2006).
Therefore, while carbon fixation may be similar between woody and graminoid species,
differences in WUE may have consequences for ecosystem water cycling and
availability.
Changes in local species composition and diversity can alter net primary
productivity (NPP) and, subsequently, ecosystem carbon uptake (Barger et al. 2011). In a
simulated drought, Malone et al. (2013) found that sawgrass respiration increased as
water stress increased. Water drawdowns reduced CO2 uptake by sawgrass and resulted
in a release of CO2 to the atmosphere (Malone et al. 2013). If willows have higher
transpiration rates than sawgrass, more water may be lost to the atmosphere in areas with
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greater willow abundance, enhancing drought events. Quantifying gas exchange for
native and encroaching plants will help estimate how carbon and water cycling may
change due to alteration to the plant community.
Hydrologic management of subtropical wetlands in Florida
In south Florida, freshwater wetlands are capable of holding large amounts of
water resulting in multiyear hydroperiods (Ogden 2005). However, regional water
management practices have reduced the heterogeneous structure of wetland landscapes
(Watts et al. 2010) and have manipulated the hydroperiod cycle in many areas. In the
early 1900s, human settlement grew rapidly in south Florida and increased the demand
for freshwater. Construction of an extensive system of canals and levees began in 1912 to
divert water from the marshes to use for human consumption, agriculture, and urban
development and provide flood control for south Florida. As a result, more than half of
the historic area of wetlands was drained (Wood and Tanner 1990).
Many of the remaining sawgrass marshes in Florida have been compartmentalized
by water-control structures like canals and levees, drastically altering wetland hydrology.
Water management practices have reduced water storage capacity, changed patterns of
water flow, direction, and volume, and shortened hydroperiods leading to drier conditions
(Ogden 2005). Effects of these stressors have altered and degraded natural plant
communities by allowing woody plants to invade sawgrass ridges (Ogden 2005, Givnish
et al. 2008) creating a more homogenous landscape. Alterations have left these marshes
vulnerable to species invasions (Light and Dineen 1994) and have given rise to concern
for the quality and quantity of water within Florida watersheds (Mitsch and Gosselink
2000).
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The Upper St. John’s River Basin (USJRB) is a watershed in Florida that feeds
into 110 river miles and has been altered by drainage for agriculture and levee
construction since the late 1800s. Historically, precipitation at the headwater promoted
sheet across river floodplain. Densely vegetated adjacent marshes store water and
naturally regulate flooding. Organic soils in these marshes have the capacity to hold
great amounts of water, and some plants can still access water through soil capillary
action when water tables are below the soil surface. However, these services have been
compromised with the greatest changes occurring over the last 60 years (SJRWMD
2007).
Blue Cypress Marsh Conservation Area (BCMCA) is one of the least altered
wetlands within the Upper St. Johns River Basin project area. However, substantial
vegetation change has been observed over the last 70 years (Kinser et al. 1997). In the
USJRB, there has been a significant expansion of Carolina willow into herbaceous
sawgrass wetlands. Two mechanisms for willow expansion have been suggested:
enhanced seedling establishment and increased survival and growth of seedlings (Kinser
et al. 1997). Seeds need exposed, moist soils to germinate, and seedlings can survive in
shallow water depths but not deep flooded conditions (Quintana Ascencio et al. 2013).
The most rapid expansions of willow occurred between 1971-1981 and 1989-1993, which
coincided with the construction of a highway and two levees, respectively. Construction
activity may have accelerated expansion by disturbing soils and changing hydrologic
conditions which promoted seed germination and survival (Kinser et al. 1997).
Measuring transpiration and photosynthesis of sawgrass and willow will help
quantify changes in wetland water cycling that may occur due to shifting plant
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community composition. The objective of this study was to assess ecophysiological
differences in leaf gas exchange (CO2 and H2O) of willow and sawgrass in the peatland
of BCMCA. I quantified leaf gas exchange and used this data to calculate plant WUE to
evaluate the impact of willow invasion on carbon and water cycling in historic sawgrass
peatlands, determine differences in water loss between willow and sawgrass, and identify
whether water loss is a consequence of lower WUE in willow. These results improve our
understanding of the consequences of land cover change on ecosystem water cycling and
inform strategies for water and invasive species management in wetlands.
13
Table 2.1: Comparison of physiological characteristics of sawgrass and willow
Species
Growth morphology
Cladium
jamaicense
Salix spp.
Perennial graminoid
Stomatal conductance Transpiration
(mmol H2O m-2 s-1)
(mm day-1)
152 ±12a
3.16 – 5.93b
Deciduous woody broadleaf
~170c
1.54 – 16.34
a Stomatal conductance of C. jamaicense from Koch and Rawlik (1993)
b Range of transpiration measured for C. jamaicense from studies by Mao et al. (2002)
and Jiang et al. (2009)
c Stomatal conductance estimated from Figure 1d in Aasamaa and Sõber (2012) for Salix
caprea
d Range of transpiration measured for Salix sp. in studies by Cermak et al. (1984), Priban
and Ondok (1986), Pauliukonis and Schnieder (2001), and Doody and Boyce (2011)
14
Methods
Field site description
Research was conducted at BCMCA (27° 39.75’ N, -80° 38.53’ W), a sawgrass
peatland within the St. Johns Water Management District (SJWMD) located in Indian
River County, FL. BCMCA is an 8,600 ha subtropical floodplain marsh with a long
hydroperiod holding water for all or most of the year (Hall 1987, Lee et al. 2005a). Soils
are primarily histosols with peat depths of 1-4 m (Lee et al. 2005a). Vegetation is
characterized by sawgrass, maidencane, and cattail marshes, cypress (Taxodium
distichum) and red maple (Acer rubrum) tree islands, willow-dominated shrub swamps,
and white water lily (Nymphaea odorata) and bladderworts (Utricularia spp.) in deep
water areas (Kinser et al. 1997, Lee et al. 2005a). BCMCA is part of the Upper St. Johns
River Basin Project controlled by SJWMD and the US Army Corps of Engineers
(SJRWMD 2014). The goal of the USJRB Project is to moderate flooding, protect water
quality, reduce the amount of freshwater diverted to the Indian River Lagoon, and
conserve and restore natural ecosystems and habitat (SJRWMD 2007).
Leaf gas exchange measurements
Photosynthetic capacity and stomatal conductance (gs) were evaluated by
measuring leaf gas exchange (CO2 and H2O) of willow and sawgrass plants in BCMCA.
Using a portable photosynthesis system (LI-6400xt, LI-COR, Lincoln, NE, USA),
photosynthetic light response data was collected by measuring net leaf carboxylation
(Anet) and gs at decreasing levels of irradiance, or photosynthetically active radiation
(PAR; 2000, 1600, 1200, 800, 600, 400, 200, 100, 50, and 0 μmol m-2 sec-1). Each leaf
was clamped inside the chamber and allowed to acclimate at the highest level of PAR for
15
1-2 minutes before measurements began. Leaf temperature, CO2 concentration, and H2O
mol fraction inside the leaf chamber were held constant according to ambient values
within the chamber at the start of each leaf measurement. At each PAR level,
measurements of net photosynthesis (Anet; the difference between total photosynthesis
and the sum of photorespiration and dark respiration), and stomatal conductance (gs; the
rates of CO2 entering and water exiting the leaf) were recorded after a minimum and
maximum lag time of 120 and 200 seconds.
On each sampling day, light response data was collected for one leaf from five
plants each of willow and sawgrass along a pre-established boardwalk within the marsh.
Measurements were collected on 19 days between June and November 2014 until
willows dropped their leaves. New leaves were selected on each sampling day, and
collection of light response data was not repeated on previously measured leaves. Data
were collected on days with full sun to ensure that plants were light acclimated prior to
measurements. Young, fully expanded leaves with no signs of wilting, folding, or other
physical damage on each plant were selected for analysis to ensure that results were from
physiological processes and not an artifact of leaf condition. Local weather and
environmental conditions were acquired from a near by micrometeorological tower.
Light response curves were generated for willow and sawgrass by fitting light
response data to non-rectangular hyperbola, rectangular hyperbola, and exponential
models using non-linear least squares regression (nlsLM; Elzhov et al. 2013) and
compared using Akaike’s information criterion (AIC; Mazerolle 2014) using R (R Core
Team 2013). The lowest AIC values indicated that the data best fit the rectangular
hyperbola:
16
Equation 2.1: Rectangular hyperbola
𝐴net = {
𝐴max ∙ Φ ∙ PPFD
} + 𝑅d
(Φ ∙ PPFD) + 𝐴max
representing net photosynthetic rate (Anet), irradiance-saturated photosynthetic rate (Amax),
photosynthetic quantum yield (Φ), photosynthetic photon flux density (PPFD), rate of
dark respiration (Rd), light compensation point (Ic; irradiance at Anet = 0) , and the light
saturation point (Is; irradiance at Amax).
Intrinsic water use efficiency (WUE) was calculated as the ratio of Anet (μmol CO2
m-2 s-1) to gs (mol H2O m-2 s-1) collected at highest PAR point (2000 μmol m-2 sec-1).
Differences between willow and sawgrass for each of the calculated parameters (Anet,
Amax, Φ, Rd, Ic and Is), gs, and WUE were analyzed using t-tests. For both species WUE
was also compared to the day of year, leaf temperature, and water depths that the plants
experienced during data collection to determine potential temporal and environmental
correlations.
To evaluate the representativeness of the plants selected for full light response
curve data collection within the local populations at BCMCA, survey measurements of
Anet and gs were also measured from 30 willow and 29 sawgrass plants distributed
throughout the marsh (Figure 2.1). Plants were selected in areas that were easily
accessed by airboat and measurements were taken in October and November 2014 on
young, undamaged leaves. Light level was set at 2000 μmol m-2 sec-1 to match the
highest set point of light for comparison with LRC data. Flow rate was kept constant at
500 μmol sec-1 and reference CO2 was held at 400 μmol CO2 mol-1. Each leaf was
clamped in the chamber for a minimum of 5 minutes and a maximum of 10 minutes and
17
final values were logged when they were stable. Sample location coordinates were used
to assess spatial autocorrelation (Moran’s I statistic) to determine if physiological
responses of each species differed based on marsh location. Values near -1 indicate
patterning or dispersal, values near 1 indicate correlation, and values near 0 indicate a
random distribution. Additionally, T-tests were performed on Anet and gs between the
boardwalk and survey populations for each species to ensure that light response data were
representative of the entire marsh.
18
Figure 2.1: Map of survey points within BCMCA. Light blue points represent sawgrass
samples. Orange points represent willow samples. The yellow star indicates the location
of the boardwalk.
19
Results
Responses of physiological parameters to light differed between willow and
sawgrass. Willow responded with higher Anet (p = 0.003), gs (p < 0.001), Amax (p =
0.003), and Φ (p < 0.001) than sawgrass (Figure 2.2). Rd (p = 0.168), Ic (p=0.959), and Is
(p=0.350) did not differ between the two species. Willow had lower WUE (46.67
±2.450) than sawgrass (55.94 ±2.292, p = 0.007).
25
Anet (μmol CO2 m-2 sec-1)
20
15
10
Photo-CJ
5
Photo-SC
0
CJ
-5
0
500
1000
1500
2000
SC
-10
-15
Iradience (μmol m-2 sec-1)
Figure 2.2 Light response curves for sawgrass (CJ; blue diamonds) and willow (SC; red
Xs). Lines represented fitted curves based on Equation 1; parameter estimates provided in
Table 2.2
20
Table 2.2: Parameters fit to Equation 2.1 derived from light response data: net
photosynthetic rate (Anet; μmol CO2 m-2 s-1), irradiance-saturated photosynthetic rate
(Amax; μmol CO2 m-2 s-1), photosynthetic quantum yield (Φ; mol CO2 [mol photons
absorbed]-1), rate of dark respiration (Rd; μmol CO2 m-2 s-1), light compensation point (Ic;
irradiance at Anet = 0; μmol m-2 sec-1), and the light saturation point (Is; irradiance at
Amax; μmol m-2 sec-1)
Species
Sawgrass
Willow
Anet
6.436
8.052
Amax
17.47
20.57
Φ
0.052
0.078
21
Rd
-2.347
-2.994
Ic
54.77
55.43
Is
3008
2562
Moran’s I statistics indicated that photosynthetic capacity and stomatal
conductance of the survey measurements did not differ based on location within the
marsh for either sawgrass (Anet: 0.005, p=0.806; gs: -0.211, p=0.289) or willow (Anet: 0.088, p=0.651; gs: 0.025 p=0.726). Similarly, Anet did not show a significant difference
between either willow or sawgrass boardwalk and survey populations (Figure 2.3).
However, while gs was similar among willow populations, it was slightly higher for the
boardwalk sawgrass population than the survey population (p=0.048; Figure 2.3 bottom).
22
Anet (μmol CO2 m-2 sec-1)
18
15
12
9
Boardwalk
Survey
6
3
0
Sawgrass
Willow
Species
0.4
gs (mol H2O m-2 sec-1)
0.35
0.3
0.25
0.2
Boardwalk
0.15
Survey
0.1
0.05
0
Sawgrass
Willow
Species
Figure 2.3 Mean (±1 SE) Anet (top) and gs (bottom) for boardwalk and survey populations
of sawgrass and willow at PAR 2000 μmol m-2 sec-1. Asterisk (*) indicates significantly
different means (p<0.05).
23
Discussion
Anthropogenic manipulation of subtropical wetlands has led to the expansion of
woody plants including Carolina willow into sawgrass marshes. Shrubs are commonly
assumed to have higher transpiration rates than herbaceous vegetation. If this pattern is
true for willow, further encroachment into sawgrass communities could reduce water
availability and enhance drought events. Not only would lower water retention affect
ecosystem function, but also less water would be available for human consumption,
which could result in socioeconomic consequences.
No differences in Anet or gs were found between survey and boardwalk
populations except in the case of sawgrass stomatal conductance. Differences in gs
between the two sawgrass populations may have been due to seasonality or water level as
water availability can influence stomatal conductance. The survey measurements of
sawgrass took place over two days in late November with little variation in water depth.
Conversely, the boardwalk measurements spanned six months, and water depth increased
steadily over this period. Even with these slight differences, the lack of spatial patterning
or clustering of Anet and gs among survey measurements and the similarity of the survey
population to the boardwalk population suggests that the physiological parameters
estimated from the data collected at the boardwalk are representative responses of the
entire marsh.
My results indicate that willow has greater rates of gross primary production and
stomatal conductance and similar autotrophic respiration compared to sawgrass
suggesting a potential for greater ecosystem carbon sequestration with willow invasion.
This is consistent with the changes in carbon storage that have been measured where
24
shrubs have encroached in grasslands. Knapp et al. (2008) found that shrubs increase
aboveground net primary production (ANPP) by an average of four times in grasslands.
Barger et al. (2011) also found that shrubs increase net primary production (NPP) and
above ground biomass in grasslands in the western United States. Increases in NPP were
related to greater leaf area as shrub cover increased (Knapp et al. 2008, Barger et al.
2011).
In the present study, stomatal conductance of sawgrass was similar to values
measured by Koch and Rawlik (1993), but stomatal conductance of Carolina willow was
much greater than that measured by Aasamaa and Sõber (2012) for Salix caprea. Greater
stomatal conductance of Carolina willow plays a role in its lower water use efficiency
than sawgrass. This response can lead to greater loss of surface water as willow
expansion into sawgrass marshes continues. My results show that willows exhibit a
higher demand for water to assimilate the same amount of carbon as sawgrass. Since
willow is also capable of greater primary production than sawgrass, continued expansion
of willow could result in greater pressure on water availability within the wetland.
Transpiration was not compared between willow and sawgrass due to the
controlled conditions inside the leaf chamber. However, higher transpirational water loss
to the atmosphere combined with greater water use from willow could lead to autogenic
drying of the wetland. Dryer conditions could promote a positive feedback where more
willow seedlings are able to establish and accelerate population spread. The effect of
water availability has some inconsistent influences on transpiration. Willows invading
riparian areas with unlimited access to water have higher transpiration rates (Doody et al.
25
2011). Moreover, as long as water is readily available, changes in water table position
have no effect on tree transpiration (Shoemaker and Sumner 2006).
Structural change in plant communities could change local wetland conditions.
Shrubby vegetation can change light regimes of the understory. Canopy structure
influences light attenuation and reflectance depending on leaf area and angle (Koch and
Rawlik 1993). Leaf area increases as shrub cover increases which shades light-limited
plants and can decrease species richness (Koch and Rawlik 1993, Knapp et al. 2008).
Differences in light attenuation would affect not only emergent plants but also submerged
aquatic vegetation. Shifts in these plant communities could compromise wetland health.
In this study gas exchange was only measured during the wet season. Differences
during the dry season could affect ecosystem gas exchange since willows are deciduous
shrubs that drop their leaves at the start of the dry season. No gas exchange occurs in
these plants without leaves. However, sawgrass photosynthesis continues year-round.
Schedlbauer et al. (2012) have measured increased respiration in sawgrass during the dry
season in short hydroperiod marshes in the Everglades. Autogenic drying due to willow
encroachment could enhance seasonal fluxes in gas exchange that are not evident in the
results of the present study.
The results obtained from this study will provide a better understanding of
ecophysiological changes that occur within marsh communities with shrub expansion,
which will have cascading impacts on soil accretion and turnover, and microclimate. The
data presented in this study can be combined with vegetation cover data to estimate
changes in ecosystem water and carbon cycling due to shrub encroachment. Future
studies including year-round field measurements, response of plants to water level
26
manipulation, and comparisons of plant populations in different soil conditions will
further expand comprehension of physiological responses to environmental drivers.
Understanding the implications of willow expansion will improve landscape models of
wetland water and carbon exchange as well as inform water management decisions in the
USJRB and similar subtropical marshes.
27
CHAPTER 3 – LANDSCAPE IMPLICATIONS OF WETLAND SHRUB EXPANSION
Land cover change in grass-dominated ecosystems can cause drastic alterations in
landscape function. Shifts from herbaceous graminoids to woody shrubs affect wildlife
habitat, plant diversity, and microclimate characteristics. Changes in vegetation cover
can impact faunal biodiversity. Shrubby species degrade habitat for some bird species
(Marko 1999, Grant et al. 2004, Boyce et al 2012). Spider diversity and abundance has
also declined in areas of expanding shrub populations, and toxins released from
decomposing leaf litter may negatively affect amphibians (Boyce et al. 2012). Shrub
expansion also changes vegetation composition by shading more heliophilic herbaceous
plants in favor of woody monocultures or shade tolerant understories (Battaglia et al.
2007). Shading from shrubs can also change understory temperatures, and branch
structure can affect air circulation patterns (Grygoruk et al. 2014) altering rates of leaf
gas exchange (Koch and Rawlik 1993).
Perhaps the greatest concern surrounding woody plant encroachment is the impact
these plants may have on water and carbon cycles. Woody plants have greater gas
exchange rates than herbaceous species resulting in more rapid carbon assimilation and
water loss. Root morphology of shrubs allows them to access water not readily available
to other plants (Doody et al. 2011, Boyce et al. 2012). This promotes higher primary
productivity and simultaneously results in a greater water loss through transpiration
(Huxman et al. 2005). These differences between herbaceous and woody vegetation
cover can directly or indirectly change landscape hydrology. Although this phenomenon
28
has more commonly been reported in terrestrial grasslands (Knapp et al. 2008, Barger et
al. 2011, Doody et al. 2011), growing attention has been directed towards shrub
encroachment in herbaceous freshwater wetlands. Examples include honeysuckle
(Lonicera maackii) in North American ephemeral wetlands (Boyce et al. 2012), shrubs
and trees in the Alaskan Kenai Peninsula (Berg et al. 2009), Mimosa pigra in Australia
(Cook et al. 1996, Marko 1999), and Melaleuca quinquenervia in the Florida everglades
(Bodel et al. 1994, Turner et al. 1998). Understanding differences in leaf-level gas
exchange is important not only in a plant physiology sense, but it can have direct
implications for landscape-level ecosystem gas exchange. As wetlands are intricately
tied to landscape hydrology (McLaughlin and Cohen 2013), identifying the ways that
land cover change impacts water cycling is important both for natural and managed
wetlands.
Here, I use Blue Cypress Management Conservation Area (BCMCA) as a case
study to estimate total ecosystem gas exchange rates attributed to sawgrass and willow
and illustrate how water availability within the marsh could change with continued shrub
encroachment. The Saint Johns River Water Management District (SJRWMD) surveyed
plant communities in BCMCA in 2001 and 2008 using aerial photographs and remote
sensing techniques (Figure 3.1; SJRWMD unpublished data). Cover classes were defined
by dominant plant species (Table 3.1; from Bryan and Miller 2004) and the areal extent
of each community was quantified (Table 3.1; SJRWMD unpublished data). From these
surveys I isolated sawgrass (SG) and willow (WS) community cover data. Each of these
communities is defined by having at least 70% coverage of sawgrass or willow
29
respectively. The total area classified as each community type was multiplied by the
70% threshold to get an estimate of species-specific plant cover (Figure 3.2).
Leaf area index (LAI) was measured from biomass collections at BCMCA.
Aboveground biomass was clipped at the soil surface from 1 m2 sawgrass (n=25) in June
and November 2013 and April 2015 and willow (n=6) plots in March 2015 in areas
representative of each community type. Leaves were measured using an area meter (LI3100C; LI-COR, Lincoln, NE, USA) to quantify the total leaf area per square meter of
ground area. The average LAI of each species was multiplied by the adjusted speciesspecific cover area estimates derived from the vegetation survey data from 2001 and
2008 to get the total leaf area in the marsh for each species. Total leaf area was
multiplied by average net photosynthetic rate (Anet) and stomatal conductance (gs)
measured at PAR values of 2000 μmol m-2 sec-1 for willow and sawgrass (described in
Chapter 2) to estimate the total gas flux for both populations (Table 3.2). Calculations
were repeated for sets of cover data from both survey years (Figures 3.3 and 3.4).
Overall, both willow and sawgrass extent in BCMCA increased from 2001 to
2008, but, proportionally, willow cover increased more rapidly than sawgrass (29%
increase versus 8% increase, respectively). The increase in willow was primarily at the
expense of mixed herbaceous marsh, mixed shrub, and sawgrass communities. The
increase in sawgrass was primarily at the expense of another herbaceous community type
(mixed herbaceous marsh). Both of these changes may be indicative of drier conditions
during that time. In fact, hydrologic data indicates that marsh exposure occurred more
frequently during the period following hydrologic management changes in 1997 (Ponzio,
pers comm). Although sawgrass covers roughly twice as much area in BCMCA, willow
30
has a bigger influence on total gas exchange from marsh vegetation. The combined rate
of carbon assimilation of sawgrass and willow increased by 72.65 mol CO2 s-1 between
2001 and 2008 with 14.59 mol CO2 s-1 increase from sawgrass and 58.06 mol CO2 s-1
from willow (Figure 3.2). In the same period, the rate of water exchange increased by
1688.8 kmol H2O s-1 with a 280.26 kmol H2O s-1 increase from sawgrass and 1408.5
kmol H2O s-1 increase from willow (Figure 3.3). The greater spread of willow in
conjunction with its greater leaf gas exchange rates suggests that total ecosystem gas
exchange is increasing more rapidly leading to net ecosystem gain in carbon at the
expense of greater water loss since willow has a lower water use efficiency than
sawgrass.
The calculations from these data represent underestimates of vegetation gas
exchange for each population at the landscape level. Greater cover of each dominant
species probably exists in many areas of BCMCA (since classification is based on at least
70% cover of each species). Additionally, other areas that have been classified as a
different community type may also include sawgrass or willow cover as a component of
that community that is not included in the above calculations. Actual ecosystem gas
fluxes due to willow and sawgrass may, therefore, be higher than these estimates.
The above calculations assume that all leaves within the willow canopy have the
same rates of gas exchange. However, leaf productivity could vary depending on leaf age
and vertical canopy position. In a survey of tree and shrub leaf traits, Wright et al. (2006)
found that light availability declined with leaf age and vertically down through the
canopy. Since light is a direct driver of photosynthesis, older, lower canopy leaves may
have different gas exchange rates than younger, upper canopy leaves. If this is the case,
31
the landscape level estimates of gas exchange presented here may overestimate actual
conditions. Future research that monitors gas exchange along a gradient of leaf age or
vertical canopy position would improve these estimates.
In addition to leaf gas exchange, changes in vegetation cover could affect soil
characteristics. Decomposition of leaves dropped from deciduous broadleaf trees and
shrubs can alter soil composition and nutrient content (Bedford et al. 1999). Differences
in root morphology and function of woody and herbaceous plants could have different
effects on soil porosity and belowground carbon storage (Lambers et al. 2006). Although
these aspects of land cover change are outside the scope of the present study,
considerations of belowground functions will further inform how shrub encroachment
may alter ecosystem gas flux.
Future climate scenarios could change water availability within subtropical
wetlands. Although predictions vary some models suggest precipitation will decline
which would reduce freshwater inputs to wetland ecosystems (Obeysekera et al. 2011).
Conversely, a few scenarios estimate that precipitation could increase (Obeysekera et al.
2011, Obeysekera et al. 2014). However, higher temperatures associated with climate
change will lead to increased evaporation of surface water (Nungesser et al. 2014,
Obeysekera et al. 2014). The estimates of landscape-scale gas exchange presented here
can be incorporated into water budget models under current conditions and future climate
scenarios. Combining vegetation water use data with precipitation and temperature
trends and projections, land managers can estimate the total ecosystem water flux
attributed to changes in willow cover.
32
Transpiration from shrubs will further dry the landscape resulting in a positive
feedback loop of decreased water availability and shrub persistence and spread. As drier
conditions promote woody plant encroachment, the presence of shrubs could change fire
regimes. Woody plants can act as fire barriers (Wade et al. 1980). Many shrubs are
resistant to fire (Wade et al. 1980, Marko 1999) including Carolina willow (Lee et a.
2005a,b). An increase of live woody biomass can lead to a reduction in ecosystem
flammability, or, alternatively, shrubs can shade flammable understory plants, reducing
fuel loads entirely (Rundel et al. 2014). As a result shrub encroachment can lengthen
fire-return intervals. In the long-term, reduced fire frequency and extreme drying
conditions could increase the likelihood of intense fires on the landscape (Nungesser et
al. 2014). Extreme fire events could open patches for recruitment by willow or other
woody species.
Hydrologic alterations and reduced fire frequency have allowed shrubs to
encroach grass-dominated wetlands where they have a positive feedback to further alter
habitat and promote their success. In subtropical peatlands Carolina willow has greater
carbon and water exchange rates than sawgrass and could account for a larger portion of
ecosystem gas flux even in areas where it occupies a smaller spatial extent such as
BCMCA. As climate change influences temperatures and precipitation rates, higher
transpirational water loss from willow could result in a net decline in water availability.
Ecophysiological changes within marsh communities due to shrub expansion will also
have cascading impacts on soil carbon storage, and microclimate. The results of this
study will improve landscape models of wetland water and carbon exchange and inform
water management decisions.
33
Figure 3.1: Vegetation surveys of Blue Cypress Management Conservation Area in (left
map) 2001 and (right map) 2008 (SJRWMD, unpublished data).
34
Table 3.1: Cover class community names and descriptions found in BCMCA with spatial
cover (ha) measured in 2001 and 2008
Community
Description
2001
cover
2.4
2008
cover
1.2
Broadleaf
emergent marsh
(BE)
Canals, ditches,
and borrow pits
(CA)
Cattail (Typha
spp.) (CT)
70% or greater coverage of emergent
wetland plants with wide leaf
structure
Small, man-made water bodies
91.1
108.6
70% or greater coverage of cattail.
Sawgrass, ferns, other herbaceous
species, and small shrubs may also be
present
Cattail/sawgrass 30%–70% cattail, less than 70%
(CTSG)
sawgrass, and less than 10% other
species.
Cypress (CY)
70% or greater cover by Taxodium spp
89.8
72.8
22.6
25.8
131.1
134.3
Free-floating
plants (FF)
0
33.2
292.8
332.4
1992.8
1254.7
648.9
913.4
9.9
39.6
Grass/sedge
marsh (GS)
Mixed
herbaceous
marsh (HM)
Hardwood
swamp (HS)
Levees with
road (LR)
Waterhyacinth (Eichhornia crassipes),
water lettuce (Pistia stratiotes), Salvinia
spp., and other species that are not
connected to the bottom of the water
body
70% or greater coverage of wetland
grass or sedge species such as
maidencane (Panicum hemitomon),
cupscale (Sacciolepis striata), and
spikerush (Eleocharis spp.,
Rhynchospora spp.)
Combination of broadleaf emergents,
semi-floating species with grasses,
sedges, cattail, and/or sawgrass;
components classified in other
categories are each less than 70%.
70% or greater coverage of mixed
wetland tree species
Levees, either mowed or paved, with
surface maintained for vehicles.
35
Ludwigia (LU)
Wooded levees
or spoil banks
(LV)
Mixed shrub
wetland (MS)
Open water
(OW)
Sawgrass
(Cladium
jamaicense) (SG)
Tree island (TI)
Water lilies
(slough) (WL)
Willow (WS)
70% or greater canopy coverage by
Ludwigia spp. that is at least twice as
tall as the herbaceous layer
Levees with surface not maintained
or cleared, or spoil banks formed
when canals were made.
1.1
10.4
4.7
7.8
Mixture of shrub species and/or trees
that are at least twice as tall as the
herbaceous layer
Natural or large impounded water
bodies with no floating or emergent
vegetation; submersed vegetation
may be present
70% or greater coverage of sawgrass
1765.2
1630.9
2633.9
2637.4
2735.0
2945.2
Relatively small patches of trees
within the marsh (tree islands),
consisting of mixed wetland tree
species including maple, cabbage
palm (Sabal palmetto), and dahoon
(Ilex cassine)
70% or greater coverage of bottomrooted species with floating leaves,
including water lily (Nymphaea spp.)
and Nuphar luteum. Also may contain
bladderworts (Utricularia spp.).
70% or greater canopy coverage of
willow, Salix caroliniana, that is at
least twice as tall as the herbaceous
layer
82.0
77.0
90.4
11.1
1272.3
1588.1
36
Table 3.2: Parameters used to estimate landscape-level gas flux of sawgrass and willow
communities; net photosynthesis (Anet; μmol CO2 m-2 sec-1) and stomatal conductance (gs;
mol H2O m-2 sec-1) measured at PAR level 2000 μmol m-2 sec-1, leaf area index (LAI; m2
leaf area per m2 ground area), and the estimated spatial extent (ha) of sawgrass and
willow species-specific coverage calculated from the 2001 and 2008 community surveys
at BCMCA.
Community
Anet
gs
LAI
Sawgrass
Willow
12.52
14.81
0.240
0.359
0.792
1.773
37
Spatial Extent
2001
1914.5
890.62
2008
2061.7
1111.7
2500
Spatial extent (ha)
2000
1500
Sawgrass
1000
Willow
500
0
2001
2008
Year
Figure 3.2 Spatial cover adjusted for species-specific estimates from plant community
data.
38
Carbon assimilation (mol CO2 s-1)
350
300
250
200
Sawgrass
150
Willow
100
50
0
2001
2008
Year
Figure 3.3: Total population carbon assimilation from willow and sawgrass
39
Water exchange (kmol H2O s-1)
8000
7000
6000
5000
4000
Sawgrass
3000
Willow
2000
1000
0
2001
2008
Year
Figure 3.4: Total population stomatal conductance from willow and sawgrass
40
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