Applied Vegetation Science 12: 23–31,
& 2008 International Association for Vegetation Science
23
Savanna dynamics in central Texas: just succession?
Fowler, Norma L.1 & Simmons, Mark T.2
1
Section of Integrative Biology, University of Texas at Austin, Austin, TX 78712, USA;
Lady Bird Johnson Wildflower Center, University of Texas at Austin, 4801 Lacrosse Ave., Austin, TX 78739, USA;
E-mail msimmons@wildflower.org;
Corresponding author; Fax 11 512 471 3878; E-mail [email protected]
2
Abstract
Question: What is the best way to model savanna dynamics? Specifically, under what conditions is a
traditional succession model, i.e., a model of ordered,
uni-directional change in the plant community, better
than a state-and-transition model?
Location: Central Texas savannas.
Methods: We describe three examples of successional
processes in central Texas savannas: (a) woody plant
encroachment, (b) invasion by a non-native grass, and (c)
establishment of different grass species in highly disturbed
sites.
Results and Conclusions: Savanna dynamics are now
commonly conceptualized with state-and-transition models. However, in some situations a traditional succession
model may be more appropriate or more useful. Succession models may better fit current ecological reality, as
found in central Texas. Succession models emphasize
transient dynamics rather than an (often unknown) endpoint, and direct us towards relevant literature from nonsavanna ecosystems. Succession models may be particularly useful for land management and restoration, and
where woody plant encroachment and/or invasions by
non-native species control vegetation dynamics.
Keywords: Bothriochloa ischaemum; Invasive species; Juniperus ashei; Savanna; State-and-transition model;
Succession model; Texas; Vegetation dynamics; Woody
plant encroachment.
Introduction
Savannas, defined as spatial mosaics of herbaceous and woody plant-dominated patches, form
15-25% of the world’s terrestrial vegetation (Asner
et al. 2004). Savanna research has focused on the
coexistence of woody and herbaceous species, especially the ways in which fire, herbivory, and
temporal and spatial differences in water uptake and
use allow their coexistence (Sankaran et al. 2004 and
references therein). Savanna dynamics are now
usually conceptualized with state-and-transition
models, where the states are different savanna,
woodland, and grassland plant communities and the
transitions between them may or may not involve
direct human manipulation (Westoby et al. 1989;
Bestelmeyer et al. 2004). Such models typically include two or more alternate stable states (Laycock
1991; Allen-Diaz & Bartolome 1998).
In contrast, succession models conceptualize
community dynamics as an ordered series of states
that follow one another in time without direct human intervention. Using examples from central
Texas savannas, particularly the savannas of the
eastern Edwards Plateau, we argue that succession
models can still be appropriate for savannas, especially in the context of land management. A stateand-transition model with alternate stable states and
reversible transitions may be essential to understand
the history of a savanna if, for example, it was once
maintained by fire, but a succession model may better describe its present dynamics.
At present, succession in central Texas savannas
occurs at two different temporal and spatial scales.
At the landscape/multi-decadal scale, savannas are
being converted to woodlands, as discussed in the
first section of this paper. At a smaller scale, succession occurs within herbaceous patches in savannas,
as discussed in the next two sections. We then consider succession and state-and-transition models
more generally, and conclude the paper with some
management applications.
Succession of Savannas to Woodlands
Landscapes in central Texas are typically mosaics of savannas and oak/juniper-dominated
woodlands. The savannas are usually dominated by
species of oak (Quercus), juniper (Juniperus), and C4
grasses. Under present conditions, these savannas
persist only if woody species are removed mechanically. Otherwise, woody species increase in
abundance until the former savanna is converted to
24
FOWLER, N.L. & SIMMONS, M.T.
woodland, a process known as woody plant encroachment.
On the eastern Edwards Plateau of central
Texas, Juniperus ashei, a native multi-stemmed
small tree, converts Q. fusiformis savannas to
woodlands (Smeins & Fuhlendorf 1997; Jessup et al.
2003). Individual J. ashei plants become established
in the open and under existing woody plants, clusters of woody plants expand and coalesce, and
eventually the remaining glades disappear and a
closed-canopy stand of extremely low biodiversity
forms. The process is often relatively rapid (Fig. 1).
Thus the ranchers, conservation managers, and research ecologists in this region are confronted with a
process that requires no direct human intervention
and is ordered (more and more J. ashei over time),
and therefore fits the definition of succession as used
in most ecology textbooks and in this paper. The
land manager trying to preserve or restore savannas
is trying to slow, stop, or reverse succession.
Similar landscapes and landscape dynamics are
found elsewhere in Texas. On the western Edwards
Plateau, J. pinchottii replaces J. ashei (McPherson et
al. 1988; McPherson & Wright 1990). East of the
Plateau, several woody species, including J. virginiana, convert Q. stellata savannas to woodlands
(Kramer & Rykiel 1996). Savannas are not restricted to a narrow ecotone in central Texas: from
the eastern edge of Q. stellata savannas to the western edge of the Edwards Plateau is 700 km. In
south Texas, mesquite (Prosopis glandulosa) is often
the dominant encroaching species (Archer et al.
1988; Archer 1995; Hamilton & Ueckert 2004), as it
is in north-central Texas (Ansley et al. 2001). Woody
plant encroachment is also important in many other
savannas and (former) grasslands elsewhere in the
USA and throughout the world (Van Auken 2000;
Archer et al. 2001).
Woody plant encroachment is often thought to
be the result of a changed disturbance regime, especially changes in fire or herbivory. Examples of such
encroachment-favoring changes include fire suppression in central US grasslands (Briggs et al.
2002a, b) and in many other parts of the world
(Bond et al. 2005), overgrazing of desert grasslands
of the southwestern US (Bahre 1991) and in many
other arid regions (Asner et al. 2004), and elephant
removal (Baxter & Getz 2005). Little evidence has
been located about pre-settlement (or pre-1492) vegetation in central Texas, but it is likely that fires,
some of them set by Native Americans, once controlled J. ashei and maintained savannas in central
Texas, perhaps with woodland as an alternate stable
state (Smeins 1980; Weniger 1984, 1988; Fuhlendorf
& Smeins 1997; Smeins et al. 1997). Crown fires,
which can kill even large J. ashei, are presently quite
rare but can occur when conditions are suitable
(Bryant et al. 1983), and the region is now considered to be at high risk of wildfires (Baum 2004;
Anon. 2008a). The intensity of prescribed surface
fires is usually low, in part because grazing reduces
fine fuels in many sites, but also because there are
countywide bans on prescribed burning when conditions are dry and windy and plant moisture
content is low (Anon. 2008b). As a result, prescribed
surface fires are often not intense enough to kill
most J. ashei, even very small plants (Noel & Fowler
2007). Neither grazing nor protection from grazing
prevents J. ashei encroachment (Fig. 1). The usual
management practice is mechanical removal of J.
ashei (Hamilton & Ueckert 2004).
If woody plant encroachment is a form of succession, the rich literature on succession in other
systems should be a good source of insights and hypotheses. For example, the classic paper by Connell
& Slatyer (1977) and its many descendants ask
Fig. 1. Sequential aerial photographs of a portion of Pedernales Falls State Park, Blanco County, Texas, from 1951 to 2004.
During this 53-year period, the increase in woody cover in this site averaged 1.1% per year (Gonzalez & Fowler 2007),
primarily due to an increase in the number and size of Juniperus ashei (Ashe juniper) plants. This site was grazed prior to
1970 and ungrazed thereafter. Aerial photographs provided by Texas Natural Resources Information System, Austin, TX.
- SAVANNA DYNAMICS IN CENTRAL TEXAS whether the interactions between species during
succession are inhibitory, facilitative, or neutral.
Facilitation of J. ashei seedling survival by adult J.
ashei has been documented (Batchelor 2004; Batchelor & Fowler 2004). This is likely to be because of a
reduction in water stress through shading by adults.
However, the effect is small, and the effect of adult
J. ashei on seedling growth is negative. The effect of
adult Q. fusiformis on seedling J. ashei was entirely
negative, apparently because damage by falling
branches negated any positive shading effect. Facilitation, therefore, is not the cause of the very
common clustering of juvenile J. ashei around adult
J. ashei and Q. fusiformis (Fowler 1988; Batchelor
et al. 2002).
Another topic addressed in the succession literature is the role of seed dispersal. It was found
elsewhere that the establishment of bird-dispersed
species is more frequent under perches (McClanahan & Wolfe 1993; Vieira et al. 1994). Fruit-eating
birds disperse the seeds of J. ashei (Chavez-Ramirez
& Slack 1994), and seed dispersal by perching birds
is the most likely explanation for the clustering of J.
ashei juveniles around adult Q. fusiformis and adult
male J. ashei. The clustering of J. ashei juveniles
around adult female J. ashei is augmented by J.
ashei seeds that fall close to parent plants.
In a classic succession model, succession terminates at a single end point, the ‘‘climax’’ community.
It is not certain what the climax community would
be on the eastern Edwards Plateau under present
conditions. Due to white-tailed deer (Odocoileus
virginicus) herbivory, oaks (Quercus spp.) are not
successfully regenerating in either savannas or
woodlands in this region at present (Russell &
Fowler 1999, 2002, 2004). The same appears to be
true for most other woody species, most of which
are more palatable than oak (Armstrong et al. 1991;
Armstrong & Young 2002). Similar effects of whitetailed deer have been reported from eastern US forests (McShea et al. 1997; Russell et al. 2001; Côté et
al. 2004). Juniperus ashei, however, is highly unpalatable to deer. Because it can establish in
woodlands as well as savannas (Van Auken et al.
2004), a nearly monospecific stand of J. ashei seems
to be the likely climax community under present
conditions.
However, we do not know what will happen as
stands of J. ashei age. If crown fires were to become
more common, the landscape might become a mosaic of J. ashei stands of different ages. Changes in
land use will also affect the future landscape. As the
urban-rural interface moves west from Austin and
San Antonio, former ranches are being converted
25
into 1-20 ha homesites. As a consequence, juniper
removal and deer hunting both decrease, increasing
the rate of succession from savanna (and from diverse woodland) to J. ashei stands.
Bothriochloa ischaemum Invasion as Succession
The vegetation dynamics of the eastern Edwards Plateau fit the definition of succession in
another way. Savannas in this region are being invaded by Bothriochloa ischaemum var songarica, a
non-native perennial C4 bunchgrass. While this
grass does not live under the canopies of woody
plants, it grows in almost every other type of upland
site in the region (Gabbard & Fowler 2007). Neither
grazing, protection from grazing, nor dormant-season fires control it, and in many sites it has formed
single-species stands, with associated reductions in
native plant diversity (Gabbard & Fowler 2007).
There is no evidence to suggest that under present
conditions this grass will not continue to spread
throughout the region and to increase in abundance
where it has not yet become dominant. The invasion
of savannas by B. ischaemum thus fits the definition
of succession: it requires no direct human intervention and is ordered in time (increasing B. ischaemum
density over time). This successional sequence appears to terminate at a single end point (a nearly
mono-specific stand of B. ischaemum) that persists
indefinitely if J. ashei encroachment is controlled.
A major goal of land managers who wish to
preserve native herbaceous plant diversity in the region is to slow, stop, or reverse the succession of
grasslands to B. ischaemum dominance. Two invaded grasslands in central Texas that were burned
during the growing season of B. ischaemum showed
significant reductions in the abundance of this species with little or no change in other native and nonnative species (Simmons et al. 2007), suggesting that
growing-season fires may be a selective method of
controlling this invasive grass. If so, then the reintroduction of occasional growing-season fires,
which was probably the historic fire regime in the
region (Fuhlendorf & Smeins 1997), might simultaneously control both B. ischaemum and J. ashei.
Restoration of the Herbaceous Component of
Savanna Communities as Succession
A succession model may also be useful in the
study and management of revegetation following
disturbance. There is little direct information about
26
FOWLER, N.L. & SIMMONS, M.T.
the development or dynamics of the native herbaceous component of savanna communities of the
eastern Edwards Plateau. It is not even known whether succession actually occurs within the native
herbaceous community, and successional sequences
have not been described. The plant colonists of
small soil disturbances created by ants or armadillos
are the same species that occur in the surrounding
herbaceous community (N.L. Fowler, unpubl.).
The best information on succession within the
herbaceous component of these savannas is indirect,
and comes from a restoration study (Tinsley et al.
2006). The objective of this study was to develop
seed mixtures of native species for roadside revegetation, with the larger goal of providing viable
substitutes for the mixtures of non-native grasses
commonly used for restoration. Native grasses have
acquired the reputation of being difficult to establish. However, the native grass species whose seed is
commercially available are mostly tall, slow-growing, long-lived species such as Schizachyrium
scoparium. To identify native species that might
have better establishment rates, Tinsley et al. used
information from studies of the effects of grazing on
the species composition of rangelands.
The literature on grazing and rangeland ecology
and management has generally used a rather different definition of succession from that used in
ecology textbooks and in this paper. In the former
literature, community composition has often been
conceptualized as changing reversibly along a continuum, with grazing intensity the primary factor
that shifts community composition between earlier
successional states (higher grazing intensity) and later successional states (lower grazing intensity)
(Stoddart et al. 1975). As grazing intensity increases,
‘‘increasers’’ are favored over ‘‘decreasers’’, and vice
versa as grazing intensity decreases. Under this definition of succession, early successional species are
increasers, and decreasers are later successional
species; we will refer to them as increasers and decreasers to avoid using two different definitions of
early and late successional species. Many of the
grasses found in central Texas have been classified as
increasers or decreasers (Fig. 2).
Palatability can affect a species’ classification:
highly palatable plants may decrease in abundance
even under light grazing. However, among grasses,
the difference between increasers and decreasers is
usually height (Fig. 2). Taller grasses lose a larger
proportion of their biomass when grazed to ‘‘bite
height’’ than do shorter grasses, and therefore are
decreasers. Where grazing is relatively heavy, the
reduction in competition from taller species allows
Fig. 2. Relationship between vegetative height and establishment success following (a) spring and (b) summer seed
sowing. Establishment success rate data from Tinsley et al.
(2006); vegetation height values from Coffey & Russell
(2004). rs, Spearman’s correlation coefficient. ARPU 5
Aristida purpurea; BOCU 5 Bouteloua curtipendula;
BOHI 5 Bouteloua hirsuta; BORI 5 Bouteloua rigdiseta;
BOLA 5 Bothriochloa laguroides; BUDA 5 Buchlöe dactyloides; CYDA 5 Cynodon dactylon; ELCA 5 Elymus
canadensis; ERPI 5 Erioneuron pilosum; HIBE 5 Hilaria
belangeri; LECO 5 Leptoloma cognatum; LEDU 5
Leptochloa
dubia;
NALE 5 Nassella
leucotricha;
PAHA 5 Panicum hallii; SCSC 5 Schizachyrium scoparium; SEIT 5 Setaria italica; SONU 5 Sorghastrum nutans.
Species are pooled (/) where their seedlings could not be
distinguished. Of these species, ARPU, BOHI, BORI,
BUDA, ERPI, HIBE, and NALE (circles) have been reported to be increasers, i.e., to increase in abundance when
grazed, while BOCU, LECO, SCSC, and SONU (squares)
have been reported to be decreasers (Dyksterhuis 1946;
Launchbaugh 1955; Smeins et al. 1976). These authors
disagree about the status of BOLA, and the others (triangles) have not been classified.
shorter species to increase. Experiments using
fenced and unfenced transplants of six common
Texas grass species verified that height is correlated
with the effects of grazing on relative growth rate
- SAVANNA DYNAMICS IN CENTRAL TEXAS (Fowler 2002). Tinsley et al. (2006) deliberately included in their study a number of species that had
been previously identified as increasers, or are of
short stature, or both (Fig. 2), an example of the use
of a succession model, together with the body of
knowledge associated with that model, to discover a
better strategy for restoration.
The hypothesis that short stature, increaser
species establish better in disturbed habitats than do
taller decreaser species is supported by analysis of
data from Table 5 of Tinsley et al. (2006). The six
short grasses Aristida purpurea, Buchlöe dactyloides,
Hilaria belangeri, Erioneuron pilosum, Leptochloa
dubia, and Panicum hallii had an average seedling
establishment of 14.6% (the ratio of live seedlings to
viable seed); in contrast, four taller species, Bothriochloa laguroides, Elymus canadensis, Schizachyrium
scoparium, and Sorghastrum nutans had an average
of only 0.13% seedling establishment. The relationship between grass height (from Coffey & Russell
2004) and establishment success was significantly
negative (Fig. 2). This suggests that there may be a
successional sequence among the native herbaceous
species of central Texas savannas, perhaps initiated
by disturbances larger than those created by localized digging.
A lack of seeds may delay succession. Seed may
be lacking where a closed canopy stand of J. ashei
has been removed or where overgrazing has been
especially severe, as well as along severely disturbed
roadsides (Kinucan & Smeins 1992). There appears
to be little recruitment of herbaceous species from
seed under mechanically cleared old stands of J.
ashei (M.T. Simmons, pers. obs.). Greatly delayed
succession due to lack of propagules of early successional species has also been documented in other
systems (Reichhardt 1982; Turnbull et al. 2000).
Some Advantages of Succession Models for Savannas
The use of succession models in central Texas
acknowledges the present reality confronting land
managers. Describing woody plant encroachment
and the invasion of Bothriochloa ischaemum
as successional processes acknowledges their directionality and the difficulty of slowing, stopping, or
reversing them. Land managers confront a situation
where small modifications of fire, grazing, and
browsing are not sufficient to stop these successional
processes. Therefore, very high inputs of resources,
such as repeated mechanical clearing, are necessary.
In general, succession models encourage us to
be realistic about the difficulties of reversing some
27
processes. Beginning with the seminal work of Westoby et al. (1989), users of state-and-transition
models have noted that, for example, when grazing
is removed the vegetation often does not revert to its
pre-grazing composition; recognition of this fact
was a motivation for abandoning the range definition of succession, with its assumption of reversible
changes along a continuum. For example, overgrazing of south Texas savannas promotes an
increase in Prosopis glandulosa (honey mesquite)
and eventual conversion to woodland; cessation of
grazing does not return such sites to savannas (Archer 1995). If the reality on the ground is a strong
directional change in the vegetation, a succession
model has the merit of realism.
Succession models emphasize transient dynamics over stable states, the process of change over
its endpoint(s). Transient dynamics, not stable
states, best describe vegetation in central Texas,
where communities are constantly changing rather
than being stable or semi-stable. Transient dynamics
characterize the vegetation of many parts of the
world, including many savannas, because many
plant communities have been greatly affected by
historical factors, especially past land management
practices. Worldwide, not only savannas are changing; forests that were once thought to represent
climax vegetation are now known to be, in part, the
product of past human land use, hurricanes, and
other events in previous decades or centuries (Foster
& Boose 1992; Foster & Aber 2004). Succession
models are particularly appropriate where the stable
states are unknown or uncertain, as is the case for
the Edwards Plateau.
Finally, succession models direct our attention
towards a rich body of scientific literature and practical experience that may, at first glance, not seem
relevant to understanding savannas. Information
from other ecosystems may provide insights into the
mechanisms underlying succession in savannas, as
in the examples discussed above.
Succession Models Versus State-and-Transition
Models
We have argued that, for the savannas of the
eastern Edwards Plateau, and by extension some
other savannas, succession models may be more
useful, especially for land management and restoration, than state-and-transition models based on the
concept of alternate stable states. Authors (e.g.,
Westoby et al. 1989; Allen-Diaz & Bartolome 1998)
who have argued against using succession models
28
FOWLER, N.L. & SIMMONS, M.T.
for savannas have usually been addressing the use of
models that assume that succession is a reversible
process driven by grazing, i.e., models that use the
range literature definition of succession. The deficiencies in that form of succession model do not
apply to models that use the definition of succession
common in the non-savanna literature, i.e., a definition that does not assume reversibility or a
controlling role for grazing (Pickett & White 1985).
We do not argue that a state-and-transition
model for these savannas would be inappropriate;
indeed, developing such a model is a long-term goal
of our research. Furthermore, we agree with other
authors (Smeins 1980; Weniger 1988; Smeins et al.
1997) that savanna was probably a quasi-stable state
in this region under pre-settlement fire regimes, at
least in some sites. We simply argue that, given present vegetation, present land management practices
and constraints, and our current level of scientific
understanding of the vegetation of this region, succession models are both appropriate and useful.
From a theoretical point of view, the two types
of models can be seen as special cases of each other.
Succession models can be viewed as state-and-transition models with a single stable state. Ecologists
long ago incorporated fire-maintained stable states
as ‘‘fire disclimaxes’’ within models of succession
(Daubenmire 1968). However, the two types of
models direct us towards different bodies of literature and practical experience, and emphasize
different aspects of ecosystem dynamics.
Savanna Management and Restoration
Savannas are desirable in central Texas for
many reasons. Where the native plant community
has not been replaced by Bothriochloa ischaemum,
savannas represent an important component of
native plant biodiversity. In this region, the endangered black-capped vireo (Vireo atricapillus)
requires savannas with a particular shrub structure
and composition (Anon. 1991). Ranching has been,
and continues to be, an important use of much
of the Edwards Plateau; ranchers routinely remove
J. ashei to increase forage for cattle. Recently,
J. ashei removal has also been used to increase water
runoff and aquifer recharge (Wu et al. 2001; Anon.
2008c).
Land management on the eastern Edwards Plateau is, therefore, often directed towards stopping,
slowing, or reversing succession towards B. ischaemum-dominated herbaceous stands and, on a
somewhat longer time scale, stopping, slowing, or
reversing succession towards J. ashei-dominated
woodland. Where savannas have been lost, whether
by conversion to woodland or by construction and
road-building, the task becomes fostering succession
in the desired direction while resisting its ‘‘natural’’
tendencies towards B. ischaemum and J. ashei dominance. Similar challenges confront managers of
savannas in many areas (e.g., Peterson & Reich
2001; Brockway et al. 2002; Baker 2006).
In labeling savanna dynamics as ‘‘just succession’’, we do not intend to minimize the challenges
and difficulties of managing savannas in central
Texas or elsewhere. It may be difficult or impossible
to reinstate the earlier disturbance regime (e.g., intense fires). Finding an acceptable substitute for the
earlier disturbance regime may also present substantial challenges, especially if the goal is
restoration of native biodiversity. Seed addition
may be required. In some situations restoration may
require very substantial interventions, such as grading to reduce erosion rates (Whisenant 1999).
Acknowledgements. We thank Craig Pease, Karen Alofs,
Ana González, Dana Price, Scott Meiners, and an anonymous reviewer for their comments on earlier versions of
this paper.
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Received 8 April 2008;
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