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Toward an old-growth concept for grasslands, savannas, and woodlands

Authors:
  • Avignon Université
  • Instituto de Pesquisas Ambientais - Governo do Estado de São Paulo

Abstract

We expand the concept of "old growth" to encompass the distinct ecologies and conservation values of the world's ancient grass-dominated biomes. Biologically rich grasslands, savannas, and open-canopy woodlands suffer from an image problem among scientists, policy makers, land managers, and the general public, that fosters alarming rates of ecosystem destruction and degradation. These biomes have for too long been misrepresented as the result of deforestation followed by arrested succession. We now know that grassy biomes originated millions of years ago, long before humans began deforesting. We present a consensus view from diverse geographic regions on the ecological characteristics needed to identify old-growth grasslands and to distinguish them from recently formed anthropogenic vegetation. If widely adopted, the old-growth grassland concept has the potential to improve scientific understanding, conservation policies, and ecosystem management.
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Many of the world’s grasslands are ancient ecosystems
composed of communities that require centuries to
assemble and perennial plants capable of living for
decades to millennia. These ecosystems are being lost to
agriculture, tree plantations, mining, and urban sprawl;
they are also being degraded by invasive species, poorly
managed domestic livestock, altered fire regimes, ele-
vated atmospheric carbon dioxide, and nitrogen deposi-
tion (Parr et al. 2014). Meanwhile, other types of herba-
ceous and grass-dominated vegetation (eg old fields and
derived savannas) are expanding (Veldman and Putz
2011). Confusion over the substantive differences
between natural grassland ecosystems and their novel,
anthropogenic counterparts has contributed to a lack of
concern over the impacts of environmental change on
biodiversity in natural grasslands. This confusion persists
in large part because we currently lack a framework for
conceptualizing the role of long periods of time in the
development of biodiverse grasslands.
To promote the recognition and conservation of nat-
ural grasslands, we extend the concept of “old growth” to
grasslands (Panel 1) and suggest that old growth is not
limited to forests. The ideas we present are drawn from
recent advances in grassland ecology and are motivated
by the threats of human-caused environmental change.
The old-growth grassland concept will help distinguish
ecosystems with high conservation values and unique
ecological attributes from vegetation that forms over
short timescales in response to human land uses. The
concept will also help us evaluate the ecological and eco-
nomic costs associated with the loss or degradation of
grassland ecosystems, and determine the rates at which
such transitions occur. Finally, questions evoked by the
old-growth grassland concept will help identify promising
areas of fundamental ecological research.
We define grasslands broadly as ecosystems in which
graminoids (ie grasses and grass-like plants), forbs, and
shrubs form a relatively continuous “herbaceous layer”
of vegetation. This classification includes grasslands
without trees, as well as savannas and woodlands that
support a range of tree densities (ie “grassy biomes”; Parr
et al. 2014). We emphasize the ecology of the herba-
ceous layer, in recognition that herbaceous species
account for the majority of plant diversity in these sys-
tems and are critical to ecosystem processes. Trees are
important to the ecology of savannas and woodlands (eg
Toward an old-growth concept for
grasslands, savannas, and woodlands
Joseph W Veldman1*, Elise Buisson2, Giselda Durigan3, G Wilson Fernandes4,5, Soizig Le Stradic6,
Gregory Mahy6, Daniel Negreiros4, Gerhard E Overbeck7, Robin G Veldman8, Nicholas P Zaloumis9,
Francis E Putz10, and William J Bond9
We expand the concept of “old growth” to encompass the distinct ecologies and conservation values of the
world’s ancient grass-dominated biomes. Biologically rich grasslands, savannas, and open-canopy woodlands
suffer from an image problem among scientists, policy makers, land managers, and the general public, that fos-
ters alarming rates of ecosystem destruction and degradation. These biomes have for too long been misrepre-
sented as the result of deforestation followed by arrested succession. We now know that grassy biomes originated
millions of years ago, long before humans began deforesting. We present a consensus view from diverse geo-
graphic regions on the ecological characteristics needed to identify old-growth grasslands and to distinguish
them from recently formed anthropogenic vegetation. If widely adopted, the old-growth grassland concept has
the potential to improve scientific understanding, conservation policies, and ecosystem management.
Front Ecol Environ 2015; 13(3): 154–162, doi:10.1890/140270
In a nutshell:
Old-growth grasslands are ancient ecosystems characterized
by high herbaceous species richness, high endemism, and
unique species compositions
Biodiversity in old-growth grasslands is maintained by fre-
quent fires, herbivores, and soil characteristics that limit tree
growth
To survive fires and grazing, many grassland herbs, forbs, and
shrubs have developed specialized underground organs that
enable them to resprout repeatedly; evidence of grassland
plant longevity and ecosystem carbon storage is often below-
ground
Old-growth grasslands are threatened by agricultural conver-
sion, fire exclusion, invasive species, surface mining, poorly
managed livestock, and misinformed afforestation projects
Initiatives to identify, conserve, and restore old-growth grass-
lands are urgently needed
1Department of Ecology, Evolution and Organismal Biology, Iowa State
University, Ames, IA *(jveldman@iastate.edu); 2IMBE, Avignon
Université, CNRS, IRD, Aix Marseille Université, Avignon, France;
3Laboratório de Ecologia e Hidrologia Florestal, Floresta Estadual de
Assis, Instituto Florestal, Assis, Brazil; continued on p 162
CONCEPTS AND QUESTIONS
JW Veldman et al.Old-growth grasslands
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© The Ecological Society of America www.frontiersinecology.org
Ratnam et al. 2011), but the vast ecological differences
between trees and herbaceous plants warrant explicit
consideration of these plant communities in concep-
tualizing old-growth grasslands (Kirkman and Mitchell
2006).
Grasslands occur across a huge range of environmen-
tal conditions, from the tropics to the Arctic, from sea
level to mountain tops, from arid to humid areas, from
sites with long hydroperiods to well-drained sites, and
from shallow rocky soils to deep clay soils (Brown and
Makings 2014). Given the global diversity of grasslands,
our intention is not to define old growth for any one
place. Instead, we provide an overview of the common
characteristics of old-growth grasslands in hopes of
improving their fates.
nConcept development
Descriptions of old-growth grasslands are scattered
throughout the literature but to our knowledge have not
previously been consolidated into a single framework.
Feller and Brown (2000) argued that an old-growth con-
cept for grasslands, similar to that for forests, was urgently
needed to provide greater protection for natural grasslands
of native species in the western US. Their article has gone
largely unnoticed by ecologists, and “old growth” is still
rarely used in the context of grasslands. Nonetheless, old-
growth themes and descriptions of grasslands with old-
growth characteristics do appear in the ecology and con-
servation literature (eg Noss 2013), as well as in literature
on old-growth “forests” in the US – maintained by fre-
quent fires – that many ecologists would consider to be
savannas (Ratnam et al. 2011). For example, in ecosystems
dominated by ponderosa pine (Pinus ponderosa) and long-
leaf pine (Pinus palustris), frequent fires and high herba-
ceous species diversity characterize old growth (Laughlin
et al. 2004; Kirkman and Mitchell 2006).
A list of old-growth grassland characteristics based on
observations of and literature about ecosystems in Africa,
Australia, Europe, and the Americas is presented in Panel
1. We acknowledge a bias toward lowland to mid-eleva-
tional, tropical, and temperate ecosystems, and hope that
experts in high-latitude and high-elevation grasslands
will expand on our ideas. These general characteristics
should serve as a guide for developing ecosystem-specific
definitions of old growth.
Fire and herbivores
Recognition of antiquity in old-growth grasslands
requires an appreciation for the roles of fire and herbivores
in grassland plant evolution (Figure 1). Grasslands com-
posed of C3plant species and grazed by ungulates devel-
oped by 20–30 million years ago (MYA; Jacobs et al.
1999). Grasslands composed of C4species, and that
burned frequently, have been prominent in the tropics
Panel 1. Characteristics of many old-growth grasslands, savannas, and woodlands
Ecosystem-level characteristics
Species assemblages that do not occur in
young, secondary grasslands
High herbaceous-layer plant species diversity
High small-scale (eg 1 m2) species richness
Presence of endemic species
Transient seed banks
Persistent bud banks
High ratio of herbaceous species to tree species
High belowground biomass
Little accumulation of litter or duff
Open, discontinuous tree canopies
Factors that maintain biodiversity
Frequent surface fires
Herbivory by native megafauna
Soil disturbance by digging animals
Low-intensity domestic livestock grazing
Shallow soils; nutrient-poor soils
Seasonal water deficits; seasonal flooding
High (toxic) concentrations of soil metals
Land uses incompatible with old growth
Surface mining or quarrying
Tillage agriculture
Plantation forestry
Intensive pasture management (eg use of exotic
forage grasses and fertilizers)
Prolonged high-intensity grazing
Life-history and functional characteristics of
old-growth indicator plant species
Slow growing
Long-lived
Strong resprouting capacity
Low success at establishing from seed
Poor colonization ability
Investment in underground storage organs
Clonal growth
High root:shoot ratio
Fire-enhanced or fire-dependent flowering and fruiting
Fire-tolerant (thick-barked) trees
Causes of degradation
Fire suppression/exclusion
Overgrazing and poorly managed domestic livestock
Declines in native megafaunal herbivores
Woody encroachment
Invasive species
Atmospheric nitrogen deposition
Elevated atmospheric CO2concentrations
Anthropogenic soil disturbances
Land uses potentially compatible with old growth
Low-intensity domestic livestock grazing
Timber or fuelwood extraction
Old-growth grasslands JW Veldman et al.
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and subtropics since 3–8 MYA (Cerling et al. 1997;
Edwards et al. 2010). Millions of years of frequent distur-
bances in grasslands, which intermittently reduced
aboveground biomass, selected for plant morphological
characteristics and life-history strategies that are funda-
mentally different from those of forest species (eg Simon
et al. 2009; Ratnam et al. 2011; Maurin et al. 2014).
Plant longevity in grassland environments requires that
the parts of the plant where growth occurs (ie meristem-
atic tissues, including buds and vascular cambia) are insu-
lated from high temperatures and protected from herbi-
vores, and that photosynthetic tissues can be rapidly
regenerated following disturbances. For most perennial
graminoids and forbs, the principal growth regions are
located near or below the soil surface, where temperatures
remain low during fires (Figure 2). In most savanna trees
(and some herbs; Figure 3), fire-resistant stems with insu-
lated buds permit a substantial portion of a plant’s above-
ground biomass to be retained through fires (Ratnam et
al. 2011). To permit resprouting, many woody plants and
perennial forbs invest in underground organs that store
starch and water (Figure 2; Simon et al. 2009; Maurin et
al. 2014). Clonal growth (eg via rhizomes, stolons, or
tillers) is common, and allows plants to regenerate after
disturbance and to colonize their local area. The preva-
lence of clonal growth likely reflects the many hurdles to
establishment from seed (eg water stress and seed preda-
tion), and apparent life-history trade-offs between persis-
tence and sexual reproduction (Benson and Hartnett
2006; Lamont et al. 2011). In sum, because disturbance
regimes and plant characteristics are dramatically differ-
ent in grasslands than in forests, many signs of antiquity
in old-growth forests (eg large diameter trees, accumu-
lated woody debris; Franklin and Spies 1991) are inap-
plicable to grasslands.
Frequent fires and herbivory are essential to the persis-
tence of most old-growth grasslands, especially where
precipitation and soil nutrient availability are sufficient
to permit forest development (Bond and Keeley 2005).
Fire exclusion or removal of herbivores in these mesic
grasslands can result in rapid transitions from grassland
to forest or shrubland, with associated losses of herba-
ceous plant diversity. Grasslands also occur where tree
growth is limited by shallow soils, low soil moisture
availability, seasonal flooding, or high concentrations of
toxic metals. Because these “edaphic” (influenced by soil
properties) grasslands are less dependent on frequent dis-
turbances than are mesic grasslands, they are often the
only representatives of old growth in severely fire-
excluded landscapes (Noss 2013). Climate-determined
grasslands occur in regions where tree cover is limited by
some aspect of climate, often low precipitation in the
tropics (Staver et al. 2011) or low temperatures at high
latitudes or elevations. While the distinctions between
mesic, edaphic, and climate-determined grasslands are
useful for identifying their primary environmental deter-
minants (ie factors that limit trees), the importance of
fire and herbivory to most old-growth grasslands cannot
be overstated. Across a wide range of climatic and soil
conditions, frequent fires and herbivores limit the abun-
dance of trees and shrubs, promote herbaceous produc-
tivity, consume dead plant material, return nutrients to
Figure 1. Intact disturbance regimes: (a) a surface fire in a Bolivian savanna; (b) megafaunal herbivores in eastern Africa.
(a) (b)
JW Veldman et al.Old-growth grasslands
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© The Ecological Society of America www.frontiersinecology.org
the soil, stimulate reproduc-
tion, and maintain plant
diversity (eg Platt et al. 1988;
Lehmann et al. 2014; Nayak et
al. 2014; Veldman et al. 2014).
Time, degradation, and
recovery
How we perceive and concep-
tualize time has an enormous
influence on how we study
and seek to conserve organ-
isms and their environments.
Questions of time are critical;
in a single day, a large tractor
can plow up and convert a
hectare or more of old-growth
grassland into an agricultural
field. If that field is aban-
doned, even if propagule sources are nearby and establish-
ment conditions are favorable, it will require a century or
longer for a comparable grassland to form (Baer et al.
2010; Redhead et al. 2014). Where invasive grass species
are present or biophysical conditions are severely altered,
ecosystems may never return to an old-growth state
(Stromberg and Griffin 1996). Indeed, a number of stud-
ies of severely degraded grasslands conclude that old-
growth indicator species are very slow to re-establish and
that grasslands on former agricultural land are ecologi-
cally distinct from old growth (Stromberg and Griffin
1996; Kirkman et al. 2004; Zaloumis and Bond 2011; Le
Stradic et al. 2014; Redhead et al. 2014). Therefore, we
suggest the term “secondary grassland” be used to describe
young ecosystems on land that historically supported old-
growth grasslands (eg Zaloumis and Bond 2011), and that
old growth be used as the reference to develop definitions
of grassland degradation and to guide restoration
(Laughlin et al. 2004).
nPromoting conservation
Cultural resonance
The old-growth grassland concept is a necessary precursor
to public support for grassland conservation. Similar con-
cepts that draw from both scientific theory and beliefs
about nature have bolstered previous conservation efforts
and achieved lasting relevance; for example, the develop-
ment and promotion of the charismatic, culturally reso-
nant concepts of “biodiversity” and – in the context of
forests – “old growth” continue to serve popular conserva-
tion movements (Takacs 1996; Proctor 2009). The ideals
embodied in these concepts are more than just tools to
guide conservation research and advocacy. For the public,
they evocatively capture the value of certain aspects of
the natural world, including antiquity, diversity, and
beauty, which some may even express in terms of sacred-
ness (Taylor 2010).
If old-growth grasslands are to be conserved, they need
to be conceptualized in a culturally resonant manner
because their biodiversity is not always evident to the
untrained eye. Even if ecologists recognize their value,
most lay observers are not yet accustomed to seeking
Figure 2. (a) Storage taproots and rhizomes of a grassland forb (Cryptosepalum maraviense) in
central Africa; (b) root tubers of a shrub (Campomanesia adamantium) in a Brazilian savanna; (c)
basal rosette and bulb of a grassland herb (Ledebouria stenophylla) from southern Africa; (d) storage
taproot (~1.1 m deep) of a savanna forb (Eriogonum tomentosum) in the southeastern US.
(a)
A Willem
(b) (c)
(d)
Old-growth grasslands JW Veldman et al.
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“Nature” – much less appreciating antiquity – in grass-
lands, where signs of old growth and associated conserva-
tion values are subtle, if not subterranean. Recognition of
grassland conservation values is further complicated by
the dependence of grasslands on disturbances that some
consider “unnatural” or “undesirable” but that must be
maintained, often through active management (Panel 2).
Given this cultural context, we suggest that the old-
growth concept can help scientists and the public to rec-
ognize the value of grasslands and to advocate for their
conservation.
Carbon, conservation, and old growth
There is growing recognition that natural grasslands
composed of native species are at risk from policies
designed to sequester carbon (C) to mitigate climate
change (Putz and Redford 2010; Parr et al. 2014). In
particular, C payment schemes under development by
the United Nations Framework Convention on
Climate Change (UNFCCC) and the program for
Reducing Emissions from Deforestation and Forest
Degradation (REDD+), as well as the definitions of
“forest” used by the United Nations Food and
Agriculture Organization, put grasslands at risk from
afforestation projects and agricultural conversion.
Grasslands are a target for C sequestration projects
because fire suppression and tree planting are straight-
forward means of dramatically increasing aboveground
biomass, at least in the short term (Canadell and
Raupach 2008). We are also concerned that a lack of
international policies recognizing natural grasslands
and including mechanisms to slow their loss will incen-
tivize the conversion of old-growth grasslands to agri-
cultural uses.
The old-growth grassland concept addresses these con-
servation concerns in several ways. First, by emphasizing
the long periods of time required for species-diverse
grasslands of native species to assemble, this concept will
raise awareness of the long-term biodiversity costs associ-
ated with a narrow focus on C and forests. Second, by
emphasizing the roles of time and disturbances in grass-
land formation, the old-growth concept requires that we
consider the permanence of sequestered C in climate
mitigation projects. With C stored in belowground plant
organs, accumulated soil organic matter, and the wood of
fire-tolerant savanna trees (eg Miranda et al. 2014), old-
growth grasslands may securely store C for long periods
of time in fire- and drought-prone landscapes. Third,
advancement of the concept depends on ecologists
developing ecosystem-specific definitions of old growth.
Once established, these definitions should help reconcile
conflicts between global agreements on C and biodiver-
sity conservation priorities at the national level (Sasaki
and Putz 2009).
Figure 3. (a) Persistent charred stem of a graminoid (Bulbostylis paradoxa) and (b) old stems of Vellozia variabilis in highland
grasslands in Brazil.
(a) (b)
Panel 2. The cattle conundrum
A challenge to conceptualizing old-growth grasslands is deter-
mining their compatibility with livestock grazing (Nayak et al.
2014). The idea of domestic livestock in an old-growth ecosys-
tem may seem counterintuitive given the role of cattle ranching
as a global driver of ecosystem degradation. Indeed, poor range-
land management (eg high stocking densities, sowing exotic
grasses) can irreparably change natural grasslands (Feller and
Brown 2000). Nevertheless, livestock grazing at appropriate
densities and intervals appears to be compatible with, or even
beneficial to, grassland biodiversity in some systems. This is the
case in southern Brazil, where cattle exclusion leads to forest
succession and declines in old-growth grassland plant diversity
(Overbeck et al. 2007). Cattle may also provide opportunities to
maintain or restore grasslands that would otherwise be con-
verted to mechanized agriculture or other land uses (Miller et
al. 2012). In grasslands highly suited to agriculture, livestock
grazing is perhaps the only land use that provides economic
returns while maintaining many old-growth grassland character-
istics. In sum, livestock represent both perils and opportunities
for grassland conservation. We need to determine where live-
stock are compatible with biodiversity and use the old-growth
concept to move beyond production-focused grassland man-
agement (Feller and Brown 2000).
JW Veldman et al.Old-growth grasslands
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nFuture ecological research
There is a great deal that we do not know about grassland
plant longevity, the time required for old-growth commu-
nities to assemble, and the rates at which other old-
growth attributes develop (eg soil C storage). The old-
growth concept challenges us to investigate basic
questions about grassland natural history, potentially
improving our ability to answer fundamental ecological
questions (Panel 3).
The few estimates of grassland plant longevity that are
available are based on anecdotal accounts of plant size,
longitudinal studies of individual plants, and modeling of
clonal plant demographics and growth. Accounts from
Brazilian grasslands, for example, suggest that Vellozia spp
require 100 years to reach reproductive maturity and can
live for over 500 years (Alves and Kolbek 1994).
Longitudinal studies provide reliable age estimates, but
are limited in their time and spatial scales. On the basis of
39 years of observations in a grassland in central North
America, Lauenroth and Adler (2008) found that forbs
are typically short-lived ( 18 years) and that most grasses
can live for at least 20 years, with individuals of two
species known to be 39 years old. Previously, Lauenroth
et al. (1994) used demographic data to estimate that blue
grama (Bouteloua gracilis), a C4bunchgrass, can live for
450 years. Indeed, clonal species (including bunch-
grasses) are likely to be the oldest grassland plants, with
the potential to live for decades to millennia (de Witte
and Stöcklin 2010). A variety of modeling techniques
have been used to estimate the ages of both genets
(colonies of genetically identical individuals) and ramets
(individual clones). For example, matrix modeling of pale
pitcher plants (Sarracenia alata) in wet savannas of the
southern US suggests that ramets typically live 59 years
(Brewer 2001), and genets can presumably be much older.
Takahashi et al. (2011) combined molecular genetics and
modeling approaches to estimate that genets of the fire-
adapted saw palmetto, Serenoa repens, which is endemic
to the southeastern US, may live for 10 000 years. In sum,
the maximum ages of old-growth grassland plants may
range widely, depending on growth form, disturbance
regimes, and historical climate stability (Panel 4). More
research is needed on the ages and demographics of long-
lived grassland plants, but short-lived species deserve
consideration as well.
Short-lived grasses and forbs, in combination with
long-lived perennials, can be indicators of old growth,
particularly in grasslands of the arid tropics and temperate
steppes (Coiffait-Gombault et al. 2012). Annual, bien-
nial, and short-lived perennial species are often among
the early colonists of sites disturbed by digging animals
(Platt 1975) or locally intense grazing (Mack and
Thompson 1982) and are thus an important part of
dynamic, old-growth grasslands. But where abundant,
annuals can indicate severe degradation associated with
altered disturbance regimes, biological invasions, and
declines in native perennial populations (Mack and
Thompson 1982; Feller and Brown 2000). Whether as
indicators of old growth or as signs of degradation it will
be important to determine the role of short-lived plants
in ecosystem-specific definitions of old-growth grasslands.
Although plant community composition is an indicator
of grassland maturity (Panel 1; Figure 4), we lack firm
estimates of the time required for species-diverse grass-
lands that include rare and endemic species to form. Our
best estimates are based on studies of secondary grasslands
following abandonment of agriculture, plantation
forestry, and surface mining (Kirkman et al. 2004;
Zaloumis and Bond 2011; Le Stradic et al. 2014; Redhead
et al. 2014). These studies clearly demonstrate that old-
growth grassland communities require many decades, and
probably several centuries, to assemble. Rates of grassland
formation are likely influenced by interactions between a
Figure 4. Unique plant communities: (a) a highland grassland
in Brazil; (b) post-fire flowering and a charred, fire-tolerant tree
in a savanna of the southeastern US.
(a) (b)
Old-growth grasslands JW Veldman et al.
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host of factors, including soil biophysical characteristics.
At grassland restoration sites on former agricultural land
in central North America, soil microbial communities are
estimated to require 30–60 years, and soil C and nitrogen
140 years, to recover to levels found in old-growth grass-
lands (Bach et al. 2010; Baer et al. 2010). In these studies,
soil recovery varied with soil texture, suggesting that rates
of grassland soil development can be quite different, even
within the same ecosystem. Additional studies are needed
to improve our understanding of the role of time in the
development of old-growth grassland soils, as well as the
assembly of plant and animal communities.
nConclusions
Having presented the ecological basis and conservation
motivations for the old-growth grassland concept, we sug-
gest that much work remains. Ecologists should establish
ecosystem-specific definitions of old growth and work to
identify and map these grasslands at local, regional, and
continental scales. Land managers should learn to recog-
nize old-growth characteristics and prioritize the mainte-
nance of disturbance regimes through prescribed fire,
wildfire, native herbivores, and, in certain cases, domes-
tic livestock. Conservation advocacy groups should seek
legal protection for old-growth grasslands and support
educational outreach to raise awareness of these imper-
iled biomes.
Policies designed to promote afforestation for any of a
variety of reasons, including climate-change mitiga-
tion, should explicitly acknowledge the conservation
values of old-growth grasslands. Specifically, these poli-
cies should recognize destruction of natural grasslands
for agriculture to be a form of “agricultural conversion”
with negative environmental consequences similar to
those that occur from deforestation. Likewise, policies
that promote fire suppression or planting exotic forage
grasses should be re-evaluated in regions that support
old-growth grasslands; non-native invasive species
should be recognized as a serious threat to old-growth
grassland biodiversity and a major hurdle to restoration.
Finally, financial mechanisms (eg payments for ecosys-
tem services) should be developed that promote the
protection and maintenance of biodiverse ancient
Panel 3. Relevance of old-growth grasslands to ecology
Adler et al. (2011) assessed the relationship between productivity
and species richness in a global dataset of herbaceous plant com-
munities; the majority of their study sites were grasslands of vari-
ous forms and origins, and these were grouped together for their
analyses. Although the authors did not specifically distinguish old-
growth grasslands from others, they did describe a subset of sites
as “anthropogenic”. They concluded that there was no consistent
pattern in the data, and thus productivity alone was a poor predic-
tor of species richness.
We reclassified the sites used by Adler et al. (2011) into two
groups (Figure 5): (1) those likely to be old-growth grasslands (ie
sites with no known history of cultivation, intact disturbance
regimes, and native plant communities; n= 13), and (2) secondary
grasslands and derived savannas (ie previously cultivated sites and
those classified by Adler et al. [2011] as anthropogenic; n= 16);
sites with insufficient information remained unclassified (n= 19).
We found a strong positive linear relationship between productiv-
ity and diversity for old-growth sites (r2= 0.40, P= 0.02), and no
apparent relationship for secondary grasslands and derived savan-
nas. We suggest that this is one example of how distinguishing old-
growth grasslands from other ecosystem states may lead to funda-
mental insights.
Panel 4. Latitudinal trends in old-growth grasslands
There are likely to be profound differences between grasslands
in regions that were transformed during Pleistocene glaciations
(2.6–0.12 MYA) and grasslands of tropical and subtropical
regions with relatively stable climates (Kirkman et al. 2014).
Post-glacial grasslands (eg in temperate North America, Europe,
and Asia) have assembled during the past 12 000 years through
dispersal from refugia where grasslands persisted during glacia-
tions. In contrast, many tropical and subtropical grasslands have
existed for >2 million years in more or less the same locations.
These trends in paleoclimatic stability have likely resulted in lat-
itudinal trends in grassland plant life-history characteristics. In
particular, we hypothesize that in post-glacial grasslands, peren-
nial plants are shorter-lived (ie years to decades; Lauenroth and
Adler 2008) and have higher dispersal and colonization poten-
tials than in climate-stable grasslands, where perennial plants
can be extremely long-lived (centuries to millennia; Alves and
Kolbek 1994; Takahashi et al. 2011) and are usually poor colo-
nizers. In looking for old-growth grasslands, we should expect
that, as in old-growth forests (Outcalt 1997; Devall 1998), the
time required for these grasslands to form – and the ages of
old-growth plants – may vary by at least an order of magnitude,
from ~100 years (Redhead et al. 2014) to perhaps >1000 years.
In many cases, these differences may be attributable to latitudi-
nal trends in climate stability.
Figure 5. The productivity–diversity relationship in old-
growth grasslands compared to secondary grasslands and
derived savannas. Modified from Adler et al. (2011).
40
30
20
10
0
Mean richness (species m–2)
0 200 400 600 800
Mean live biomass (g m–2)
Old-growth grassland
Secondary grassland
and derived savanna
Unclassified
JW Veldman et al.Old-growth grasslands
161
© The Ecological Society of America www.frontiersinecology.org
grasslands. It is our hope that if our proposed concept is
widely adopted by ecologists, policy makers and others
will follow suit, thereby improving the prospects for
conservation of the world’s old-growth grasslands,
savannas, and woodlands.
nAcknowledgements
We thank A Willem for Figure 2a and the Nutrient
Network for the data used in Figure 5.
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4Ecologia Evolutiva e Biodiversidade, Universidade Federal de Minas
Gerais, Belo Horizonte, Brazil; 5Department of Biology, Stanford
University, Stanford, CA; 6Gembloux Agro-Bio tech, Biodiversity and
Landscape Unit, University of Liege, Gembloux, Belgium;
7Department of Botany, Universidade Federal do Rio Grande do Sul,
Porto Alegre, Brazil; 8Department of Philosophy and Religious Studies,
Iowa State University, Ames, IA; 9Department of Biological Sciences,
University of Cape Town and South African Environmental
Observation Network, NRF, Rondebosch, South Africa;
10Department of Biology, University of Florida, Gainesville, FL
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