Partitioning of root and shoot competition and the stability of savannas

A classic problem in coexistence theory is how grasses and trees coexist in savannas. A popular deterministic model of savannas, the rooting niche separation model, is based on an assumption that is not empirically supported in many savannas. Alternative models that do not rely on the rooting niche assumption invoke intricate stochastic mechanisms that limit their attractiveness as general models of savannas. In this article we develop an alternative deterministic model of grass-tree interactions and use it to analyze the conditions under which grass-tree coexistence is possible. The novel feature of this model is that it partitions aboveground and belowground competition and simulates the fact that fire and herbivory remove only aboveground biomass. The model predicts that stable coexistence of grasses and trees is possible, even when grasses and trees do not have separate rooting niches. We show that when aboveground competition is intense, grasses can be excluded by trees; under such conditions, fire can prevent grasses from exclusion and induce a stable savanna state. The model provides a general framework for exploring the interactive effects of competition, herbivory, and fire on savanna systems.     

Buffering the savanna: fire regimes and disequilibrium ecology in West Africa

According to contemporary ecological theory, the mechanisms governing tree cover in savannas vary by precipitation level. In tropical areas with mesic rainfall levels, savannas are unstable systems in which disturbances, such as fire, determine the ratio of trees to grasses. Precipitation in these so-called “disturbance-driven savannas” is sufficient to support forest but frequent disturbances prevent transition to a closed canopy state. Building on a savanna buffering model we argue that a consistent fire regime is required to maintain savannas in mesic areas. We hypothesize that the spatiotemporal pattern of fires is highly regular and stable in these areas. Furthermore, because tree growth rates in savannas are a function of precipitation, we hypothesize that savannas with the highest rainfall levels will have the most consistent fire pattern and the most intense fires—thus the strongest buffering mechanisms. We analyzed the spatiotemporal pattern of burning over 11 years for a large subset of the West African savanna using a moderate resolution imaging spectroradiometer active fire product to document the fire regime for three savanna belts with different precipitation levels. We used LISA analysis to quantify the spatiotemporal patterns of fires, coefficient of variance to quantify differences in peak fire dates, and center or gravity pathways to characterize the spatiotemporal patterns of the fires for each area. Our analysis confirms that spatiotemporal regularity of the fire regime is greater for mesic areas that for areas where precipitation is lower and that areas with more precipitation have more regular fire regimes.     

Modelling responses of pine savannas to climate change and large‐scale disturbance

Global warming can potentially influence ecological communities through altered disturbance regimes in addition to increased temperatures. We investigate the response of pine savannas in the southeastern United States to global warming using a simple Lotka‐Volterra competition model together with predicted changes to fire and hurricane disturbance regimes with global climate change. In the southeastern United States, decreased frequency of both fires and hurricanes with global warming will shift pine savannas toward a forested state. A CO₂ fertilization effect that increases the growth rate of tree populations will also push southeastern landscapes from open savannas towards closed forests. Transient dynamics associated with climate driven changes in vegetation will last on the order of decades to a century. In our model, the sensitivity of savannas to relative changes in the frequency of fire versus hurricanes is linearly dependent on the growth rate and mortality of trees in fire and hurricane disturbances.     

A resource ratio model of the effects of changes in CO<Subscript>2</Subscript> on woody plant invasion

It is expected that elevated CO₂ levels may have an important positive effect on the dominance of woody plants over grasses in savannas and grasslands. I propose that these changes in the relative abundance of trees and shrubs over grasses may be explained by Tilman’s resource ratio models. This change will occur because C₃ trees will have higher net photosynthetic rates than C₄ grasses which predominate in savannas. This will cause trees to have higher growth rates than grasses. An additional factor in trees and shrubs with carbon-based defences (such as tannins or other polyphenols) is that they may be better defended and, thus, lose less material to herbivory. Consequently, trees and shrubs should invade savannas and grasslands because their R* values will be lower. I compare this model to another less parsimonious model based on fire and carbon storage and allocation. Although these models are not necessarily mutually exclusive, the resource ratio model may be differentiated from the fire-carbon model on the basis of the presence of fire and/or the sensitivity to elevations in global CO₂ levels.     

Self‐thinning and Tree Competition in Savannas

This paper examines the feasibility of applying self‐thinning concepts to savannas and how competition with herbaceous vegetation may modify self‐thinning patterns among woody plants in these ecosystems. Competition among woody plants has seldom been invoked as a major explanation for the persistence of herbaceous vegetation in mixed tree–grass ecosystems. On the contrary, the primary resource‐based explanations for tree–grass coexistence are based on tree–grass competition (niche‐separation) that assumes that trees are inferior competitors unless deeper rooting depths provide them exclusive access to water. Alternative nonresource‐based hypotheses postulate that trees are the better competitors, but that tree populations are suppressed by mortality related to fire, herbivores, and other disturbances. If self‐thinning of woody plants can be detected in savannas, stronger evidence for resource‐limitation and competitive interactions among woody plants would suggest that the primary models of savannas need to be adjusted. We present data from savanna sites in South Africa to suggest that self‐thinning among woody plants can be detected in low‐disturbance situations, while also showing signs that juvenile trees, more so than adults, are suppressed when growing with herbaceous vegetation in these ecosystems. This finding we suggest is evidence for size‐asymmetric competition in savannas.     

Seasonality and facilitation drive tree establishment in a semi-arid floodplain savanna

A popular hypothesis for tree and grass coexistence in savannas is that tree seedlings are limited by competition from grasses. However, competition may be important in favourable climatic conditions when abiotic stress is low, whereas facilitation may be more important under stressful conditions. Seasonal and inter-annual fluctuations in abiotic conditions may alter the outcome of tree–grass interactions in savanna systems and contribute to coexistence. We investigated interactions between coolibah (Eucalyptus coolabah) tree seedlings and perennial C₄ grasses in semi-arid savannas in eastern Australia in contrasting seasonal conditions. In glasshouse and field experiments, we measured survival and growth of tree seedlings with different densities of C₄ grasses across seasons. In warm glasshouse conditions, where water was not limiting, competition from grasses reduced tree seedling growth but did not affect tree survival. In the field, all tree seedlings died in hot dry summer conditions irrespective of grass or shade cover, whereas in winter, facilitation from grasses significantly increased tree seedling survival by ameliorating heat stress and protecting seedlings from herbivory. We demonstrated that interactions between tree seedlings and perennial grasses vary seasonally, and timing of tree germination may determine the importance of facilitation or competition in structuring savanna vegetation because of fluctuations in abiotic stress. Our finding that trees can grow and survive in a dense C₄ grass sward contrasts with the common perception that grass competition limits woody plant recruitment in savannas.     

Tree–grass coexistence in savannas revisited – insights from an examination of assumptions and mechanisms invoked in existing models

Several explanations for the persistence of tree–grass mixtures in savannas have been advanced thus far. In general, these either concentrate on competition‐based mechanisms, where niche separation with respect to limiting resources such as water lead to tree–grass coexistence, or demographic mechanisms, where factors such as fire, herbivory and rainfall variability promote tree–grass persistence through their dissimilar effects on different life‐history stages of trees. Tests of these models have been largely site‐specific, and although different models find support in empirical data from some savanna sites, enough dissenting evidence exists from others to question their validity as general mechanisms of tree–grass coexistence. This lack of consensus on determinants of savanna structure and function arises because different models: (i) focus on different demographic stages of trees, (ii) focus on different limiting factors of tree establishment, and (iii) emphasize different subsets of the potential interactions between trees and grasses. Furthermore, models differ in terms of the most basic assumptions as to whether trees or grasses are the better competitors. We believe an integration of competition‐based and demographic approaches is required if a comprehensive model that explains both coexistence and the relative productivity of the tree and grass components across the diverse savannas of the world is to emerge. As a first step towards this end, we outline a conceptual framework that integrates existing approaches and applies them explicitly to different life‐history stage of trees.     

Effects of frequent fires and grazing on stable nitrogen isotope ratios of vegetation in northern Australia

The ratios of stable nitrogen isotopes expressed as δ¹⁵N values can indicate the openness of nitrogen cycles in ecosystems. Southwards through the Northern Territory, values of foliar δ¹⁵N in savanna trees increase as mean annual rainfall decreases from approximately 1800 mm to approximately 750 mm, with foliar δ¹⁵N thereafter decreasing toward arid central Australia. Recent literature argues that this pattern is caused by higher grazing intensity in semi‐arid savannas, but counter views have attributed the pattern more directly to variations in aridity. In this paper, grazed and ungrazed sites in a semi‐arid savanna are compared, and it is shown that grazing has a relatively small effect on the positive foliar δ¹⁵N values of grasses, but no effect on δ¹⁵N values of trees. This gives little support to the argument that variations in grazing pressure at the scale of hundreds of kilometres could result in detectable differences in the foliar δ¹⁵N values of trees. I then compare the semi‐arid savannas with mesic savannas, where fires are frequent, and with mesic rainforests, which are rarely burnt. Greater foliar δ¹⁵N values in rainforest and fire‐excluded mesic savannas than in frequently burnt savannas suggests that fire regimes affect foliar δ¹⁵N. The previously observed pattern in δ¹⁵N values along the rainfall gradient in the Northern Territory is consistent with trends in fire frequency and possible direct effects of fire, but further work is required to determine the relative impacts of aridity and fire. Within a particular rainfall regime, foliar δ¹⁵N values may indicate historical fire frequencies.     

Variable rainfall has a greater effect than fire on the demography of the dominant tree in a semi‐arid Eucalyptus savanna

Rainfall, fire and competition are emphasized as determinants of the density and basal area of woody vegetation in savanna. The semi‐arid savannas of Australia have substantial multi‐year rainfall deficits and insufficient grass fuel to carry annual fire in contrast to the mesic savannas in more northern regions. This study investigates the influence of rainfall deficit and excess, fire and woody competition on the population dynamics of a dominant tree in a semi‐arid savanna. All individuals of Eucalyptus melanophloia were mapped and monitored in three, 1‐ha plots over an 8.5 year period encompassing wet and dry periods. The plots were unburnt, burnt once and burnt twice. A competition index incorporating the size and distance of neighbours to target individuals was determined. Supplementary studies examined seedling recruitment and the transition of juvenile trees into the sapling layer. Mortality of burnt seedlings was related to lignotuber area but the majority of seedlings are fire resistant within 12 months of germination. Most of the juveniles (≤1 cm dbh) of E. melanophloia either died in the dry period or persisted as juveniles throughout 8.5 years of monitoring. Mortality of juveniles was positively related to woody competition and was higher in the dry period than the wet period. The transition of juveniles to a larger size class occurred at extremely low rates, and a subsidiary study along a clearing boundary suggests release from woody competition allows transition into the sapling layer. From three fires the highest proportion of saplings (1–10 cm dbh) reduced to juveniles was only 5.6% suggesting rates of ‘top‐kill’ of E. melanophloia as a result of fire are relatively low. Girth growth was enhanced in wet years, particularly for larger trees (>10 cm dbh), but all trees regardless of size or woody competition levels are vulnerable to drought‐induced mortality. Overall the results suggest that variations in rainfall, especially drought‐induced mortality, have a much stronger influence on the tree demographics of E. melanophloia in a semi‐arid savanna of north‐eastern Australia than fire.     

An idealized model for tree–grass coexistence in savannas: the role of life stage structure and fire disturbances

1.  We discuss a simple implicit-space model for the competition of trees and grasses in an idealized savanna environment. The model represents patch occupancy dynamics within the habitat and introduces life stage structure in the tree population, namely adults and seedlings. A tree can be out-competed by grasses only as long as it is a seedling.
2.  The model is able to predict grassland, forest, savanna and bistability between forest and grassland, depending on the different characteristics of the ecosystem, represented by the model's parameters.
3.  The inclusion of stochastic fire disturbances significantly widens the parameter range where coexistence of trees and grasses is found. At the same time, grass-fire feedback can induce bistability between forest and grassland.
4.   Synthesis . These results suggest that tree–grass coexistence in savannas can be either deterministically stable or stabilized by random disturbances, depending on prevailing environmental conditions and on the types of plant species present in the ecosystem.     

A lepidopteran (<Emphasis Type="Italic">Imbrasia belina</Emphasis>) might influence tree-grass balance of <Emphasis Type="Italic">Colophospermum mopane</Emphasis> savanna

Traditional explanations of tree-grass coexistence in African savannas are based on competition between these growth forms or demographic bottlenecks of trees maintained by fire or mammalian browsers. Perturbation of their “balance” may result in an alternate system state of woody encroachment. Invertebrate herbivory has never been offered as an explanation. We developed a consumer-resource model which illustrated that annual irruptions of a lepidopteran (Imbrasia belina), known as mopane worm, can determine the tree-grass balance of semi-arid Colophospermum mopane savanna in southern Africa. Model performance was sensitive to the abundance, hence mortality, of mopane worms, owing to their complete defoliation of tree leaf biomass resulting in altered competitive relations between trees and grasses. Invertebrate herbivores have been recognized in other systems as agents for effecting a state change of host tree populations; this modeling study offers a first indication of such a role for the well-researched tree-grass relations of African savannas.     

The effects of fire on woody plant encroachment are exacerbated by succession of trees of decreased palatability

The ingression of woody plants into the grassy layer of savannas and grasslands has become a global concern. The increase of woody plants has been primarily attributed over grazing, fire and more recently to the increase of atmospheric CO₂. We used long-term observations and analyses to assess changes in woody vegetation in Ithala Game Reserve (IGR), South Africa. Textural analysis of aerial photographs was used to detect changes in woody vegetation, from 1943 to 2007 in Ithala Game Reserve (IGR), South Africa. Daily rainfall data from 1905 to 2009 were used in a time-series analysis to determine if rainfall patterns have changed. The time-series analysis showed that the low magnitude (0–10 mm) rainfall events decreased from 1916 to 2009 and high magnitude rainfall events increased (10–20 and >20 mm). The mean annual rainfall increased from ～700 to ～850 mm from the 1930s to the 2000s. This change in rainfall was a key factor in the increase in woody vegetation from 1943 to 2009. We also used field data from the same reserve collected over 30 years to assess the increases in tree cover. Tree cover and density increased significantly by 32.5% and 657.9 indiv ha^?1 respectively, over 64 years. Before the proclamation of IGR in 1972, increases in woody vegetation from 1943 were non-significant. After the proclamation of IGR, herbivore population numbers and spatial distribution influenced the accumulation of grassy biomass required to fuel fires. In areas with reduced fuel loads, the consequential suppression of fire accelerated the rate of woody plant invasion into savannas. The increase in woody vegetation coincided with a decrease in palatable (e.g. Acacia gerrardii and Acacia davyi) and an increase in unpalatable woody plants. The avoidance of the unpalatable trees (e.g. Euclea and Searsia species) by large mammalian herbivores has allowed these trees to increase in density relatively unhindered.     

Walter’s two-layer hypothesis revisited: back to the roots!

Walter (Jahrb Wiss Bot 87:750–860, 1939) proposed a two-layer hypothesis, an equilibrium explanation for coexistence of savanna trees and grasses. This hypothesis relies on vertical niche partitioning and assumed that grasses are more water-use efficient than trees and use subsurface water while trees also have access to deeper water sources. Thus, in open savannas, grasses were predicted to predominate because of their water use efficiency and access to subsurface water. This hypothesis has been a prominent part of the savanna literature since first proposed. We review the literature on Walter’s hypothesis and reconsider his original intentions. Walter intended this hypothesis to be restricted to dry savannas. In his opinion, mesic and humid savannas were controlled by biotic factors and disturbances. We surveyed the global savanna literature for records of vertical niche partitioning by grasses and trees. We find that, within the scope of Walter’s original intentions, this hypothesis works remarkably well, and in some cases is appropriate for deserts as well as for dry temperate systems and even some mesic savannas.     

Root dynamics influence tree–grass coexistence in an Australian savanna

Both resource and disturbance controls have been invoked to explain tree persistence among grasses in savannas. Here we determine the extent to which competition for available resources restricts the rooting depth of both grasses and trees, and how this may influence nutrient cycling under an infrequently burned savanna near Darwin, Australia. We sampled fine roots <2 mm in diameter from 24 soil pits under perennial as well as annual grasses and three levels of canopy cover. The relative proportion of C₃ (trees) and C₄ (grasses) derived carbon in a sample was determined using mass balance calculations. Our results show that regardless of the type of grass both tree and grass roots are concentrated in the top 20 cm of the soil. While trees have greater root production and contribute more fine root biomass grass roots contribute a disproportional amount of nitrogen and carbon to the soil relative to total root biomass. We postulate that grasses maintain soil nutrient pools and provide biomass for regular fires that prevent forest trees from establishing while savanna trees, are important for increasing soil N content, cycling and mineralization rates. We put forward our ideas as a hypothesis of resource‐regulated tree–grass coexistence in tropical savannas.     

Changing dominance of key plant species across a Mediterranean climate region: implications for fuel types and future fire regimes

Herbaceous and woody plants represent different fuel types in flammable ecosystems, due to contrasting patterns of growth and flammability in response to productivity (moisture availability). However, other factors, such as soil type, fire regimes and competitive interactions may also influence the relative composition of herbaceous and woody plants within a community. The Mediterranean climate region of south eastern Australia is transitional between two contrasting fuel systems; herbaceous dominated in the dry north, versus woody plant dominated shrublands in the relatively moist south. Across the rainfall gradient of the region, there are confounded changes in dominant soil types and fire frequency. We used model-subset selection using Akaike’s Information Criterion to examine potential driving mechanisms of community compositional change from herbaceous (e.g. Triodia scariosa, Austrostipa sp.) to woody plants (e.g. Beyeria opaca, Leptospermum coriaceum, Acacia ligulata) by measuring relative cover across combinations of rainfall, time since the last fire (TSF) and soil type. We examined the relative influence of environmental versus competitive interactions on determining the cover of perennial hummock grass, T. scariosa, and co-occurring woody shrubs. Rainfall and soil types, rather than competition, were the over-arching determinants of the relative cover of grasses and shrubs. Given the sensitivity to rainfall, our results indicate there is strong potential for the nature of fuel, flammability and fire regimes to be altered in the future via climate change in this region.     

When is a ‘forest’ a savanna,and why does it matter?

Savannas are defined based on vegetation structure, the central concept being a discontinuous tree cover in a continuous grass understorey. However, at the high‐rainfall end of the tropical savanna biome, where heavily wooded mesic savannas begin to structurally resemble forests, or where tropical forests are degraded such that they open out to structurally resemble savannas, vegetation structure alone may be inadequate to distinguish mesic savanna from forest. Additional knowledge of the functional differences between these ecosystems which contrast sharply in their evolutionary and ecological history is required. Specifically, we suggest that tropical mesic savannas are predominantly mixed tree–C₄ grass systems defined by fire tolerance and shade intolerance of their species, while forests, from which C₄ grasses are largely absent, have species that are mostly fire intolerant and shade tolerant. Using this framework, we identify a suite of morphological, physiological and life‐history traits that are likely to differ between tropical mesic savanna and forest species. We suggest that these traits can be used to distinguish between these ecosystems and thereby aid their appropriate management and conservation. We also suggest that many areas in South Asia classified as tropical dry forests, but characterized by fire‐resistant tree species in a C₄ grass‐dominated understorey, would be better classified as mesic savannas requiring fire and light to maintain the unique mix of species that characterize them.     

Herbaceous vegetation responses to experimental fire in savannas and forests depend on biome and climate

Fire–vegetation feedbacks potentially maintain global savanna and forest distributions. Accordingly, vegetation in savanna and forest ecosystems should have differential responses to fire, but fire response data for herbaceous vegetation have yet to be synthesized across biomes. Here, we examined herbaceous vegetation responses to experimental fire at 30 sites spanning four continents. Across a variety of metrics, herbaceous vegetation increased in abundance where fire was applied, with larger responses to fire in wetter and in cooler and/or less seasonal systems. Compared to forests, savannas were associated with a 4.8 (±0.4) times larger difference in herbaceous vegetation abundance for burned versus unburned plots. In particular, grass cover decreased with fire exclusion in savannas, largely via decreases in C₄ grass cover, whereas changes in fire frequency had a relatively weak effect on grass cover in forests. These differential responses underscore the importance of fire for maintaining the vegetation structure of savannas and forests.     

Cascading biodiversity and functional consequences of a global change–induced biome switch

Aim At a regional scale, across southern Africa, woody thickening of savannas is becoming increasingly widespread. Using coupled vegetation and faunal responses (ants), we explore whether major changes in woody cover in savannas represent an increase in the density of savanna trees (C₄ grass layer remains intact) or a ‘regime shift’ in system state from savanna to thicket (=dry forest) where broad‐leaved, forest‐associated trees shade out C₄ grasses. Location Hluhluwe Game Reserve, South Africa. Methods We sampled paired open (low woody cover) and closed (high cover that have undergone an increase in tree density) sites. Vegetation was sampled using belt transects, and a combination of pitfall trapping and Winkler sampling was used for ants. Results Closed habitats did not simply contain a higher density of woody savanna species, but differed significantly in structure, functional composition (high prevalence of broad‐leaved trees, discontinuous C₄ grasses) and system properties (e.g. low flammability). Ant assemblage composition reflected this difference in habitat. The trophic structure of ant assemblages in the two habitats revealed a functional shift with much higher abundances of predatory species in the closed habitat. Main conclusions The predominance of species with forest‐associated traits and concomitant reduction of C₄ grasses in closed sites indicate that vegetation has undergone a shift in fundamental system state (to thicket), rather than simply savanna thickening. This biome shift has cascading functional consequences and implications for biodiversity conservation. The potential loss of many specialist savanna plant species is especially concerning, given the spatial extent and speed of this vegetation switch. Although it is not clear how easily the habitat switch can be reversed and how stable the thicket habitats are, it is likely in the not‐too‐distant future that conservation managers will be forced to make decisions on whether to actively maintain savannas.     

Fire and fire-adapted vegetation promoted C4 expansion in the late Miocene

Large proportions of the Earth's land surface are covered by biomes dominated by C(4) grasses. These C(4)-dominated biomes originated during the late Miocene, 3-8 million years ago (Ma), but there is evidence that C(4) grasses evolved some 20 Ma earlier during the early Miocene/Oligocene. Explanations for this lag between evolution and expansion invoke changes in atmospheric CO(2), seasonality of climate and fire. However, there is still no consensus about which of these factors triggered C(4) grassland expansion. We use a vegetation model, the adaptive dynamic global vegetation model (aDGVM), to test how CO(2), temperature, precipitation, fire and the tolerance of vegetation to fire influence C(4) grassland expansion. Simulations are forced with late Miocene climates generated with the Hadley Centre coupled ocean-atmosphere-vegetation general circulation model. We show that physiological differences between the C(3) and C(4) photosynthetic pathways cannot explain C(4) grass invasion into forests, but that fire is a crucial driver. Fire-promoting plant traits serve to expand the climate space in which C(4)-dominated biomes can persist. We propose that three mechanisms were involved in C(4) expansion: the physiological advantage of C(4) grasses under low atmospheric CO(2) allowed them to invade C(3) grasslands; fire allowed grasses to invade forests; and the evolution of fire-resistant savanna trees expanded the climate space that savannas can invade.     

Increased tree densities in South African savannas: >50 years of data suggests CO2 as a driver

For the past century, woody plants have increased in grasslands and savannas worldwide. Woody encroachment may significantly alter ecosystem functioning including fire regimes, herbivore carrying capacity, biodiversity and carbon storage capacity. Traditionally, increases in woody cover and density have been ascribed to changes in the disturbance regime (fire and herbivores) or rainfall. Increased atmospheric CO₂ concentrations may also contribute, by increasing growth rates of trees relative to grasses. This hypothesis is still heavily debated because usually potential CO₂ effects are confounded by changes in land use (disturbance regime). Here we analyse changes in woody density in fire experiments at three sites in South African savannas where the disturbance regime (fire and herbivores) was kept constant for 30 and 50 years. If global drivers had significant effects on woody plants, we would expect significant increases in tree densities and biomass over time under the constant disturbance regime. Woody density remained constant in a semiarid savanna but tripled in a mesic savanna between the 1970s and 1990s. At the third site, a semiarid savanna near the southern limits of the biome, tree density doubled from the mid 1990s to 2010. Interpretation of the causes is confounded by population recovery after clearing, but aerial photograph analysis on adjacent non‐cleared areas showed an accompanying 48% increase in woody cover. Increased CO₂ concentrations are consistent with increased woody density while other global drivers (rainfall) remained constant over the duration of the experiments. The absence of a response in one semiarid savanna could be explained by a smaller carbon sink capacity of the dominant species, which would therefore benefit less from increased CO₂. Understanding how savannas and grasslands respond to increased CO₂ and identifying the causes of woody encroachment are essential for the successful management of these systems.