Seasonal patterns in biomass burning emissions from southern African vegetation fires for the year 2000

A modeling framework has been developed to examine the spatial and temporal aspects of biomass burning emissions from southern African savanna fires. The complexity of the fire emissions processes is described using a spatially and temporally explicit model that integrates recently published satellite‐driven fuel load amounts, the GBA‐2000 satellite burned area time series and empirically derived parameterizations of combustion completeness and emission factors (EFs). To represent fire behavior characteristics, land cover is classified into grasslands and woodlands using the MODIS percent tree cover product. The combustion completeness is modeled as a function of grass fuel moisture and the EFs as a function of grass fuel moisture in grasslands and fuel mixture in woodlands. Fuel moisture is derived from satellite vegetation index time series. The analysis at the regional scale shows that early burning in grasslands may lead to higher amounts of products of incomplete combustion, despite the lower amounts of fuel consumed, compared with late dry season burning. In contrast, early burning in woodlands results in lower emissions, in both products of complete and incomplete combustion, because less fuel is consumed than in the late dry season when the fuels are drier. Overall, burning in woodlands dominates the regional emission budgets. Emissions estimates for various atmospheric species, many of which are modeled for the first time, are reported. The modeled estimates for 2000 are (in Tg) 296 CO₂, 11.7 CO, 0.350 CH₄, 0.348 NMHC and 1.1 particulates (<2.5 μm). Especially high is the previously undetermined contribution of oxygenated volatile organic compounds (0.915 Tg). A sensitivity analysis of fixed vs. seasonally variable EFs and combustion completeness demonstrates the importance of accounting for the seasonal variations of these two variables in emissions modeling.     

Global grass (Poaceae) success underpinned by traits facilitating colonization,persistence and habitat transformation

Poaceae (the grasses) is arguably the most successful plant family, in terms of its global occurrence in (almost) all ecosystems with angiosperms, its ecological dominance in many ecosystems, and high species richness. We suggest that the success of grasses is best understood in context of their capacity to colonize, persist, and transform environments (the “Viking syndrome”). This results from combining effective long‐distance dispersal, efficacious establishment biology, ecological flexibility, resilience to disturbance and the capacity to modify environments by changing the nature of fire and mammalian herbivory. We identify a diverse set of functional traits linked to dispersal, establishment and competitive abilities. Enhanced long‐distance dispersal is determined by anemochory, epizoochory and endozoochory and is facilitated via the spikelet (and especially the awned lemma) which functions as the dispersal unit. Establishment success could be a consequence of the precocious embryo and large starch reserves, which may underpin the extremely short generation times in grasses. Post‐establishment genetic bottlenecks may be mitigated by wind pollination and the widespread occurrence of polyploidy, in combination with gametic self‐incompatibility. The ecological competitiveness of grasses is corroborated by their dominance across the range of environmental extremes tolerated by angiosperms, facilitated by both C₃ and C₄ photosynthesis, well‐developed frost tolerance in several clades, and a sympodial growth form that enabled the evolution of both annual and long‐lived life forms. Finally, absence of investment in wood (except in bamboos), and the presence of persistent buds at or below ground level, provides tolerance of repeated defoliation (whether by fire, frost, drought or herbivores). Biotic modification of environments via feedbacks with herbivory or fire reinforce grass dominance leading to open ecosystems. Grasses can be both palatable and productive, fostering high biomass and diversity of mammalian herbivores. Many grasses have a suite of architectural and functional traits that facilitate frequent fire, including a tufted growth form, and tannin‐like substances in leaves which slow decomposition. We mapped these traits over the phylogeny of the Poales, spanning the grasses and their relatives, and demonstrated the accumulation of traits since monocots originated in the mid‐Cretaceous. Although the sympodial growth form is a monocot trait, tillering resulting in the tufted growth form most likely evolved within the grasses. Similarly, although an ovary apparently constructed of a single carpel evolved in the most recent grass ancestor, spikelets and the awned lemma dispersal units evolved within the grasses. Frost tolerance and C₄ photosynthesis evolved relatively late (late Palaeogene), and the last significant trait to evolve was probably the production of tannins, associated with pyrophytic savannas. This fits palaeobotanical data, suggesting several phases in the grass success story: from a late Cretaceous origin, to occasional tropical grassland patches in the later Palaeogene, to extensive C₃ grassy woodlands in the early–middle Miocene, to the dramatic expansion of the tropical C₄ grass savannas and grasslands in the Pliocene, and the C₃ steppe grasslands during the Pleistocene glacial periods. Modern grasslands depend heavily on strongly seasonal climates, making them sensitive to climate change.     

The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas

The distribution and abundance of trees can be strongly affected by disturbance such as fire. In mixed tree/grass ecosystems, recurrent grass‐fuelled fires can strongly suppress tree saplings and therefore control tree dominance. We propose that changes in atmospheric [CO₂] could influence tree cover in such metastable ecosystems by altering their postburn recovery rates relative to flammable herbaceous growth forms such as grasses. Slow sapling recovery rates at low [CO₂] would favour the spread of grasses and a reduction of tree cover. To test the possible importance of [CO₂]/fire interactions, we first used a Dynamic Global Vegetation Model (DGVM) to simulate biomass in grassy ecosystems in South Africa with and without fire. The results indicate that fire has a major effect under higher rainfall conditions suggesting an important role for fire/[CO₂] interactions. We then used a demographic model of the effects of fire on mesic savanna trees to test the importance of grass/tree differences in postburn recovery rates. We adjusted grass and tree growth in the model according to the DGVM output of net primary production at different [CO₂] relative to current conditions. The simulations predicted elimination of trees at [CO₂] typical of the last glacial period (180 ppm) because tree growth rate is too slow (15 years) to grow to a fire‐proof size of ca. 3 m. Simulated grass growth would produce an adequate fuel load for a burn in only 2 years. Simulations of preindustrial [CO₂] (270 ppm) predict occurrence of trees but at low densities. The greatest increase in trees occurs from preindustrial to current [CO₂] (360 ppm). The simulations are consistent with palaeo‐records which indicate that trees disappeared from sites that are currently savannas in South Africa in the last glacial. Savanna trees reappeared in the Holocene. There has also been a large increase in trees over the last 50–100 years. We suggest that slow tree recovery after fire, rather than differential photosynthetic efficiencies in C₃ and C₄ plants, might have been the significant factor in the Late Tertiary spread of flammable grasslands under low [CO₂] because open, high light environments would have been a prerequisite for the spread of C₄ grasses. Our simulations suggest further that low [CO₂] could have been a significant factor in the reduction of trees during glacial times, because of their slower regrowth after disturbance, with fire favouring the spread of grasses.     

The origin of the savanna biome

Savannas are a major terrestrial biome, comprising of grasses with the C₄ photosynthetic pathway and trees with the C₃ type. This mixed grass–tree biome rapidly appeared on the ecological stage 8 million years ago with the near‐synchronous expansion of C₄ grasses around the world. We propose a new hypothesis for this global event based on a systems analysis that integrates recent advances in how fire influences cloud microphysics, climate and savanna ecology in a low carbon dioxide (CO₂) world. We show that fire accelerates forest loss and C₄ grassland expansion through multiple positive feedback loops that each promote drought and more fire. A low CO₂ atmosphere amplifies this cycle by limiting tree recruitment, allowing the ingress of C₄ grasses to greatly increase ecosystem flammability. Continued intensification of land use could enhance or moderate the network of feedbacks that have initiated, promoted and sustained savannas for millions of years. We suggest these alterations will overprint the effects of anthropogenic atmospheric change in coming decades.     

A native C<Subscript>3</Subscript> grass alters fuels and fire spread in montane grassland of South Africa

Although most fire research in plant ecology focuses on vegetation responses to burning, shifts in plant community composition wrought by climate change can change wildland fuelbeds and affect fire behaviour such that the nature of fire in these systems is altered. Changes that introduce substantially different fuel types can alter the spatial extent of fire, with potential impacts on community succession and biodiversity. Montane grasslands of sub-Saharan Africa are threatened by climate change because species distributions can shift with climatically determined ranges. We studied the impact of patches of the temperate C₃ grass Festuca costata in C₄-dominated grassland at the transition between their subalpine ranges in South Africa’s Drakensberg. We used empirical data on fuel moisture and fuel load across F. costata-dominated patches in a C₄-dominated matrix in fire spread models to predict the effect of larger, higher-moisture F. costata patches on the spatial extent of fire. Results indicate F. costata reduces fire spread and burn probability in F. costata patches, and the effect increases as live fuel moisture increases and patches get larger. However, as a native species, F. costata does not appear to have the extreme, fire-suppressing effect of non-native C₃ grasses in other C₄ grasslands. Instead, F. costata patches likely increase variability in the spatial extent of fire in this C₄-dominated grassland, which likely translates to spatial variability on vegetation succession.     

Elevation‐induced climate change as a dominant factor causing the late Miocene C4 plant expansion in the Himalayan foreland

During the late Miocene, a dramatic global expansion of C₄ plant distribution occurred with broad spatial and temporal variations. Although the event is well documented, whether subsequent expansions were caused by a decreased atmospheric CO₂ concentration or climate change is a contentious issue. In this study, we used an improved inverse vegetation modeling approach that accounts for the physiological responses of C₃ and C₄ plants to quantitatively reconstruct the paleoclimate in the Siwalik of Nepal based on pollen and carbon isotope data. We also studied the sensitivity of the C₃ and C₄ plants to changes in the climate and the atmospheric CO₂ concentration. We suggest that the expansion of the C₄ plant distribution during the late Miocene may have been primarily triggered by regional aridification and temperature increases. The expansion was unlikely caused by reduced CO₂ levels alone. Our findings suggest that this abrupt ecological shift mainly resulted from climate changes related to the decreased elevation of the Himalayan foreland.     

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.     

Restoration of C4 grasses with seasonal fires in a C3/C4 grassland invaded by Prosopis glandulosa,a fire‐resistant shrub

Questions: Can prescribed fire restore C₄ perennial grasses in grassland ecosystems that have become dominated by fire‐resistant C₃ shrubs (Prosopis glandulosa) and C₃ grasses? Do fires in different seasons alter the direction of change in grass composition? Location: Texas, USA. Methods: We quantified short‐ and long‐term (12 yr post‐fire) herbaceous functional group cover and diversity responses to replicated seasonal fire treatments: (1) repeated‐winter fires (three in 5 yr), (2) repeated‐summer fires (two in 3 yr), and (3) alternate‐season fires (two winter and one summer in 4 yr), compared with a no‐fire control. Results: Summer fires were more intense than winter fires, but all fire treatments temporarily decreased Prosopis and C₃ annual grass cover. The alternate‐season fire treatment caused a long‐term increase in C₄ mid‐grass cover and functional group diversity. The repeated‐summer fire treatment increased C₄ short‐grass cover but also caused a long‐term increase in bare ground. The repeated winter fire treatment had no long‐term effects on perennial grass cover. Mesquite post‐fire regrowth had increasingly negative impacts on herbaceous cover in all fire treatments. Conclusions: Summer fire was necessary to shift herbaceous composition toward C₄ mid‐grasses. However, the repeated‐summer fire treatment may have been too extreme and caused post‐fire herbaceous composition to “over‐shift” toward less productive C₄ short‐grasses rather than C₄ mid‐grasses. This study provides some of the first long‐term data showing a possible benefit of mixing seasonal fires (i.e., the alternate‐season fire treatment) in a prescribed burning management plan to restore C₄ mid‐grass cover and enhance overall herbaceous diversity.     

Long‐term variability and rainfall control of savanna fire regimes in equatorial East Africa

Fires burning the vast grasslands and savannas of Africa significantly influence the global carbon cycle. Projecting the impacts of future climate change on fire‐mediated biogeochemical processes in these dry tropical ecosystems requires understanding of how various climate factors influence regional fire regimes. To examine climate–vegetation–fire linkages in dry savanna, we conducted macroscopic and microscopic charcoal analysis on the sediments of the past 25 000 years from Lake Challa, a deep crater lake in equatorial East Africa. The charcoal‐inferred shifts in local and regional fire regimes were compared with previously published reconstructions of temperature, rainfall, seasonal drought severity, and vegetation dynamics to evaluate millennial‐scale drivers of fire occurrence. Our charcoal data indicate that fire in the dry lowland savanna of southeastern Kenya was not fuel‐limited during the Last Glacial Maximum (LGM) and Late Glacial, in contrast to many other regions throughout the world. Fire activity remained high at Lake Challa probably because the relatively high mean‐annual temperature (~22 °C) allowed productive C₄ grasses with high water‐use efficiency to dominate the landscape. From the LGM through the middle Holocene, the relative importance of savanna burning in the region varied primarily in response to changes in rainfall and dry‐season length, which were controlled by orbital insolation forcing of tropical monsoon dynamics. The fuel limitation that characterizes the region's fire regime today appears to have begun around 5000–6000 years ago, when warmer interglacial conditions coincided with prolonged seasonal drought. Thus, insolation‐driven variation in the amount and seasonality of rainfall during the past 25 000 years altered the immediate controls on fire occurrence in the grass‐dominated savannas of eastern equatorial Africa. These results show that climatic impacts on dry‐savanna burning are heterogeneous through time, with important implications for efforts to anticipate future shifts in fire‐mediated ecosystem processes.     

Impact of storm‐burning on Melaleuca viridiflora invasion of grasslands and grassy woodlands on Cape York Peninsula,Australia

This paper examines invasion of grasslands on Cape York Peninsula, Australia, by Melaleuca viridiflora and other woody species, and the role of storm‐burning (lighting fires after the first wet season rains) in their maintenance. Trends in disturbance features, fuel characteristics, ground layer composition, and woody plants dynamics under combinations of withholding fire and storm‐burning over a 3‐year period were measured on 19 plots in three landscape settings. Population dynamics of M. viridiflora are described in detail and 20‐year population projections based on transition matrices under different fire regimes generated. Numerous M. viridiflora suckers occurred within the grass layer, increasing each year regardless of fire regime, and were rapidly recruited to the canopy in the absence of fire. Storm‐burning had little impact on fuel, ground layer or woody plant composition, but maintained open vegetation structure by substantially reducing recruitment of M. viridiflora suckers to the sapling layer, and by reducing the above‐grass‐layer abundance of several other invasive woody species. Population projections indicated that withholding fire for 20 years could cause a sevenfold increase of M. viridiflora density on Ti‐tree flats, and that annual to triennial storm‐burning should be effective at maintaining a stable open vegetation structure. These findings argue against vegetation thickening being an inevitable consequence of climate change. We conclude that a fire regime that includes regular storm‐burning can be effective for maintaining grasslands and grassy woodlands being invaded by M. viridiflora.     

Upward expansion of fire‐adapted grasses along a warming tropical elevation gradient

High mountain regions in the tropics have thus far been impacted relatively little by anthropogenic activity or plant invasions, however, they are unlikely to be immune to impacts of global change, including climate change and other anthropogenic disturbances. Changes in fire regimes are known to accelerate the spread of invasive C₄ grasses and interactions between changes in fire and climate can alter species distributions. The aim of this study was to compare grass distributions along an elevational gradient in Hawai‘i between 1966–1967 and 2008 to determine whether C₄ and C₃ grass distributions are shifting upward in response to alterations in fire and climate patterns. Field plots at Hawai‘i Volcanoes National Park were surveyed for grass species and cover at ?150 m elevation intervals and compared to previous surveys done in 1966–1967. We found that the transition elevation, marking a shift in dominance between C₄ and C₃ grasses based on relative cover, shifted upward over 40 yr (95% confidence interval = 1476 m ± 130 m in 2008 versus 1200 m ± 106 m in 1966–1967). On the other hand, maximum elevations of all C₄ or C₃ grasses as a group were not significantly greater than 1966–1967 elevations; however, a subset of C₄ (and fewer C₃) grasses moved to substantially higher elevations, and these were the species adapted to fire. 100% of fire‐adapted grasses moved up in elevation compared to 29% of non‐fire adapted species, and the change in elevation of those species (=+454 m) was significantly greater than the change in elevation of non‐fire adapted species (p = 0.003). Our study documents an upward expansion of fire‐adapted grasses at high elevations in the tropics as an important threat that seems to be compounded by warming trends.     

Aboveground Biomass Removal by Burning and Raking Increases Diversity in a Reconstructed Prairie

Prescribed spring burning often contributes to a predominance of C₄ grasses and low forb abundance and is impractical at many sites, especially near development. We tested raking after mowing as an alternative to prescribed burning in a reconstructed Minnesota prairie. We also tested mowing without raking as a possible means of maintaining prairie communities. Frequency, flowering stem abundance, and cover were measured for all plant species and native functional groups (C₄ grasses, C₃ graminoids, forbs, legumes, and annual or biennial forbs). Mowing alone did not differ from the control in its effect on any functional groups of plants. Round‐headed bush clover (Lespedeza capitata), a legume, and Black‐eyed Susan (Rudbeckia hirta), a biennial, increased in frequency with treatments that removed biomass (i.e., fire or raking), but they did not have significantly more flowering stems. Thus, new plants established well from seed, whereas the vitality of mature plants did not change. Raking had similar effects to burning on most functional groups, although flowering stems of C₄ grasses were significantly more abundant after fire than after raking. Burning reduced some C₃ forbs and grasses and favored the dominance of C₄ grasses. Therefore, raking after mowing in the spring provides an alternative to prescribed burning that has many of the same positive aspects as fire but does not promote aggressive C₄ grasses to the same extent.     

Will global change improve grazing quality of grasslands? A call for a deeper understanding of the effects of shifts from C4 to C3 grasses for large herbivores

The current dramatic increase in atmospheric CO₂ concentration favours C₃ versus C₄ photosynthesis, and although other aspects of environmental conditions come into play, it implies an uncertain future for C₄ grasses. If it has been suggested that the poor quality of C₄ grasses contributed to large mammalian herbivores declines as C₄ grasslands spread from the late Miocene, these investigations of the past have not been matched by a similar attention focused on the future implications of C₄ to C₃ shifts. Here we discuss how these may affect grazing systems, also considering other aspects of C₃/C₄ differences (productivity, phenology) which might affect herbivore performance. Current knowledge suggests that important changes in herbivore performance could be observed, but is too fragmentary to allow general quantitative conclusions. We urge plant and herbivore ecologists to collectively address these limitations, as the future of grazing systems has important implications for biodiversity and human livelihoods.     

Post‐fire resprouting responses of native and exotic grasses from Cumberland Plain Woodland (Sydney,Australia) under elevated carbon dioxide

This study investigated the effect of elevated CO₂ on the post‐fire resprouting response of a grassland system of perennial grass species of Cumberland Plain Woodland. Plants were grown in mixtures in natural soil in mesocosms, each containing three exotic grasses (Nassella neesiana, Chloris gayana, Eragrostis curvula) and three native grasses (Themeda australis, Microlaena stipoides, Chloris ventricosa) under elevated (700 ppm) and ambient (385 ppm) CO₂ conditions. Resprouting response after fire at the community‐ and species‐level was assessed. There was no difference in community‐level biomass between CO₂ treatments; however, exotic species made up a larger proportion of the community biomass under all treatments. There were species‐level responses to elevated CO₂ but no significant interactions found between CO₂ and burning or plant status. Two exotic grasses (N. neesiana and E. curvula, a C₃ and a C₄ species respectively), and one native grass (M. stipoides, a C₃ species) significantly increased in biomass, and a native C₄ grass (C. ventricosa) significantly decreased in biomass under elevated CO₂. These results suggest that although overall productivity of this community may not change with increases in CO₂ and fire frequency, the community composition may alter due to differential species responses.     

Rise of the grassland biome,central North America

Fossil floras and mammalian faunas from the Great Plains indicate that as aridity increased during the Miocene and Pliocene, forests and woodlands were confined gradually to moister valleys as grassland spread on the interfluves which were covered earlier with park-like openings. The initial rise of extensive grasslands probably commenced in the Miocene-Pliocene transition (7-5 m.y. ago), the driest part of the Tertiary, which restricted forests and woodlands and encouraged the explosive evolution of grasses and forbs. Following the fluctuation of Pleistocene climatevegetation zones, warm, dry Altithermal climate restricted wooded tracts at the expense of spreading grasslands. The rise of the grassland biome was thus due to occasional periods of increased aridity that restricted forests and woodlands and favored grasses and forbs; to increasing drought west of the 100th meridian which created a flammable source (dry grass); to natural and man-made fires on the relatively flat plains over which fire could spread uninterruptedly; to fire that destroyed relict trees and groves on the flat grasslands, restricting them to rocky ridges removed from fire; and probably also to large browsing mammals (many now extinct) that may have destroyed scattered trees and shrubs on the interfluves during the Altithermal. Youthfulness of the grassland biome agrees with a) the occurrence of most of its species in the bordering forests and woodlands, b) the presence of few endemic plants in it, a relation shown also by insects and birds, c) the relict occurrence of diverse trees over the region, and d) the invasion of grassland by woody vegetation.     

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.     

C3 grasses have higher nutritional quality than C4 grasses under ambient and elevated atmospheric CO2

Grasses with the C₃ photosynthetic pathway are commonly considered to be more nutritious host plants than C₄ grasses, but the nutritional quality of C₃ grasses is also more greatly impacted by elevated atmospheric CO₂ than is that of C₄ grasses; C₃ grasses produce greater amounts of nonstructural carbohydrates and have greater declines in their nitrogen content than do C₄ grasses under elevated CO₂. Will C₃ grasses remain nutritionally superior to C₄ grasses under elevated CO₂ levels? We addressed this question by determining whether levels of protein in C₃ grasses decline to similar levels as in C₄ grasses, and whether total carbohydrate : protein ratios become similar in C₃ and C₄ grasses under elevated CO₂. In addition, we tested the hypothesis that, among the nonstructural carbohydrates in C₃ grasses, levels of fructan respond most strongly to elevated CO₂. Five C₃ and five C₄ grass species were grown from seed in outdoor open‐top chambers at ambient (370 ppm) or elevated (740 ppm) CO₂ for 2 months. As expected, a significant increase in sugars, starch and fructan in the C₃ grasses under elevated CO₂ was associated with a significant reduction in their protein levels, while protein levels in most C₄ grasses were little affected by elevated CO₂. However, this differential response of the two types of grasses was insufficient to reduce protein in C₃ grasses to the levels in C₄ grasses. Although levels of fructan in the C₃ grasses tripled under elevated CO₂, the amounts produced remained relatively low, both in absolute terms and as a fraction of the total nonstructural carbohydrates in the C₃ grasses. We conclude that C₃ grasses will generally remain more nutritious than C₄ grasses at elevated CO₂ concentrations, having higher levels of protein, nonstructural carbohydrates, and water, but lower levels of fiber and toughness, and lower total carbohydrate : protein ratios than C₄ grasses.     

Environmental and Evolutionary Preconditionsfor the Origin and Diversification of the C4 PhotosyntheticSyndrome

Abstract: C₄ photosynthesis is an evolutionary solution to high rates of photorespiration and low kinetic efficiency of Rubisco in CO₂‐depleted atmospheres of recent geologic time. About 7500 plant species are C₄, in contrast to 30 000 CAM and 250 000 C₃ species. All C₄ plants occur in approximately 90 genera from 18 angiosperm families. In all of these families, the C₄ pathway evolved independently. In many, multiple independent origins have occurred, such that over 30 distinct evolutionary origins of the C₄ pathway are recognized. Fossil and carbon isotope evidence show that the C₄ syndrome is at least 12 to 15 million years old, although estimates based on molecular sequence comparisons indicate it is over 20 million years old. The evolutionary radiation of herbaceous angiosperms may have been required for C₄ plant evolution. All C₄ species occur in advanced angiosperm families that appeared in the fossil record in the past 70 million years. Most of these families diversified in terms of genera and species numbers between 20 to 40 million years ago, during a period of global cooling, atmospheric CO₂ reduction and aridification. During the period of diversification, numerous traits arose in the C₃ flora that enhanced their performance in arid environments and atmospheres of reduced CO₂. Some of these traits may have predisposed certain taxa to develop the C₄ pathway once atmospheric CO₂ levels declined to a point where the ability to concentrate CO₂ had a selective advantage. Leading traits in C₃ plants that may have facilitated the initial transition to C₄ photosynthesis include close vein spacing and an enlargement of the bundle sheath cell layer to form a Kranz‐like anatomy. Ecological factors not directly connected with photosynthesis probably also played a role. For example, extensive ecological disturbance may have been needed to convert C₃‐dominated woodlands into open, high‐light habitats where herbaceous C₄ plants could succeed. Disturbances in the form of fire, and browsing by large mammals, increase during the time of C₄ plant evolution and diversification. Fire increased because of the drying climate, while browsing increased with the evolutionary diversification of the mammalian megafauna in the Oligocene and Miocene epochs. In summary, the origin of C₄ plants is hypothesized to have resulted from a novel combination of environmental and phylogenetic developments that, for the first time, established the preconditions required for C₄ plant evolution.     

The antiquity of Madagascar’s grasslands and the rise of C4 grassy biomes

Aim Grasslands and savannas, which make up > 75% of Madagascar’s land area, have long been viewed as anthropogenically derived after people settled on the island c. 2 ka. We investigated this hypothesis and an alternative – that the grasslands are an insular example of the post‐Miocene spread of C₄ grassy biomes world‐wide. Location Madagascar, southern Africa, East Africa. Methods We compared the number of C₄ grass genera in Madagascar with that in southern and south‐central African floras. If the grasslands are recent we would expect to find fewer species and genera in Madagascar relative to Africa and for these species and genera to have very wide distribution ranges in Madagascar. Secondly, we searched Madagascan floras for the presence of endemic plant species or genera restricted to grasslands. We also searched for evidence of a grassland specialist fauna with species endemic to Madagascar. Plant and animal species endemic to C₄ grassy biomes would not be expected if these are of recent origin. Results Madagascar has c. 88 C₄ grass genera, including six endemic genera. Excluding African genera with only one or two species, Madagascar has 86.6% of southern Africa’s and 89.4% of south‐central Africa’s grass genera. C₄ grass species make up c. 4% of the flora of both Madagascar and southern Africa and species : genus ratios are similar (4.3 and 5.1, respectively). Turnover of grasses along geographical gradients follows similar patterns to those in South Africa, with Andropogoneae dominating in mesic biomes and Chlorideae in semi‐arid grassy biomes. At least 16 monocot genera have grassland members, many of which are endemic to Madagascar. Woody species in frequently burnt savannas include both Madagascan endemics and African species. A different woody flora, mostly endemic, occurs in less frequently burnt grasslands in the central highlands, filling a similar successional niche to montane C₄ grasslands in Africa. Diverse vertebrate and invertebrate lineages have grassland specialists, including many endemic to Madagascar (e.g. termites, ants, lizards, snakes, birds and mammals). Grassland use of the extinct fauna is poorly known but carbon isotope analysis indicates that a hippo, two giant tortoises and one extinct lemur ate C₄ or CAM (crassulacean acid metabolism) plants. Main conclusions The diversity of C₄ grass lineages in Madagascar relative to that in Africa, and the presence of plant and animal species endemic to Madagascan grassy biomes, does not fit the view that these grasslands are anthropogenically derived. We suggest that grasslands invaded Madagascar after the late Miocene, part of the world‐wide expansion of C₄ grassy biomes. Madagascar provides an interesting test case for biogeographical analysis of how these novel biomes assembled, and the sources of the flora and fauna that now occupy them. A necessary part of such an analysis would be to establish the pre‐settlement extent of the C₄ grassy biomes. Carbon isotope analysis of soil organic matter would be a feasible method for doing this.