Timing of C4 grass expansion across sub-Saharan Africa

The emergence of C(4) grass biomes is believed to have first taken place in the upper Miocene, when a series of events modified global climate with long-lasting impacts on continental biotas. Changes included major shifts in floral composition-characterized in Africa by shrinking of forests and emergence of C(4) grasses and more open landscapes-followed by large-scale evolutionary shifts in faunal communities. The timing of the emergence of C(4) grasses, and the subsequent global expansion of C(4) grass-dominated biomes, however, is disputed, leading to contrasting views of the patterns of environmental changes and their links to faunal shifts, including those of early hominins. Here we evaluate the existing isotopic evidence available for central, eastern, and southern Africa, and review interpretations in light of these data. Pedogenic and biomineral carbonate delta(13)C data suggest that clear evidence for C(4) biomass in low latitudes exists only from 7-8 Ma. This likely postdates the emergence of C(4) plants, whose physiology is adapted to low atmospheric carbon dioxide concentrations. Biomes with C(4) grasses appeared later in mid-latitude sites. Moreover, C(4) grasses apparently remained a relatively minor component of most environments until the late Pliocene and early Pleistocene. Hence establishment of C(4) grasses, even as minor components of African biomes, precedes the very earliest evidence for bipedalism by two million years, and the more abundant and secure evidence by some three to four million years. This may suggest a protracted process of hominin adaptation to these emerging, more open landscapes.     

Fire and the Miocene expansion of C4 grasslands

C₄ photosynthesis had a mid‐Tertiary origin that was tied to declining atmospheric CO₂, but C₄‐dominated grasslands did not appear until late Tertiary. According to the ‘CO₂‐threshold’ model, these C₄ grasslands owe their origin to a further late Miocene decline in CO₂ that gave C₄ grasses a photosynthetic advantage. This model is most appropriate for explaining replacement of C₃ grasslands by C₄ grasslands, however, fossil evidence shows C₄ grasslands replaced woodlands. An additional weakness in the threshold model is that recent estimates do not support a late Miocene drop in pCO₂. We hypothesize that late Miocene climate changes created a fire climate capable of replacing woodlands with C₄ grasslands. Critical elements were seasonality that sustained high biomass production part of year, followed by a dry season that greatly reduced fuel moisture, coupled with a monsoon climate that generated abundant lightning‐igniting fires. As woodlands became more open from burning, the high light conditions favoured C₄ grasses over C₃ grasses, and in a feedback process, the elevated productivity of C₄ grasses increased highly combustible fuel loads that further increased fire activity. This hypothesis is supported by paleosol data that indicate the late Miocene expansion of C₄ grasslands was the result of grassland expansion into more mesic environments and by charcoal sediment profiles that parallel the late Miocene expansion of C₄ grasslands. Many contemporary C₄ grasslands are fire dependent and are invaded by woodlands upon cessation of burning. Thus, we maintain that the factors driving the late Miocene expansion of C₄ were the same as those responsible for maintenance of C₄ grasslands today.     

The global distribution of ecosystems in a world without fire

This paper is the first global study of the extent to which fire determines global vegetation patterns by preventing ecosystems from achieving the potential height, biomass and dominant functional types expected under the ambient climate (climate potential). To determine climate potential, we simulated vegetation without fire using a dynamic global-vegetation model. Model results were tested against fire exclusion studies from different parts of the world. Simulated dominant growth forms and tree cover were compared with satellite-derived land- and tree-cover maps. Simulations were generally consistent with results of fire exclusion studies in southern Africa and elsewhere. Comparison of global 'fire off' simulations with landcover and treecover maps show that vast areas of humid C(4) grasslands and savannas, especially in South America and Africa, have the climate potential to form forests. These are the most frequently burnt ecosystems in the world. Without fire, closed forests would double from 27% to 56% of vegetated grid cells, mostly at the expense of C(4) plants but also of C(3) shrubs and grasses in cooler climates. C(4) grasses began spreading 6-8 Ma, long before human influence on fire regimes. Our results suggest that fire was a major factor in their spread into forested regions, splitting biotas into fire tolerant and intolerant taxa.     

Orbital forcing and the spread of C4 grasses in the late Neogene: stable isotope evidence from South African speleothems

Reconstructing Plio-Pleistocene African paleoenvironments is important for models of early hominin evolution, but is often hampered by low-resolution or discontinuous climatic data. Here, we present high-resolution stable oxygen and carbon isotope time series data from two flowstones (secondary cave deposits) from the South African hominin-bearing Makapansgat Valley. The age of the older of the two flowstones (Collapsed Cone) is constrained by magnetostratigraphy to approximately 4-5 Ma; the younger flowstone (Buffalo Cave) grew between 2.0-1.5 Ma, as determined by magnetostratigraphy and orbital tuning of the isotopic data. The carbon isotope data is used as a proxy for the proportion of C(4) grasses in the local environment and the oxygen isotope data reflects monsoon rainfall intensity. The carbon isotope evidence indicates that in the late Miocene/early Pliocene, the local environment was dominated by C(3) vegetation, whereas, in the Plio-Pleistocene, it was composed of a mixture of C(3) and C(4) vegetation. This suggests that C(4) grasses became a significant part of the Makapansgat Valley ecosystem at approximately 4-5 Ma, towards the end of the late Neogene global expansion of C(4) grasses. After this initial expansion, South Africa experienced further fluctuations in the proportion of C(3) and C(4) vegetation during the Plio-Pleistocene, in response to regional and global climatic changes. Most notably, the Buffalo Cave flowstone provides evidence for C(4) grass expansion at ca. 1.7 Ma that we suggest was a response to African aridity caused by the onset of the Walker Circulation in the Pacific Ocean at this time.     

Quaternary Ice-Age dynamics in the Colombian Andes: developing an understanding of our legacy

Pollen records from lacustrine sediments of deep basins in the Colombian Andes provide records of vegetation history, the development of the floristic composition of biomes, and climate variation with increasing temporal resolution. Local differences in the altitudinal distribution of present-day vegetation belts in four Colombian Cordilleras are presented. Operating mechanisms during Quaternary Ice-Age cycles that stimulated speciation are discussed by considering endemism in the asteraceous genera Espeletia, Espeletiopsis and Coespeletia. The floristically diverse lower montane forest belt (1000-2300 m) was compressed by ca. 55% during the last glacial maximum (LGM) (20 ka), and occupied the slopes between 800 m and 1400 m during that period. Under low LGM atmospheric pCO2 values, C4-dominated vegetation, now occurring below 2200 m, expanded up to ca. 3500 m. Present-day C3-dominated paramo vegetation is therefore not an analogue for past C4-dominated vegetation (with abundant Sporobolus lasiophyllus). Quercus immigrated into Colombia 478 ka and formed an extensive zonal forest from 330 ka when former Podocarpus-dominated forest was replaced by zonal forest with Quercus and Weinmannia. During the last glacial cycle the ecological tolerance of Quercus may have increased. In the ecotone forests Quercus was rapidly and massively replaced by Polylepis between 45 and 30 ka illustrating complex forest dynamics in the tropical Andes.     

The spread of grass-dominated habitats in Turkey and surrounding areas during the Cenozoic: Phytolith evidence

The arrival of hipparionine horses in the eastern Mediterranean region around 11 Ma was traditionally thought to mark the simultaneous westward expansion of savanna vegetation across Eurasia. However, recent paleoecological reconstructions based on tooth wear, carbon isotopes, and functional morphology indicate that grasses played a minor role in Late Miocene ecosystems of the eastern Mediterranean, which were more likely dry woodlands or forests. The scarcity of grass macrofossils and pollen in Miocene floras of Europe and Asia Minor has been used to support this interpretation. Based on the combined evidence, it has therefore been suggested that Late Miocene ungulate faunal change in the eastern Mediterranean signals increased aridity and landscape openness, but not necessarily the development of grass-dominated habitats.

To shed new light on the Miocene evolution of eastern Mediterranean ecosystems, we used phytolith assemblages preserved in direct association with faunas as a proxy for paleovegetation structure (grassland vs. forest). We extracted phytoliths and other biogenic silica from sediment samples from well-known Early to Late Miocene ( 20–7 Ma) faunal localities in Greece, Turkey, and Iran. In addition, a Middle Eocene sample from Turkey yielded phytoliths and served as a baseline comparison for vegetation inference.

Phytolith analysis showed that the Middle Eocene assemblage consists of abundant grass phytoliths (grass silica short cells) interpreted as deriving from bambusoid grasses, as well as diverse forest indicator phytoliths from dicotyledonous angiosperms and palms, pointing to the presence of a woodland or forest with abundant bamboos. In contrast, the Miocene assemblages are dominated by diverse silica short cells typical of pooid open-habitat grasses. Forest indicator phytoliths are also present, but are rare in the Late Miocene (9–7 Ma) assemblages. Our analysis of the Miocene grass community composition is consistent with evidence from stable carbon isotopes from paleosols and ungulate tooth enamel, showing that C₄ grasses were rare in the Mediterranean throughout the Miocene. These data indicate that relatively open habitats had become common in Turkey and surrounding areas by at least the Early Miocene ( 20 Ma), > 7 million years before hipparionine horses reached Europe and arid conditions ensued, as judged by faunal data.     

Explaining the end of the hominoid experiment in Europe

The Vallesian Crisis involved the extinction of most of the hominoids that settled successfully in Europe during the middle and early Late Miocene, including Dryopithecus, Ankarapithecus and Graecopithecus. This event has been dated at 9.6 Ma, predating by more than one million years the spread of the C4 grasses and the retreat of forests over large parts of the globe at 7-8 Ma. The finding of macrofloral remains in the Terrassa section (Vallès-Penedès Basin) sheds new light on the nature of vegetational change associated with the hominoid extinction. This section presents an abundant late Vallesian vertebrate fauna and has been accurately dated at 9.2 - 9 Ma by paleomagnetism. Therefore, it provides the best indication of the kind of vegetation that occupied the area after the Vallesian Crisis. It is suggested that the extinction of the late Miocene Western European hominoids was not related to the spread of grasses, but to a significant increase of a floral association dominated by deciduous trees.     

HISTORY OF ARIDLAND VEGETATION AND CLIMATE: A GLOBAL PERSPECTIVE

1. The origin and history of aridlands and their vegetational cover is closely related to geological history, especially in relation to plate tectonics, mountain building, land-and sea-level changes and ice ages, and the arrival of modern humans and their subsequent development. 2. A close relationship exists between world temperatures and precipitation. Therefore, the development of present-day zonal, aridland vegetation and climate cannot be divorced from the history of the latest ice age. 3. The combined geological and palaeobotanical evidence demonstrate that whilst the origin of modern openland and aridland vegetation went back to the time of the origin of angiosperms during the Cretaceous, its subsequent expansion went hand in hand with the lowering of world temperatures during the Palaeogene and Neogene, when a series of angiospermous families having dominantly herbaceous and openland taxa, as members, appeared successively in the stratigraphic record. 4. The successive lowering of world temperatures had the overall effect of reducing precipitation levels all over the globe. Consequently, high rainfall areas, bearing closed forests, became progressively smaller and smaller and the decreased rainfall promoted the evolution and expansion of low biomass, openland and aridland vegetation. 5. The break-up of regional closed forests had started from the middle Miocene but the main expansion of zonal, aridland vegetation, in low and middle latitudes, appears to have taken place, together with the expansion of tundra vegetation, at high latitudes, from the late Pliocene. Glaciations of a magnitude of at least two-thirds that of the late Pleistocene glacial maxima started to occur about that time, 25 Ma ago. 6. The alternations between cold, glacial and warm, interglacial periods, during the late Neogene, especially with increased amplitude during the last 0.4 Ma, allowed less and less time for forest vegetation to expand and stabilize during the warm intervals, with the result that openland and aridland vegetation was able to expand to unprecedented levels. 7. Further, as man increased fire frequencies during the late Pleistocene, the relatively fire-sensitive and mesophytic taxa were selectively eliminated and more and more forests were opened to invasion by openland taxa in low and middle latitudes. Later on, with the clearance of forests for agriculture, the overall effect on vegetation was to create open landscapes which favoured the expansion of openland taxa at low, middle and high latitudes, during an interglacial, that is, the Holocene, a feature that is unprecedented in the entire earlier geological record.     

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.     

Climate and CO2 controls on global vegetation distribution at the last glacial maximum: analysis based on palaeovegetation data, biome modelling and palaeoclimate simulations

The global vegetation response to climate and atmospheric CO₂ changes between the last glacial maximum and recent times is examined using an equilibrium vegetation model (BIOME4), driven by output from 17 climate simulations from the Palaeoclimate Modelling Intercomparison Project. Features common to all of the simulations include expansion of treeless vegetation in high northern latitudes; southward displacement and fragmentation of boreal and temperate forests; and expansion of drought‐tolerant biomes in the tropics. These features are broadly consistent with pollen‐based reconstructions of vegetation distribution at the last glacial maximum. Glacial vegetation in high latitudes reflects cold and dry conditions due to the low CO₂ concentration and the presence of large continental ice sheets. The extent of drought‐tolerant vegetation in tropical and subtropical latitudes reflects a generally drier low‐latitude climate. Comparisons of the observations with BIOME4 simulations, with and without consideration of the direct physiological effect of CO₂ concentration on C₃ photosynthesis, suggest an important additional role of low CO₂ concentration in restricting the extent of forests, especially in the tropics. Global forest cover was overestimated by all models when climate change alone was used to drive BIOME4, and estimated more accurately when physiological effects of CO₂ concentration were included. This result suggests that both CO₂ effects and climate effects were important in determining glacial‐interglacial changes in vegetation. More realistic simulations of glacial vegetation and climate will need to take into account the feedback effects of these structural and physiological changes on the climate.     

Long-term patterns of shrub expansion in a C4-dominated grassland: fire frequency and the dynamics of shrub cover and abundance

Worldwide, grassland ecosystems have experienced a major shift in growth-form dominance as woody plant species have expanded and replaced native grasses. In the C(4)-dominated grasslands of central North America, a reduction in fire frequency is the most cited cause of this shift in growth forms as fire both enhances grass productivity and constrains the establishment and expansion of native woody vegetation. Using an 18-yr plant species composition data set, we quantified patterns of change in shrub cover, frequency, and species richness associated with three distinct fire regimes. During the study period (1983-2000), shrub cover increased most dramatically in sites in which the frequency of fire was once every 4 yr (intermediate frequency; 28.6%) followed by sites in which fire occurred only once during the 18-yr period (low frequency; 23.7%). Annual fire effectively prevented the recruitment of new woody species, but even with this high fire frequency, shrub cover increased slightly (3.7%). Comparatively, shrub species richness increased by three and six, respectively, in the intermediate- and low-frequency fire sites. These data indicate that within this grassland, periods without fire are necessary for recruitment of both new individuals and additional shrub species; however, once established, shrub cover will increase regardless of fire frequency and even annual fire will not reduce shrub abundance.     

Carbon dioxide starvation,the development of C4 ecosystems,and mammalian evolution.

The decline of atmospheric CO2 over the last 65 million years (Ma) resulted in the ''CO2-starvation'' of terrestrial ecosystems and led to the widespread distribution of C4 plants, which are less sensitive to CO2 levels than are C3 plants. Global expansion of C4 biomass is recorded in the diets of mammals from Asia, Africa, North America, and South America during the interval from about 8 to 5 Ma. This was accompanied by the most significant Cenozoic faunal turnover on each of these continents, indicating that ecological changes at this time were an important factor in mammalian extinction. Further expansion of tropical C4 biomass in Africa also occurred during the last glacial interval confirming the link between atmospheric CO2 levels and C4 biomass response. Changes in fauna and flora at the end of the Miocene, and between the last glacial and interglacial, have previously been attributed to changes in aridity; however, an alternative explanation for a global expansion of C4 biomass is CO2 starvation of C3 plants when atmospheric CO2 levels dropped below a threshold significant to C3 plants. Aridity may also have been a factor in the expansion of C4 ecosystems but one that was secondary to, and perhaps because of, gradually decreasing CO2 concentrations in the atmosphere. Mammalian evolution in the late Neogene, then, may be related to the CO2 starvation of C3 ecosystems.     

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.     

A new global biome reconstruction and data-model comparison for the Middle Pliocene

Aim To produce a robust, comprehensive global biome reconstruction for the Middle Pliocene (c. 3.6–2.6 Ma), which is based on an internally consistent palaeobotanical data set and a state‐of‐the‐art coupled climate–vegetation model. The reconstruction gives a more rigorous picture of climate and environmental change during the Middle Pliocene and provides a new boundary condition for future general circulation model (GCM) studies. Location Global. Methods Compilation of Middle Pliocene vegetation data from 202 marine and terrestrial sites into the comprehensive GIS data base TEVIS (Tertiary Environmental Information System). Translation into an internally consistent classification scheme using 28 biomes. Comparison and synthesis of vegetation reconstruction from palaeodata with the outputs of the mechanistically based BIOME4 model forced by climatology derived from the HadAM3 GCM. Results The model results compare favourably with available palaeodata and highlight the importance of employing vegetation–climate feedbacks and the anomaly method in biome models. Both the vegetation reconstruction from palaeobotanical data and the BIOME4 prediction indicate a general warmer and moister climate for the Middle Pliocene. Evergreen taiga as well as temperate forest and grassland shifted northward, resulting in much reduced tundra vegetation. Warm‐temperate forests (with subtropical taxa) spread in mid and eastern Europe and tropical savannas and woodland expanded in Africa and Australia at the expense of deserts. Discrepancies which occurred between data reconstruction and model simulation can be related to: (1) poor spatial model resolution and data coverage; (2) uncertainties in delimiting biomes using climate parameters; or (3) uncertainties in model physics and/or geological boundary conditions. Main conclusions The new global biome reconstruction combines vegetation reconstruction from palaeobotanical proxies with model simulations. It is an important contribution to the further understanding of climate and vegetation changes during the Middle Pliocene warm interval and will enhance our knowledge about how vegetation may change in the future.     

Early Pleistocene vegetation change in upland south‐eastern Australia

Aim This study aims to improve our understanding of the late Cenozoic history of Australian rain forest and sclerophyll biomes by presenting a detailed pollen record demonstrating the floristic composition and orbital‐scale patterns of change in forest communities of upland south‐eastern Australia, during the Early Pleistocene. The record is examined in order to shed light on the nature of the transition from rain forest‐dominated ‘Tertiary’ Australian vegetation to open‐canopied ‘Quaternary’ vegetation. Location Stony Creek Basin (144.13° E, 37.35° S, 550 m a.s.l), a small, infilled palaeolake in the western uplands of Victoria, Australia. Methods A c. 40‐m‐long sediment core was recovered from the infilled palaeolake. Palynology was used to produce a record of changing vegetation through time. Multivariate analyses provided a basis for interpreting the composition of rain forest and sclerophyll forest communities and for identifying changes in these communities over successive insolation cycles. Results Early Pleistocene upland south‐eastern Australian vegetation was characterized by orbital‐scale, cyclic alternation between rain forest and sclerophyll forests. Individual intervals of forest development underwent patterns of sequential taxon expansion that recurred in successive vegetation cycles. Diverse rain forests included a number of angiosperm and gymnosperm taxa now extinct regionally to globally. Sclerophyll forests were also diverse, and occurred under warm and wet climate conditions. Main conclusions The Stony Creek Basin record demonstrates that as recently as c. 1.5 Ma diverse rain forests persisted in southern Australia beyond the modern continental range of rain forest. The importance of conifers in these rain forests emphasizes that they have no modern Australian analogue. Alternation in dominance between these forests and diverse, sclerophyllous open canopied forests was apparently driven by changes in seasonality, and may have been promoted by fire.     

土地利用约束下中国东部南北样带生产力和植被分布对全球变化的响应

对现有的区域植被动态模拟模型进行了改进,使之包含了土地利用分布格局对植被和生态系统相关过程的影响。改进后的模型被用地研究中国东部南北样带(NSTEC)植被和净第一性生产力对未来气候变化的响应。模拟结果显示土地利用格局对未来气候条件下植被分布的变迁和生产力形成过程有非常显著的影响。与没有土地利用约束的情形相比较,土地利用作为限制条件缓减了植被类型之间的竞争,从而减少了模拟的样带区域内常绿阔叶林,但增加了模拟灌木和草地的分布。土地利用约束使得模拟得到的当前条件下的净第一性生产力更为接近实际情况,且未来气候条件下的生产力改变量更为可信。对未来CO2倍增条件下7个大气环流模型预测的气候情景的模拟结果表明：落叶阔叶林将显著增加,但针叶林、灌木和草原的分布将下降。未来气候条件下NSTEC样带的净第一性生产力总量将增加。预测样带北部的净第一性生产力的变化范围大于样带南部。温度变化比降水变化对样带的生产力具有更强的控制。     

Climate‐driven rampant speciation of the Cape flora

Aim To test whether the radiation of the extremely rich Cape flora is correlated with marine‐driven climate change. Location Middle to Late Miocene in the south‐east Atlantic and the Benguela Upwelling System (BUS) off the west coast of South Africa. Methods We studied the palynology of the thoroughly dated Middle to Late Miocene sediments of Ocean Drilling Program (ODP) Site 1085 retrieved from the Atlantic off the mouth of the Orange River. Both marine upwelling and terrestrial input are recorded at this site, which allows a direct correlation between changes in the terrestrial flora and the marine BUS in the south‐east Atlantic. Results Pollen types from plants of tropical affinity disappeared, and those from the Cape flora gradually increased, between 10 and 6 Ma. Our data corroborate the inferred dating of the diversification in Aizoaceae c. 8 Ma. Main conclusions Inferred vegetation changes for the Late Miocene south‐western African coast are the disappearance of Podocarpus‐dominated Afromontane forests, and a change in the vegetation of the coastal plain from tropical grassland and thicket to semi‐arid succulent vegetation. These changes are indicative of an increased summer drought, and are in step with the development of the southern BUS. They pre‐date the Pliocene uplift of the East African escarpment, suggesting that this did not play a role in stimulating vegetation change. Some Fynbos elements were present throughout the recorded period (from 11 Ma), suggesting that at least some elements of this vegetation were already in place during the onset of the BUS. This is consistent with a marine‐driven climate change in south‐western Africa triggering substantial radiation in the terrestrial flora, especially in the Aizoaceae.     

土地利用约束下中国东部南北样带生产力和植被分布对全球变化的响应

对现有的区域植被动态模拟模型进行了改进,使之包含了土地利用分布格局对植被和生态系统相关过程的影响.改进后的模型被用于研究中国东部南北样带(NSTEC)植被和净第一性生产力对未来气候变化的响应.模拟结果显示土地利用格局对未来气候条件下植被分布的变迁和生产力形成过程有非常显著的影响.与没有土地利用约束的情形相比较,土地利用作为限制条件缓减了植被类型之间的竞争,从而减少了模拟的样带区域内常绿阔叶林,但增加了模拟灌木和草地的分布.土地利用约束使得模拟得到的当前条件下的净第一性生产力更为接近实际情况,且未来气候条件下的生产力改变量更为可信.对未来CO2倍增条件下7个大气环流模型预测的气候情景的模拟结果表明:落叶阔叶林将显著增加,但针叶林、灌木和草原的分布将下降.未来气候条件下NSTEC样带的净第一性生产力总量将增加.预测样带北部的净第一性生产力的变化范围大于样带南部.温度变化比降水变化对样带的生产力具有更强的控制.