Neotropical vegetation response to rapid climate changes during the last glacial period: Palynological evidence from the Cariaco Basin

We present new palynological information from the anoxic Cariaco Basin, off Venezuela, that provides insight into the response of northernmost South American vegetation to rapid climate changes between 68 and 28 ka, specifically during North Atlantic Heinrich events (HEs) and Dansgaard/Oeschger cycles. We defined three different vegetation modes: (1) an interstadial mode characterized by the highest pollen concentration and the maximum extension of semi-deciduous and evergreen forests; (2) a stadial mode characterized by increases of salt marshes, herbs, and montane forests; and (3) a Heinrich event mode characterized by the lowest pollen concentrations, abrupt increases of salt marshes, and decreased forest abundance. Similarly, indices of C4/C3 plants show increases during stadials with clear peaks during the onset of HEs, though grasslands did not become dominant during these periods. We alternatively propose that these expansions of C4 plants are associated with the expansion of coastal salt marshes. Our vegetation record suggests the prevalence of humid conditions during interstadials, dry and cold conditions during stadials, and dry and cold conditions together with changes in sea level during HEs. This new palynological evidence supports previous interpretations that main environmental changes in northernmost South America were driven by latitudinal displacements of the Intertropical Convergence Zone and sea-level changes.     

The stratigraphy and fire history of the Kutai Peatlands, Kalimantan, Indonesia

The equatorial peatlands of the Kutai lowland of eastern Kalimantan are generally 4–10 m in thickness but some sections exceed 16 m in depth. The deposition of peat commenced about 8000 yrs ago after shallow flooding of the basin by the Mahakam River. The earliest vegetation is a Pandanus swamp which grades upwards to swamp forest dominated by dipterocarps. The peatland has expanded laterally and rivers have maintained narrow levee-channel tracks through the swamp, which has grown vertically in balance with river accretion. Historical fires are associated with extreme El Niño years of drought, but human agency is important. The fires of 1982–1983 and 1997–1998 burnt up to 85% of the vegetation on the peatland. Although charcoal analyses show that fire has occurred throughout the history of the peatland, it is rare in forests remote from rivers until the last 3000 years and only common within the last millennium. Fires are earlier and more frequent in sites accessible from waterways, and floodplains have been widely burnt down to water table or below, forming extensive lakes.     

Fire and Vegetation History from the Coastal Rain Forest of the Western Oregon Coast Range

High-resolution charcoal and pollen analyses were used to reconstruct a 4600-yr-long history of fire and vegetation near Taylor Lake in the wettest forests of coastal Oregon. Today, fires in these forests are rare because the season of ignition does not coincide with months of dry fuels. From ca. 4600 to 2700 cal yr B.P. fire episodes occurred at intervals of 140±30 yr while forest vegetation was dominated by disturbance-adapted taxa such as Alnus rubra. From ca. 2700 cal yr B.P. to the present, fire episodes have become less common, occurring at intervals of 240±30 yr, and fire-sensitive forest taxa, such as Tsuga heterophylla and Picea sitchensis, have become more prominent. Fire occurrence during the mid-Holocene was similar to that of the more xeric forests in the eastern Coast Range and suggests that summer drought was widespread. After ca. 2700 cal yr B.P., a decrease in fire episode frequency suggests that cooler conditions and possibly increased summer fog allowed the establishment of present-day Picea sitchensis forests within the watershed. These results provide evidence that fire has been an important disturbance agent in the Coast Range of Oregon, and variations in fire frequency and climate have led to the establishment of present-day forests.     

Climate variability over the last 9900 cal yr BP from a swamp forest pollen record along the semiarid coast of Chile

We present a fossil pollen analysis from a swamp forest in the semiarid coast of Chile (32°05′S; 71°30′W), at the northern influence zone of southern westerly wind belt. A ∼10,000 cal yr BP (calendar years before 1950) palynological sequence indicates a humid phase characterized by dense swamp forest taxa dated between ∼9900 and 8700 cal yr BP. The presence of pollen-starved sediments with only scant evidence for semiarid vegetation indicates that extreme aridity ensued until ∼5700 cal yr BP. The swamp forest recovered slowly afterwards, helped by a significant increase in moisture at ∼4200 cal yr BP. A new swamp forest contraction suggests that another slightly less intense drought occurred between ∼3000 and 2200 cal yr BP. The swamp forest expansion begins again at ∼2200 cal yr BP, punctuated by a highly variable climate. Comparisons between the record presented here with other records across the region imply major variations in the extent of the southern westerlies during the Holocene. This variability could have been caused either by latitudinal displacements from the present mean position of southern westerlies wind belt or by changes in the intensity of the South Pacific Subtropical Anticyclone, both of which affect winter precipitation in the region.     

Mid- to late Holocene environmental and climatic changes in New Caledonia, southwest tropical Pacific, inferred from the littoral plain Gouaro-Déva

Multiproxy analysis of three littoral cores from western New Caledonia supports the hypothesis that the main controlling factors of environmental changes are sea-level change, ENSO variability and extra-tropical phenomena, such as the Medieval Warm Period (MWP) marked by a tendency for La Niña-like conditions in the tropical Pacific. The record starts during the late Holocene sea-level rise when the terrestrial vegetation indicated wet and cool conditions. The site was a coastal bay definitely transformed into a freshwater swamp at around 3400 cal yr BP, after the rapid drawdown of sea level to its current level. Sediments and foraminiferal assemblages indicated subsequent episodes of freshwater infillings, emersion or very high-energy conditions, likely related to climatic changes and mostly controlled by ENSO variability. Between 2750 and 2000 cal yr BP, relatively dry and cool climate prevailed, while wetter conditions predominated between ca. 1800 and 900 cal yr BP. The Rhizophoraceae peak between ca. 1080 and 750 cal yr BP, coeval with the MWP, may indicate a global phenomenon. Microcharcoal particles present throughout the record increased after 1500 cal yr BP, suggesting an anthropogenic source. From ca. 750 cal yr BP the appearance of current type of vegetation marks the human impact.     

Late Oligocene–Early Miocene palaeoecology based on pollen data from the Kalkım-Gönen Basin (Northwest Turkey)

Radiometric and palynological data of the Upper Oligocene–Lower Miocene Soma Formation from the Kalk?m-Gönen Basin yield new results related to age and palynological contents. In this study, Upper Oligocene strata from the Dani?ment and Linfa areas and Lower Miocene strata from the Bengiler area were sampled palynologically and for radiometric dating. The Dani?ment assemblage, which is older than the Linfa assemblages, mainly contains coniferous and evergreen to deciduous mixed mesophytic forest elements. Relatively high quantities of the altitudinal plants Picea and Abies, indicate a cooler palaeoclimate. The Linfa associations mainly include coniferous and riparian elements. Pollen of the riparian plant Alnus and Taxodiaceae indicative for the swamp forest community was predominant, probably as a result of a high lake level. There is a hiatus during the Oligocene–Miocene transition, probably showing a non-depositional phase and sea-level fall indicating the Mi-1 glaciation event. Higher in the sequence, the Aquitanian Bengiler sediments include high amounts of coniferous forest elements as well as components indicative for the evergreen and deciduous mesophytic forest and also riparian forest and swamp forest. Due to presence of thermophilous taxa Reveesia, Mastixiaceae and Arecaceae, a warm and humid palaeoclimate is inferred according to quantitative analyses using the Coexistence Approach.     

A Holocene pollen and diatom record from Vanderlin Island, Gulf of Carpentaria, lowland tropical Australia

Sedimentary, palynological and diatom data from a dunefield lake deposit in the interior of Vanderlin Island in the Gulf of Carpentaria are presented. Prior to the formation of present perennial lake conditions, the intensified Australian monsoon associated with the early Holocene marine transgression allowed Cyperaceae sedges to colonise the alluvial margins of an expansive salt flat surrounded by an open Eucalyptus woodland. As sea level stabilised between 7500 and 4500 cal yr B.P. coastal dunes ceased to develop allowing dense Melaleuca forest to establish in a Restionaceae swamp. Dune-sand input into the swamp was diminished further as the increasingly dense vegetation prevented fluvial and aeolian transported sand arriving from coastal sources. This same process impounded the drainage basin allowing a perennial lake to form between 5500 and 4000 cal yr B.P. Myriophyllum and other aquatic taxa colonised the lake periphery under the most extensive woodland recorded for the Holocene. The palynological data support an effective precipitation model proposed for northern Australia that suggests more variable conditions in the late Holocene. A more precise measure of effective precipitation change is provided by diatom-based inferences that indicate few changes in lake hydrology. Such interpretations are explained in terms of palynological sensitivity to adjustments in local fire regimes where regional precipitation change may only be recorded indirectly through fire promoting mechanisms, including intensified ENSO periodicity and human impact.     

Expansion of Magellanic Moorland during the late Pleistocene: Palynological evidence from northern Isla de Chiloé, Chile

The late Quaternary vegetation of northern Isla de Chiloé is inferred from palynological analysis of a section in the Río Negro drainage (42°03′S, 73°50′W). At ca. 30,500 yr B.P., maxima of Astelia and Donatia occurred, suggesting wetland development. From that time until ca. 27,000 yr B.P., steppe indicators such as Compositae/Gramineae dominated, suggesting drier conditions. After 27,000 yr B.P., the moorland shrub Dacrydium gradually increased, reaching a maximum by 18,000 yr B.P. At this time Astelia increased again, suggesting development of cushion bog during cold and wet conditions. The glacial-postglacial transition is characterized by a marked change from peaty sediments to clays, a decrease in the cushion bog flora, and the prevalence of Gramineae/ Compositae and swamp taxa. This vegetation prevailed until ca.7000 yr B.P. when forest taxa became dominant. The floristic pattern inferred from the pollen spectra of the Rio Negro section suggests that the late Pleistocene vegetation of Chiloé resembled modern Magellanic Moorland vegetation (52°–56°lat S). Based on climatic conditions presently associated with Magellanic Moorland, its occurrence in Chiloé at low elevations during the late Pleistocene implies a decrease in average temperature of at least 4°C and an increase in annual precipitation of at least 1500 mm.     

Depositional model of a macrotidal estuary and floodplain, South Alligator River, Northern Australia

The South Alligator River, Northern Territory of Australia, has a macrotidal estuary. Tidal influence (spring tidal range 5–6 m at the mouth) extends 105 km up the channel. It is dominated by freshwater in the wet season (December-April) with a salt wedge near the mouth, but is well mixed and becomes saline throughout the dry season. The tidal channel can be divided into four different channel types: an estuarine funnel, a sinuous meandering segment, a cuspate meandering segment (in which the inside of bends are pointed) and an upstream tidal channel. The distribution of morphologically defined land classes and morphological units within each land class on the floodplain flanking the estuary differs from one channel type to another. Several stratigraphic and morphostratigraphic units have been recognized from drill holes on the coastal and deltaic-estuarine plains, and a model of development is proposed on the basis of extensive radiocarbon chronology and palynology. The coastal plain has prograded with most rapid sedimentation between 5000 and 3000 yr BP. A similar pattern of progradation is identified in the estuarine funnel. In the sinuous segment of the estuary the channel has migrated laterally across the floodplain. Previous channel positions are indicated by palaeochannels and the meander tract is occupied by laminated channel sediments. Within the cuspate segment there are numerous sinuous palaeochannels on the plains. In the upstream segment, the channel and palaeochannels have long straight reaches with irregular bends and discontinuous levées, and channel avulsion is indicated. Mangrove mud is a widespread stratigraphic unit throughout the plains. The initial phase of development is a transgressive phase. 8000–6800 yr BP, when mangrove forests extended landwards into a pre-existing valley as sea-level rose. As sea-level stabilized, the transgressive phase was followed by a widespread mangrove phase, termed the ‘big swamp’ 6800–5300 yr BP. The mangrove forests disappeared from most of the plains as vertical accretion continued, and were replaced by grass and sedge-covered floodplains. During the sinuous phase, about 5300–2500 yr BP, the channel migrated laterally and eroded the deltaicestuarine plain and deposited lateral accretion deposits (laminated channel sediments). Part of the channel of the South Alligator River has then progressed from sinuous to cuspate in form, and erosion of river banks has occurred. Transgressive and big swamp phases occurred under rising and stabilizing sea-level, respectively. Later morphodynamic channel adjustments occurred under conditions of stable sea-level. The depositional model has direct application to other estuaries in northern Australia, and may be applied to other areas where sea-level change has been similar.     

Towards a chronology of the Late Pleistocene in the northern Alpine Foreland

Dating results from terrestrial records in the northern foreland of the Alps have been compiled in order to establish an independent chronostratigraphy for the climate history of this region. U/Th dates of peat deposited during the final phase of the Last Interglacial indicate that it lasted until at least c. 115 000 yr ago. The Early Würmian started with a period of severe cold climate causing a substitution of forest by tundra-like vegetation. It is assumed that during this period glaciers advanced to the margin of the foreland of at least the Western Alps. Sediments attributed to this glaciation are dated to about 103 000 yr. Three subsequent interstadials, all characterized by coniferous forest, were interrupted by cold stadials with steppe to tundra-like vegetation. The first interstadial is dated to about 95 000 yr. There is evidence for an interstadial with open coniferous woodland and three phases of steppe vegetation during the Middle Würmian, between c. 50000 and 30 000 yr ago. The last glaciation of the Alpine Foreland reached its maximum extension between 24 000 and 21 500 yr and glaciers rapidly collapsed before ˜17 500 yr ago. A series of minor re-advances during the Lateglacial is reported from within the Alps, but the glaciers barely reached the main Alpine valleys during this time. The last of these advances formed the Egesen moraine and occurred at about 11 800 yr ago during the Younger Dryas.     

Middle Weichselian environments on western Yamal Peninsula, Kara Sea based on pollen records

Pollen data from two sections from a coastal cliff on the western Yamal Peninsula (69°43.27′N, 66°48.80′E) document the environmental history during the Karginsky (Middle Weichselian) interstadial. Low pollen concentrations, high amounts of redeposited pollen, and relatively high presence of Artemisia pollen characterize sediments deposited at about 33,000 ¹⁴C yr B.P. Grass-sedge plant associations with few other herbs occupied the area during the late Karginsky interstadial. Artemisia pollen may indicate rather xerophytic vegetation and disturbed soils in the area. The dominance of redeposited pollen reflects scarce (disturbed) vegetation cover and low pollen productivity. The climate was relatively cold and dry. Sediments dated to 32,400 ¹⁴C yr B.P. contain fewer redeposited pollen and concentration of non-redeposited pollen is significantly higher. Pollen contents indicate the dominance of tundra-like grass-sedge vegetation and more humid conditions. Pollen records dated between 30,100 and 25,100 ¹⁴C yr B.P. also reflect scarce tundra-like vegetation during this interval. The presence of Betula nana and Salix pollen may reflect limited presence of shrub communities. This suggests that the climate was somewhat warmer during the latter part of the interstadial. However, generally the pollen records show that harsh environmental conditions prevailed on the Yamal Peninsula during the Karginsky interstadial.     

Late Holocene sea-level changes in the northwest Tuamotu islands, French Polynesia

Field surveys of several sea-level indicators (exposed in situ reef framework, conglomerates, coral colonies and Tridacna shells in a growth position, sea-corrosion notches) carried out on six atolls from the NW Tuamotus (Mataiva, Rangiroa, Arutua, Kaukura, Apataki and Takapoto) and data from four subsurface boreholes drilled through Mataiva show that during the late Holocene mean sea level (MSL) reached a maximum elevation at approximately + 0.9 m. It remained above the present MSL from between 6000 and 5500 yr B.P. until at least 1200 yr B.P. Human settlements on the atolls were extremely unlikely and probably impossible throughout this time. The area investigated seems to have been tectonically quite stable during the late Holocene. A local curve of MSL variations may be representative of the regional eustatic pattern.     

Determination of mangrove forest performance in reducing tsunami run-up using physical models

Coastal ecosystems such as mangroves fringing tropical coastlines have been recognized as natural protectors of the coastal areas against destructive attack of a tsunami. In this paper, the authors aim to investigate the interaction of a tsunami wave on a typical mangrove forest and to determine its performance in reducing the run-up. A laboratory experiment using a hydraulic flume with a mangrove forest model was carried out in which tests were conducted by varying the vegetation widths of 0, 1, 2 and 3?m and average densities of 8, 6 and 4 trees per 100?cm² using a scale ratio of 1:100. Two conditions of water levels were considered in the experiments at several tsunami wave heights between 2.4 and 14?cm. The dam break method used in the experiments produced two types of waves. At low water condition, a bore was developed and subsequently, a solitary wave was produced during high water. The results of the experiments showed that in general, vegetation widths and densities demonstrate a dampening effect on tsunami run-up. A larger vegetation width was found to be more effective in dissipating the wave energy. The first 1?m width of mangrove forest could reduce 23?C32?% during high water and 31?C36?% during low water. Increasing the mangrove forest width to 2 and 3?m could further increase the average percentage of run-up reduction by 39?C50?% during high water and 34?C41?% during low water condition. It was also observed that densities of the mangrove forest do not influence the run-up reduction as significantly as the forest widths. For mangrove forest densities to be significantly enough to reduce more tsunami run-up, an additional density of 4 trees/100?m² needs to be provided. The experiments also showed that mangrove roots are more effective in reducing the run-up compared to the trunks and canopies. The experiments managed to compare and present the usefulness of mangrove forests in dissipating wave energy and results produced are beneficial for initiating design guidelines in determining setback limits or buffer zones for development projects in mangrove areas.     

Geology,palynology, and climatic significance of two pre-Pinedale Lake sediment sequences in and near Yellowstone National Park

Pollen analysis of a section of lake sediments at Grassy Lake Reservoir indicates a vegetational sequence changing from tundra, to spruce-fir-pine forest, to pine forest, to tundra at the top. Pollen analysis of a section of lake sediments on Beaverdam Creek indicates a tundra vegetation at the base, followed by a brief episode of spruce-fir forest and a return to a tundra vegetation at the top. The analyses of both sections suggest a cold to cool to cold climatic sequence, interpreted as interstadial in character. However, differences suggest that they represent separate interstadials. Pinedale Till disconformably overlies the lake deposits at Grassy Lake Reservoir. The upper sediments contain wood ¹⁴C dated at >42,000 yr; the lowermost interfinger with till shown to be more than about 70,000 yr old. The deposits at Beaverdam Creek grade upward into proglacial Pinedale deposits, contain an ash that is probably about 70,000 yr old near their base, and rest comformably on gravel that grades down into lake sediments containing wood debris suggestive of an older climatic amelioration. We conclude that the warmest part of the interstadial at Grassy Lake Reservoir is probably more than 70,000 yr old, and that the warmest part of the interstadial analyzed at Beaverdam Creek is slightly younger than 70,000 yr old.     

High-resolution last deglaciation record from the Congo fan reveals significance of mangrove pollen and biomarkers as indicators of shelf transgression

High abundances of mangrove pollen have been associated with transgressive cycles on tropical margins, but the detailed relations between systems tracts and the taphonomy of the pollen are unclear. We report here the occurrence and high abundance of Rhizophora pollen, in association with taraxerol, a Rhizophora-sourced biomarker, from a high-resolution Congo fan core covering the last deglaciation. An age model based on ¹⁴C dates enables the temporal changes in taraxerol content and the percentage frequencies and flux (pollen grains (pg) cm⁻² (10³ yr)⁻¹) of mangrove pollen to be compared quantitatively with the lateral rate of transgression across the flooding surface (derived from glacio-hydro-isostatic model output and the bathymetry of the margin). Rhizophora pollen concentrations and taraxerol content of the sediment are very strongly positively correlated with the lateral rate of transgression and indicate, independently of any sequence stratigraphic context, that mangrove pollen spikes are associated with the transgressive systems tract rather than the highstand systems tract or maximum flooding surface. Lower-resolution longer-term records from this margin indicate an association between taraxerol concentrations and transgressive rather than regressive phases. The flux of these materials to the Congo fan is interpreted as a function of the erosion of flooded mangrove swamp on the shelf and, less importantly, changing extent of mangrove habitat, during sea-level rise. Congo River palaeoflood events also result in reworking of mangrove pollen and supply to the fan, but this mechanism is subdominant. Rhizophora pollen has been underestimated in many palynological studies undertaken on cores from the African margin because of inappropriate sieve mesh size used during laboratory preparation.     

Holocene vegetation and volcanic activity,Auckland Isthmus,New Zealand

A 12 000 to 4000 yr BP pollen and tephra-bearing profile from Auckland, New Zealand, provides insights into the vegetation history and evidence for early Holocene volcanic activity in this area centred on the Mount Wellington basaltic volcano. Possibly 500 yr separated initial scoriaceous ash deposition (ca. 9500 yr ago) and subsequent major lava flows (ca. 9000 yr ago) from Mount Wellington. The local vegelation, topography, and drainage patterns were substantially modified during this time, and damming by the lava flows resulted in the formation of Lake Waiatarua in a shallow valley head ca. 9000 yr ago. Diatom evidence indicates that this lake was initially deep (> 5 m) but was shallowing around 4000 yr ago. In contrast to the Mount Wellington eruptions, tephra deposition resulting from distant rhyolitic volcanic activity of the central North Island and Mayor Island has had little effect on the Auckland vegetation during this time interval (12 000–4000 yr ago). Between ca. 12 000 and 10 000 yr ago, conifer-angiosperm forest was the predominant vegetation cover on Auckland Isthmus, but during the early Holocene, forest dominated by Metrosideros expanded, probably on to fresh volcanic surfaces resulting from the Mount Wellington eruptions. At this time, swamp forest communities developed in Waiatarua valley basin, and included species indicative of moist, mild, relatively frost-free climates. Some taxa show histories consistent with other records from the northern New Zealand region, including the rise of Ascarina lucida ca. 11 000 to 9000 yr ago, and its subsequent decline, and the expansion of Agathis australis (kauri) forest communities from ca. 6000 yr ago. Taken together the history of local and regional vegetation points to a mild, moist and weakly seasonal early Holocene climate, which subsequently became drier with greater seasonal temperature extremes.     

Re-envisioning the structure of last glacial vegetation in New Zealand using beetle fossils

A series of 18 fossil beetle assemblages are used to reconstruct the paleoenvironment of the northwest West Coast, New Zealand, over the period of the last interstadial-stadial transition (ca. 37,000-21,300 cal yr BP). The samples were recovered from an in-filled hollow within a dune field ca. 9 km south west of Westport (41°47′S, 171°30′E). This fossil beetle reconstruction is compared to an existing palynological reconstruction from the same site. The beetle assemblages indicate an environment of marshy shrubland interspersed with closed canopy coastal vegetation prior to glacial onset, and a mosaic of closed canopy patches and open tussock grassland during full glacial conditions. These interpretations, contrast with the palynologically based interpretation which indicates subalpine shrubland prior to glacial onset and widespread grassland with little woody vegetation during the period of maximum glacial cooling. This study is consistent with other non-pollen studies in New Zealand and indicates that the palynological interpretation of the paleoenvironment of the Westport region downplays the importance of closed canopy vegetation in the area during the transition from interstadial to full glacial (stadial) conditions. It challenges the interpretation of open vegetation at low elevations during glacial periods from pollen studies.     

北京天竺晚上新世以来植被演替与气候变迁

研究了北京天竺TZ01钻孔岩心孢粉样品239块，孢粉组合反映了北京天竺晚上新世以来(约3.580 Ma BP)植被演替和气候变迁大致经历了10个发展阶段：(1) 3.580～3.050 Ma BP：以针叶林植被为主，气候寒冷偏湿；(2) 3.050～2.555 Ma BP：植被为落叶阔叶和常绿阔叶混交林-草原类型，该阶段开始升温，但气候较为温暖湿润；(3)2.555～2.300 Ma BP：植被为温凉偏干的疏林草原，湿度开始下降；(4) 2.300～1770 Ma BP：植被类型仍为疏林草原，该阶段温度上升，但幅度并不大，湿度基本不变，气候温凉干爽；(5) 1.770～1.070 Ma BP：植被类型为针阔混交林-草原，气候温暖湿润；(6) 1.070～0.922 Ma BP：植被演替为较暖湿略干的针阔混交林-草原；(7)0.922～0.800 Ma BP：植被为较寒冷干燥的疏林草原，温度降低，降水减少；(8) 0.800～0.650 Ma BP：植被为温暖偏干的以松为主的疏林草原，温度有所上升，但湿度变化较小；(9)0.650～0.160 Ma BP：植被为温凉湿润的针阔叶混交林-草原；(10) 0.160 Ma BP至今：植被以针阔叶混交林-草原为主，前期气候冷湿，后期较为温暖湿润。     

Coupling near-surface geomorphology with mangrove community diversity at the estuarine scale: A case study at Dongzhaigang Bay,China

Coastal wetlands are key features of the Earth's surface and are characterized by a diverse array of coupled geomorphological and biological processes. However, the links between the distribution of biodiversity (for example, species and structural diversity) and the formation of coastal geomorphology are not well-understood on a landscape scale most useful to coastal zone managers. This study describes the relationship between select geomorphological and biological mangrove community features (i.e. species composition and functional root type) in a landscape-distributed coastal zone of Dongzhaigang Bay, north-eastern Hainan Island, China. A total of 11 mangrove species and five functional aerial root types were encountered, with the location of species by root types being controlled by the elevation of the soil surface. Plank roots, prop roots and pneumatophores occupied the lowest intertidal elevations, while knee roots and fibrous roots of the mangrove fern, Acrostichum aureum, preferred the highest intertidal elevations. Surface sediment deposition in areas with mangroves was greater than deposition in non-mangrove forest zones, establishing an important biological mechanism for this large-area response because surface erosion/compaction was also more prominent within mangrove roots. Indeed, functional root type influenced the magnitude of deposition, erosion and compaction, with knee roots and pneumatophores being more effective in promoting deposition and preventing surface erosion/compaction than prop roots. These results indicate a potential role for vegetation type (especially functional root type) to influence coastal geomorphological processes at large landscape scales. While soil surface elevation is correlated to the distribution of mangrove species and functional root types, a significant feedback exists between elevation change and the capacity of those root types to influence coastal geomorphological differentiation within sustainable intertidal elevations. An enhanced understanding of geomorphological development, mangrove species distribution and functional root type may improve management to support nature-based solutions that adjust more effectively to sea-level rise through feedbacks.     

Seasonality change and the third millennium BP rainforest crisis in southern Cameroon (Central Africa)

The third millennium BP crisis of the central African rainforest is not sufficiently understood. The low resolution of most pollen profiles and a large plateau of the calibration curve aggravate the exact dating of the event, and its causal climatic parameters are debated. We present a high-resolution pollen profile from the swamp site Nyabessan in the southern Cameroonian rainforest, covering the period 3100-2300 cal yr BP. Between 3100 and 2500 cal yr BP, the climate was favourable for a regional evergreen forest with Caesalpiniaceae and Lophira and a local Raphia swamp forest. Around 2500/2400 cal yr BP, a significant decrease of mature forest and swamp forest taxa and an increase of pioneers indicate that the rain forest was seriously disturbed and replaced by secondary formations. The dominance of Trema orientalis, a pioneer well adapted to seasonal desiccation, points to a much more accentuated seasonality after 2500 cal yr BP, which seems to be linked to a southwards shift of the ITCZ during the northern hemisphere winter months. We propose that the rain forest crisis between 2500 and 2200 cal BP created favourable conditions for farming and paved the way for a major expansion of Bantu speaking populations.