Past Vegetation Changes Research Articles

Morphology and ecological significance of Holocene palynomorphs from the Indian Ocean Coastal Belt Biome of South Africa

Palynomorphs are indicators of past vegetation change. Since it is accepted that vegetation reflects the local climate, based on modern analogues, palynological analyses are a good proxy for climates of the past. In view of a demand for information on palynomorph types to support palaeoclimatic studies, we present an atlas of palynomorph taxa from the Indian Ocean Coastal Belt of eastern South Africa and Mozambique. Photomicrographs of the fossils are provided as well as brief descriptions and ecological information obtained from the literature. The palynomorphs in this atlas represent a vast variety of pollen, spores (algae, pteridophytes and fungi) and other palynomorphs such as types that were derived from the marine environment and incorporated into the lagoonal deposits. The holistic analysis of all microfossils found here is intended to assist palaeoecological interpretations of wetland dynamics, fire regimes, climate fluctuations and anthropogenic disturbances in the Indian Ocean Coastal Belt.

Phytolith characterization of culturally salient plant species of the Pacific Northwest of North America

Palaeoecological and archaeological studies often use soil phytoliths to gain insight into past vegetation changes and the long-term use of plants by people, particularly domesticated cultivars. However, to make accurate interpretations and analyses using plant phytoliths, regionally-specific reference collections are warranted. There is ample research documenting phytolith morphotypes observed in grass species worldwide (due to their high phytolith production), and of dicots in tropical regions, but there is dearth of phytolith references for dicot species in temperate regions like the Pacific Northwest of North America. This study aims to fill part of this gap by documenting phytolith morphotypes observed in 60 culturally important plant foods, medicines, and textiles, all predominantly dicot species. Phytoliths are classified by abundance in specific plant parts (e.g., stem, leaf, flower) and the findings are compared to closely related species studied elsewhere. Results indicate that many of the dicot plant species in the Pacific Northwest produce more phytoliths than closely related species worldwide, likely due to differing environmental factors which affect phytolith production in plants. These findings could be useful in palaeoecological contexts to assess vegetation changes or identify dicot-dominated ecosystems, like Indigenous cultivated and managed forests and other perennial fruit tree/shrub dominated landscapes. Additionally, three of the species studied here showed possible diagnostic phytoliths but further confirmation is warranted. Future studies should consider localized environmental patterns affecting phytolith production, and therefore, our understanding of phytolith taxonomy and deposition/taphonomy.

Lateral migration differs between phytolith morphotypes on sand dune surfaces

AbstractArid and semi‐arid lands are exceptionally sensitive to climate change. However, the application of phytolith analysis to these environments is hindered by the potential for lateral migration of phytoliths during wind erosion, which may affect the reliability of phytolith‐based paleoenvironmental reconstructions. Moreover, there is a lack of quantitative studies of the dispersion and deposition of phytoliths by wind erosion. Here we apply Sutton's equation and theoretical models from the field of blown sand physics and engineering to quantify the lateral migration of various phytolith morphotypes in the surface soil of sand dunes in the Horqin Sandy Land in China. Phytolith morphotypes and concentrations were determined in addition to sedimentary organic matter content and grain size. Combined with the analysis of plant communities, these measurements were used to quantify the lateral migration of phytolith morphotypes, and the results were compared with theoretical models. We found that phytolith concentrations decreased exponentially under an annual average wind speed with distance from the surface source; specifically, a large proportion of lateral phytolith migration occurred within the distance of ~3–5 m. There were significant linear correlations between the phytolith concentration and other environmental factors. A comprehensive form of Sutton's equation was used to estimate that a relatively large proportion (8.35%) of short‐cell phytoliths may migrate laterally on dunes that are vulnerable to wind erosion. However, large phytoliths are deposited almost in situ, and relatively limited lateral migration of wind‐transported phytoliths occurs in the Horqin Sandy Land overall. Our results provide a theoretical model and practice template for the application of phytolith analysis to soil and sediments, especially as a proxy of past vegetation and ecological change in the Horqin Sandy Land, and other areas affected by wind erosion. Additionally, short‐cell phytoliths in palaeoenvironmental contexts satisfy the criteria necessary to investigate the extent with frequent aeolian activity.

Expansion of C4 plants in the tropical Leizhou Peninsula during the Last Glacial Maximum: Modulating effect of regional sea-level change

Due to the lack of relatively long-term, high-resolution terrestrial records in tropical southern China, there is limited published research on terrestrial vegetation changes and their responses to regional and/or global climate forcings since the last glacial period. In this study, a 170-cm-long peat core (covering the interval from ~44.1 to 9.3 cal kyr BP) recovered from the Xialu peatland in Leizhou Peninsula, South China, was analyzed for organic carbon isotope (δ13Corg), along with total organic carbon, total nitrogen, and bulk dry density, to investigate past vegetation and hydroclimatic changes. Our results showed that C4 plants dominated the study region during Marine Isotope Stage (MIS) 2 (29–14 cal kyr BP), indicating generally cooler and drier conditions during MIS 2 relative to late MIS 3 (~ 44.1–29 cal kyr BP) and early MIS 1 (14–9.3 cal kyr BP). In particular, the driest conditions occurred during the Last Glacial Maximum (~ 25–19 cal kyr BP) when sea level was at its lowest. In addition, several millennial-scale climatic events associated with the expansion of C4 plants were clearly identified. Our record is sensitive to a variety of glacial-interglacial forcings, including regional processes and global forcing, among which the inundation history of Beibu Gulf due to sea-level change during the late Quaternary, which has been neglected in previous studies, may have played an important role in modulating paleo-hydroclimatic changes in tropical southern China.

Past Fire and Vegetation Change in the Hyperdiverse Forests of the Ecuadorian Amazon.

The Ecuadorian Amazon holds more biodiversity than most other places on Earth. Palms are a particularly dominant component of the vegetation; however, it remains unknown to what degree the pattern has persisted through time. Here, we investigate the persistence of palm dominance through time and the degree to which past human activities (e.g., fire, cultivation, and forest opening) have affected changes in palm abundances across five regions of the Ecuadorian Amazon. We analyzed soil cores (40-80 cm depth) from each region for charcoal (evidence of past fire) and phytoliths (evidence of past vegetation change). The timings of fires (based on 14C radiocarbon dates), the occurrence, recurrence, and number of fires (based on charcoal presence and abundance in samples), and the amount of change in palm abundances (based on phytoliths) varied within and between the studied regions. The charcoal and phytolith results indicate the presence of low levels of past human activity at all sites. Our results show that patterns of modern palm hyperdominance found in Amazonian forests have not been persistent through time, and that even low levels of past human activities can affect palm abundance.

Holocene biome reconstruction on the Chinese loess plateau based on pollen

Reconstruction of past vegetation changes is critical for sustainable development in the fragile and highly eroded ecosystems on the Chinese Loess Plateau (CLP) under global change, particularly in light of future warming events. In this study, we collected fossil pollen data from 40 sites to reconstruct the Holocene (since 11 ka) vegetation pattern of the CLP using the biomization method. The results demonstrate that the CLP vegetation during the entire Holocene was dominated by grassland landscapes in most areas and periods, whereas the forest landscape only developed in some areas during certain periods. Steppe vegetation rather than forest vegetation was distributed in the central region of the CLP. Six forest biomes were primarily distributed in the southwestern and northeastern plateau high-altitude areas, showing notable expansion toward low latitudes and longitudes during the early to mid-Holocene. The steppe biome showed a rather wide distribution, primarily in the center and south, and especially in the expanded western region during the Early Holocene. Minor variations were observed for the widely distributed steppe biomes that shifted to high longitudes and latitudes during the Holocene. The desert biome was identified in the relatively restricted western and northern regions of China. Reconstructed biome spatial distributions showed a large vegetation gradient on the CLP. This suggests that variations in forest biomes on the CLP were potentially influenced by changes in precipitation. Different topographies within the same area led to different biomes. In addition, increased human activity may cause vegetation destruction.

Postglacial vegetation and climate change in the Lake Onega region of eastern Fennoscandia derived from a radiocarbon-dated pollen record

With its numerous environmental archives stored in lake and peat sediments and relatively low human pressure, the Lake Onega region in eastern Fennoscandia is regarded as a particularly promising area for studying past changes in vegetation and climate since the Lateglacial period. The 885-cm-long sediment core RZ19 (62°27′53″N, 34°26′4″E) was collected from Razlomnoe Peat on the northern shore of Lake Onega in 2019, radiocarbon-dated and analysed for pollen and cryptogam spores. The age-depth model suggests continuous sedimentation since ca. 11,800 a BP (all ages given in years (a) or kiloyears (ka) before present (BP) with BP referring to 1950 CE). The results of pollen analysis and pollen-based biome reconstruction show rapid afforestation of the area in the Early Holocene, although the scores of the tundra biome remain relatively high prior to ca. 11,450 a BP, suggesting that the vegetation was likely more open than today. Between 8300 and 8000 a BP, Betula sect. Albae shows a marked increase in pollen percentage, while Pinus sylvestris experiences a marked decrease. These changes coinciding with the 8.2 ka BP cooling event indicate less favourable conditions for Scots pine while being beneficial for fast-growing birch. The transition from the Early to Middle Holocene (i.e. from Greenlandian to Northgrippian) is marked by an increase in pollen productivity, spread of Picea and further afforestation of the area. The decrease in arboreal and Picea pollen percentages and the abrupt increase in landscape openness after ca. 4000 a BP mark the onset of the Late Holocene (i.e. Northgrippian-Meghalayan transition) and likely reflect the combined effect of insolation-induced temperature decrease and associated paludification and forest retreat rather than a decrease in atmospheric precipitation and/or spread of Late Neolithic agriculture.

Alternation between terrestrial and aquatic plants dominated organic matter sources in the Tiaoshu wetland (south China) and its response to late Pleistocene environmental changes

Widely distributed wetlands in China play an important role in the global carbon cycle. However, the long-term response of wetland evolution and carbon storage in tropical southern China to climate change remains unclear. Here, we present a sediment core record from the Tiaoshu Wetland in the northern Leizhou Peninsula and multiple proxy indicators are investigated, with the aim of better understanding past vegetation and environmental changes. The following conclusions were drawn: (i) The organic matter deposited in the Tiaoshu Wetland was mainly derived from terrestrial C3 plants and aquatic plants growing in and/or around the catchment. The temporal variability in the relative proportions of organic matter contributed by various sources revealed paleoenvironmental changes in the study area, generally consistent with regional and global climate records. (ii) The peat formation pattern in the Tiaoshu Wetland is in contrast to some individual sites reported in subtropical China, suggesting that there may be different peat formation patterns in the tropical and subtropical regions of China, which may be closely related to specific hydrothermal conditions in different regions. In addition, by comparison with atmospheric CH4 concentrations, our results suggest potential relationships between the evolution of wetlands in tropical China and global atmospheric CH4 cycles.

Modern pollen rain reveals differences across forests, open and mosaic landscapes in Madagascar

Societal Impact StatementPalaeoecological investigations are crucial in understanding millennial to centennial land use and land cover change. By analysing the modern pollen rain from four main vegetation types across Madagascar, this research provides baselines for improving the interpretation of pollen records in palaeoecological studies. This study determines the modern pollen‐vegetation relationship in Madagascar and gives a better understanding of the island's landscapes. Despite some spatial limitations, this approach contributes to resolving the debated topic related to the origin of Madagascar's open ecosystems. Knowing the vegetation history prior to and after human settlement would help guide biodiversity management and its associated ecosystem services.Summary Modern pollen rain contributes to calibrating and defining palaeo‐records relative to the current vegetation of diverse landscapes. Quantifying past vegetation change is important to better frame sustainable and resilient management methods of biodiversity considering human needs. In this paper, we aim to assess modern pollen assemblages in different vegetation types in Madagascar and inform how these could contribute to the interpretation of palaeo‐records. We identified modern pollen assemblages from 21 samples across four major vegetation types in Madagascar, including seven (N = 7) newly generated samples from the tropical dry forest in the Northwest region and 14 datasets downloaded from the African Pollen Database website. The downloaded datasets are from core tops representing modern vegetation previously analysed in the tropical dry forests and spiny thickets, the wooded grassland–bushland mosaic and the littoral humid forest. We evaluated the frequency of different taxa in the samples and conducted multivariate analyses to evaluate the similarities and ecological affinities between samples. Pollen rain reflected the main vegetation types across Madagascar and provided information on the current structure and state of the landscape. Regarding the interpretation of pollen records, pollen rain samples revealed that: (i) Open mosaic ecosystems should contain a percentage of at least 40% grass pollen, informing on the origin of the vegetation in older samples and the state of degradation in more recent vegetation reconstructions; and (ii) Anthropogenic vegetation such as plantations are marked by the abundance of pioneer taxa such as Pinus and/or Eucalyptus spp. with a frequency >20%. Our findings improve the interpretations of pollen records and help differentiate past distribution of forest, open and mosaic vegetation, in Madagascar. It will contribute to further research on human practices, land use and into scenarios of conservation planning not only in Madagascar but also across the tropics.

Rate of vegetation change in southeast China during the Holocene and its potential drivers

Understanding the factors behind past vegetation changes is instructive to anticipate how vegetation will react to future climate change and human activities. The factors that contribute to the changes in vegetation during the Holocene in the topographically diverse southeast China are still not well understood. This study presents a recently acquired subalpine pollen record and combines it with pollen records from low-lying areas and mountains in southeast China. We utilized a Rate of Change (RoC) analysis on the pollen sequences to detect shifts in vegetation composition. Our findings indicate a significant increase in vegetation rate of change in southeast China over the past 3000 years, comparable to or exceeding maximum levels seen during the early Holocene, a period characterized by greater climate variability. By analyzing available paleorecords of climate, fire, and human activity, it becomes clear that vegetation in southeast China was significantly affected by climate before c. 3 ka BP. However, after this time, human impacts began to play a prominent role, first in the lowlands and later in the mountains, in addition to the influence of climate variations. Our study emphasizes the significance of taking into account the rate of vegetation compositional change when examining the factors that influence long-term vegetation changes.

Plant macrofossil data for 48-0 ka in the USGS North American Packrat Midden Database, version 5.0

Plant macrofossils from packrat (Neotoma spp.) middens provide direct evidence of past vegetation changes in arid regions of North America. Here we describe the newest version (version 5.0) of the U.S. Geological Survey (USGS) North American Packrat Midden Database. The database contains published and contributed data from 3,331 midden samples collected in southwest Canada, the western United States, and northern Mexico, with samples ranging in age from 48 ka to the present. The database includes original midden-sample macrofossil counts and relative-abundance data along with a standardized relative-abundance scheme that makes it easier to compare macrofossil data across midden-sample sites. In addition to the midden-sample data, this version of the midden database includes calibrated radiocarbon (14C) ages for the midden samples and plant functional type (PFT) assignments for the midden taxa. We also provide World Wildlife Fund ecoregion assignments and climate and bioclimate data for each midden-sample site location. The data are provided in tabular (.xlsx), comma-separated values (.csv), and relational database (.mdb) files.

A palynological atlas of the Cerrado-Caatinga ecotone in northeastern Brazil

Describing the diversity of Brazilian plant species, as well as the wide variety in the associated pollen grains, is crucial to understanding their evolution and ensuring effective conservation. The accurate identification of pollen is key to successful pollen studies and the consistent paleoecological interpretation of past vegetation changes. Pollen analysts usually identify plant taxa from reference collections built from herbaria material. In Brazil, pollen atlases primarily focus on three main vegetation domains, namely, the Amazon Forest, the Atlantic Forest, and the Cerrado, while information about Caatinga pollen taxa is dispersed across multiple studies; this significantly impedes efforts to further develop palynological research. To address this gap, we built a palynological atlas for the state of Piauí which is covered by the biomes Caatinga and Cerrado along with their transitional zones. These domains encompass various vegetation types, including dense shrubland, grassland, palm swamp formation, riverine forest, seasonal deciduous and semi-deciduous forests, as well as valley-side marshes. We collected flower buds from the Herbarium Prisco Bezerra (EAC) at the Federal University of Ceará and extracted the pollen grains for the pollen reference slides. The atlas comprises 398 species from 99 families, with the most represented families being Asteraceae, Bignoniaceae, Euphorbiaceae, Fabaceae, Lamiaceae, Malvaceae, Malpighiaceae, and Rubiaceae. Each taxa is presented with a description of the pollen grain, a photo showing polar and equatorial views, and ecological information as life form (liana/scandent/vine, herb, subshrub, shrub or tree), substrate, and vegetation type to support further studies.

Fire‐regime variability and ecosystem resilience over four millennia in a Rocky Mountain subalpine watershed

Abstract Wildfires strongly influence forest ecosystem processes, including carbon and nutrient cycling, and vegetation dynamics. As fire activity increases under changing climate conditions, the ecological and biogeochemical resilience of many forest ecosystems remains unknown. To investigate the resilience of forest ecosystems to changing climate and wildfire activity over decades to millennia, we developed a 4800‐year high‐resolution lake‐sediment record from Silver Lake, Montana, USA (47.360° N, 115.566° W). Charcoal particles, pollen grains, element concentrations and stable isotopes of C and N serve as proxies of past changes in fire, vegetation and ecosystem processes such as nitrogen cycling and soil erosion, within a small subalpine forest watershed. A published lake‐level history from Silver Lake provides a local record of palaeohydrology. A trend towards increased effective moisture over the late Holocene coincided with a distinct shift in the pollen assemblage c. 1900 yr BP, resulting from increased subalpine conifer abundance. Fire activity, inferred from peaks in macroscopic charcoal, decreased significantly after 1900 yr BP, from one fire event every 126 yr (83–184 yr, 95% CI) from 4800 to 1900 yr BP, to one event every 223 yr (175–280 yr) from 1900 yr BP to present. Across the record, individual fire events were followed by two distinct decadal‐scale biogeochemical responses, reflecting differences in ecosystem impacts of fires on watershed processes. These distinct biogeochemical responses were interpreted as reflecting fire severity, highlighting (i) erosion, likely from large or high‐severity fires, and (ii) nutrient transfers and enhanced within‐lake productivity, likely from lower severity or patchier fires. Biogeochemical and vegetation proxies returned to pre‐fire values within decades regardless of the nature of fire effects. Synthesis. Palaeorecords of fire and ecosystem responses provide a novel view revealing past variability in fire effects, analogous to spatial variability in fire severity observed within contemporary wildfires. Overall, the palaeorecord highlights ecosystem resilience to fire across long‐term variability in climate and fire activity. Higher fire frequencies in past millennia relative to the 20th and 21st century suggest that northern Rocky Mountain subalpine ecosystems could remain resilient to future increases in fire activity, provided continued ecosystem recovery within decades.

Fire in the clouds: How changing land use shaped an Andean biodiversity hotspot

Past land use, particularly fire, affects modern tropical forests. Charcoal from lake sediments is commonly used to estimate past fire parameters such as burn severity and frequency, but fire intensity also plays a major role in shaping vegetation and vegetation change. Past fire intensity has remained elusive using common paleoecological approaches. We present a new approach to reconstruct past fire (pyrolysis) temperature, a metric of fire intensity, and reveal how human fire use changed and shaped biodiverse Andean montane forests over the last 2100 years. We use spectra obtained from micro-Fourier Transformed Infrared Spectroscopy (FTIR) of individual charcoal particles recovered from the sediments of Lagua de los Condores (Peru) to characterize its chemical composition. We then compare the spectra generated from the sedimentary charcoal fragments with a modern reference dataset to infer the pyrolysis temperature at which they were formed. Reconstructed maximum pyrolysis temperature varied with changes in land use and changes in precipitation. Mid-temperature fires (500–600 °C) dominated the record, and co-occurred with maize cultivation. After 1200 CE the Chachapoya people, referred to as cloud warriors by the Incas, started to use the site for ceremonial purposes as the climate got wetter. We demonstrate a concomitant change in the complete fire regime with fires becoming less severe, less frequent and burning at a lower temperature after this transition. This change in land use resulted in the first forest recovery in 2000 years, which was mainly composed of species with low bark thickness, a trait of fire sensitivity. Our reconstruction of pyrolysis temperature demonstrates that the analysis of fire severity, frequency, and our added metric of intensity, is needed to understand the drivers of past vegetation change.

Late Holocene riparian vegetation dynamics, environmental changes, and human impact in the Harapan forest of Sumatra, Indonesia

Riparian wetland ecosystems provide important ecological services but are also vulnerable to climate change and human activities. To understand the influence of natural factors (e.g. climate change, flooding, drought) and human activities (e.g. agriculture) as well as to support management strategies, reconstructions of past vegetation and environmental changes are needed. To achieve this, we conducted a multi-proxy paleoecological analysis, including pollen and spores, macro-charcoal and radiocarbon dating, on a sediment core taken from a riparian area in the Harapan forest of Sumatra. Three distinct periods were identified: i) AD 1100 – 1400: Upland and swamp forest with riparian and herbaceous vegetation, possibly part of a riparian buffer zone (e.g. riverbank), was present in the study area under a stronger dry season regime; ii) AD 1400 – 1870: freshwater swamps expanded to the study site; iii) later, from AD 1870 to present, upland forests dominated in the study area with a strong dry season. The presence of cereal cultivation from AD 1300 – 1450, and oil palm (Elaeis guineensis) since the mid-19th century AD indicates the presence of small-scale agriculture in the study area. This study of riparian vegetation dynamics and environmental changes in the Harapan forest of Sumatra shows the development from a riparian forest to a freshwater swamp and upland forest under the impact of climate change and human activities.

RETRACTED: Reconstructing past vegetation based on fossil and modern pollen data in Sougna mountain (Western Rif mountains - Northern Morocco)

The process of quantifying and reconstructing past vegetation changes is a crucial step towards comprehending the dynamics of human-environment interactions throughout time. In this study, we present a reconstruction of the vegetation cover of the Sougna mountain (Western Rif Mountain) over the last 6000 yr based on multivariate statistical analysis (cluster analysis and non-metric multidimensional scaling, NMDS) of 31 modern pollen samples, and detailed fossil pollen record. The integrated approach, used in this study was able to identify and differentiate six distinct modern vegetation communities. Moreover, the analysis linked fossil pollen records with these modern vegetation communities as their analogues. The information on the contemporary vegetation cover of the modern pollen sites was used to determine the past vegetation changes of the fossil pollen site in the Sougna mountain. The vegetation changes estimated from this analysis reveal a higher degree of landscape openness than what was initially indicated by the fossil pollen percentages. The results of this study demonstrate a marked decline in forest ecosystems during the last millennium, indicating a significant transformation in vegetation over time. Additionally, the analysis indicates that some species have diminished their distribution range and relocated to more suitable environments. The methodology employed in this study, which combines fossil pollen data with modern pollen data and vegetation in a single NMDS analysis, permits reconstruction of the past vegetation by identifying statistically their modern analogues.

Refining data–data and data–model vegetation comparisons using the Earth mover's distance (EMD)

Abstract. Comparing temporal and spatial vegetation changes between reconstructions or between reconstructions and model simulations requires carefully selecting an appropriate evaluation metric. A common way of comparing reconstructed and simulated vegetation changes involves measuring the agreement between pollen- or model-derived unary vegetation estimates, such as the biome or plant functional type (PFT) with the highest affinity scores. While this approach based on summarising the vegetation signal into unary vegetation estimates performs well in general, it overlooks the details of the underlying vegetation structure. However, this underlying data structure can influence conclusions since minor variations in pollen percentages modify which biome or PFT has the highest affinity score (i.e. modify the unary vegetation estimate). To overcome this limitation, we propose using the Earth mover's distance (EMD) to quantify the mismatch between vegetation distributions such as biome or PFT affinity scores. The EMD circumvents the issue of summarising the data into unary biome or PFT estimates by considering the entire range of biome or PFT affinity scores to calculate a distance between the compared entities. In addition, each type of mismatch can be given a specific weight to account for case-specific ecological distances or, said differently, to account for the fact that reconstructing a temperate forest instead of a boreal forest is ecologically more coherent than reconstructing a temperate forest instead of a desert. We also introduce two EMD-based statistical tests that determine (1) if the similarity of two samples is significantly better than a random association given a particular context and (2) if the pairing between two datasets is better than might be expected by chance. To illustrate the potential and the advantages of the EMD as well as the tests in vegetation comparison studies, we reproduce different case studies based on previously published simulated and reconstructed biome changes for Europe and capitalise on the advantages of the EMD to refine the interpretations of past vegetation changes by highlighting that flickering unary estimates, which give an impression of high vegetation instability, can correspond to gradual vegetation changes with low EMD values between contiguous samples (case study 1). We also reproduce data–model comparisons for five specific time slices to identify those that are statistically more robust than a random agreement while accounting for the underlying vegetation structure of each pollen sample (case study 2). The EMD and the statistical tests are included in the paleotools R package (https://github.com/mchevalier2/paleotools, last access: 3 May 2023).

Influences of sea level changes and volcanic eruptions on Holocene vegetation in Tonga

AbstractHere, we investigate Mid‐ to Late‐Holocene vegetation changes in low‐lying coastal areas in Tonga and how changing sea levels and recurrent volcanic eruptions have influenced vegetation dynamics on four islands of the Tongan archipelago (South Pacific). To investigate past vegetation and environmental change at Ngofe Marsh (‘Uta Vava’u), we examined palynomorphs (pollen and spores), charcoal (fire), and sediment characteristics (volcanic activity) from a 6.7‐m‐long sediment core. Radiocarbon dating indicated the sediments were deposited over the last 7700 years. We integrated the Ngofe Marsh data with similar previously published data from Avai’o’vuna Swamp on Pangaimotu Island, Lotofoa Swamp on Foa Island, and Finemui Swamp on Ha’afeva Island. Plant taxa were categorized as littoral, mangrove, rainforest, successional/ disturbance, and wetland groups, and linear models were used to examine relationships between vegetation, relative sea level change, and volcanic eruptions (tephra). We found that relative sea level change has impacted vegetation on three of the four islands investigated. Volcanic eruptions were not identified as a driver of vegetation change. Rainforest decline does not appear to be driven by sea level changes or volcanic eruptions. From all sites analyzed, vegetation at Finemui Swamp was most sensitive to changes in relative sea level. While vegetation on low‐lying Pacific islands is sensitive to changing sea levels, island characteristics, such as area and elevation, are also likely to be important factors that mediate specific island responses to drivers of change.

The distribution of fossil pollen and charcoal in stalagmites

AbstractPollen preserved in caves provides a little-appreciated opportunity to study past vegetation and climate changes in regions where conventional wetland sediments are either unavailable, contain little organic matter, and/or are difficult to date accurately. Most palynology in caves has focused on clastic infill sediments, but pollen preserved in growing speleothems provides important new opportunities to develop vegetation and climatic records that can be dated accurately with radiometric methods. However, when pollen is present in speleothems, concentrations can vary by orders of magnitude, highlighting how little we know about the processes that transport pollen into caves and onto speleothem surfaces, and that determine the pollen's preservation probability. To explore these aspects of speleothem pollen taphonomy, we investigated the distribution of pollen and microscopic charcoal within several stalagmites from southwest Australia. We examined spatial patterns in pollen and charcoal preservation in order to distinguish whether observed gradients result from preservation or are products of systematic transport processes working along stalagmite surfaces. We find that pollen grains and charcoal fragments are located preferentially on the flanks of most stalagmites. This suggests that pollen grain and charcoal deposition on speleothems is influenced by transport and accumulation of detrital debris on growing surfaces. These insights will assist in future sampling campaigns focusing on speleothem pollen and charcoal contents.

Super resolution of historic Landsat imagery using a dual generative adversarial network (GAN) model with CubeSat constellation imagery for spatially enhanced long-term vegetation monitoring

Detailed spatial representations of terrestrial vegetation are essential for precision agricultural applications and the monitoring of land cover changes in heterogeneous landscapes. The advent of satellite-based remote sensing has facilitated daily observations of the Earth’s surface with high spatial resolution. In particular, a data fusion product such as Planet Fusion has realized the delivery of daily, gap-free surface reflectance data with 3-m pixel resolution through full utilization of relatively recent (i.e., 2018-) CubeSat constellation data. However, the spatial resolution of past satellite sensors (i.e., 30–60 m for Landsat) has restricted the detailed spatial analysis of past changes in vegetation. In order to overcome the spatial resolution constraint of Landsat data for long-term vegetation monitoring, we propose a dual remote-sensing super-resolution generative adversarial network (dual RSS-GAN) approach combining Planet Fusion and Landsat 8 data to simulate spatially enhanced long-term time-series of the normalized difference vegetation index (NDVI) and near-infrared reflectance from vegetation (NIRv). We evaluated the performance of the dual RSS-GAN against in situ tower-based continuous measurements (up to 8 years) and remotely piloted aerial system-based maps of cropland and deciduous forest in the Republic of Korea. The dual RSS-GAN enhanced spatial representations in Landsat 8 images and captured seasonal variation in vegetation indices (R2 > 0.90, for the dual RSS-GAN maps vs in situ data from all sites). Overall, the dual RSS-GAN reduced Landsat 8 vegetation index underestimations compared with in situ measurements; relative bias values of NDVI ranged from − 5.8 % to 0.3 % and − 12.4 % to − 3.7 % for the dual RSS-GAN and Landsat 8, respectively. This improvement was caused by spatial enhancement through the dual RSS-GAN, which reflects fine-scale information from Planet Fusion. Finally, the dual RSS-GAN maps showed both spatial enhancement and reducing the underestimation of vegetation index in historic Landsat dataset from 1984. This study presents a new approach for resolving sub-pixel spatial information in Landsat images.