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1. Subregional variability in the response of New England vegetation to postglacial climate change

   https://doi.org/10.1111/jbi.13407  

   Oswald, W. Wyatt ; Foster, David R. ; Shuman, Bryan N. ; Doughty, Elaine D. ; Faison, Edward K. ; Hall, Brian R. ; Hansen, Barbara C. S. ; Lindbladh, Matts ; Marroquin, Adriana ; Truebe, Sarah A. ( September 2018 , Journal of Biogeography)

   We analysed a dataset composed of multiple palaeoclimate and lake‐sediment pollen records from New England to explore how postglacial changes in the composition and spatial patterns of vegetation were controlled by regional‐scale climate change, a subregional environmental gradient, and landscape‐scale variations in soil characteristics.

   The 120,000‐km²study area includes parts of Vermont and New Hampshire in the north, where sites are 150–200 km from the Atlantic Ocean, and spans the coastline from southeastern New York to Cape Cod and the adjacent islands, including Block Island, the Elizabeth Islands, Nantucket, and Martha's Vineyard.

   We analysed pollen records from 29 study sites, using multivariate cluster analysis to visualize changes in the composition and spatial patterns of vegetation during the last 14,000 years. The pollen data were compared with temperature and precipitation reconstructions.

   Boreal forest featuringPiceaandPinus banksianawas present across the region when conditions were cool and dry 14,000–12,000 calibrated¹⁴C years before present (ybp).Pinus strobusbecame regionally dominant as temperatures increased between 12,000 and 10,000 ybp. The composition of forests in inland and coastal areas diverged in response to further warming after 10,000 ybp, whenQuercusandPinus rigidaexpanded across southern New England, whereas conditions remained cool enough in inland areas to maintainPinus strobus. Increasing precipitation allowedTsuga canadensis,Fagus grandifolia, andBetulato replacePinus strobusin inland areas during 9,000–8,000 ybp, and also led to the expansion ofCaryaacross the coastal part of the region beginning at 7,000–6,000 ybp. Abrupt cooling at 5,500–5,000 ybp caused sharp declines inTsugain inland areas andQuercusat some coastal sites, and the populations of those taxa remained low until they recovered around 3,000 ybp in response to rising precipitation. Throughout most of the Holocene, sites underlain by sandy glacial deposits were occupied byPinus rigidaandQuercus.

   Postglacial changes in the composition and spatial pattern of New England forests were controlled by long‐term trends and abrupt shifts in temperature and precipitation, as well as by the environmental gradient between coastal and inland parts of the region. Substrate and soil moisture shaped landscape‐scale variations in forest composition.

    

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2. Vegetation response to climatic changes in western Amazonia over the last 7,600 years

   https://doi.org/10.1111/jbi.13704  

   Nascimento, Majoi N. ; Martins, Gabriel S. ; Cordeiro, Renato C. ; Turcq, Bruno ; Moreira, Luciane S. ; Bush, Mark B. ( September 2019 , Journal of Biogeography)

   Ongoing and future anthropogenic climate change poses one of the greatest threats to biodiversity, affecting species distributions and ecological interactions. In the Amazon, climatic changes are expected to induce warming, disrupt precipitation patterns and of particular concern, to increase the intensity and frequency of droughts. Yet the response of ecosystems to intense warm, dry events is not well understood. In the Andes the mid‐Holocene dry event (MHDE),c. 9,000 to 4,000 years ago, was the warmest and driest period of the last 100,000 years which coincided with changes in evaporation and precipitation that caused lake levels to drop over most of tropical South America. This event probably approximates our near‐climatic future, and a critical question is:How much did vegetation change in response to this forcing?

   Lake Pata, Brazilian Western Amazonia.

   Terrestrial and aquatic plants.

   We used pollen, charcoal, total organic carbon (TOC), total nitrogen (TN), δ¹³C and δ¹⁵N data from a new high‐resolution core that spans the lastc. 7,600 years history of Lake Pata.

   We found that in the wettest section of Amazonia changes associated with the MHDE were detected in the geochemistry analysis but that vegetation changed very little in response to drought during the Holocene. This is the first high‐resolution core without apparent hiatuses that spans most of the Holocene (last 7,600 cal yrbp) from Lake Pata, Brazil. Changes in the organic geochemistry of sediments indicated that between c. 6,500 and 3,600 cal yrbplake levels dropped. Vegetation, however, showed little change as near‐modern forests were seen throughout the record, evidencing the substantial resilience of this system. Only a few species replacements and minor fluctuations in abundance were observed in the pollen record.

   The mid‐Holocene warming and reduced precipitation had a limited impact on western Amazonian forests. We attribute much of the resilience to a lack of fire in this system, and that if human‐set fires were to be introduced, the forest destruction from that cause would override that induced by climate alone.

    

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3. Assessing the influence of vegetation structure and phenological variability on pollen‐vegetation relationships using a 15‐year Neotropical pollen rain record

   https://doi.org/10.1111/jvs.12897  

   Haselhorst, Derek S. ; Moreno, J. Enrique ; Punyasena, Surangi W. ; Giesecke, ed., Thomas ( May 2020 , Journal of Vegetation Science)

   Pollen collected from aerial pollen traps are used to interpret compositional changes in the fossil pollen record. Relationships between pollen abundances and vegetation are commonly measured using the ratio between the abundance of a pollen type and its corresponding basal area in a plant community (R‐rel). Pollen–vegetation relationships have been extensively studied in temperate and boreal latitudes using surface sediment, moss polster and pollen trap samples, representing long‐term accumulations of pollen. In contrast, pollen–vegetation relationships are not well‐resolved in diverse tropical habitats because of a lack of modern long‐term pollen and vegetation datasets. In the present study, we address two primary questions: 1. How variable are tropical pollen–vegetation relationships measured using R‐rel over time? 2. To what extent are differences in pollen abundances among traps related to variability in basal area in the surrounding community?

   Barro Colorado Island, Panama.

   Variability in an annually sampled, 15‐year pollen rain was compared with annual basal area data collected over the same timeframe using R‐rel.

   R‐rel values were highly variable on a year‐to‐year basis and more consistent over time‐averaged sampling windows ≥3 years. A strong positive relationship between pollen abundances and local basal area surrounding pollen traps was determined in eight taxa.

   The pollen–vegetation relationships provide a summary of the pollen representation of Neotropical taxa common in paleoecological samples over an extended sampling duration that approaches the timespan represented in lake sediment samples (~20 years). The results highlight taxa in which differences in pollen abundances among traps were strongly correlated with changes in surrounding basal area.

    

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4. Pollen-based evidence of extreme drought during the last Glacial (32.6–9.0 ka) in coastal southern California

   https://doi.org/10.1016/j.quascirev.2015.08.029  

   Heusser, Linda E. ; Kirby, Matthew E. ; Nichols, Jonathan E. ( October 2015 , Quaternary Science Reviews)

   High resolution pollen analyses of sediment core LEDC10-1 from Lake Elsinore yield the first well-dated, terrestrial record of sub-centennial-scale ecologic change in coastal southern California between ~32 and 9 ka. In the Lake Elsinore watershed, the initial, mesic montane conifer forests dominated by Pinus, and Cupressaceae with trace amounts of Abies and Picea were replaced by a sequence of multiple, extended severe mega-droughts between ~27.5 and ~25.5 ka, in which halophytic and xerophytic herbs and shrubs occupied an ephemeral lake. This prolonged and extended dry interval, which corresponds with warm waters offshore, imply strengthening of the North Pacific High and persistent below-average winter precipitation. The subsequent, contrasting monotonic occurrence of montane conifers reflects little variation in cold, mesic climate until ~15 ka. Postglacial development of Quercus woodland and chaparral mark the return to more xeric, warmer conditions at this time. A brief reversal at ~13.1e~12.1 ka, as reflected by an expansion of Pinus, is correlative with the Younger Dryas and interrupts development of warm, postglacial climate. Subsequent gradual expansion of xeric vegetation post e Younger Dryas denotes the establishment of a winter hydroclimate regime in coastal southern California that is more similar to modern conditions. Pollen-based reconstructions of temperature and precipitation at Lake Elsinore are generally correlative with pollen-based paleoclimatic reconstructions and foraminifera based sea surface temperatures from Santa Barbara Basin in marine core ODP 893. The conspicuous absence of the ~27.5e~25.5 ka glacial “mega-drought” in the Santa Barbara Basin pollen record highlights the sensitivity of Lake Elsinore to hydroclimate change, and thus, the importance of this new record that indicates that mega-drought can occur during the full glacial when climatic boundary conditions and forcings differed substantially from the present. 

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5. A neotropical perspective on the uniqueness of the Holocene among interglacials

   https://doi.org/10.1038/s41467-023-43231-0  

   Schiferl, J. ; Kingston, M. ; Åkesson, C. M. ; Valencia, B. G. ; Rozas-Davila, A. ; McGee, D. ; Woods, A. ; Chen, C. Y. ; Hatfield, R. G. ; Rodbell, D. T. ; et al ( November 2023 , Nature Communications)

   Understanding how tropical systems have responded to large-scale climate change, such as glacial-interglacial oscillations, and how human impacts have altered those responses is key to current and future ecology. A sedimentary record recovered from Lake Junín, in the Peruvian Andes (4085 m elevation) spans the last 670,000 years and represents the longest continuous and empirically-dated record of tropical vegetation change to date. Spanning seven glacial-interglacial oscillations, fossil pollen and charcoal recovered from the core showed the general dominance of grasslands, although during the warmest times some Andean forest trees grew above their modern limits near the lake. Fire was very rare until the last 12,000 years, when humans were in the landscape. Here we show that, due to human activity, our present interglacial, the Holocene, has a distinctive vegetation composition and ecological trajectory compared with six previous interglacials. Our data reinforce the view that modern vegetation assemblages of high Andean grasslands and the presence of a defined tree line are aspects of a human-modified landscape.

    

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