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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. More than one way to kill a spruce forest: The role of fire and climate in the late‐glacial termination of spruce woodlands across the southern Great Lakes

   https://doi.org/10.1111/1365-2745.13517  

   Jensen, Allison M. ; Fastovich, David ; Watson, Ben I. ; Gill, Jacquelyn L. ; Jackson, Stephen T. ; Russell, James M. ; Bevington, Joseph ; Hayes, Katherine ; Lininger, Katherine B. ; Rubbelke, Claire ; et al ( November 2020 , Journal of Ecology)

   In the southern Great Lakes Region, North America, between 19,000 and 8,000 years ago, temperatures rose by 2.5–6.5°C and sprucePiceaforests/woodlands were replaced by mixed‐deciduous or pinePinusforests. The demise ofPiceaforests/woodlands during the last deglaciation offers a model system for studying how changing climate and disturbance regimes interact to trigger declines of dominant species and vegetation‐type conversions.

   The role of rising temperatures in driving the regional demise ofPiceaforests/woodlands is widely accepted, but the role of fire is poorly understood. We studied the effect of changing fire activity onPiceadeclines and rates of vegetation composition change using fossil pollen and macroscopic charcoal from five high‐resolution lake sediment records.

   The decline ofPiceaforests/woodlands followed two distinct patterns. At two sites (Stotzel‐Leis and Silver Lake), fire activity reached maximum levels during the declines and both charcoal accumulation rates and fire frequency were significantly and positively associated with vegetation composition change rates. At these sites,Piceadeclined to low levels by 14 kyr BP and was largely replaced by deciduous hardwood taxa like ashFraxinus, hop‐hornbeam/hornbeamOstrya/Carpinusand elmUlmus. However, this ecosystem transition was reversible, asPiceare‐established at lower abundances during the Younger Dryas.

   At the other three sites, there was no statistical relationship between charcoal accumulation and vegetation composition change rates, though fire frequency was a significant predictor of rates of vegetation change at Appleman Lake and Triangle Lake Bog. At these sites,Piceadeclined gradually over several thousand years, was replaced by deciduous hardwoods and high levels ofPinusand did not re‐establish during the Younger Dryas.

   Synthesis. Fire does not appear to have been necessary for the climate‐driven loss ofPiceawoodlands during the last deglaciation, but increased fire frequency accelerated the decline ofPiceain some areas by clearing the way for thermophilous deciduous hardwood taxa. Hence, warming and intensified fire regimes likely interacted in the past to cause abrupt losses of coniferous forests and could again in the coming decades.

    

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3. Human influence on late Holocene fire history in a mixed-conifer forest, Sierra National Forest, California

   https://doi.org/10.1186/s42408-023-00245-9  

   Klimaszewski-Patterson, Anna ; Dingemans, Theodore ; Morgan, Christopher T. ; Mensing, Scott A. ( January 2024 , Fire Ecology)

   Understanding pre-1850s fire history and its effect on forest structure can provide insights useful for fire managers in developing plans to moderate fire hazards in the face of forecasted climate change. While climate clearly plays a substantial role in California wildfires, traditional use of fire by Indigenous people also affected fire history and forest structure in the Sierra Nevada. Disentangling the effects of human versus climatically-induced fire on Sierran forests from paleoecological records has historically proved challenging, but here we use pollen-based forest structure reconstructions and comparative paleoclimatic-vegetation response modeling to identify periods of human impact over the last 1300 years at Markwood Meadow, Sierra National Forest.

   We find strong evidence for anthropogenic fires at Markwood Meadow ca. 1550 – 1750 C.E., contemporaneous with archaeological evidence for fundamental shifts in Indigenous lifeways. When we compare our findings to five other paleoecological sites in the central and southern Sierra Nevada, we find evidence for contemporaneous anthropogenic effects on forest structure across a broad swath of cismontane central California. This is significant because it implies that late 19th and early twentieth century forest structure – the structure that land managers most often seek to emulate – was in part the result anthropogenic fire and precolonial resource management.

   We consequently suggest that modern management strategies consider (1) further incorporating traditional ecological knowledge fire practices in consultation with local tribal groups, and (2) using pollen-based reconstructions to track how forest composition compares to pre-1850 C.E. conditions rather than the novel forest states encountered in the late 20th and early twenty-first centuries. These strategies could help mitigate the effects of forecast climate change and associated megafires on forests and on socio-ecological systems in a more comprehensive manner.

    

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

   https://doi.org/10.1111/1365-2745.14201  

   Clark‐Wolf, Kyra D. ; Higuera, Philip E. ; McLauchlan, Kendra K. ; Shuman, Bryan N. ; Parish, Meredith C. ( October 2023 , Journal of Ecology)

   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.

    

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5. Vegetation response to wildfire and climate forcing in a Rocky Mountain lodgepole pine forest over the past 2500 years

   https://doi.org/10.1177/0959683620941068  

   Chileen, Barrie V ; McLauchlan, Kendra K ; Higuera, Philip E ; Parish, Meredith ; Shuman, Bryan N ( November 2020 , The Holocene)

   null (Ed.)

   Wildfire is a ubiquitous disturbance agent in subalpine forests in western North America. Lodgepole pine ( Pinus contorta var. latifolia), a dominant tree species in these forests, is largely resilient to high-severity fires, but this resilience may be compromised under future scenarios of altered climate and fire activity. We investigated fire occurrence and post-fire vegetation change in a lodgepole pine forest over the past 2500 years to understand ecosystem responses to variability in wildfire and climate. We reconstructed vegetation composition from pollen preserved in a sediment core from Chickaree Lake, Colorado, USA (1.5-ha lake), in Rocky Mountain National Park, and compared vegetation change to an existing fire history record. Pollen samples ( n = 52) were analyzed to characterize millennial-scale and short-term (decadal-scale) changes in vegetation associated with multiple high-severity fire events. Pollen assemblages were dominated by Pinus throughout the record, reflecting the persistence of lodgepole pine. Wildfires resulted in significant declines in Pinus pollen percentages, but pollen assemblages returned to pre-fire conditions after 18 fire events, within c.75 years. The primary broad-scale change was an increase in Picea, Artemisia, Rosaceae, and Arceuthobium pollen types, around 1155 calibrated years before present. The timing of this change is coincident with changes in regional pollen records, and a shift toward wetter winter conditions identified from regional paleoclimate records. Our results indicate the overall stability of vegetation in Rocky Mountain lodgepole pine forests during climate changes and repeated high-severity fires. Contemporary deviations from this pattern of resilience could indicate future recovery challenges in these ecosystems. 

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