More Like this

1. 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.

    

   more » « less
2. Less fuel for the next fire? Short‐interval fire delays forest recovery and interacting drivers amplify effects

   https://doi.org/10.1002/ecy.4042  

   Braziunas, Kristin H. ; Kiel, Nathan G. ; Turner, Monica G. ( April 2023 , Ecology)

   As 21st‐century climate and disturbance dynamics depart from historic baselines, ecosystem resilience is uncertain. Multiple drivers are changing simultaneously, and interactions among drivers could amplify ecosystem vulnerability to change. Subalpine forests in Greater Yellowstone (Northern Rocky Mountains, USA) were historically resilient to infrequent (100–300 year), severe fire. We sampled paired short‐interval (<30‐year) and long‐interval (>125‐year) post‐fire plots most recently burned between 1988 and 2018 to address two questions: (1) How do short‐interval fire, climate, topography, and distance to unburned live forest edge interact to affect post‐fire forest regeneration? (2) How do forest biomass and fuels vary following short‐interval versus long‐interval severe fires? Mean post‐fire live tree stem density was an order of magnitude lower following short‐interval versus long‐interval fires (3240 vs. 28,741 stems ha⁻¹, respectively). Differences between paired plots were amplified at longer distances to live forest edge. Surprisingly, warmer–drier climate was associated with higher seedling densities even after short‐interval fire, likely relating to regional variation in serotiny of lodgepole pine (Pinus contortavar.latifolia). Unlike conifers, density of aspen (Populus tremuloides), a deciduous resprouter, increased with short‐interval versus long‐interval fires (mean 384 vs. 62 stems ha⁻¹, respectively). Live biomass and canopy fuels remained low nearly 30 years after short‐interval fire, in contrast with rapid recovery after long‐interval fire, suggesting that future burn severity may be reduced for several decades following reburns. Short‐interval plots also had half as much dead woody biomass compared with long‐interval plots (60 vs. 121 Mg ha⁻¹), primarily due to the absence of large snags. Our results suggest differences in tree regeneration following short‐interval versus long‐interval fires will be especially pronounced where serotiny was high historically. Propagule limitation will also interact with short‐interval fires to diminish tree regeneration but lessen subsequent burn severity. Amplifying driver interactions are likely to threaten forest resilience under expected trajectories of a future fire.

    

   more » « less
3. Climate–fire–vegetation interactions and the rise of novel landscape patterns in subalpine ecosystems, Colorado

   https://doi.org/10.1111/1365-2745.13138  

   Calder, William John ; Stefanova, Ivanka ; Shuman, Bryan ; McMichael, ed., Crystal ( February 2019 , Journal of Ecology)

   Interactions at multiple scales can shape how forest ecosystems respond to both climate change and disturbance. At landscape scales, feedbacks cause vegetation and wildfire regimes to alter one another, while forest patterns at local scales can result from feedbacks between plants and their abiotic environment. Consequently, disturbances and abiotic changes may give rise to forest patterns that further affect processes like wildfire.

   Examples may exist in the history of subalpine ribbon forests, which are alternating areas of forest and meadow where snow drifting and linear bands of trees depend upon each other. Just as ongoing climate changes may change snow levels and the dynamics in these forests, past climate changes and wildfires may have generated the conditions suitable for ribbon forests.

   To examine feedbacks in subalpine forests and the potential that climate changes and fires gave rise to ribbon forests, we obtained six fossil pollen records from a subalpine landscape in Colorado. Forests there may have responded to climate change and widespread wildfires ˜1,000 years ago when >80% of sites on the landscape burned within a century. The fires coincided with regional warming, but the extent of burning declined before the climate cooled, possibly driven by changes in fuel structure and composition.

   The pollen records indicate that large vegetation changes coincided with the widespread wildfires at five of the six sites. Pollen assemblages consistent with ribbon forests first became important at this time as sagebrush (Artemisia) and other meadow taxa increased and conifers, especially spruce (Picea) declined. The ribbon forests emerged as novel fuel breaks at the time when temperatures rose ˜0.5°C but the number of sites burned per century declined. Cooling during the Little Ice Age then expanded the openings and extent of ribbon forests, probably helping to reduce the frequency of fires across our sites to its minimum.

   Synthesis. The rise of ribbon forests in northern Colorado illustrates how climate and fire can interact to rapidly transform landscapes and disturbance regimes. The interactions can produce forest patterns with unexpected consequences such as reduced wildfire despite regional warming.

    

   more » « less
4. 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.

    

   more » « less
5. 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.

    

   more » « less