An official website of the United States government
Abstract 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
Interacting effects of fire and hydroclimate on oak and beech community prevalence in the southern Great Lakes region
Abstract Rising temperatures, increasing hydroclimate variability and intensifying disturbance regimes increase the risk of rapid ecosystem conversions. We can leverage multi‐proxy records of past ecosystem transformations to understand their causes and ecosystem vulnerability to rapid change.Prior to Euro‐American settlement, northern Indiana was a mosaic of prairie, oak‐dominated forests/woodlands and beech‐dominated hardwood forests. This heterogeneity, combined with well‐documented but poorly understood past beech population declines, make this region ideal for studying the drivers of ecosystem transformations.Here, we present a new record from Story Lake, IN, with proxies for vegetation composition (pollen), fire (charcoal) and beech intrinsic water use efficiency (δ13C of beech pollen; δ13Cbeech). Multiple proxies from the same core enable clear establishment of lead–lag relationships. Additionally, δ13Cbeechenables direct comparisons between beech population abundance and physiological responses to changing environments. We compare Story Lake to a nearby lake‐level reconstruction and to pollen records from nearby Pretty and Appleman Lakes and the distal Spicer Lake, to test hypotheses about synchrony and the spatial scale of governing processes.The 11.7 ka sediment record from Story Lake indicates multiple conversions between beech‐hardwood forest and oak forest/woodland. Beech pollen abundances rapidly increased between 7.5 and 7.1 ka, while oak declined. Oak abundances increased after 4.6 ka and remained high until 2.8 ka, indicating replacement of mesic forests by oak forest/woodland. At 2.8 ka, beech abundances rapidly increased, indicating mesic forest reestablishment. Beech and oak abundances correlate with charcoal accumulation rates but beech abundance is not correlated with δ13Cbeech.Fluctuations in beech abundances are synchronous among Story, Appleman and Pretty Lakes, but asynchronous between Story and Spicer Lakes, suggesting regulation by local‐scale vegetation‐fire‐climate feedbacks and secondarily by regional‐scale drivers.Holocene forest composition and fire dynamics appear to be closely co‐regulated and may be affected by local to regional climate variations. The importance of extrinsic drivers and positive/negative feedbacks changes over time, with higher ecoclimate sensitivity before 2.8 ka and greater resilience afterwards.Synthesis: Overall, oak‐ and beech‐dominated ecosystems were highly dynamic over the Holocene, with multiple ecosystem conversions driven by shifting interactions among vegetation, hydroclimate and fire regime.
more »
« less
- PAR ID:
- 10539551
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Ecology
- Volume:
- 112
- Issue:
- 5
- ISSN:
- 0022-0477
- Format(s):
- Medium: X Size: p. 1101-1122
- Size(s):
- p. 1101-1122
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
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.more » « less
-
Abstract Global change is shifting disturbance regimes that may rapidly change ecosystems, sometimes causing ecosystems to shift between states. Interactions between disturbances such as fire and disease could have especially severe effects, but experimental tests of multi‐decadal changes in disturbance regimes are rare. Here, we surveyed vegetation for 35 years in a 54‐year fire frequency experiment in a temperate oak savanna–forest ecotone that experienced a recent outbreak of oak wilt. Different fire regimes determined whether plots were savanna or forest by regulating tree abundance (r2 = 0.70), but disease rapidly reversed the effect of fire exclusion, increasing mortality by 765% in unburned forests, but causing relatively minor changes in frequently burned savannas. Model simulations demonstrated that disease caused unburned forests to transition towards a unique woodland that was prone to transition to savanna if fire was reintroduced. Consequently, disease–fire interactions could shift ecosystem resilience and biome boundaries as pathogen distributions change.more » « less
-
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.more » « less
-
Summary Rising atmospheric carbon dioxide concentrations (CO2) and atmospheric nitrogen (N) deposition have contrasting effects on ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) symbioses, potentially mediating forest responses to environmental change.In this study, we evaluated the cumulative effects of historical environmental change on N concentrations and δ15N values in AM plants, EM plants, EM fungi, and saprotrophic fungi using herbarium specimens collected in Minnesota, USA from 1871 to 2016. To better understand mycorrhizal mediation of foliar δ15N, we also analyzed a subset of previously published foliar δ15N values from across the United States to parse the effects of N deposition and CO2rise.Over the last century in Minnesota, N concentrations declined among all groups except saprotrophic fungi. δ15N also declined among all groups of plants and fungi; however, foliar δ15N declined less in EM plants than in AM plants. In the analysis of previously published foliar δ15N values, this slope difference between EM and AM plants was better explained by nitrogen deposition than by CO2rise.Mycorrhizal type did not explain trajectories of plant N concentrations. Instead, plants and EM fungi exhibited similar declines in N concentrations, consistent with declining forest N status despite moderate levels of N deposition.more » « less
