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Author: Ronda L. Little
Publisher:
ISBN:
Category : Conifers
Languages : en
Pages : 216
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Author: Ronda L. Little
Publisher:
ISBN:
Category : Conifers
Languages : en
Pages : 216
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Book Description
Author: Winslow D. Hansen
Publisher:
ISBN:
Category : Climatic changes
Languages : en
Pages : 21
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Robust tree regeneration following high-severity wildfire is key to the resilience of subalpine and boreal forests, and 21st century climate could initiate abrupt change in forests if postfire temperature and soil moisture become less suitable for tree seedling establishment. Using two widespread conifer species, lodgepole pine (Pinus contorta var. latifolia) and Douglas-fir (Pseudotsuga menziesii var. glauca), we conducted complementary experiments to ask (1) How will projected early- to mid-21st-century warming and drying affect postfire tree seedling establishment and mortality? (2) How does early seedling growth differ between species and vary with warming and drying? With a four-year in situ seed-planting experiment and a one growing season controlled-environment experiment, we explored effects of climate on tree seedling establishment, growth, and survival and identified nonlinear responses to temperature and soil moisture. In our field experiment, warmer and drier conditions, consistent with mid21st-century projections, led to a 92% and 76% reduction in establishment of lodgepole pine and Douglas-fir. Within three years, all seedlings that established under warmer conditions died, as might be expected at lower elevations and lower latitudes of species? ranges. Seedling establishment and mortality also varied with aspect; approximately 1.7 times more seedlings established on mesic vs. xeric aspects, and fewer seedlings died. In the controlled-environment experiment, soil temperatures were 2.0°?5.5°C cooler than the field experiment, and warming led to increased tree seedling establishment, as might be expected at upper treeline or higher latitudes. Lodgepole pine grew taller than Douglas-fir and produced more needles with warming. Douglas-fir grew longer roots relative to shoots, compared with lodgepole pine, particularly in dry soils. Differences in early growth between species may mediate climate change effects on competitive interactions, successional trajectories, and species distributions. This study demonstrates that climate following high-severity fire exerts strong control over postfire tree regeneration in subalpine conifer forests. Climate change experiments, such as those reported here, hold great potential for identifying mechanisms that could underpinfundamental ecological change in 21st-century ecosystems.
Author: Winslow D. Hansen
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Climate and disturbance regimes are rapidly changing in earth's forests, and these trends are expected to continue through the 21st century. It remains unresolved whether and where forests will absorb increased perturbations without changing qualitatively and where forest resilience might erode. This dissertation provides a foundation to begin addressing these uncertainties. I combined field observation, experiments, and process-based simulation to study effects of changing climate and wildfire on postfire tree regeneration and forest resilience in Yellowstone National Park, the largest intact wildland area of the contiguous United States. Chapter 1 quantifies effects of ecological filters on a colonizing cohort of aspen. These aspen trees established from seed after the 1988 fires and survived at higher elevations than their prefire distribution. I then conducted a long-term field experiment and shorter controlled-environment experiment to determine how temperature and soil moisture consistent with 21st-century projections may alter postfire seedling establishment of two widespread conifers (Chapter 2). In chapters 3 and 4, I used a forest simulation model to test multiple mechanisms of regeneration failure and to explore how suppression may alter 21st-century fire and forests. Long-term study of colonizing aspen demonstrated how wildfire can catalyze rapid shifts in tree-species distributions. Aspen seedlings were initially favored at lower elevations close to their prefire distribution. By 25yrs postfire, aspen was favored to survive at higher elevations, likely due to warming. From the experiments, it appears postfire drought may be a powerful force for change in subalpine forests because regeneration was drastically reduced under hotter-drier conditions. Simulations, where multiple climate-fire drivers could be considered over longer periods, however, indicate the potential for remarkable resilience. Regeneration failure was the exception, not the rule. Suppression of fire also had little impact on 21st-century fire or forests. Collectively, this research demonstrates that multiple streams of quantitative inquiry are necessary to better resolve how changing climate and disturbance will alter forests. Management steps could be taken to bolster vulnerable forests (e.g. reseeding after fires), if mechanisms of change are understood. However, forest-management strategies should not discount the inherent resilience of the system.
Author: Tyler John Hoecker
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Wildfires shaped subalpine forests in the northern US Rocky Mountains for millennia. However, recent climate-driven increases in fire frequency and severity are constraining postfire tree regeneration in forests dominated by fire-sensitive obligate-seeder conifers via reduced propagule pressure and in xeric areas via elevated aridity. Changes in tree regeneration are projected to drive large-scale shifts in the extent and character of future forests, which will cascade to effect critical ecosystem services. This dissertation explored controls on tree regeneration in diverse subalpine forest settings and disturbance contexts and implications for future ecosystem function. I investigated (i) effects of interactions between short-interval fires and topographic position on postfire regeneration using a seed-planting experiment in the Greater Yellowstone Ecosystem (GYE); (ii) consequences of future landscape change for three wildlife species in the GYE using the simulation model iLand and the Maxent algorithm; and (ii) shifts in forest composition and structure after recent fires in Glacier National Park using a field study. Postfire conifer regeneration was low on south-facing aspects affected by high-severity fires at short intervals (30 yrs). Soils on south-facing slopes were 2℗ʻC warmer and >20% drier than north-facing slopes after short-interval burns, and residual fire-killed trees after long-interval fires (>150 yrs) provided microclimate buffering of a similar magnitude. Distribution modeling from 2017-2100 revealed extensive loss of old forest in climate scenarios with substantial warming (RCP 8.5) and drying (HadGEM2-ES). Habitat for three vertebrates (Picoides arcticus, Martes caurina, Tamiasciurus hudsonicus) depended on spatial overlap of persistent forest with suitable climate, which were frequently mismatched. Mesic forests in Glacier remain resilient to fire, but short-interval fire reduced stand density and simplified composition. Postfire regeneration after long-interval fires is dominated by fire-adapted conifers; a second fire erodes the ability of fire-sensitive species to establish. If these shifts persist, the range of mesic conifers is likely to be reduced. Findings indicate climate- and fire-catalyzed changes in forest ecosystems are already emerging in the northern Rockies. Managing forests to preserve historical structure and composition will be increasingly difficult, requiring approaches that direct change toward desired outcomes where possible and accept alternatives where change is unavoidable.
Author: Winslow D. Hansen
Publisher:
ISBN:
Category : Climatic changes
Languages : en
Pages : 12
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Environmental change is accelerating in the 21st century, but how multiple drivers may interact to alter forest resilience remains uncertain. In forests affected by large high-severity disturbances, tree regeneration is a resilience linchpin that shapes successional trajectories for decades. We modeled stands of two widespread western U.S. conifers, Douglas-fir (Pseudotsuga menziesii var. glauca), and lodgepole pine (Pinus contorta var. latifolia), in Yellowstone National Park (Wyoming, USA) to ask (1) What combinations of distance to seed source, fire return interval, and warming-drying conditions cause postfire tree-regeneration failure? (2) If postfire tree regeneration was successful, how does early tree density differ under future climate relative to historical climate? We conducted a stand-level (1 ha) factorial simulation experiment using the individual-based forest process model iLand to identify combinations of fire return interval (11?100 yr), distance to seed source (50?1,000 m), and climate (historical, mid-21st century, late-21st century) where trees failed to regenerate by 30-yr postfire. If regeneration was successful, we compared stand densities between climate periods. Simulated postfire regeneration were surprisingly resilient to changing climate and fire drivers. Douglas-fir regeneration failed more frequently (55%) than lodgepole pine (28% and 16% for nonserotinous and serotinous stands, respectively). Distance to seed source was an important driver of regeneration failure for Douglas-fir and non-serotinous lodgepole pine; regeneration never failed when stands were 50 m from a seed source and nearly always failed when stands were 1 km away. Regeneration of serotinous lodgepole pine only failed when fire return intervals were ?20 yr and stands were far (1 km) from a seed source. Warming climate increased regeneration success for Douglas-fir but did not affect lodgepole pine. If regeneration was successful, postfire density varied with climate. Douglasfir and serotinous lodgepole pine regeneration density both increased under 21st-century climate but in response to different climate variables (growing season length vs. cold limitation). Results suggest that, given a warmer future with larger and more frequent fires, a greater number of stands that fail to regenerate after fires combined with increasing density in stands where regeneration is successful could produce a more coarse-grained forest landscape.
Author: Suzanne M. Owen
Publisher:
ISBN:
Category : Trees
Languages : en
Pages : 177
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Wildfires in southwestern US ponderosa pine (Pinus ponderosa Lawson & C. Lawson) forests have recently increased in size and severity, leaving large, contiguous patches of tree mortality, and raising concerns about post-fire recovery. Ponderosa pines are a dominant species in the Southwest and they evolved with low- to moderate-severity fire regimes. They are poorly adapted to regenerate after large, high-severity fires because they do not have serotinous cones, re-sprouting capabilities, or long-lived seed banks. Additionally, high-severity fires can favor competing understory plants or induce long-term changes to soil nutrient dynamics and surface fuel loads, potentially altering ponderosa pine regeneration niches. Furthermore, high-severity wildfires and the loss of ponderosa pines may alter fungal community composition, including pine-symbiotic ectomycorrhizal (EM) fungi and saprotrophic fungi, which are important for forest recovery and productivity. My research objectives were to understand the effects of fire severity > 10 years post-fire on: (1) the spatial patterns, and interactions of regenerating ponderosa pine and sprouting tree species, (2) ponderosa pine regeneration niches and seedling growth, and (3) fungal sporocarp and root tip EM community composition and colonization. My study sites for the first objective included large, 4-ha plots located in two types of high-severity (100% tree mortality) burn, either adjacent to residual live forest edges (edge plots) or > 200 m from any residual live trees (interior plots) in two Arizona wildfires, the 2000 Pumpkin and 2002 Rodeo-Chediski Fires.
Author: Nathan S. Gill
Publisher:
ISBN:
Category : Forest ecology
Languages : en
Pages : 13
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Subalpine forests that historically burned every 100?300 yr are expected to burn more frequently as climate warms, perhaps before trees reach reproductive maturity or produce a serotinous seedbank. Tree regeneration after short-interval (30-yr) high-severity fire will increasingly rely on seed dispersal from unburned trees, but how dispersal varies with age and structure of surrounding forest is poorly understood. We studied wind dispersal of three conifers (Picea engelmannii, Abies lasiocarpa, and Pinus contorta var. latifolia, which can be serotinous and nonserotinous) after a stand-replacing fire that burned young (?30 yr) and older (100 yr) P. contorta forest in Grand Teton National Park (Wyoming, USA). We asked how propagule pressure varied with time since last fire, how seed delivery into burned forest varied with age and structure of live forest edges, what variables explained seed delivery into burned forest, and how spatial patterns of delivery across the burned area could vary with alternate patterns of surrounding live forest age. Seeds were collected in traps along 100-m transects (n = 18) extending from live forest edges of varying age (18, 30, and >100 yr) into areas of recent (2-yr) high-severity fire, and along transects in live forests to measure propagule pressure. Propagule pressure was low in 18-yr-old stands (~8 seeds/m2) and similarly greater in 30- and 100-yr-old stands (~32 seeds/m2). Mean dispersal distance was lowest from 18-yr-old edges and greatest from >100-yr-old edges. Seed delivery into burned forest declined with increasing distance and increased with height of trees at live forest edges, and was consistently higher for P. contorta than for other conifers. Empirical dispersal kernels revealed that seed delivery from 18-yr-old edges was very low (?2.4 seeds/m2) and concentrated within 10 m of the live edge, whereas seed delivery from >100-yr-old edges was >4.9 seeds/m2 out to 80 m. When extrapolated throughout the burned landscape, estimated seed delivery was low (49,400 seeds/ha) in 70% of areas that burned in short-interval fire (
Author:
Publisher:
ISBN:
Category : Conifers
Languages : en
Pages : 0
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Shifting wildfire patterns and climate conditions, magnified by anthropogenic climate change, are threatening the resilience of conifer forests in North America and more specifically, the western US. If native conifer species are functionally maladapted to novel fire patterns and post-fire climate conditions, large-scale shifts in conifer forest structure, composition, and extent may occur as warming intensifies. Forest resilience in the context of fire and climate can be understood and quantified by the survival of trees through fire events and success of trees to regenerate post-fire and maintain population levels. In this dissertation, I use field observations and remote sensing to examine patterns of fire-induced tree mortality and post-fire tree regeneration as proxies of conifer forest resilience in the western US, across a range of environments and forest types, and particularly within the context of expansive high-severity, stand-replacing wildfires. In Chapter 1, I evaluate the interactions between climate-environment conditions and the spatial, structural, and temporal characteristics of fire refugia as drivers of subalpine forest recovery in the cool and moist Cascade Range of Oregon and Washington. In Chapter 2, I quantify large-scale patterns of post-fire delayed conifer tree mortality across three ecoregions and two broad forest types in the western US using high-resolution satellite imagery, and I evaluate whether post-fire delayed conifer tree mortality is a ubiquitous process across broad geographies, and if so, I ask i) what drives it? and ii) can it meaningfully affect seed dispersal and thus forest regeneration processes? Finally, in Chapter 3, I use an aggregated database of post-fire conifer establishment responses, across over 1800 sites and four ecoregions in the western US, to challenge the generalized notion that conifer species' shade-tolerance dictates their regenerative capacity within exposed early seral post-fire environments.
Author: Linda L. Wallace
Publisher: Yale University Press
ISBN: 0300127758
Category : Nature
Languages : en
Pages : 400
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Americans currently choose their president through the electoral college, an extraordinarily complex mechanism that may elect a candidate who does not receive the most votes. In this provocative book, George Edwards III argues that, contrary to what supporters of the electoral college claim, there is no real justification for a system that might violate majority rule. Drawing on systematic data, Edwards finds that the electoral college does not protect the interests of small states or racial minorities, does not provide presidents with effective coalitions for governing, and does little to protect the American polity from the alleged harms of direct election of the president. In fact, the electoral college distorts the presidential campaign so that candidates ignore most small states and some large ones and pay little attention to minorities, and it encourages third parties to run presidential candidates and discourages party competition in many states. Edwards demonstrates effectively that direct election of the president without a runoff maximizes political equality and eliminates the distortions in the political system caused by the electoral college.
Author: Thomas T. Veblen
Publisher:
ISBN:
Category : History
Languages : en
Pages : 204
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