Geology topics

Dylan W. Schwilk

Publications and source records attributed to Dylan W. Schwilk.

7 recordsLinked to original sources

USGS2022

Reimagine fire science for the anthropocene

Fire is an integral component of ecosystems globally and a tool that humans have harnessed for millennia. Altered fire regimes are a fundamental cause and consequence of global change, impacting people and the biophysical systems on which they depend. As part of the newly emerging Anthropocene, marked by human-caused climate change and radical changes to ecosystems, fire danger is increasing, and fires are having increasingly devastating impacts on human health, infrastructure, and ecosystem services. Increasing fire danger is a vexing problem that requires deep transdisciplinary, trans-sector, and inclusive partnerships to address. Here, we outline barriers and opportunities in the next generation of fire science and provide guidance for investment in future research. We synthesize insights needed to better address the long-standing challenges of innovation across disciplines to (i) promote coordinated research efforts; (ii) embrace different ways of knowing and knowledge generation; (iii) promote exploration of fundamental science; (iv) capitalize on the “firehose” of data for societal benefit; and (v) integrate human and natural systems into models across multiple scales. Fire science is thus at a critical transitional moment. We need to shift from observation and modeled representations of varying components of climate, people, vegetation, and fire to more integrative and predictive approaches that support pathways toward mitigating and adapting to our increasingly flammable world, including the utilization of fire for human safety and benefit. Only through overcoming institutional silos and accessing knowledge across diverse communities can we effectively undertake research that improves outcomes in our more fiery future.

PNAS Nexus↗

USGS2020

Biogeography of fire regimes in western US conifer forests: A trait-based approach

Aim Functional traits are a critical link between species distributions and the ecosystem processes that structure those species’ niches. Concurrent increases in the availability of functional trait data and our ability to model species distributions present an opportunity to develop functional trait biogeography, i.e. the mapping of functional traits across space. Functional trait biogeography can improve process-based predictions about the resistance of certain species assemblages to changing environmental conditions across landscape scales. We illustrate this concept by developing the first trait-based, quantitative ranking of fire resistance (adult tree survival) in North American conifer species, and mapping that fire resistance across space. Location and Time period Western Continental United States, present-day. Major taxa studied 29 common conifer tree species. Methods We compiled six traits for each species: three relating to tree morphology and three relating to litter flammability. We combined these traits into a single fire resistance score, and used community-weighted averaging to estimate the fire resistance scores of different forest communities, using interpolated species distribution and relative abundance data. Results Species associated with historically frequent fire have high fire resistance scores (e.g., Pinus ponderosa), reflected by thick bark, tall crowns, and flammable litter. Species associated with subalpine or arid conditions have low fire resistance scores (e.g., Picea engelmannii and Pinus edulis), reflected by thin bark, short stature, poor self-pruning and low litter flammability. A map of forest community fire resistance across the western US reveals agreement with independent assessments of historical fire regimes, while also identifying areas where community-wide species traits may be mismatched with historical fire regimes. Main conclusions Quantifying the functional traits that confer resistance to tree-killing fire provides a direct link between ecosystem disturbance and community resistance. Understanding this link is critical to evaluating long-term resilience of different forest types under dynamic fire regimes. Our work represents the first known spatial representation of fire-resistance traits at a regional scale, and as such provides a link between functional traits and biogeography relevant to a critical ecosystem process.

Global Ecology and Biogeography↗

USGS2016

Flammability as an ecological and evolutionary driver

We live on a flammable planet yet there is little consensus on the origin and evolution of flammability in our flora. We argue that part of the problem lies in the concept of flammability, which should not be viewed as a single quantitative trait or metric. Rather, we propose that flammability has three major dimensions that are not necessarily correlated: ignitability, heat release and fire spread rate. These major axes of variation are controlled by different plant traits and have differing ecological impacts during fire. At the individual plant scale, these traits define three flammability strategies observed in fire-prone ecosystems: the non-flammable, the fast-flammable and the hot-flammable strategy (with low ignitability, high flame spread rate and high heat release, respectively). These strategies increase the survival or reproduction under recurrent fires, and thus, plants in fire-prone ecosystems benefit from acquiring one of them; they represent different (alternative) ways to live under recurrent fires. Synthesis . This novel framework based on different flammability strategies helps us to understand variability in flammability across scales, and provides a basis for further research.

Journal of Ecology↗

USGS2012

A plant distribution shift: temperature, drought or past disturbance?

Simple models of plant response to warming climates predict vegetation moving to cooler and/or wetter locations: in mountainous regions shifting upslope. However, species-specific responses to climate change are likely to be much more complex. We re-examined a recently reported vegetation shift in the Santa Rosa Mountains, California, to better understand the mechanisms behind the reported shift of a plant distribution upslope. We focused on five elevational zones near the center of the gradient that captured many of the reported shifts and which are dominated by fire-prone chaparral. Using growth rings, we determined that a major assumption of the previous work was wrong: past fire histories differed among elevations. To examine the potential effect that this difference might have on the reported upward shift, we focused on one species, Ceanothus greggii : a shrub that only recruits post-fire from a soil stored seedbank. For five elevations used in the prior study, we calculated time series of past per-capita mortality rates by counting growth rings on live and dead individuals. We tested three alternative hypotheses explaining the past patterns of mortality: 1) mortality increased over time consistent with climate warming, 2) mortality was correlated with drought indices, and 3) mortality peaked 40–50 years post fire at each site, consistent with self-thinning. We found that the sites were different ages since the last fire, and that the reported increase in the mean elevation of C. greggii was due to higher recent mortality at the lower elevations, which were younger sites. The time-series pattern of mortality was best explained by the self-thinning hypothesis and poorly explained by gradual warming or drought. At least for this species, the reported distribution shift appears to be an artifact of disturbance history and is not evidence of a climate warming effect.

California↗

USGS2006

Fire decreases arthropod abundance but increases diversity: Early and late season prescribed fire effects in a Sierra Nevada mixed-conifer forest

Prior to fire suppression in the 20th century, the mixed-conifer forests of the Sierra Nevada, California, U.S.A., historically burned in frequent fires that typically occurred during the late summer and early fall. Fire managers have been attempting to restore natural ecosystem processes through prescription burning, and have often favored burning during the fall in order to mimic historical fire regimes. Increasingly, however, prescription burning is also being done during the late spring and early summer in order to expand the window of opportunity for needed fuel reduction burning. The effect of prescribed fires outside of the historical fire season on forest arthropods is not known. The objective of this study was to compare the short-term effects of prescribed fires ignited in the early and late fire season on forest floor arthropods. Arthropod abundance and diversity were assessed using pitfall trapping in replicated burn units in Sequoia National Park, California. Overall, abundance of arthropods was lower in the burn treatments than in the unburned control. However, diversity tended to be greater in the burn treatments. Fire also altered the relative abundances of arthropod feeding guilds. No significant differences in arthropod community structure were found between early and late season burn treatments. Instead, changes in the arthropod community appeared to be driven largely by changes in fuel loading, vegetation, and habitat heterogeneity, all of which differed more between the burned and unburned treatments than between early and late season burn treatments.

California↗

USGS2006

Tree mortality from fire and bark beetles following early and late season prescribed fires in a Sierra Nevada mixed-conifer forest

Over the last century, fire exclusion in the forests of the Sierra Nevada has allowed surface fuels to accumulate and has led to increased tree density. Stand composition has also been altered as shade tolerant tree species crowd out shade intolerant species. To restore forest structure and reduce the risk of large, intense fires, managers have increasingly used prescription burning. Most fires prior to EuroAmerican settlement occurred during the late summer and early fall and most prescribed burning has taken place during the latter part of this period. Poor air quality and lack of suitable burn windows during the fall, however, have resulted in a need to conduct more prescription burning earlier in the season. Previous reports have suggested that burning during the time when trees are actively growing may increase mortality rates due to fine root damage and/or bark beetle activity. This study examines the effects of fire on tree mortality and bark beetle attacks under prescription burning during early and late season. Replicated early season burn, late season burn and unburned control plots were established in an old-growth mixed conifer forest in the Sierra Nevada that had not experienced a fire in over 120 years. Although prescribed burns resulted in significant mortality of particularly the smallest tree size classes, no difference between early and late season burns was detected. Direct mortality due to fire was associated with fire intensity. Secondary mortality due to bark beetles was not significantly correlated with fire intensity. The probability of bark beetle attack on pines did not differ between early and late season burns, while the probability of bark beetle attack on firs was greater following early season burns. Overall tree mortality appeared to be primarily the result of fire intensity rather than tree phenology at the time of the burns. Early season burns are generally conducted under higher fuel moisture conditions, leading to less fuel consumption and potentially less injury to trees. This reduction in fire severity may compensate for relatively modest increases in bark beetle attack probabilities on some tree species, ultimately resulting in a forest structure that differs little between early and late season prescribed burning treatments.

California↗

USGS2006

The role of fire refugia in the distribution of Pinus sabiniana (Pinaceae) in the southern Sierra Nevada

Although widespread throughout the interior foothills of central and northern California, Pinus sabiniana Dougl. has a disjunct distribution in the southern Sierra Nevada, where it is abundant in the Kern River and Tule River watersheds, but is absent from the Kaweah River watershed between 36° and 37°N. This gap in the pine's distribution has long intrigued botanists and ecologists and has elicited a number of hypotheses for this anomalous biogeographical pattern. Here we propose a new hypothesis that couples unique features of the southern Sierra Nevada topography with unique features of P. sabiniana 's response to fire. This low elevation pine is widely distributed in grassland and chaparral, and where it occurs with the latter vegetation, it is extremely vulnerable to high intensity wildfires. Under these conditions, meta-populations persist over time in refugia in riparian areas and during fire-free intervals expand outwards re-colonizing shrubland dominated slopes. The lack of such refugia in the very steep and narrow Kaweah drainage is hypothesized to explain the absence of this pine in that area. To test this hypothesis, we studied the age-structure of P. sabiniana in the area of the 2002 McNally Fire in the Kern drainage to compare age distributions of trees and tree skeletons along a gradient up slope away from riparian zones. Maximum age declined significantly with distance from riparian areas, suggesting that past fires have eliminated P. sabiniana from the slopes and that the pines have re-colonized during fire-free intervals. The relationship was strongest when our data were restricted to areas that had a previously recorded fire. We also found that the riparian areas in the Kern drainage were significantly wider than those in Kaweah drainage, suggesting that fewer such fire refugia exist in the latter watershed, and providing an explanation for the lack of P. sabiniana between 36° and 37°.

California↗