Low-severity fire increases tree defense against bark beetle attacks

Induced defense is a common plant strategy in response to herbivory. Although abiotic damage, such as physical wounding, pruning, and heating, can induce plant defense, the effect of such damage by large-scale abiotic disturbances on induced defenses has not been explored and could have important consequences for plant survival facing future biotic disturbances. Historically, low-severity wildfire was a widespread, frequent abiotic disturbance in many temperate coniferous forests. Native Dendroctonus and Ips bark beetles are also a common biotic disturbance agent in these forest types and can influence tree mortality patterns after wildfire. Therefore, species living in these disturbance-prone environments with strategies to survive both frequent fire and bark beetle attack should be favored. One such example is Pinus ponderosa forests of western North America. These forests are susceptible to bark beetle attack and frequent, low-severity fire was common prior to European settlement. However, since the late 1800s, frequent, low-severity fires have greatly decreased in these forests. We hypothesized that non-lethal, low-severity, wildfire induces resin duct defense in P. ponderosa and that lack of low-severity fire relaxes resin duct defense in forests dependent on frequent, low-severity fire. We first compared axial resin duct traits between trees that either survived or died from bark beetle attacks. Next, we studied axial ducts using tree cores with crossdated chronologies in several natural P. ponderosa stands before and after an individual wildfire and, also, before and after an abrupt change in fire frequency in the 20th century. We show that trees killed by bark beetles invested less in resin ducts relative to trees that survived attack, suggesting that resin duct-related traits provide resistance against bark beetles. We then show low-severity fire induces resin duct production, and finally, that resin duct production declines when fire ceases. Our results demonstrate that low-severity fire can trigger a long-lasting induced defense that may increase tree survival from subsequent herbivory

RESEARCH REPORT CONDITION OF LIVE FIRE-SCARRED PONDEROSA PINE ELEVEN YEARS AFTER REMOVING PARTIAL CROSS-SECTIONS

ABSTRACT Our objective is to report mortality rates for ponderosa pine trees in Oregon ten to eleven years after removing a fire-scarred partial cross-section from them, and five years after an initial survey of post-sampling mortality. We surveyed 138 live trees from which we removed fire-scarred partial crosssections in 1994/95 and 387 similarly sized, unsampled neighbor trees of the same species. These trees were from 78 plots distributed over about 5,000 ha at two sites in northeastern Oregon. The annual mortality rate for sectioned trees from 1994/95 to 2005 was 3.6% compared to 2.1% for the neighbor trees. However, many of the trees that died between 2000 and 2005 were likely killed by two prescribed fires at one of the sites. Excluding all trees in the plots burned by these fires (regardless of whether they died or not), the annual mortality rate for sectioned trees was 1.4% (identical to the rate from 1994/95 to 2000) compared to 1.0% for neighbor trees. During these fires, a greater proportion of sectioned trees died than did catfaced neighbor trees (80% versus 64%) but the difference was not significant

Multicentury Fire and Forest Histories at 19 Sites in Utah and Eastern Nevada

Our objective is to provide site-specific fire and forest histories from Utah and eastern Nevada that can be used for land management or additional research. We systematically sampled fire scars and tree-recruitment dates across broad gradients in elevation and forest type at 13 sites in Utah and 1 in eastern Nevada to characterize spatial and temporal variation in historical fire regimes as well as forest structure and composition. We collected similar data non-systematically at five additional sites in Utah. These 19 sites include a broad range of forest types (from pinyon-juniper woodlands to spruce-fir forests) and fire regime types. In this report, we summarize local-scale spatial and temporal variation with site-specific details of historical fire regimes and forests that will be useful for local natural resource and fire management of the individual sites. For each site, we report topography, chronologies of fire and tree recruitment, and properties derived from those chronologies such as time-averaged fire regime parameters (mean fire interval and fire severity) and changes in forest composition and structure that have occurred since the late 1800s

Western Spruce Budworm and Wildfire: Is There a Connection?

In the interior Pacific Northwest, extensive defoliation of mixed conifer forests during outbreaks of western spruce budworm (WSB) may leave the visual impression of a tinderbox with trees primed to burst into flame. But is this the case? We addressed this question with funding from the USDA/U.S. Department of the Interior Joint Fire Science Program (project 09– 1–06–5). Here we summarize our three recent publications exploring the potential relationship between WSB outbreaks and fire. We used a multimethod approach to explore potential disturbance interactions that might cause one disturbance to change the occurrence or severity of the other. We used tree-ring records to see whether WSB and fire are related in time and computer modeling to see how defoliation could affect crown fire behavior

Characteristics of atmospheric organic and elemental carbon particle concentrations in Los Angeles

A fine particle air monitoring network was operated in the Los Angeles area during 1982. It was found that carbonaceous aerosols accounted for typically 40% of total fine particle mass loadings at most monitoring sites. The ratio of total carbon (TC) to elemental carbon (EC) in ambient samples and in primary source emissions was examined as an indicator of the extent of secondary organic aerosol formation. It was found that TC to EC ratios at all sites on average are no higher than recent estimates of the TC to EC ratio in primary source emissions. There is little evidence of the sustained summer peak in the ratio of TC to EC that one might expect if greatly enhanced secondary organics production occurs during the photochemical smog season. The TC to EC ratio does rise by the time that air masses reach the prevailing downwind edge of the air basin as would be expected if secondary organics are being formed during air parcel transport, but the extent of that increase is modest. These results suggest that primary particulate carbon emissions were the principal contributor to long-term average fine aerosol carbon concentrations in the Los Angeles area during 1982

Western Spruce Budworm Outbreaks Did Not Increase Fire Risk over the Last Three Centuries: A Dendrochronological Analysis of Inter-Disturbance Synergism

Insect outbreaks are often assumed to increase the severity or probability of fire occurrence through increased fuel availability, while fires may in turn alter susceptibility of forests to subsequent insect outbreaks through changes in the spatial distribution of suitable host trees. However, little is actually known about the potential synergisms between these natural disturbances. Assessing interdisturbance synergism is challenging due to the short length of historical records and the confounding influences of land use and climate changes on natural disturbance dynamics. We used dendrochronological methods to reconstruct defoliator outbreaks and fire occurrence at ten sites along a longitudinal transect running from central Oregon to western Montana. We assessed synergism between disturbance types, analyzed long-term changes in disturbance dynamics, and compared these disturbance histories with dendroclimatological moisture availability records to quantify the influence of moisture availability on disturbances. After approximately 1890, fires were largely absent and defoliator outbreaks became longer-lasting, more frequent, and more synchronous at our sites. Fires were more likely to occur during warm-dry years, while outbreaks were most likely to begin near the end of warm-dry periods. Our results show no discernible impact of defoliation events on subsequent fire risk. Any effect from the addition of fuels during defoliation events appears to be too small to detect given the overriding influence of climatic variability. We therefore propose that if there is any relationship between the two disturbances, it is a subtle synergistic relationship wherein climate determines the probability of occurrence of each disturbance type, and each disturbance type damps the severity, but does not alter the probability of occurrence, of the other disturbance type over long time scales. Although both disturbance types may increase in frequency or extent in response to future warming, our records show no precedent that western spruce budworm outbreaks will increase future fire risk

Interactions of Insects, Fire and Climate on Fuel Loads and Fire Behavior in Mixed Conifer Forest

Mixed-conifer forests in the interior Pacific Northwest are subject to sporadic outbreaks of the western spruce budworm, the most destructive defoliator in western North America. Such outbreaks usually occur synchronously over broad regions and lead to widespread decreases in growth rates and low to moderate levels of mortality. In the last century, changing land use and fire suppression have led to an increase in the amount and density of host tree species, and changed fire regimes. This has altered the severity and frequency of both fire and western spruce budworm. In spite of the ecological and economic significance of these disturbances, their interactions with each other and with climate are not fully understood. We used two approaches to examine these interactions across a range of temporal and spatial scales. First, we used dendrochronological methods to examine the climatic drivers of budworm outbreaks and fires and to assess the association of fire and budworm over three centuries in 13 stands across Oregon, Idaho, and Montana. Second, we used a mechanistic fire behavior model, the Wildland-urban interface Fire Dynamics Simulator (WFDS) to examine the sensitivity of crown fire to multiple aspects of defoliated crown fuels, including changing crown bulk density and branchwood moisture. The dendrochronological reconstructions revealed repeated western spruce budworm outbreaks and fires over the past several centuries, with different climate events associated with each disturbance. Outbreaks sometimes persisted more than a decade and were often synchronous among sites. An average of 12 outbreaks occurred at each site, each lasting an average of 12 years in length, with an average of 15 years between outbreaks. Outbreak initiation was often regionally synchronous. Synchrony was higher in the second half of the record (since 1900), possibly due to increased abundance and continuity of host trees during the fire suppression era. Outbreak duration and frequency were also somewhat higher after approximately 1890. We found that warm-dry conditions occurred one to three years preceding outbreak initiation, suggesting that drought-stressed trees permit population growth to a level at which predators no longer strongly limit the budworm population. The mean fire return interval in these mixed-conifer stands was 34 years (range: 16 – 53 years). Fires tended to occur during warm-dry years. We found no evidence of a consistent relationship between the timing of fires and western spruce budworm outbreaks. Western spruce budworm is associated with the ends of droughts and fire is simply associated with single drought years. The simulation study found that defoliation reduces both torching and crowning potential, requiring greater surface fire intensity for crown ignition than undefoliated tree crowns with the same crown base height. Single, highly defoliated trees (80%) experienced little or no torching, and moderately defoliated trees (50%) required about twice the surface fire intensity of undefoliated trees to produce the same heat output. For example, at a surface fire intensity of 700 kW/m2 , 99% of the canopy fuel from the undefoliated tree was consumed, leaving 2 kg of foliage on the tree, compared to 81% consumption of a moderately (50%) defoliated tree, leaving 15 kg of foliage. The effects of defoliation were somewhat mitigated by canopy fuel heterogeneity and potential branchwood drying, but these effects were less pronounced than defolation itself. Our study suggests that areas heavily defoliated by western spruce budworm may inhibit crown fire spread and may thus promote non-lethal surface fires

Advancing dendrochronological studies of fire in the United States

© 2018 by the authors. Licensee MDPI, Basel, Switzerland. Dendroecology is the science that dates tree rings to their exact calendar year of formation to study processes that influence forest ecology (e.g., Speer 2010 [1], Amoroso et al., 2017 [2]). Reconstruction of past fire regimes is a core application of dendroecology, linking fire history to population dynamics and climate effects on tree growth and survivorship. Since the early 20th century when dendrochronologists recognized that tree rings retained fire scars (e.g., Figure 1), and hence a record of past fires, they have conducted studies worldwide to reconstruct [2] the historical range and variability of fire regimes (e.g., frequency, severity, seasonality, spatial extent), [3] the influence of fire regimes on forest structure and ecosystem dynamics, and [4] the top-down (e.g., climate) and bottom-up (e.g., fuels, topography) drivers of fire that operate at a range of temporal and spatial scales. As in other scientific fields, continued application of dendrochronological techniques to study fires has shaped new trajectories for the science. Here we highlight some important current directions in the United States (US) and call on our international colleagues to continue the conversation with perspectives from other countries

Spatial and temporal variation in historical fire regimes of the Blue Mountains, Oregon and Washington: the influence of climate

Thesis (Ph. D.)--University of Washington, 1997To identify the influence of climate on spatial and temporal variation in fire, low- to high-severity fire regimes were reconstructed from tree-rings in the Blue Mountains. Fire recurrence, extent, severity and seasonality (low-severity fires only) were determined from fire scars and ages of 1426 trees sampled on 2914-8585 ha grids in 4 watersheds. Before 1900, fire regimes varied at regional (among watersheds) and local (within watersheds) spatial scales, although not all parameters of fire varied at both scales. Regionally, fires in ponderosa pine-dominated forests burned more frequently and earlier in the growing season in southern than northern watersheds, consistent with the occurrence of longer and drier fire seasons in the southern Blue Mountains. Fire extent did not vary regionally. Locally, fire recurrence varied with topography (aspect or elevation) in steep terrain but not in gentle terrain, while local variation in fire extent was unrelated to topography in any watershed. Temporal variation in the extent of low-severity fires was compared to existing tree-ring reconstructions of regional precipitation and an index of the Southern Oscillation (SOI). In southern watersheds, fire extent varied inversely with precipitation on annual and longer time scales. In northern watersheds, SOI tended to be low (El Nino conditions) during fire years, consistent with shorter snow-cover duration to the north during El Nino years. Prior year's climate (regional precipitation or SOI) did not influence fire extent in any watershed. Despite some regional synchrony in precipitation, fires rarely burned in more than one of the sampled watersheds during a given year, probably because processes that influence the ignition and/or spread of fire operate at sub-regional spatial scales (e.g., lightning strikes, precipitation from convective storms). After about 1900, few fires occurred in any of the watersheds. These results suggest that to predict spatial variation in fire regimes within a given forest type, the spatial scales at which the controls of fire (e.g., climate and topography) operate must be considered. These results also imply that future fire regimes could be affected by changes in the duration of snow cover or by changes in the amount or timing of ignition