Title: Fires Following Bark Beetles: Factors Controlling Severity and Disturbance Interactions in Ponderosa Pine

Abstract

Previous studies have suggested that bark beetles and fires can be interacting disturbances, whereby bark beetle–caused tree mortality can alter the risk and severity of subsequent wildland fires. However, there remains considerable uncertainty around the type and magnitude of the interaction between fires following bark beetle attacks, especially in drier forest types such as those dominated by ponderosa pine (Pinus ponderosa Lawson & C. Lawson). We used a full factorial design across a range of factors thought to control bark beetle-fire interactions, including the temporal phase of the outbreak, level of mortality, and wind speed. We used a three-dimensional physics-based model, HIGRAD/FIRETEC, to simulate fire behavior in fuel beds representative of 60 field plots across five national forests in northern Arizona, USA. The plots were dominated by ponderosa pine, and encompassed a gradient of bark beetle–caused mortality due to a mixture of both Ips and Dendroctonus species. Non-host species included two sprouting species, Gambel oak (Quercus gambelii Nutt.) and alligator juniper (Juniperus deppeana Steud.), as well as other junipers and pinyon pine (Pinus edulis Engelm.). The simulations explicitly accounted for the modifications of fuel mass and moisture distribution caused by bark beetle–caused mortality. We first analyzed the influence of themore » outbreak phase, level of mortality, and wind speed on the severity of a subsequent fire, expressed as a function of live and dead canopy fuel consumption. We then computed a metric based on canopy fuel loss to characterize whether bark beetles and fire are linked disturbances and, if they are, if the linkage is antagonistic (net bark beetle and fire severity being less than if the two disturbances occurred independently) or synergistic (greater combined effects than independent disturbances). Both the severity of a subsequent fire and whether bark beetles and fire are linked disturbances depended on the outbreak phase of the bark beetle mortality and attack severity, as well as the fire weather (here, wind). Greater fire severity and synergistic interactions were generally associated with the “red phase” (when dead needles remain on trees). In contrast, during the “gray phase” (when dead needles had fallen to the ground), fire severity was either similar to, or less than, green-phase fires and interactions were generally antagonistic, but included both synergistic and neutral interactions. The simulations also revealed that the magnitude of the linkage between these two disturbances was smaller for fires occurring during high wind conditions, especially in the red phase. This complexity might be a reason for the contrasted or controversial perception of bark beetle-fire interactions reported in the literature, since both fire severity and the type and magnitude of the linkage can vary strongly among studies. These results suggest that, for fires burning in the gray phase following moderate levels of mortality, bark beetle–caused mortality may buffer rather than exacerbate fire severity. However, for fires burning under high wind speeds, regardless of the outbreak phase or level of mortality, the near complete loss of canopy fuels may push this ecosystem into an alternative state dominated by sprouting species.« less

Authors:

Sieg, Carolyn H. ^[1]; Linn, Rodman R. ^[2]; Pimont, Francois ^[3]; Hoffman, Chad M. ^[4]; McMillin, Joel D. ^[5]; Winterkamp, Judith ^[2]; Baggett, L. Scott ^[6]

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+ Show Author Affiliations

1. USDA Forest Service, Flagstaff, AZ (United States). Rocky Mountain Research Station
2. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
3. Ecologie des Forets Mediterraneennes (UFRM), Avignon (France)
4. Colorado State Univ., Fort Collins, CO (United States)
5. USDA Forest Service, Boise, ID (United States). Forest Health Protection
6. USDA Forest Service, Fort Collins, CO (United States). Rocky Mountain Research Station

Publication Date:

Fri Dec 01 00:00:00 EST 2017

Research Org.:

Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

Sponsoring Org.:

USDA Forest Service; USDOE

OSTI Identifier:

1463539

Report Number(s):

LA-UR-17-22606
Journal ID: ISSN 1933-9747

Grant/Contract Number:  

AC52-06NA25396

Resource Type:

Accepted Manuscript

Journal Name:

Fire Ecology (Online)

Additional Journal Information:

Journal Name: Fire Ecology (Online); Journal Volume: 13; Journal Issue: 3; Journal ID: ISSN 1933-9747

Publisher:

Association for Fire Ecology (AFE)

Country of Publication:

United States

Language:

English

Subject:

54 ENVIRONMENTAL SCIENCES; Earth Sciences; antagonism, canopy fuel consumption, Dendroctonus, fire severity, HIGRAD/FIRETEC, interacting disturbances, Ips, linked disturbances, Pinus ponderosa, synergism