Mixed-severity fire in lodgepole pine dominated forests: are historical regimes sustainable on Oregon’s Pumice Plateau, USA?

Abstract

In parts of central Oregon, coarse-textured pumice substrates limit forest composition to low-density lodgepole pine (Pinus contorta Douglas ex Loudon var. latifolia Engelm. ex S. Watson) with scattered ponderosa pine (Pinus ponderosa Lawson & C. Lawson) and a shrub understory dominated by antelope bitterbrush (Purshia tridentata (Pursh) DC.). We reconstructed the historical fire regime from tree rings and simulated fire behavior over 783 ha of this forest type. For centuries , extensive mixed-severity fires occurred every 26 to 82 years, creating a multi-aged forest and shrub mosaic. Simulation modeling suggests that the historical mix of surface and passive crown fire were primarily driven by shrub biomass and wind speed. However, a century of fire exclusion has reduced the potential for the high-severity patches of fire that were common historically, likely by reducing bitterbrush cover, the primary ladder fuel. This reduced shrub cover is likely to persist until fire or insects create new canopy gaps. Crown fire potential may increase even with current fuel loadings if the climate predicted for midcentury lowers fuel moistures, but only under rare extreme winds. This study expands our emerging understanding of complexity in the disturbance dynamics of lodgepole pine across its broad North American range.Résumé : Dans certaines parties du centre de l'Oregon, la forêt est seulement composée de pin tordu latifolié (Pinus contorta Douglas ex Loudon var. latifolia Engelm. ex S. Watson) à faible densité avec la présence sporadique du pin ponderosa (Pinus ponderosa Lawson & C. Lawson) et d'un sous-étage dominé par la purshie tridentée (Purshia tridentata (Pursh) DC.) à cause de substrats à texture grossière composés de pierre ponce. Nous avons reconstitué le régime des feux passé à partir des cernes annuels et nous avons simulé le comportement du feu sur 783 ha de ce type de forêt. Pendant des siècles (1650-1900), de vastes incendies d'intensité mixte sont survenus à tous les 26-82 ans, créant une forêt inéquienne et une mosaïque d'arbustes. Les simulations indiquent que la biomasse des arbustes et la vitesse du vent étaient les principaux facteurs responsables du mélange historique de feux de surface et de feux de cime intermittents. Cependant, l'exclusion du feu depuis un siècle a réduit la possibilité que des parcelles de forêt soient brûlées par des feux de sévérité élevée, fréquents dans le passé, probablement en réduisant le couvert de purshie tridentée, le principal combustible étagé. Ce faible couvert d'arbuste persistera probablement jusqu'à ce que le feu ou les insectes créent de nouvelles trouées dans le couvert forestier. La possibilité qu'il y ait des feux de cime pourrait augmenter, même avec les charges actuelles de combustibles, si le climat qui est prédit pour le milieu du siècle réduit la teneur en humidité des combustibles et à condition qu'il y ait de forts vents, ce qui est rare. Cette étude enrichit les connaissances récemment acquises au sujet de la complexité de la dynamique des perturbations...

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“…Several of the lodgepole pine stands we sampled did have establishment dates around 1900 and were located in or near mapped fire perimeters from the same time period (Leiberg 1900). Gara et al (1985) and Heyerdahl et al (2014) also found cohorts of lodgepole pine tree recruitment coincident with natural disturbance years (i.e., fire, windthrow, and beetle outbreak). However, their fire scar evidence suggested that high severity fire was spatially limited and low severity fire was more common across the study area (Heyerdahl et al 2014).…”

“…Gara et al (1985) and Heyerdahl et al (2014) also found cohorts of lodgepole pine tree recruitment coincident with natural disturbance years (i.e., fire, windthrow, and beetle outbreak). However, their fire scar evidence suggested that high severity fire was spatially limited and low severity fire was more common across the study area (Heyerdahl et al 2014). Some studies have hypothesized that lodgepole pine follows two successional pathways following fire: (1) self-replacement on flat, cold-air drainages with poor soils; and (2) succession to mountain hemlock and red fir forests on better sites (Zeigler 1978, Agee 1993.…”

“…Lodgepole pine stands are characterized by even-aged establishment following high severity fire. Although lodgepole pine in the southern Cascade Range is not serotinous, it establishes in high severity postfire gaps via seed from surviving trees, and post-fire stand density is related to the proximity of these seed trees (Zeigler 1978, Pierce and Taylor 2011, Heyerdahl et al 2014.…”

“…The CLNP management documents, including the Fire Management Plan, describe the fire regime of these forests as ranging from mixed severity (mean fire return interval of 40 to 70 years) in mid-elevation red fir forests to a low frequency, high severity regime (mean fire return interval greater than 100 years) for high elevation mountain hemlock forests (National Park Service 2007). However, observed fire behavior and effects in recent managed fires in this region suggest that fires may be more mixed in severity (Agee 1993, Chappell and Agee 1996, McNeil and Zobel 1980, Perry et al 2011, Heyerdahl et al 2014; C. Farris, National Park Service, Klamath Falls, Oregon, USA, personal communication). For example, the 2006 Bybee Fire burned over 1100 ha of lodgepole pine, red fir, and mountain hemlock forests within the study area.…”

“…Although mixed-severity fire regimes are likely widespread throughout the western United States, they are poorly documented (Perry et al 2011, Heyerdahl et al 2014, Hessberg et al 2016. In addition, the term "mixed severity" has not been well defined in the literature.…”