Forest plant cover across multiple forest types following a 61,000 Ha megafire

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Devri Adams Tanner, Kordan Kildew, Brian Brown, Noelle V. Zenger and Samuel B. St. Clair, Plant and Wildlife Sciences, Brigham Young University, Provo, UT

Background/Question/Methods
Fires are becoming larger, more intense, and more frequent due to human activity, which necessitates a further understanding of novel fire regimes and their impacts. Novel fires have the capacity to expand past historical fire boundaries leaving patches of varying burn severity that may significantly alter plant regeneration. To study these phenomena, we asked the following question: How do burn conditions from a novel fire affect plant cover across multiple forest types? We installed transects across three forest types in central Utah’s 2018 Pole Creek Megafire burn scar: aspen-conifer, oak-maple, and pinyon-juniper. We geographically paired each fifty-meter transect between a burned and an unburned site, with six replications per forest type. Plant cover was measured using the pin drop method to determine foliar layers every five meters along each transect.
Results/Conclusions
Our results suggest that high severity fires differentially affect post-fire plant cover in adjacent forest-types because plant cover loss varied by an order of magnitude between forest-types. Aspen-conifer (AC) forests and pinyon-juniper (PJ) showed the most distinct difference in plant cover loss, where “no cover” measurements increased by sixteen times in AC but only two times in PJ following the fire. The post-fire plant cover loss was lower in PJ forests, likely due to environmental conditions that cause PJ forests to be less dense than AC forests, and thus there would be less fuel to carry fire in the understory. PJ forests also saw a five-fold increase in post-fire forb canopy cover that may have mitigated cover loss due to their rapid post-fire regeneration. AC and oak-maple (OM) forests also saw an increase in post-fire forbs, but only two times unburned forb cover. Aside from the total cover loss, the largest trend in OM forests was the altered ratio of oak to maple canopy cover between burned and unburned sites. In the absence of fire, the canopy cover of OM forests was maple dominant, with an approximate ratio of 70:5 (maple: oak). Following the fire, canopy cover flipped to be oak dominant with a ratio of about 5:15 (maple: oak). The switch was likely due to oak trees’ ability to rapidly send up sprouts from their existing roots following disturbance. Our study suggests that high severity fires alter plant cover differentially by forest type, which may have implications for successional pathways taken during regeneration.