Needing a quick fix? Exploring the potential of pre-fire fertilization to mitigate fire severity in ponderosa pine saplings

Abstract Background Under shifting fire regimes, forested ecosystems within the western US are predicted to experience a shorter time interval between wildland fire events, leading to increased fire exposure to younger tree cohorts. Consequently, the assessment of silvicultural approaches to help increase the resistance of these younger trees to wildland fires may help inform land management actions to promote growth and sustain healthy forests. Methods We assessed the structural overshoot hypothesis in the context of vegetation fire severity by applying supplemental fertilizer to Pinus ponderosa saplings in advance of experimental fires. We explored three different treatments: no fertilizer, 7 weeks prior to fires, and 1 week prior to fire. We then used pyro-ecophysiology experiments to assess whether saplings subjected to fertilization treatments differed from untreated control groups across a range of fire intensities and assessed post-fire growth, physiology, tissue nutrient content, and mortality. Results We demonstrated that the addition of nitrogen, either 1 week or 7 weeks prior to the fires, did not change the relationship between fire intensity and percentage mortality observed in the unfertilized fire treatments. Fertilization across all groups caused a decline in pre-fire leaf area and caused a more rapid decline of the change of chlorophyll fluorescence post-fire in the highest intensity treatment. Separability in the spectral indices were pronounced in the 7-week fertilization treatment, suggesting cellular changes occurred over this period. The 0.8 MJ m −2 FRE level that was associated with 100% mortality across all treatments was associated with increases in root aluminum concentrations. Conclusions Pre-fire fertilization does not increase resistance to fire-induced mortality in ponderosa pine saplings, but this should be evaluated in other species with improved ability to effectively use supplemental nutrients. Further research is also warranted to assess how fertilization is altering the physiology properties of saplings and how this in turn impacts fire effects under fires of varying intensity. The increase in root aluminum concentration in the 0.8 MJ m −2 treatment may indicate an additional mechanism contributing to fire-induced tree mortality, but this should be reassessed in a variety of soil mediums. Further research is warranted to assess this mechanism and other soil chemistry processes and characteristics (e.g., pH) that may affect nutrient uptake and post-fire survival and recovery.