Physiological drivers of the live foliar moisture content ‘spring dip’ in Pinus resinosa and Pinus banksiana and their relationship to foliar flammability

Abstract

A phenomenon known as the ‘Spring Dip’ in conifer live foliar moisture content (LFMC) has been documented and monitored for decades. This period also corresponds with intense crownfire activity in areas dominated by Pinus resinosa (Red pine) or Pinus banksiana (Jack pine). Despite a long-standing tradition of measuring LFMC during the dip period, the drivers of these variations have been the source of much speculation but little investigation and the actual causes of foliar flammability change have received even less attention. Here we assess the seasonal drivers of LFMC variations and their impact on foliar flammability. Foliar samples were collected for an entire year from both Red pine and Jack pine at a site in Central Wisconsin. New and previous year’s foliage were sampled separately when both were present. From these samples, we determined LFMC, foliar chemistry and foliar density. We also ignited samples in an open flame burner to assess seasonal changes in their flammability. We verified that there is indeed a drop in the foliar moisture content during the spring. However, foliar density changes explained 96.7% of the variation in LFMC across both species and both needle age categories. These density changes were driven by an accumulation of starch and sugar in the previous year’s foliage, most likely as a result of the onset of photosynthesis in the spring. Foliar starch, sugar and crude fat content explained 86.4% of the variation in foliar density. Foliar flammability followed the same trend as LFMC, reaching its period of highest flammability during the time of the lowest LFMC. However, these changes were strongly related to changes in foliar density, where density explained 51% and 77.4% of the variations in foliar flammability. Our results challenge the assumption that live conifer foliage flammability is limited by its water content and this study has led to a new theory of the factors that dominate live fuel flammability.