17The 'Spring Dip' in conifer live foliar moisture content (LFMC) has been well documented but 18 the actual drivers of these variations have not been fully investigated. Here we span this 19 knowledge gap by measuring LFMC, foliar chemistry, foliar density and foliar flammability on 20 new and old foliage for an entire year from both Pinus resinosa (red pine) and Pinus banksiana 21 (jack pine) at a site in Central Wisconsin. We found that needle dry mass increased by up to 70% 22 in just three weeks and these increases were manifested as strong seasonal variations in foliar 23 moisture content and foliar density. These needle dry mass changes were driven by an 24 accumulation of starch in old foliage, likely resulting from springtime photosynthesis onset, and 25 also by accumulations of sugar and crude fat in new needles as they fully matured. Foliar starch, 26 sugar and crude fat content accounted for 84% of the variation in foliar density across both 27 species. Flammability differences were also strongly related to changes in foliar density, where 28 density accounted for 39% and 25% of the variations in foliar time-to-ignition of jack pine and 29 red pine respectively. Finally, we use the computational fluid dynamics-based wildland fire 30 model FIRETEC to examine how these foliar physio-chemical changes may influence wildland 31 fire behavior. Under the lowest canopy density and windspeed, simulated fires in dormant 32 condition stands did not propagate as crown fires while spring dip stands successfully spread as 33 crown fires as a result of the higher potential energy content of the canopy. Simulated wildland 34 fire spread rates increased by as much as 63%, nominal fireline width increased by as much as 35 89% and active fire area more than doubled relative to dormant season fuel conditions and the 36 most significant changes occurred in areas with low canopy cover and low within-tree bulk 37 density. Our results challenge the assumption that live conifer foliage flammability is limited 38 only by its water content; this study suggests a new theory and an expanded view of the factors 39 that dominate live fuel flammability and that subsequently influence larger scale wildland fire 40 behavior. 41 44 4 1. Introduction 53 54 Jack pine (Pinus banksiana) and red pine (Pinus resinosa) are distributed throughout much of the 55 high latitude and temperate North American forests; collectively they cover parts of eleven US 56 states and eight Canadian provinces. Wildfires are an integral component of their ecology 57 (Ahlgren and Ahlgren , 1960). Fires that occur in these areas can vary from low intensity surface 58 fire to high intensity crown fires. Fire severity significantly affects the ecological succession and 59 subsequent distribution of these trees throughout the boreal region (Arseneault, 2001). It is 60 therefore important to develop a complete understanding of the factors that drive fire severity in 61 these forests. 62Fire behavior in these forests is a crucial component of the development of manage...
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.
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