Seasonal variations in red pine (Pinus resinosa) and jack pine (Pinus banksiana) foliar physio-chemistry and their potential influence on stand-scale wildland fire behavior

Abstract

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...

Linking Forest Flammability and Plant Vulnerability to Drought

“…LMA also increases during leaf maturation [55]. This effect of leaf age has been associated with seasonal declines in conifer LFMC [56]. Therefore, we suggest that LFMC models may be improved by taking seasonal variation in LMA into account.…”

Pyro-Ecophysiology: Shifting the Paradigm of Live Wildland Fuel Research

“…The flawed assumption was made that live fuels behaved similarly and that only water weight varied over time. However, both the water weight and dry weight of live fuels changes diurnally [10,11], seasonally [12,13], and inter-annually [14]. Thus, FMC is derived from two metrics that vary independently over space and time.…”

“…It is likely that these drought metrics are proxies for plant water status, which is better measured using physiological water stress indicators such as water potential or relative water content [17]. Dry weight changes (FMC denominator) are most directly related to carbon cycle processes such as photosynthesis, respiration, carbon allocation, and canopy phenology, and dry weight changes alone have been shown to heavily influence live fuel ignitability [13]. Live FMC dynamics are most likely a combination of seasonal changes in actual water content and dry matter change [12,18,19].…”

Numerical Investigation of Aggregated Fuel Spatial Pattern Impacts on Fire Behavior

“…Finer scale investigations suggest that tree architecture and fuel distribution within a tree crown can significantly impact how individual trees burn [60,66,82]. It is likely that variability in fine-scale fuel properties, chemical composition [83] and size class all play key roles in how trees burn and how fire propagates.…”