De-coupling seasonal changes in water content and dry matter to predict live conifer foliar moisture content

Abstract

2014. Decoupling seasonal changes in water content and dry matter to predict live conifer foliar moisture content. Int. J. Wildland Fire. 23(4):480-489.Abstract. Live foliar moisture content (LFMC) significantly influences wildland fire behaviour. However, characterising variations in LFMC is difficult because both foliar mass and dry mass can change throughout the season. Here we quantify the seasonal changes in both plant water status and dry matter partitioning. We collected new and old foliar samples from Pinus contorta for two growing seasons and quantified their LFMC, relative water content (RWC) and dry matter chemistry. LFMC quantifies the amount of water per unit fuel dry weight whereas RWC quantifies the amount of water in the fuel relative to how much water the fuel can hold at saturation. RWC is generally a better indicator of water stress than is LFMC. We separated water mass from dry mass for each sample and we attempted to best explain the seasonal variations in each using our measured physiochemical variables. We found that RWC explained 59% of variation in foliar water mass. Additionally, foliar starch, sugar and crude fat content explained 87% of the variation in seasonal dry mass changes. These two models combined explained 85% of the seasonal variations in LFMC. These results demonstrate that changes to dry matter exert a stronger control on seasonal LFMC dynamics than actual changes in water content, and they challenge the assumption that LFMC variations are strongly related to water stress. This methodology could be applied across a range of plant functional types to better understand the factors that drive seasonal changes in LFMC and live fuel flammability.

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

“…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]. Dry weight changes can be further decomposed into various chemical components, such as structural, non-structural, photosynthetic, and defensive compounds, and each of these sets of compounds can have a differing influence on leaf flammability [20].…”

“…Dry weight changes can be further decomposed into various chemical components, such as structural, non-structural, photosynthetic, and defensive compounds, and each of these sets of compounds can have a differing influence on leaf flammability [20]. Ultimately, decoupling the influences of water and dry matter dynamics and exploring their independent influences on combustion might be the best approach [12]. Ideally, it would be best for flammability studies to quantify both plant water stress with strong physiological indicators such as relative water content or leaf water potential as well as dry matter content variations in standard metrics such as leaf mass area that can capture within and between species variations in leaf mass (ovals, Figure 1).…”

“…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.…”

Reliability of leaf relative water content (RWC) measurements after storage: consequences for in situ measurements

“…During recent years, progress has been made in methods to measure plant water status noninvasively using foliar reflectance (e.g., Jördens et al 2009;Cheng et al 2012), but such a method is currently not easily available for a wide use, especially for field investigations (Sancho-Knapik et al 2013). The gravimetric method that has been described as optimal, with respect to reliability and simplicity (Lösch 2001), remains widely used (Fernández-García et al 2014;Jolly et al 2014;Liu et al 2014;Thameur et al 2014;Chimungu et al 2015, Gorai et al 2015Yang et al 2015). Accurate measurement of the different leaf masses is not trivial (Rachmilevitch et al 2006).…”

“…In the field, however, the quick loss of water from excised leaves poses a problem, requiring protective measures against water loss. Tin cans (Stocker 1929;Jolly et al 2014), plastic vials (Hadley and Smith 1983;Armas et al 2010;Aref et al 2013;Fernández-García et al 2014), and sealed plastic bags (Smart and Bingham 1974;Gonzalez and Gonzalez-Vilar 2001;Davidson et al 2006;Gonçalves et al 2011;Ryser et al 2011;Walter et al 2011;Teszlák et al 2013) have been used to protect the collected leaves against water loss during the transportation from field to laboratory. Govender et al (2009) stress the importance of a proper sampling procedure to prevent water loss by keeping leaves in plastic bags and in a cool dark place.…”