Shoot flammability of shrubs in Texas

Date

2023-05

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Abstract

Flammability is a trait of land plants, and an understanding of this trait is important to comprehend the ecological and evolutionary consequences of fire on plant species in fire-prone ecosystems. Numerous plant traits can influence flammability. Although many studies have examined the flammability of individual leaves,fewer have examined how entire canopies behave as fuel because quantifying plant flammability by burning whole plants is expensive. However, a recent study has suggested that burning shoots can predict plant flammability. In this study, I used a new plant flammability device that allows standardized measurements of canopy flammability of portions up to 70 cm long. I burned 70 cm branches from at least three samples per species of 16 native shrubs of Texas and measured four canopy traits: total dry mass per 70 cm, canopy density, leaf: stem (in dry mass basis) and canopy moisture content and four common leaf traits: leaf mass per area (LMA), leaf area per leaflet, leaf length per leaflet and leaf moisture content. I used these data to answer two questions: 1) Are canopy traits more important than leaf traits in shoot flammability? 2) Are heat release and flame spread rate independent axes of flammability in shrub fuels? I found that canopy traits are more important in determining flammability than are leaf traits; total dry mass per 70 cm branch and canopy density together were the best predictors of temperature integration in shoot flammability in shrub fuels. Furthermore, I found that shoot flammability in shrub fuel is mostly described by a single axis, represented by flammability metrics related to duration. This finding illustrates the potential for incorporating canopy traits in fire behavior models and might improve the understanding of fire-fuel feedback in the shrublands of Texas. Grazing and fire have a long history in Texas. Fire and herbivores can both increase plant flammability and alter the composition of biomes. Flammability and the extent of herbivores’ ingestion are influenced by plant functional traits. A plant can defend itself from herbivores’ that browse by using physical defense, chemical defense, or a combination of the two and can persist in fire-prone ecosystems through their different life history strategies. In light of the recent advancement in our understanding of the trade-off among different types of plant defense against herbivores and the relationship between plant flammability and palatability, I tested the difference in flammability between armed and unarmed species as well as two groups of shrubs based on white-tailed deer preference to answer two questions: 1) Are the least preferred shrubs for white-tailed deer more flammable than moderate to low preferred shrubs? 2) Do armed plants have lower flammability than unarmed plants? I found that the least preferred shrubs are more flammable than moderate to low preferred shrubs. However, I haven’t found any significant difference in flammability between unarmed and armed species. This study might help to improve the understanding of the trade-off between the physical and chemical defense of plants against herbivores and the unified framework for fire and herbivores’ effects on plant life history.

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Keywords

Shoot Flammability, Shrub, Canopy Traits, Plant Defense, Herbivore Preference

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