Grass Canopy Architecture Influences Temperature Exposure at Soil Surface

Abstract

There is increasing recognition that plant traits contribute to variations in fire behavior and fire regime. Diversity across species in litter flammability and canopy flammability has been documented in many woody plants. Grasses, however, are often considered homogeneous fuels in which any flammability differences across species are attributable to biomass differences alone and therefore are of less ecological interest, because biomass is hugely plastic. We examined the effect of grass canopy architecture on flammability across eight grass species in short grass steppe of New Mexico and Texas. To characterize grass canopy architecture, we measured biomass density and “biomass-height ratio” (the ratio of canopy biomass above 10 cm to that of biomass below 10 cm). Indoor flammability experiments were performed on air-dried individual plants. As expected, plant biomass influenced all flammability measures. However, biomass-height ratio had additional negative effect on temperature exposure at soil surface (accumulation of mean temperature >100 °C) in well-cured grasses, which is an important fire behavior metric predicting soil heating and meristem survival. This canopy architecture effect, however, needs further investigation to be isolated from biomass density due to correlation of these two traits. This result demonstrates the potential for species-specific variation in architecture to influence local fire effects in grasses.

A handbook for the standardised sampling of plant functional traits in disturbance-prone ecosystems, with a focus on open ecosystems

“…These samples then need to be dried to a constant mass and weighed, with biomass ratio calculated as the ratio of mass for material above 10 cm to the mass of material below 10 cm. The trait is fairly stable within a species (Gao and Schwilk 2018) and measuring five individuals per species at a location should be adequate. Biomass ratios should ideally be measured on individuals in the first-year post-fire, once peak flowering has been reached, so as to standardise for the effect of biomass accumulation over time.…”

“…Plants with higher biomass ratios (i.e. relatively more biomass above 10 cm) vent heat upwards, which results in lower soil-surface temperatures when burned once total aboveground biomass is accounted for (Gao and Schwilk 2018). Lower soil-surface temperatures during fire increases the likelihood of basal meristems and other crown tissues surviving the fire, allowing the plant to survive and resprout after fires (Choczynska and Johnson 2009).…”

“…High BRs have been found to reduce soil-surface temperatures in fully dry grass fuels; however, when grasses are actively growing, high fuel moisture can lead to inefficient combustion and smouldering, which can override the effect of high biomass ratios on reducing soil-surface temperatures (Gao and Schwilk 2018). For laterally spreading species, such as mat-forming grazing lawn grasses, where individuals are difficult to isolate, a 20-Â 20-cm section of the grazing lawn can be clipped as a representative sample.…”

“…the depth of the grass crown and basal buds below the soil surface) can be effective against both fire and grazing, serving both to insulate basal meristematic tissue from intense heat when burned, and providing rooting strength when grazed (Lemon 1949). Crown and basal buds can be protected from fire by high biomass ratios, which elevate the combustible material and, thus, vent the heat of the fire away from the soil surface (Gao and Schwilk 2018). Alternatively, dense packing of leaf sheaths around culm bases provides insulation against the heat of the fire (Lemon 1949).…”

Corrigendum to: A handbook for the standardised sampling of plant functional traits in disturbance-prone ecosystems, with a focus on open ecosystems

“…Biomass ratio (BR) is the ratio of canopy biomass above 10-cm height to canopy biomass below 10-cm height (i.e. from the soil surface to 10 cm; Gao and Schwilk 2018). This trait quantifies how canopy architecture influences temperature at the soil surface when a plant burns.…”