Burn hot or tolerate trees: flammability decreases with shade tolerance in grasses

Abstract

In ecosystems where trees and grasses coexist, some grass species are found only in open habitats and others persist under trees. The persistence of shade intolerant grasses in ecosystems such as open woodlands and savannas depends on recurrent fires to open the tree canopy. Therefore, grasses that depend on open sites might benefit from high flammability. We tested if shade intolerant grasses are more flammable than shade tolerant grasses and if flammability differences affected post-fire grass growth. We examined the relationship between shade tolerance and flammability by determining individual-level flammability and species shade tolerance of 17 grass species. We also measured grass traits to determine trait effects on flammability and the post-fire response. Grass species varied in flammability, mainly in the amount of heat produced during burning. Shade tolerant species produced less heat at 50 cm above the ground. Biomass and live fuel moisture had the greatest effects on heat release. However, the negative effect of live fuel moisture on heat release at the soil surface was weakened in plants with high specific leaf area. In addition, grass bulk density negatively influenced heat release at 50 cm height. Heat release at the soil surface negatively influenced postfire growth. However, the influences of soil heating and species-specific traits on individual survival were more complex with 2-and 3-way interactions. Shade tolerance was negatively correlated with a major axis of flammability variation: shade tolerant grasses produced less heat where that heat could damage tree boles. Such heterogeneity in grass flammability may help maintain the tree-grass mixture in natural plant communities. If shade tolerant grasses near trees cause less fire damage to woody plants, especially tree saplings, this may weaken positive grass-fire feedbacks and thus aid the long-term coexistence of trees and grasses.

Accounting for herbaceous communities in process‐based models will advance our understanding of “grassy” ecosystems

“…Conversely, the occurrence of fire often increases herbivore pressure due to short‐term increases in nutrients after fire (Ojima et al, 1994) that are incorporated into plant tissues, thereby increasing forage quality. These effects may also vary depending on the plant growth form or PFT under consideration (Gao & Schwilk, 2018, 2022).…”

Savanna fire regimes depend on grass trait diversity

“…In woody plants, it is well-established that flammability varies significantly among and within species [21][22][23] and it has been repeatedly demonstrated that small changes in woody species composition may significantly alter vegetation flammability and fire regime [24][25]. However, variation in the flammability of grass or other herbaceous species has only received attention recently, despite its global importance [26][27][28][29][30]. This oversight likely originates with a historical assumption that grasses vary little in their flammability, which has led to the diversity of grassy fuels being reduced to one or few classes in fire behaviour modelling [31].…”

“…Since trees shade grasses and compete with them for limited water and soil nutrients, woody plant encroachment alters grass community trait and species composition [50][51]. Recent work shows shading decreases grass flammability by reducing grass biomass, increasing grass moisture content and favouring less-flammable shade-tolerant species [30,[52][53]. Therefore, fire-driven changes to grass communities may cause a positive feedback on fire regimes, further reinforcing reduced fire activity via changes in grass traits.…”

Shoot flammability patterns among plant species of the wildland–urban interface in the fire-prone Greater Blue Mountains World Heritage Area

“…Plants provide fuel for wildfires, and there is considerable variation among species in the flammability of leaves (Murray et al 2013), canopy shoots (Kraaij et al 2022) and whole plants (Gao and Schwilk 2022). This variation plays an important role in dictating wildfire dynamics (Weise et al 2005;Plucinski et al 2010;Schwilk 2015;Fares et al 2017), and species flammability is now a fundamental component of models predicting wildfire behaviour (Zylstra et al 2016;Prince et al 2017;Tumino et al 2019; For full list of author affiliations and declarations see end of paper Burton et al 2021;Popović et al 2021;Zylstra and Liow 2021).…”