Changes to Cretaceous surface fire behaviour influenced the spread of the early angiosperms

Abstract

Summary Angiosperms evolved and diversified during the Cretaceous period. Early angiosperms were short‐stature weedy plants thought to have increased fire frequency and mortality in gymnosperm forest, aiding their own expansion. However, no explorations have considered whether the range of novel fuel types that diversified throughout the Cretaceous also altered fire behaviour, which should link more strongly to mortality than fire frequency alone.We measured ignitability and heat of combustion in analogue Cretaceous understorey fuels (conifer litter, ferns, weedy and shrubby angiosperms) and used these data to model palaeofire behaviour.Variations in ignition, driven by weedy angiosperms alone, were found to have been a less important feedback to changes in Cretaceous fire activity than previously estimated. Our model estimates suggest that fires in shrub and fern understories had significantly greater fireline intensities than those fuelled by conifer litter or weedy angiosperms, and whilst fern understories supported the most rapid fire spread, angiosperm shrubs delivered the largest amount of heat per unit area.The higher fireline intensities predicted by the models led to estimates of enhanced scorch of the gymnosperm canopy and a greater chance of transitioning to crown fires. Therefore, changes in fire behaviour driven by the addition of new Cretaceous fuel groups may have assisted the angiosperm expansion.

Biological and geophysical feedbacks with fire in the Earth system

“…Conceptually, the flammability of a system is controlled by a number of factors: temperature, wind speed, relative humidity, O 2 concentrations, moisture, and the amount and structure of fuels, each of which can compensate or exacerbate others. For example, it is clear from experimental work that non-flammable fuel types become flammable as O 2 concentrations increase (Belcher et al 2010b), such that even moist forests would have been easy to ignite in the high O 2 atmospheres of the Carboniferous and Cretaceous periods (Belcher and Hudspith 2017). An example at much shorter timescales is the impact that a sudden rise in relative humidity can have by extinguishing a fire burning through a uniform fuel bed (Cheney et al 1993).…”

“…increased flammability results in less above-ground biomass. Therefore, under high O 2 we would anticipate more frequent and intense fires (Belcher and Hudspith 2017) that would suppress large land plant biomass, slowing biological weathering rates, and the rate at which key nutrients like phosphorus are released from rocks (Lenton 2001). This is because plant roots actively mine soils for phosphorus, and can increase weathering rates by an order of magnitude (Quirk et al 2012).…”

“…There are dramatic examples in the paleo-record of changes in global patterns of vegetation and fire linked to the evolution of new leaf properties and plant architecture-e.g. early angiosperms, and subsequently the grass family-Poaceae (Belcher and Hudspith 2017, Bond and Scott 2010, Keeley and Rundel 2005. At shorter time-scales, changes in dominant plant species can also significantly alter fire activity and Earth system feedbacks (D'Antonio and Vitousek 1992, Girardin et al 2013.…”

Does fuel type influence the amount of charcoal produced in wildfires? Implications for the fossil record

“…). This might be reconciled by considering that, although each fire in a high oxygen world must produce less charcoal, fire frequency itself must have been significantly increased owing to the ease of ignition, and rapid spread of fires (Belcher & Hudspith ). Therefore this increased fire activity, potentially producing larger burned areas, might account for the higher abundance of charcoal found in the fossil record during these periods.…”

“…Therefore this increased fire activity, potentially producing larger burned areas, might account for the higher abundance of charcoal found in the fossil record during these periods. It is also likely that fuel moisture played a role, because under higher oxygen concentrations increasingly wetter fuels can burn (Watson & Lovelock ), and spread rates and fire intensity were both found to be higher for wet fuels as atmospheric oxygen increased, compared to those ignited under ambient conditions (Belcher & Hudspith ). To reconcile our observations with the charcoal fossil record, future experiments should seek to study the influence of charcoal production across a range of fuel moistures and atmospheric oxygen concentrations.…”