Litter Flammability of 50 Southeastern North American Tree Species: Evidence for Mesophication Gradients Across Multiple Ecosystems

Widespread fire exclusion and land-use activities across many southeastern United States forested ecosystems have resulted in altered species composition and structure. These changes in composition and structure have been implicated in positive fire-vegetation feedbacks termed “mesophication” where fire spread and intensity are diminished. In forests and woodlands, inherent flammability of different species is the mechanistic driver of mesophication. To date, there has been limited work on documenting the high diversity of flammability among species in the region, limiting the ability to differentiate among species to restore fuels that sustain fire regimes. Here, we coalesce disparate flammability data and add missing species across the spectrum from species that facilitate fire (so called “pyrophytes”) to those that dampen fire (so called “mesophytes”). We present data on 50 important tree species from across the southeast, all burned using identical laboratory methods. We divide our results for four dominant ecosystems: Coastal Plain uplands, oak-hickory woodlands, Appalachian forests, and bottomland forests. Across ecosystems, the most flammable species were American chestnut (Castanea dentata), a suite of pines (Pinus palustris, P. elliottii, P. serotina, and P. rigida), several oaks (Q. laevis, Q. falcata, Q. margaretta, and Q. alba), and sourwood (Oxydendrum arboreum). At the mesophytic end, the least flammable species were Tsuga canadensis, Acer rubrum, and several other hardwoods previously implicated in mesophication. Each of the four ecosystems we studied contained species that spanned the pyrophytic to mesophytic gradient. These data fill in some key holes in our understanding of southeastern fire adaptations, but also provide context for restoration decisions and fire management prioritization efforts to restore and sustain fire-prone ecosystems of the region.

Mediterranean Heathland as a Key Habitat for Fire Adaptations: Evidence from an Experimental Approach

Some fire ecology studies that have focused on garrigue-like vegetation suggest a weak selective pressure of fire in the Mediterranean Basin compared to other Mediterranean-type regions. However, fire-prone Mediterranean heathland from the western end of the Mediterranean Basin has been frequently ignored in the fire ecology literature despite its high proportion of pyrogenic species. Here, we explore the evolutionary ecology of seed traits in the generalist rockrose Cistus salviifolius L. (Cistaceae) aiming to ascertain the role of the Mediterranean heathland for fire adaptations in the Mediterranean Region. We performed a germination experiment to compare the relationship of seed size to (i) heat-stimulated germination, (ii) dormancy strength, and (iii) heat survival in plants from ‘high-fire’ heathland vs. ‘low-fire’ coastal shrubland. Germination after heat-shock treatment was higher in large seeds of both ‘high-fire’ and ‘low-fire’ habitats. However, dormancy was weaker in small seeds from ‘low-fire’ habitats. Finally, seed survival to heat shock was positively related to seed size. Our results support that seed size is an adaptive trait to fire in C. salviifolius, since larger seeds had stronger dormancy, higher heat-stimulated germination and were more resistant to heat shock. This seed size–fire relationship was tighter in ‘high-fire’ Mediterranean heathland than ‘low-fire’ coastal shrubland, indicating the existence of differential fire pressures and evolutionary trends at the landscape scale. These findings highlight the Mediterranean heathland as a relevant habitat for fire-driven evolution, thus contributing to better understand the role of fire in plant evolution within the Mediterranean region.

Pine Species That Support Crown Fire Regimes Have Lower Leaf-Level Terpene Contents Than Those Native to Surface Fire Regimes

Fire is increasingly being recognised as an important evolutionary driver in fire-prone environments. Biochemical traits such as terpene (volatile isoprenoid) concentration are assumed to influence plant flammability but have often been overlooked as fire adaptations. We have measured the leaf-level flammability and terpene content of a selection of Pinus species native to environments with differing fire regimes (crown fire, surface fire and no fire). We demonstrate that this biochemical trait is associated with leaf-level flammability which likely links to fire-proneness and we suggest that this contributes to post-fire seedling survival. We find that surface-fire species have the highest terpene abundance and are intrinsically the most flammable, compared to crown-fire species. We suggest that the biochemical traits of surface fire species may have been under selective pressure to modify the fire environment at the leaf and litter scale to moderate fire spread and intensity. We indicate that litter flammability is driven not only by packing ratios and bulk density, but also by terpene content.

Fire: Mimicking Nature

This chapter opens with a discussion naturally ignited fires and fire adaptations, which have evolved over millions of years. It then considers the role people have played over time in manipulating fire regimes, both locally and on a broad scale. Examples from diverse biomes such as the savannas and grasslands of South Africa and Madagascar, the forests of northern Europe and the grasslands and forests of North America provide evidence of the interactions between climate and human-set ignitions. The studies of the systems include analysis of a diverse range of evidence, including sediments, documents, and field evidence, analyzed using models that focus on patterns and processes of fire regimes under differing climates and human activities. The importance of perceptions of the role of fire is also discussed in terms of using fire for management, with examples of changes in attitude in North America from the 19th to the 21st centuries, which have led from seeing all fires as bad to valuing fire as a management tool. Analysis of the historical importance of human-set and natural fires has been critical to arriving at current management decisions.

Homage to L. M. Coutinho: fire adaptations in cerrado plants

Professor Coutinho (1934–2016; Sao Paulo, Brazil) studied fire adaptations in Brazilian savannas during the 1970s, when very few researchers recognised fire as an evolutionary force. His main contributions were on fire-stimulated flowering, serotiny and nutrient cycling. However, he is little known, partly because he was not Anglo-Saxon but also because he was ahead of his time, when fire and evolution were still distant concepts.

Deep history of wildfire in Australia

Australian plant species vary markedly in their fire responses, and the evolutionary histories of the diverse range of traits that lead to fire tolerance and fire dependence almost certainly involves both exaptation and traits that evolved directly in response to fire. The hypothesis that very long-term nutrient poverty in Australian soils led to intense fires explains many of the unusual responses to fire by Australian species, as does near global distribution of evidence for fire during the Cretaceous, possibly driven by high atmospheric oxygen concentration. Recent descriptions of leaf fragments from a Late Cretaceous locality in central Australia have provided the first fossil evidence for ancient and possibly ancestral fire ecology in modern fire-dependent Australian clades, as suggested by some phylogenetic studies. The drying of the Australian climate in the Neogene allowed the rise to dominance of taxa that had their origin in the Late Cretaceous, but had not been prominent in the rainforest-dominated Paleogene. The Neogene climatic evolution meant that fire became an important feature of that environment and fire frequency and intensity began to grow to high levels, and many fire adaptations evolved. However, many plant species were already in place to take advantage of this new fire regime, and even though the original drivers for fire may have changed (possibly from high atmospheric oxygen concentrations, to long, hot, dry periods at different times in different parts of the continent), the adaptations that these species had for fire tolerance meant they could become prominent over much of the Australian continent by the time human colonisation began.