Jon E. Keeley

Fire in the Earth system.

Burn, Baby, Burn Wildfires can have dramatic and devastating effects on landscapes and human structures and are important agents in environmental transformation. Their impacts on nonanthropocentric aspects of the environment, such as ecosystems, biodiversity, carbon reserves, and climate, are often overlooked. Bowman et al. (p. 481) review what is known and what is needed to develop a holistic understanding of the role of fire in the Earth system, particularly in view of the pervasive impact of fires and the likelihood that they will become increasingly difficult to control as climate changes. Fire is a worldwide phenomenon that appears in the geological record soon after the appearance of terrestrial plants. Fire influences global ecosystem patterns and processes, including vegetation distribution and structure, the carbon cycle, and climate. Although humans and fire have always coexisted, our capacity to manage fire remains imperfect and may become more difficult in the future as climate change alters fire regimes. This risk is difficult to assess, however, because fires are still poorly represented in global models. Here, we discuss some of the most important issues involved in developing a better understanding of the role of fire in the Earth system.

Effects of Invasive Alien Plants on Fire Regimes

Abstract Plant invasions are widely recognized as significant threats to biodiversity conservation worldwide. One way invasions can affect native ecosystems is by changing fuel properties, which can in turn affect fire behavior and, ultimately, alter fire regime characteristics such as frequency, intensity, extent, type, and seasonality of fire. If the regime changes subsequently promote the dominance of the invaders, then an invasive plant–fire regime cycle can be established. As more ecosystem components and interactions are altered, restoration of preinvasion conditions becomes more difficult. Restoration may require managing fuel conditions, fire regimes, native plant communities, and other ecosystem properties in addition to the invaders that caused the changes in the first place. We present a multiphase model describing the interrelationships between plant invaders and fire regimes, provide a system for evaluating the relative effects of invaders and prioritizing them for control, and recommend ways to restore pre-invasion fire regime properties.

Fire intensity, fire severity and burn severity: a brief review and suggested usage

Several recent papers have suggested replacing the terminology of fire intensity and fire severity .P art of the problem with fire intensity is that it is sometimes used incorrectly to describe fire effects, when in fact it is justifiably restricted to measures of energy output. Increasingly, the term has created confusion because some authors have restricted its usage to a single measure of energy output referred to as fireline intensity. This metric is most useful in understanding fire behavior in forests, but is too narrow to fully capture the multitude of ways fire energy affects ecosystems. Fire intensity represents the energy released during various phases of a fire, and different metrics such as reaction intensity, fireline intensity, temperature, heating duration and radiant energy are useful for different purposes. Fire severity, and the related term burn severity, have created considerable confusion because of recent changes in their usage. Some authors have justified this by contending that fire severity is defined broadly as ecosystem impacts from fire and thus is open to individual interpretation. However, empirical studies have defined fire severity operationally as the loss of or change in organic matter aboveground and belowground, although the precise metric varies with management needs. Confusion arises because fire or burn severity is sometimes defined so that it also includes ecosystem responses. Ecosystem responses include soil erosion, vegetation regeneration, restoration of community structure, faunal recolonization, and a plethora of related response variables. Although some ecosystem responses are correlated with measures of fire or burn severity, many important ecosystem processes have either not been demonstrated to be predicted by severity indices or have been shown in some vegetation types to be unrelated to severity. This is a critical issue because fire or burn severity are readily measurable parameters, both on the ground and with remote sensing, yet ecosystem responses are of most interest to resource managers.

The Origins of C4 Grasslands: Integrating Evolutionary and Ecosystem Science

Grassland Emergence The evolution of the C4 photosynthetic pathway from the ancestral C3 pathway in grasses led to the establishment of grasslands in warm climates during the Late Miocene (8 to 3 million years ago). This was a major event in plant evolutionary history, and their high rates of foliage production sustained high levels of herbivore consumption. The past decade has seen significant advances in understanding C4 grassland ecosystem ecology, and now a wealth of data on the geological history of these ecosystems has accumulated and the phylogeny of grasses is much better known. Edwards et al. (p. 587) review this multidisciplinary research area and attempt to synthesize emerging knowledge about the evolution of grass species within the context of plant and ecosystem ecology. The evolution of grasses using C4 photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C4 grassland expansion were considerably more complex than originally assumed. Previous research has benefited substantially from dialog between geologists and ecologists, but current research must now integrate fully with phylogenetics. A synthesis of grass evolutionary biology with grassland ecosystem science will further our knowledge of the evolution of traits that promote dominance in grassland systems and will provide a new context in which to evaluate the relative importance of C4 photosynthesis in transforming ecosystems across large regions of Earth.

A Burning Story: The Role of Fire in the History of Life

Ecologists, biogeographers, and paleobotanists have long thought that climate and soils controlled the distribution of ecosystems, with the role of fire getting only limited appreciation. Here we review evidence from different disciplines demonstrating that wildfire appeared concomitant with the origin of terrestrial plants and played an important role throughout the history of life. The importance of fire has waxed and waned in association with changes in climate and paleoatmospheric conditions. Well before the emergence of humans on Earth, fire played a key role in the origins of plant adaptations as well as in the distribution of ecosystems. Humans initiated a new stage in ecosystem fire, using it to make the Earth more suited to their lifestyle. However, as human populations have expanded their use of fire, their actions have come to dominate some ecosystems and change natural processes in ways that threaten the sustainability of some landscapes.

Plant functional traits in relation to fire in crown-fire ecosystems

Disturbance is a dominant factor in many ecosystems, and the disturbance regime is likely to change over the next decades in response to land-use changes and global warming. We assume that predictions of vegetation dynamics can be made on the basis of a set of life-history traits that characterize the response of a species to disturbance. For crown-fire ecosystems, the main plant traits related to postfire persistence are the ability to resprout (persistence of individuals) and the ability to retain a persistent seed bank (persistence of populations). In this context, we asked (1) to what extent do different life- history traits co-occur with the ability to resprout and/or the ability to retain a persistent seed bank among differing ecosystems and (2) to what extent do combinations of fire- related traits (fire syndromes) change in a fire regime gradient? We explored these questions by reviewing the literature and analyzing databases compiled from different crown-fire ecosystems (mainly eastern Australia, California, and the Mediterranean basin). The review suggests that the pattern of correlation between the two basic postfire persistent traits and other plant traits varies between continents and ecosystems. From these results we predict, for instance, that not all resprouters respond in a similar way everywhere because the associated plant traits of resprouter species vary in different places. Thus, attempts to generalize predictions on the basis of the resprouting capacity may have limited power at a global scale. An example is presented for Australian heathlands. Considering the com- bination of persistence at individual (resprouting) and at population (seed bank) level, the predictive power at local scale was significantly increased.

Seed germination and life history syndromes in the California chaparral

Syndromes are life history responses that are correlated to environmental regimes and are shared by a group of species (Stebbins, 1974). In the California chaparral there are two syndromes contrasted by the timing of seedling recruitment relative to wildfires. One syndrome, here called the fire-recruiter or refractory seed syndrome, includes species (both resprouting and non-resprouting) which share the feature that the timing of seedling establishment is specialized to the first rainy season after fire. Included are woody, suffrutescent and annual life forms but no geophytes have this syndrome. These species are linked by the characteristic that their seeds have a dormancy which is readily broken by environmental stimuli such as intense heat shock or chemicals leached from charred wood. Such seeds are referred to as “refractory” and dormancy, in some cases, is due to seed coat impermeability (such seeds are commonly called hardseeded), but in other cases the mechanism is unknown. Seeds of some may require cold stratification and/or light in addition to fire related stimuli. In the absence of fire related cues, a portion or all of a species’ seed pool remains dormant. Most have locally dispersed seeds that persist in the soil seed bank until the site burns. Dispersal of propagules is largely during spring and summer which facilitates the avoidance of flowering and fruiting during the summer and fall drought. Within a life form (e.g., shrub, suffrutescent, etc.), the seeds of these species have less mass than those of species with non-refractory seeds and this possibly reflects the environmental favorableness of the postfire environment for seedling establishment. Regardless of when fire occurs, germination is normally delayed until late winter or early spring. In the absence of fire, or other disturbance, opportunities for population expansion are largely lacking for species with this syndrome.The other syndrome, here called the fire-resister or non-refractory seed syndrome, includes species that are resilient to frequent fires (mostly by vegetative resprouting), but require fire-free periods for recruiting new seedlings. Included are shrubs, subshrubs, suffrutescents, lianas, geophytes and annuals. All are linked by the characteristic that their seeds germinate in the absence of cues related to wildfires. In many cases no form of seed dormancy is present and the seeds germinate soon after dispersal; consequently these species do not accumulate a persistent seed bank. Germination and seedling establishment is independent of fire and thus opportunities for population expansion are also independent of fire. The demographic pattern of seedling recruitment varies with the life form. For shrubs, seedling recruitment may be restricted to sites free of fire for periods of a hundred years or more. Recruitment appears to require relatively mesic conditions and this may account for the patchy distribution of these species within the matrix of relatively arid sites. Finding such sites has selected for propagules specialized for wind or animal dispersal; the majority are bird dispersed. These shrub species all disperse fruits in fall and winter and this may have been selected to take advantage of migratory birds as well as to time dispersal to the winter rains typical of the mediterranean-climate. Germination typically occurs within several weeks of the first fall or winter rains. Maturation of flowers and fruits during the summer and fall drought may account for the distribution of these species on more mesic sites. Seed mass of these species is large and this may have been selected to provide an advantage to seedlings establishing under the canopy of this dense shrub community.ResumenSíndromes son las respuestas de ciclos biológicos correlacionados con régimenes ambientales y compartidos por un grupo de especies (Stebbins, 1974). En el Chaparral de California se encuentran dos síndromes, los cuales contrastan por el tiempo requerido para el restablecimiento de retonos en relación con los incendios forestales. Uno de estos síndromes, aquí denominado “síndrome fuego-restablecedor” o “semilla refractaria,” incluye especies (tanto retoāntes como no retonantes) que comparten la característica de que el tiempo para el establecimiento de plántulas está especializado en la primera estación de lluvias después del incendio Incluídas se encuentran formas de vida anuales, sufrutescentes y lenosas, sin embargo ninguna hierba perenne presenta este síndrome. Estas especies están unidas por la característica de poseer semillas con una latencia fácilmente interrumpida por estímulos ambientales, tales como un intenso shock de calor o productos químicos lixiviados de madera carbonizada. A este tipo de semillas se les conoce como refractarias. En ocasiones, su latencia se debe a la impermeabilidad de la capa de la semilla. En otros casos, el mecanismo es desconocido. Además de los estímulos relacionados con el fuego, las semillas de algunas especies pueden requerir de una estratificación fría y/o de luz. En ausencia de indicadores relacionados con el fuego, una parte o todo el grupo de semillas de una especie permanece latente. La mayoría cuenta con semillas dispersadas localmente, las cuales permanecen en el banco de semillas en el suelo, hasta que el lugar sufre de un incendio. La dispersión de semillas ocurre principalmente durante la primavera y el verano, lo cual impide el florecimiento y producción de frutos durante la sequía del verano y otono. Dentro de una forma de vida, las semillas de estas especies tienen una masa menor a la de aquellas especies con semillas no refractarias, lo cual refleja posiblemente el favorecimiento ambiental del ambiente posterior a un incendio en cuanto al establecimiento de plántulas. Sin tomar en cuenta cuando ocurre el incendio, la germinación se retrasa normalmente hasta finales del invierno o principios de la primavera. En ausencia de incendios o cualquier otra alteración, las oportunidades de expansión de la población son casi nulas para las especies con este síndrome.El otro síndrome, aquí denominado “resistidorde fuego” o “semilla no refractaria,” incluye especies resistentes a incendios fréquentes (por retono vegetativo en su mayoría), las cuales requieren, sin embargo, períodos libres de incendios para restablecer nuevas plántulas. Entre las especies incluídas se encuentran arbustos, subarbustos, sufrutescentes, lianas, hierbas perennes y anuales. Todas están unidas por la característica de tener semillas que germinan en ausencia de indicadores relacionados con incendios. En muchas ocasiones, las semillas no presentan latencia alguna y germinan poco después de su dispersión. Por consiguiente, estas especies no acumulan un banco de semillas persistente. La germinación y establecimiento de plántulas es independiente del fuego y, por lo tanto, las oportunidades de expansión para la población son asimismo independientes del fuego. El patrón demográfico de restablecimiento de plántulas varía de acuerdo a la forma de vida. En el caso de los arbustos, el restablecimiento de plántulas puede estar restringido a sitios libres de incendios por períodos de cien anos o más. El restablecimiento parece requérir condiciones relativamente mésicas, lo cual puede ser la causa de la distribución desigual de estas especies dentro de una matriz de sitios relativamente áridos. El encontrar estos sitios ha seleccionado en favor de las semillas especializadas en dispersión por viento o a través de animales. La mayoría son dispersadas por aves. Todas estas especies de arbustos dispersan sus frutos durante el otono e invierno, lo cual puede haber sido seleccionado para obtener ventaja de las aves migratorias, así como para programar la dispersión de acuerdo con las lluvias de invierno, típicas del clima mediterráneo. La germinacion ocurre típicamente por varias semanas durante las primeras lluvias de otono o invierno. La maduración de las flores y frutos durante la sequía del verano y otono puede explicar la distribución de estas especies en sitios mas mésicos. Las semillas de estas especies son de masa considerable, lo cual puede haber sido seleccionado para proporcionar una ventaja al establecimiento de plántulas bajo el follaje de esta densa comunidad de arbustos.

Reproduction of chaparral shrubs after fire: a comparison of sprouting and seeding strategies

The relative ability of sprouting and nonsprouting chaparral shrubs to recover from fire was studied by examining popula- tion of congeneric pairs of species in burned and adjacent unburned areas. The pairs of species selected, with the nonsprouting species named first, were Arctostaphylos glauca ? A. glandulosa and Ceanothus greg- gll - C. leucodermls. Data were also obtained on certain associated species, particularly Adenostoma fasclculatum. The numbers, sizes and condition of the component species at each of the sites were measured. Both sprouting and nonsprouting species showed vigorous recovery from fire. It seemed likely that the burned stands would eventually reach a state of development comparable to that shown in the preburn stand without any significant shifts in composition Mortality of shrubs resulting from the fire was complete for nonsprouters but varied in sprouting species. In some there was essentially no mortality, while in others, especially Adenostoma, it was rather high. Seedling establish- ment in the shrub species varied markedly and seemed to be correlated with the degree of fire-caused mortality. A profound difference was observed in the life histories of the two nonsprouting shrubs. Whereas Ceanothus greggll produced a very high number of seedlings after fire, Arctostaphylos glauca produced substan- tially fewer. A comparison of the density-size distribution of live and dead stems indicated that C. greggll suffers high mortality early in suc- cession, but A. glauca loses very few individuals even after 90 years without fire. In light of these results a model is proposed which we be- lieve explains the adaptive significance of the obligate-seeding strategy in the southern California chaparral.

The human dimension of fire regimes on Earth

Humans and their ancestors are unique in being a fire-making species, but ‘natural’ (i.e. independent of humans) fires have an ancient, geological history on Earth. Natural fires have influenced biological evolution and global biogeochemical cycles, making fire integral to the functioning of some biomes. Globally, debate rages about the impact on ecosystems of prehistoric human-set fires, with views ranging from catastrophic to negligible. Understanding of the diversity of human fire regimes on Earth in the past, present and future remains rudimentary. It remains uncertain how humans have caused a departure from ‘natural’ background levels that vary with climate change. Available evidence shows that modern humans can increase or decrease background levels of natural fire activity by clearing forests, promoting grazing, dispersing plants, altering ignition patterns and actively suppressing fires, thereby causing substantial ecosystem changes and loss of biodiversity. Some of these contemporary fire regimes cause substantial economic disruptions owing to the destruction of infrastructure, degradation of ecosystem services, loss of life, and smoke-related health effects. These episodic disasters help frame negative public attitudes towards landscape fires, despite the need for burning to sustain some ecosystems. Greenhouse gas-induced warming and changes in the hydrological cycle may increase the occurrence of large, severe fires, with potentially significant feedbacks to the Earth system. Improved understanding of human fire regimes demands: (1) better data on past and current human influences on fire regimes to enable global comparative analyses, (2) a greater understanding of different cultural traditions of landscape burning and their positive and negative social, economic and ecological effects, and (3) more realistic representations of anthropogenic fire in global vegetation and climate change models. We provide an historical framework to promote understanding of the development and diversification of fire regimes, covering the pre-human period, human domestication of fire, and the subsequent transition from subsistence agriculture to industrial economies. All of these phases still occur on Earth, providing opportunities for comparative research.

Fire as an evolutionary pressure shaping plant traits

Traits, such as resprouting, serotiny and germination by heat and smoke, are adaptive in fire-prone environments. However, plants are not adapted to fire per se but to fire regimes. Species can be threatened when humans alter the regime, often by increasing or decreasing fire frequency. Fire-adaptive traits are potentially the result of different evolutionary pathways. Distinguishing between traits that are adaptations originating in response to fire or exaptations originating in response to other factors might not always be possible. However, fire has been a factor throughout the history of land-plant evolution and is not strictly a Neogene phenomenon. Mesozoic fossils show evidence of fire-adaptive traits and, in some lineages, these might have persisted to the present as fire adaptations.