Some like it hot

6 03 2023

Wildfires transform forests into mosaics of vegetation. What, where, and which plants thrive depends on when and how severely a fire affects different areas of a forest. Such heterogeneity in the landscape is essential for animal species that benefit from fire like woodpeckers.

The black-backed woodpecker (Picoides arcticus) lives in the coniferous forests of North America’s boreal-Mediterranean region. Thanks to a powerful and sharp bill, this bird can excavate nests inside the trunks of (mainly dead) trees, and those cavities will be re-used later by many species of birds, mammals, and invertebrates in fire-prone landscapes (22). The images show a male with the characteristic black plumage of his back that serves as camouflage against the dark bark of a dead tree three years after a wildfire in Montana (USA). Being omnivores, the diet of this bird largely relies on the larvae of woodboring coleoptera like jewell and longhorn beetles. These insects are abundant post-fire, the champion being the fire beetle (Melanophila spp.). The thorax of fire beetles is equipped with infrared-light receptors that can detect a wildfire from tens of kilometres away (23). These fascinating little beasts are the first to arrive at a burned forest and, of course, woodpeckers follow soon after. The preference of the blackbacked woodpecker for burned forests and their cryptic feathers and pyrophilic diet reflect a long evolutionary history in response to fires. Courtesy of Richard Hutto.

Anyone raised in rural areas will have vivid recollections of wildfires: the thick, ashy smell, the overcast sky on a sunny day, and the purring of aerial firefighters dropping water from their hanging tanks. The reality is that wildfires are natural events that shape biodiversity and ecosystem function (1) — to the extent that fire is intimately linked to the appearance and evolution of terrestrial plants (2). Since the Palaeolithic, our own species has used fire at will, to cook, hunt, melt metals, open cropland or paths, or tell stories in front of a hearth (3).

Where there are regular wildfires (fire-prone ecosystems), different areas of the landscape burn in different seasons and years under different weather patterns. Therefore, each region has a unique fire biography in terms of how frequently, how much, and how long ago wildfires occurred. All those factors interact will one another and with topography.

For instance, a wildfire will behave differently on a windy versus calm day along a flat valley compared to the slopes of a mountain, and might occur in areas that experienced fires in a previous summer versus last decade. This spatial and temporal variability of fires is called pyrodiversity (4, 5), because it shapes the flora and fauna we see in fire-prone ecosystems.

It is in this context that we can understand quotes like “fires can kindle biodiversity, sparking new approaches to conservation” or “let natural fires burn, while preventing out-of-control blazes” (6), and that some argue that fire in the Antropocene could be the ecological-force equivalent of ice in the Pleistocene (3).

BOX 1: What increases the risk of a wildfire?

It all boils down to four factors (19):

— a prolonged period of drought;
— an area of continuous flammable vegetation;
— an ignition source (human activity or infrastructure, lightning);
— a heat wave or an episode of high temperatures accompanied by strong winds

Drought, vegetation, and ignition are essential ingredients for the occurrence of wildfires. As their magnitude increases (drier conditions, denser vegetation, multiple ignition triggers), so does the risk and severity of wildfire. Fire weather (high temperatures, low humidity, strong wind) lower the thresholds of drought, vegetation, and ignition above which the risk of wildfire is high (see blog in Spanish).

Once a wildfire starts, changes in the wind influence the direction and speed of fire as it passes through complex landscapes, including patches of natural vegetation (fuel) between residential areas and barriers such as agricultural fields and roads. Human resources for putting out fires also vary across regions, topography, and the time of day, all affecting firefighting access.

Consequently, no two wildfires are equal.
Camp Fire (20, 21) was active between 08 and 25/11/2018, burning 621 km2 in California (USA) — see California Department of Forestry and Fire Protection’s Green Sheet. It killed 86 people and burnt > 14,000 homes and businesses. Risk factors were: (i) 2012-2016 drought (dead wood and dry vegetation), (ii) dense conifer forest with brush understory (fire-setting point / higher elevation) and oak forest with dense and above-average-dry grass (primary carrier of fire / lower elevation), (iii) ignition = electric-power transmission line, and (iv) humidity lowered by wind events in October and early November, and zero rainfall for 7 months pre-fire, along with strong winds (40-80 km/hour) on 07 to 09/11/2018. Firefighting was challenged by orography (steep river canyons surrounded by flat volcanic benches) and initial speed of fire (222 km2 burnt in the first 12 hours following ignition time at 06:25). Photo courtesy of US Geological Survey and Pierre Markuse.

Adapting to fire

Over the past three decades, Andrew Stillman and colleagues have investigated the relationship between pyrodiversity and black-backed woodpeckers (Picoides arcticus) in the coniferous forests of the western United States (7-9). This bird (one of more than 300 species of woodpeckers known worldwide) builds its nest in the cavities of fire-trees or snags (10), and forages on a burnout buffet, i.e., insect larvae living in charred wood (11). Therefore, fire is an ally for black-backed woodpeckers, but Stillman has revealed a not-so-simple tale by tagging juveniles and adults with teletransmitters.

Stillman and his team estimated that nesting probability on dead trees increases with the extent of wildfires, but nests are more common near the border between forest fragments subjected to low and high fire severity (8). Further, when juveniles leave their nests, they use fragments of intact or mildly-burned forest much more frequently than adults (9). And when juveniles use forest areas with less than 80% tree mortality, their chance of surviving within the first month after birth increases by a factor of four (7).

Clearly, having access to dead forest where food abounds is as important as living near unburned forest to increase reproductive success. So, the life cycle of these birds depends on the way that fire severity varies across the landscape – see video here that portrays such a dependence for Californian forests.

To eat and not to be eaten

Feeding in the wild implies making decisions to search for food without encountering predators (12). Thus, black-backed woodpecker juveniles are more visible in the open areas left behind by wildfires, so they can avoid raptors in the shelter provided by healthy fragments of forests (7). All the more important, these birds have evolved to live in burned environments for millennia (10), so salvage logging and removing dead wood after a wildfire decreases their access to food and nesting sites (13).

Indeed, burned wood is not a useless waste (14) because it feeds and/or serves as habitat for many species, fertilises soils and prevents their erosion, and encourages the growth of herbs, shrubs and trees that reemerge after a wildfire (14).

Overall, wildfire should be perceived as an ecosystem service (15), at least if people were better informed.

Reproduction and demographic surveys of black-backed woodpecker (Picoides arcticus) in coniferous forests of California and Washington (USA). Top panel:  blue line shows that nesting probability (118 nests) in burnout trees increases as those trees are closer to forest fragments surviving a wildfire (8). Middle panel: bars quantify counts of woodpeckers carrying a teletransmitter (49 adults + 22 juveniles) found in forest fragments affected by four classes of fire severity. This bird prefers fragments with 25-80% tree mortality, but juveniles use healthy fragments (0-25%) more than adults (9). Bottom bars indicate survival > 1 month after birth (84 juveniles) is lower in fragments with > 80% tree mortality (7); in fact, 15 of 16 juveniles living in fragments with > 50% tree mortality perished before becoming independent from parents. Data: 10 wildfires taking place between 2007 and 2014, and burning 450 to 16,000 ha per wildfire (average = 6,700 ha). Silhouettes courtesy of Andrew Stillman.

Mediterranean forests, like those present in large stretches of western USA, southern Australia, and eastern Spain, are fire-prone due to the dry and hot summers characteristic of these regions. The problem is that we humans continue to burn fossil fuels, which warm the atmosphere and prolong droughts, making forests drier for longer periods of time.

In western Europe, farms and traditional forest activities are being abandoned because of the unrelenting migration of people from rural to urban areas — this phenomenon has been named “España vaciada” [emptied Spain] in our country (see testimonies in a documentary here). In doing so, we are promoting a continuous layer of vegetation that acts as an ever-expanding, highly flammable wick.

Altogether, we have created a ‘fire climate’ that increases the risk of wildfires over this century (16). A contemporary concern is that wildfires are gradually becoming larger in extent and stronger in severity, leading to the term “megafire” (17, 18) — see videos here about the physics of megafires and here about how megafires are reshaping forests.

If megafires lower pyrodiversity because everything burns a lot, then megafires might threaten the fine connections that many species have with pyrodiversity. It seems necessary to take managerial and policy measures to decarbonise our energy system and repopulate rural areas to prevent fire from playing a more massive role than the Earth’s history has evolved naturally.

Salvador Herrando-Pérez & Juli Pausas

Acknowledgements: Supported by the Spanish Ministry of Science and Innovation through project FIROTIC PGC2018-096569-B-I00. A Spanish version of this article has been published in Volume 444 of Quercus (Feb 2023).


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    One response

    13 03 2023

    Thanks for sharing this knowledge, that was an excellent post. I studied biology but never learned much about fires in nature. I thought the link to woodpeckers particularly interesting.
    Kind regards


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