(Tomasz B. Falkowski/SUNY-ESF)
The Phoenix: How fire and disturbance can revitalize ecosystems
Morning mist glistened like diamonds upon emerald leaves as Nach-nik and I walked down a narrow path cut through the dense forest vegetation. My companion had lived and farmed in this small Lacandon Maya community in the Lacandon rainforest of Chiapas, Mexico for his entire life, as did his forefathers before him. Upon arriving at our destination a small clearing amidst the rainforest, Nach-nik told me a traditional Lacandon legend. “Long ago, Hachakyum, our creator god, destroyed the earth. He thought it was imperfect and needed to be remade. Just before Hachakyum set his creation ablaze, Akinchob, the god of agriculture, saved our people in a canoe.” Every year for the past five centuries, smoke has risen from the rainforest like incense to the heavens. Immediately before the rains arrive to conclude the dry season in May, Lacandon Maya farmers like Nach-nik reenact their people’s salvation by burning small patches of secondary forest to clear land to plant their gardens.
Slash-and-burn agriculture is justifiably criticized as one of the leading causes of deforestation in the tropics. Because fire is also the primary tool used to clear land in swidden agricultural systems managed by indigenous populations around the world, including the Lacandon, these systems are often associated with and suppressed along with destructive slash-and-burn practices. However, the burns used in traditional agroecosystems are quite different from those used to clear land for large pastures and monoculture commercial farms. Indigenous farmers who practice swidden agriculture carefully monitor environmental conditions and are guided by centuries of knowledge that is adapted to place and passed on from generation to generation over centuries. They only burn when weather conditions can suppress fires. They meticulously control the fire intensity, frequency, and extent through the use of fire breaks. The fire is maintained at low-intensities, thereby pyrolizing carbon in wood, converting it from labile forms that would otherwise be emitted into the atmosphere into recalcitrant carbon rings that are difficult to decompose. So, rather than exacerbating climate change, swidden agriculture may help mitigate it. Swidden burns also produce biochar, which improve soil quality, improving soil structure, nutrient holding capacity, and moisture retention.
Ecological disturbances, events that suddenly disrupt an ecosystem’s function or change its structure, get a bum rap, as demonstrated by Smokey the Bear’s crusade to suppress forest fires. While it may be distressing to see the charred remains of trees after a forest fire, these disturbances are important for supporting productive and diverse biological communities.
Ecosystems do not reach a stable climax state; they are dynamic. If disturbance events that shake up the system are infrequent, ecosystems stagnate and become dominated by a few species which can exploit the available resources. Conversely, if disturbances occur too often or if they are too intense, ecosystems can only be colonized by a few opportunist species which grow quickly to the detriment of longer-lived specialist species that cannot establish themselves. The intermediate disturbance hypothesis holds that regular, intermediate-intensity disturbances are critical to maintain the balance between these two states and, in turn, maximize the system’s diversity.
This principle also explains why intermediate disturbances can enhance the productivity of ecosystems, or the amount of energy harnessed by plants to support other organisms. Infrequent disturbances can result in stasis, while high intensity, excessively frequent disturbances can suppress growth. Intermediately frequent and intense disturbances can reinvigorate a system with a pulse of nutrients and provide space for new species to grow, much like pruning a bush or tree encourages new shoots.
Intermediate sized disturbances also have the benefit of generating habitat heterogeneity across a region. A patchwork mosaic of many ecosystems at different stages of recovery from disturbances will provide more niches for more species than uniform landscapes. Thus, diversity begets further diversity. Fast-growing pioneer species can coexist with long-lived species that tend to only survive in highly developed ecosystems if there are habitats for both.
Regular disturbances are also critical in maintaining ecosystem resilience. Resilient systems can self-organize to restructure themselves and recover function after disturbances. Broken bones can mend. Towns leveled by a storm can rebuild. Disturbed ecosystems will regrow. The diversity that is supported and enhanced by disturbances is essential to maintain systems’ adaptive capacities so they can respond to environmental changes. Intermediate disturbances can also help prevent high intensity disturbance events that can severely undermine ecosystems’ abilities to restore themselves. For instance, small fires burn off decaying detritus regularly, so it does not accumulate and burn in a catastrophic inferno that might fundamentally undermine an ecosystem’s ability to recover.
Nach-nik had spent the previous month chopping the thick tangle of vines, ferns, and saplings in a small patch of jungle. He waited for the perfect conditions. It rained a few days ago and was threatening to rain again, so the vegetation surrounding the clearing was verdant and lush, contrasting the dry, withered branches and leaves on the ground. The air was humid and still‒not a breath of wind. We gathered buckets of water from a stream. Nach-nik strode into the clearing and put a match to a small pile of brush. The flames leapt to life, feeding on the slash left behind in the wake of Nach-nik’s machete. Nach-nik walked just ahead of the line of fire, as though guiding it on its path, dumping water where the flames became too intense. Sickly sweet smoke wafted through the trees and stung my nostrils. In no more than half an hour, the fire burned its way across the clearing and dwindled to embers upon reaching the intact vegetation blocking its progress. A few larger logs smoldered and the blackened earth radiated warmth. In a few days, the clearing we burned would be cool enough to plant corn, squash, beans, chayote, papaya, and dozens more crops. Just as the Maya were reborn from a corn kernel planted in the earth, so too will the farmers’ corn rise from the ashes, and give them the gift of life in return for the chance to reach toward the sun.