Friday, December 6, 2019

Good fire, bad fire

sometimes seems that all wildfires are bad.  Forests burn down, homes and whole towns are destroyed, carbon dioxide is released into the air, valuable wood is destroyed, and wild animals are killed or driven from their habitat..

So it is surprising to hear for the first time that wildfires are natural and necessary in many ecosystems.  They become bad basically only because of our own interference.

As in many pines, the seed cones of  Banksia in
Australia, open only after a fire to release their
Ecosystems in which fires are a normal part of maintenance or renewal are those in which there are distinct wet seasons and dry seasons.  During the wet season, there is abundant growth of trees, shrubs, grasses, and other herbs  During the dry season, leaves and twigs fall from the woody plants and grasses dry out.  Typically, this debris accumulates faster than it can decay, so builds up from year to year.  Sooner or later a lightning strike will ignite the accumulated debris, causing a wildfire.

 Burning removes the debris, releases nutrients back into the soil, clears out the undergrowth, trims dead branches from the trees.  In some cases, shrubs are burnt to the ground, but re-sprout quickly at the beginning of the next wet season.

Plants in these areas are adapted to these periodic fires. Pine trees, for example, survive moderate fires, and require the ground to be cleared for seeds to germinate.  In many species, seed cones will not even open until heated by fire.  Where fires are prevented for a number of years, ground vegetation becomes thick and pines do not reproduce, and when fire inevitably strikes, it is more intense and trees die.  For these reasons, foresters often conduct regular controlled burning to prevent more intense fires later.

In the pine flatwoods of Florida, fires remove the undergrowth and debris, clearing the way for 
germination and growth of pine seedlings.  Without fires, pines would gradually disappear.  
Their thick bark protects the trunks, and the upper branches are spared as well, as long as fires  
are frequent and not too intense. Saw palmettos, Serenoa repens, cover much of the ground  
here, but can be seen here recovering quickly after a fire.  
Bulb plants, like this Florida native Lilium catesbaei,
survive fires below ground.  Plants that sit out
the dry fire season are particularly common in
California, South Africa, and Australia.

The California chaparral and other forms of Mediterranean vegetation are adapted to winter rains and long summer droughts, and are also fire-maintained. It is here where we see shrubs well-adapted for re-sprouting after burning to the ground.  Grasses, and wildflowers also thrive after fires, and are suppressed if the shrubby overgrowth becomes too thick.  Between the pine forests and the chaparral, much of California is thus prone to natural fires, setting up an unfortunate conflict between nature and people building homes on vegetated hillsides.  The same tragic conflict can be seen in many parts of Australia and southern Africa.

Wildflowers, such as this Liatris, flourish where
fires are frequent in Florida.

The only places where wildfires are rare are in areas with reliable, year-round rainfall, or in areas of practically no rainfall.  So rain forests, temperate deciduous forests, and deserts do not normally experience fires.  In the rain forest, and temperate forests with precipitation all year long, vegetation rarely dries out, and debris is decomposed quickly.  In the deserts, vegetation is sparse, and very little debris is produced. It's the areas between these extremes that rely on fires.  Aside from the chaparral and pine forests mentioned above, this would include the grasslands and deciduous tropical forests that cover vast areas of Africa and tropical America.

So how do we humans turn good fires into bad fires?  There are several ways.

First, by overzealous prevention of fires where fires should normally be occurring, we allow more debris to build up, allow opportunistic undergrowth vegetation to run rampant, setting up for a more disastrous fire when lightning eventually strikes.  In these disastrous fires, pines are not only pruned, but burned to the ground.  Such fires may be so hot that even the root systems of normally resilient shrubs are destroyed, and then do not re-sprout.
After the big fire in Yellowstone National Park in 1989, grasses and wildflowers, such as the pink fireweed, Epilobium angustifoliumgrow abundantly, a boon to local herbivores. 

Second, climate change is resulting in the expansion of dry seasons into formerly wet forests in many parts of the world. The intensity of droughts, as well as floods, hurricanes, and blizzards is increasing. This is not currently seen as a significant factor in the catastrophic fires in the Amazon Basin, but are a factor in the desertification of the African savannas.

Third, clearing and burning of forest for conversion to farm or grazing land, which is occurring in the Amazon Basin at an increasing rate, can get out of hand during dry periods and burn more extensive areas than normal. Fires are normal only where rain forest transitions into deciduous tropical forest, mostly along the southern fringe of the Amazon forest. Clearing of the forest, apparently supported by the current government, is also fragmenting the forest, causing it to get drier and less able to sustain itself.

So the burning of the Amazon rain forest, unlike the routine burning of the chaparral and pine forests, is a tragically bad fire.  It is resulting in a significant loss of biodiversity and loss of photosynthetic activity that could help offset climate change.
Rauch steigt in Brasilien auf - die Feuer haben sich in den vergangenen Tagen ausgebreitet
The fires in the Amazon Basin are largely due to human activity.  They are a tragedy because of the huge loss of  biodiversity, release of CO2 into the atmosphere, and loss of photosynthetic oxygen replenishment.