Forests can take decades to recover from fire

Devastating fires raging through the World Heritage-listed Mount Kenya National Park this month have destroyed more than 80,000 hectares — almost half of the site. Fires here aren't uncommon, but this year alone more than 100 have been reported, with many of them severe. The second tallest mountain in Africa is home to lakes, dense forest, glaciers and rare animals, all of which are at risk.

As Greece gradually comes to terms with the wildfires that left 80 people dead last summer, researchers from the Australian National University (ANU) have found that forests themselves need a long time to recover from such a devastating blaze. Scientists are only just beginning to understand how and to what extent fires affect life below the forest floor.

In a recent study, Australian scientists found that it takes as long as 80 years after a wildfire and 30 years after logging for forest soils to fully recover. Working in more than 80 locations in southeast Australia, researchers tested over 700 soil samples that had suffered nine different types of damage – including wildfires, clearcutting and post-fire salvage logging.

Six months after a forest fire struck an area southwest of Berlin in summer 2018, large sections of the affected land have been cleared away. Many of the remaining trees are blackened. Scientists believe that besides the wildfires themselves, post-fire clearing can cause the loss of key soil nutrients and have long-lasting impacts on forest soils.

In many places, wildfires are an important part of a natural cycle. Some trees, like the eucalyptus, even need fire to release their seeds. Post-fire ash can actually inject large amounts of nutrients into the soil immediately after a fire, scientists say. But there can be too much of a good thing, and extended droughts, logging and fire suppression can all disrupt natural cycles.

Portugal's 700 year-old Pinhal de Leiria forest was destroyed during the wildfires that swept across Europe in summer 2018. A staggering 80 percent of the forest – home to all manner of bugs, birds and mammals – was destroyed. Despite help from local volunteers, it could take as long as half a century for the soil to regenerate.

During a fire, soil temperatures can reach 500 degrees Celsius (932 degrees Fahrenheit), leading to a loss of growth-promoting nutrients, such as phosphorus, organic carbon and nitrate. When fires occur repeatedly in the same place, it's even harder for the forest floor to recover. Without these nutrients, soils are unable to support plant growth or store carbon.

Soils are a vital part of forest ecology. They are the basis of almost all terrestrial life, scientists point out – influencing plant growth and survival, nurturing communities of beneficial fungi and bacteria, and cycles of key nutrients. They also store enormous amounts of carbon.

It's not just excessive fires that disrupt soil composition. Clearing forests with machinery and burning the remaining debris also has an impact. Logging can expose the forest floor, compact the soil and deplete its nutrients, as well as release a lot of carbon dioxide into the atmosphere.

Germany's Treuenbrietzen forest is a shadow of its former glory. Most of the remaining trees are ashen skeletons, and the forest floor is blackened. It faces an enormous struggle to regenerate. And given the predicted increase in the number, frequency and intensity of wildfires prompted by drought, it could be another lifetime before the seeds of restoration emerge.