Afforestation can increase forest cover supply and aid in addressing the issue of Climate Change. However, transforming open land to afforested land can alter the behavior of wildfires. According to a study published in the Proceedings of the National Academy of Sciences (PNAS), afforestation was associated with a 26% higher probability of fire occurrence and a 23% larger burned area.

The study analyzed 67,248 land areas that were transformed from non-forest land to planted forests between 2001 and 2023. Through satellite data analysis, the researchers looked at changes in fire incidence before and after these transformations, taking into consideration climate, accessibility, and land area. The findings varied considerably across biomes, suggesting that the effects of tree planting depend on the location and type of landscape.

There were also increases in fire activity in Mediterranean forests, woodlands, and scrublands. The study found that afforestation was associated with a 38% rise in fire occurrence and a 9% increase in expected burned area in this biome. These landscapes experience periodic drought, contain flammable vegetation, and are frequently affected by human-caused fires. The researchers suggest that increased biomass from planted forests may add to the available fuel, potentially contributing to greater fire activity.

On the other hand, temperate broadleaf and mixed forests showed a reverse pattern. Afforestation in these landscapes was associated with a 23% decline in fire occurrence but a 20% increase in expected burned area, indicating that fires became less frequent but larger. This suggests that although fires occurred less frequently, the overall area affected by fire increased. Since nearly half of the world’s planted forests are located within this biome, even modest changes in fire behavior could have substantial ecological and socioeconomic consequences.

The only biome that did not show an increase in wildfire activity was temperate grasslands, savannas, and shrublands. Here, afforestation was associated with an 18% reduction in fire occurrence, without a significant increase in burned area. This is likely due to the natural fires experienced in such landscapes, as well as the long-term human experience in using fire.

Geographically, regions where wildfire activity increased following afforestation included western North America, central Chile, southern Europe, southwestern Australia, and parts of South Africa. In those regions, plantation development coincides with a climate that becomes more favorable for extreme wildfires.

Moreover, high temperatures and low precipitation were related to increased fire incidence and burned areas. Climate change causes more droughts, heat waves, and dry vegetation, leading to conditions under which fire can occur and develop easily. The researchers accounted for climate variables when examining the relationship between afforestation and wildfire activity, but their findings also point to the importance of local fire regimes and land-use patterns.

There are consequences beyond the immediate damage caused by wildfires. Forests are often planted to store carbon dioxide over the long term, but fires can undermine this function by destroying trees and releasing stored carbon. Higher wildfire activity could also have economic consequences for countries that depend on the forestry industry, including Chile and Portugal. The authors warn that fire-related timber losses could affect future wood supplies and economies reliant on forestry.

This does not mean that afforestation will always lead to an increased risk of fires. The contrasting results across temperate biomes show why a one-size-fits-all afforestation strategy does not work. The researchers suggest evaluating fire risks alongside carbon storage and wood production when converting nonforest land into planted forests.