Excessive rain and strong winds: the dangerous combination that can knock down many trees

BarcelonaThe abundant and continuous rains of recent weeks have saturated the ground. This situation was unthinkable just a year ago, when Catalonia was still suffering the effects of a historic drought. We have gone from one extreme to the other in a matter of months, and, as is often the case, excesses are never good. Although it may seem unbelievable, so much water is also not good for some trees, as they can fall more easily if a storm with strong winds arrives with gusts of more than 80 or 100 km/h.

"When the ground softens from the rain, trees with high canopies, few branches at the base, and shallow roots are the most likely to fall in the event of strong winds," explained biologist Daniel Siscart, who has killed [a tree with a very shallow root system], in statements to ARA.

This makes it more resistant during droughts, since it quickly absorbs water from the surface with even a little rain. However, in the case of heavy rain, the stump is exposed to unstable ground and can collapse entirely in a strong wind. Furthermore, the pine tree is not balanced, as all the weight of the branches and crown is concentrated at the top. All of this puts the tree's stability at risk. In contrast, other Mediterranean species, such as the holm oak, are more resistant because their roots are deeper. They may suffer branch breakage, but the wind is unlikely to topple the entire tree.

Considering that stone pines and white pines are the dominant trees in our Mediterranean forests, we are talking about many hectares at risk. However, Siscart clarifies that pines living in deciduous forests are the ones best able to withstand strong winds. "When pine trees are closer together, they act as a group and are much less likely to be knocked down by the wind, as they help each other to counteract its effects," he explains. Conversely, if the pines are spaced further apart, the risk of them falling in strong winds is higher. This occurs, for example, in urban parks or with forest clearing to prevent fires. Clearing is a necessary measure to prevent the spread of flames in case of fire, but it also exposes the pines more to strong winds and excessive rain. "The most delicate time for a tree is the two or three years following forest clearing or logging, as they lose their neighboring trees; if they are isolated, they can fall very easily in the wind," notes Siscart, who recommends avoiding these cleared forest areas during windy periods. The worst-case scenario: snow at low elevations with wind

Siscart asserts that, given the current state of the forests, the worst-case scenario would be snowfall at low elevations combined with wind. The snow would add more weight to the Mediterranean pines, which would then fall much more easily in the wind. This is a scenario that already occurred with the heavy snowfall of March 8, 2010. "This combination would be fatal, as it would cause trees to burst," he explains.

In contrast, the high-mountain pines found in the Pyrenees, such as the Scots pine, are more accustomed to and prepared for this winter scenario, as they have flexible branches that release accumulated snow when they detect its weight. Despite these risks, Siscart explains that the current scenario is much more positive than that of drought. "From the perspective of the climate crisis, tree falls due to excess water are always more favorable than those caused by drought; the vegetation is recovering, and the trees will have a good spring, since the ecosystem benefits much more from a wet period than a dry one," the expert assures. Despite the abundant rainfall and the recovery of the forests, it should be noted that the drought killed 28% of the trees in Catalonia, and these dead trees can easily fall if the ground is damp and windy. "When hiking in forested areas with trees killed by drought, we must be very careful of the risk of them falling, especially if the wind is strong," warns Siscart.