After a windstorm, what do we do with the fallen trees?

Last February, storm Pedro left winds of more than 120km/h in some parts of Catalonia. In some forests, the extreme wind toppled a large number of trees, as was the case in Ripollès.

In situations like these, several questions arise: what should be done with all these fallen trees? Should they be removed or is it better to leave them in the forest? CREAF experts agree that there is no single answer, since the management of fallen trees generates tensions between ecological and economic priorities.

Trees can break or fall naturally, either because they are very old or because they have suffered from some disease that has weakened them. However, in situations of extreme wind, large masses of trees can fall and cause a domino effect , with a much greater visual impact.

The probability of reaching these extremes increases when several factors coincide that make trees more vulnerable. These factors are:

• Extreme events: there are episodes of very violent wind that alone can topple trees, even if they are healthy and well-rooted, such as downpours or hurricane-force gusts associated with storms or phenomena such as DANA.
• Soils saturated with water due to heavy rains: when it coincides with a strong wind before, the soil is very wet and soft. This means that the roots cannot hold on to the ground well and the tree can more easily give way to the force of the wind.
• Steep slopes and thin, shallow soils: in mountain areas the soil can be quite thin and this makes the roots more superficial and unstable. In addition, steep slopes increase this instability and any wind or heavy snowfall can easily uproot a tree.
• Shallow roots: Some tree species (such as pines) have shallower roots than others (such as oaks and holm oaks) and are therefore more likely to fall in windstorms. In addition, when trees grow in very stony soils, they also tend to have shallower roots than usual and can fall more easily in windstorms.
• Wet snow: Snow that falls at temperatures around 0°C is called wet snow and weighs much more than powder snow. This heavy snow can break tree branches and leave them weak and more vulnerable to stronger wind episodes. These situations are very common in sleet situations in spring, especially in the Pyrenees and Pre-Pyrenees.

Strong winds leave a messy-looking landscape that we easily associate with the negative effects of tree loss. But, surprisingly, winds can also be an opportunity for the forest . After an episode of extreme wind in which many trees have fallen, open spaces or clearings are often created that provide ecological benefits . On the one hand, these spaces become habitats for many species, such as birds, small mammals, insects and other pollinators, that require low forest density. In addition, these clearings help break the continuity of the forest , which helps slow the spread of wildfires.

This relevant ecological role of the winds connects with a more modern vision of renaturalization : understanding that natural phenomena such as wind, snow, drought or pests are part of the functioning of ecosystems and, therefore, must be taken into account in landscape management. “The most interesting thing about these phenomena is that they occur randomly and often complement the work done by active forest management,” says CREAF researcher Josep Maria Espelta .

Along these same lines, experts agree that in areas affected by a windstorm , nothing needs to be done to recover, as nature already does it on its own. "Oaks and many other species will quickly regrow without the need for intervention. Other species are good germinators, such as the Scots pine, and they also regenerate after a few years," explains CREAF researcher Jordi Vayreda .

After a windstorm, the management of fallen trees must be assessed on a case-by-case basis. First, the priority is often to remove accumulated wood to maintain paths and access routes to the territory , especially near roads or critical infrastructure. In these locations, fallen wood can be a physical obstacle that must be removed and, if left to dry, can easily become a large amount of fuel too close to traffic areas, thus increasing the risk of fire.

In these and other cases, the wood from fallen trees can be marketed, although it is usually difficult to obtain a profit from it. Often, this marketing is only viable with government aid, and this wood is only intended for uses such as the manufacture of chipboard or the production of biomass. Therefore, despite having a certain value, it is often insufficient to cover the costs of felling and subcontracting . “This situation is similar to what happens after a fire: some of the wood can be marketed, but at a lower price,” explains CREAF researcher Lluís Brotons , “so the final economic viability will depend on subsidies, the owner's ability to bear the expense, negotiation with the logging company or whether the action is a priority for reasons of safety or forest health.”

Finally, when assessing what to do with fallen trees, it is also necessary to take into account the benefits that dead wood provides to the ecosystem . Leaving some wood in the forest can provide nutrients, create new habitats and hiding places for various species, and in general, promote forest biodiversity . The problem appears when, after an episode of intense wind, there is a massive accumulation of fallen wood and this can become a focus for some pests . Therefore, it is necessary to assess whether it is appropriate to remove some of this wood or leave it in the forest depending on each situation.

It is impossible to completely avoid the effects of windstorms in forests, but it is possible to achieve forests that are less vulnerable to the effects of these phenomena. One of the main problems is that forest management is often not carried out when it is needed because it is not economically profitable. If it is intervened too late or the thinning is too intense , the trees are left unprotected, without “companion trees” around them, and much more exposed to the wind. CREAF researcher Lluís Comas explains this with a very clear example: “in forests where the trees look like clones because they are all long, thin and separated by the same distance, the wind passes everywhere and it is easier for them to fall en masse”.

Therefore, the best prevention strategy against gales is to have heterogeneous forests , both in terms of structure and composition of species and sizes, so that they protect each other and thus avoid a "domino effect" when episodes of extreme wind occur.

In general, there is little data available on windstorms and their effects on forests, which is why it is important to monitor them. Faced with this need, the Forest Alert project allows citizens to record photos of the effects of windstorms on forests, and become an active part of this monitoring.

As explained by the project coordinator, Mireia Banqué , “each contribution is valuable in understanding a phenomenon that, according to climate models, will increase. Since 2017, the platform has been collecting observations of different impacts on forests, including windstorms.” Although there is a lot of meteorological information on extreme wind episodes, detailed data on their real effects on forests is often lacking. “In this sense, the photographs and observations provided by citizens can help build, over time, a valuable record of the real consequences of these episodes on forests,” adds Banqué.

Therefore, the Forest Alert project calls on citizens to detect forests affected by the wind, sending a photograph to the citizen science platform Forest Alert , with the pertinent precaution of not going into the forest during or immediately after a strong wind, to avoid accidents. With this simple gesture, people can contribute to documenting a phenomenon that we will probably see more and more often in Catalan forests.