In July 2002, the Pisz Forest (north-eastern Poland) was hit by a huge hurricane that destroyed about 30,000 hectares of forest. Over the next two years, foresters cut and cleared 3 million m3 of timber, but a fragment of the destroyed forest was left without human interference. This area of about 460 ha (including all broken and fallen trees), called the Protective Forest "Szast", was a rare opportunity to observe the natural renewal processes of a commercial forest, scientists emphasise in the Journal of Vegetation Science.

The authors of the publication are Jerzy Szwagrzyk and Anna Gazda from the Department of Forest Biodiversity at the University of Agriculture in Krakow, Dorota Dobrowolska from the Forest Research Institute in Sękocin, Ewa Chećko from the University of Warmia and Mazury in Olsztyn, Jakub Zaremba from the Białowieża Geobotanical Station of the University of Warsaw and Andrzej Tomski from the University of Silesia in Katowice.

In a natural young forest, where economic operations are not carried out, forest restoration is governed by natural processes related, among other things, to young trees competing for light and water. The effect - the final shape of the forest - is different in this area than in the commercial forest, if only because the trees are of different ages and the species more diverse. In forests planted by humans, the trees are subject to interference - they are thinned, protected from being grazed on by animals, and "weeds" appearing in their vicinity, including undesirable species of trees, are combated.

In order to determine how a commercial forest regenerates itself, 12 and 13 years after the hurricane researchers examined 111 plots in the Pisz Forest, distributed regularly in the Szast Forest. They were interested in the species of trees that had grown there, their height and thickness of trunks. It turned out that the species of trees that grew spontaneously were much more diverse than the trees growing there before the disaster. Before the hurricane, pine dominated in the Szast forest. The exception were the areas located closer to the Pisa River, where there was also black alder, as well as some spruce and birch. The number of pine trees dropped most after the hurricane. In the young generation of trees that spontaneously grew after the disaster, pine is clearly less represented than before 2002. It is still the most numerous tree, but its current advantage over other species is small. On the other hand, the populations of spruce and silver birch have grown slightly, scientists report.

Common oak, the species previously practically absent in the studied area, gained most after the hurricane. The first author of the publication, Prof. Szwagrzyk explains that jays that spread acorns from the edges of the forest over long distances are responsible for its appearance. "In the young generation, the oaks grow and survive. They do well despite the grazing deer. We did not find any dead trees in the studied area. In the future, oak can play an important role in this ecosystem" - the professor says.

Professor Szwagrzyk also notes that a larger number of trees survived the hurricane of 2002 than it would result from a cursory estimation shortly after the disaster (this was shown by previous studies with his participation). "People used to say that +the hurricane came and destroyed everything+. But when we made a detailed inventory, it turned out that more than half of the trees survived" - he emphasises.

The professor reminds that the consequences of hurricanes in a natural forest are well known. "Here, however, we are dealing with a typical commercial forest, growing largely on former farmland. This forest is an artificial creation - it was not clear how it would regenerate itself. The foresters were afraid to leave the destroyed forest to itself" - the scientist says.

He reminds that the common view is that after a hurricane trees should be removed, because they will die anyway. "But in the Szast Forest more than half of the trees from the older generation remain alive. Trees are crooked, bent; no one gave them a chance to survive for more than one season. But after a dozen years they are still standing - says Prof. Szwagrzyk. - Our research also shows that nature is doing very well, and in many respects the newly formed forest is much more interesting than before, and than the one that arises as a result of planting".

Research conducted at Szast Forest offers a fresh look on the consequences of damage caused by natural disasters, such as the hurricanes in the Pisz Forest or in Tuchola Forest (in 2017), and routine activities related to renewing forests. "Clearing the forest after a natural disaster involves huge costs and a great effort. When the summary of the activities carried out in the Pisz Forest was done, it turned out that clearing the forest - removal of damaged trees, planting new ones and subsequent care - cost more than the timber obtained during that time was worth - reminds the scientist. - We show one of the possible directions: some things can be left to nature, because there is an alternative to forest cleaning and management after a natural disaster. But we do not say on what scale it should be used".

Professor Szwagrzyk reminds that the forest management model is uniform in Poland and many people think that it works well. In the long term, however, this may change, for example, when labour costs increase or there are not enough hands to work. "Then some activities, obvious today, may turn out to be too expensive and we will have to look for alternatives" - he suggests. The professor notes that the forest that regenerates itself does not guarantee the production of timber on the scale that would be provided by human-planted young forests. "However, if we look at the forest as something more than just as a source of timber, the benefits of leaving the forest to natural, spontaneous processes seem to be much larger" - he says.

Plantings have even been proposed in place of trees felled as part of the forest beetle control in the Białowieża Primeval Forest, in natural areas. At the end of May, scientists from the Committee for Environmental and Evolutionary Biology of the Polish Academy of Sciences warned against such action in a letter to the Minister of the Environment. The authors of the publication in Science also consider plantings to be a threat to the natural forest.

According to the Intergovernmental Panel on Climate Change, climate changes occurring on the Earth mean an increase in the frequency of hurricanes and other extreme climatic events. Knowledge about forest regeneration can help deal with the consequences of such events in the future. "Thanks to the research in the Szast Forest, we know that after passing of a hurricane and leaving a commercial forest to itself, the forest becomes much more diverse and more interesting in terms of nature. The forest, which arises there, regenerates gradually, this process is stretched for years. A dozen years have passed, during which new trees appear every year, some of them survive, and some do not. This process and its development over time is worth following" - suggests Prof. Szwagrzyk.

PAP - Science in Poland, Anna Ślązak

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