Climate change is disrupting the forests of the northern hemisphere. Whether in America or Eurasia, forest stands are responding almost madly to warming. Expansion to higher latitudes due to rising temperatures is widespread and will continue until the end of the century. But, in parallel, the thermal improvement is making trees more vulnerable to their usual enemies, fire, wind and pests.
In principle, trees do very well with skyrocketing carbon dioxide (CO₂) emissions to grow. On the one hand, the warming they generate extends the time and rate of growth. And the more CO₂ in the atmosphere, they do so at a higher rate. At the individual level, it means a faster life cycle. At the concrete forest level, it implies its expansion, increasing the density (more units per area) or the wooded area. However, all this depends on the type of forest stand. In the Mediterranean, for example, due to the great fertilizing effect of CO₂, the scarcity of water neutralizes it. And in the northernmost regions, it’s the temperature that rules.
A research led by Spanish scientists has analyzed the evolution of the limits of the northernmost forests, the boreal ones, and those of high mountains. To do this, they measured the growth of tree rings in 37 locations throughout the northern hemisphere (plus the Andes mountain range) since 1950 and compared them with climatic evolution, particularly temperature and rainfall. “The evolution of the width of the rings is related to temperature”, explains the researcher from the Pyrenean Institute of Ecology (IPE-CSIC) and coordinator of the study Jesús Julio Camarero.
Temperature was the main limiting factor in the expansion of the forest to the north, but warming is eliminating it
They found that practically all of the 20 species analyzed had accelerated their growth, especially since the 1980s. This coincides with an increase in global warming. Furthermore, climate change is lengthening the summer precisely where it is shortest. The growing season of forests in Scandinavian regions, northern Canada or in the Alpine ranges was limited to the months of June and July. But it has not stopped increasing and they estimate that the trees could enjoy up to twice the time to grow.
The authors of the work, published in Global Change Biology, rely on these data to model how the extreme north of the forests will move as CO₂ emissions go. In the most pessimistic scenario, by after 2050, scientists fear that a threshold will be exceeded at which growth and forest expansion decouple from warming. “In the 21st century, trees could stop responding as they have done until now, ceasing to function as thermometers,” Waiter hopes.
Another striking phenomenon is the uneven impact of climate change depending on the age of the tree. In North America they have just verified that eastern trees are becoming more fertile, producing more seeds than western ones. The only explanation they have found is that warming favors young forests more than (in forest terms).
“This explains the east-west divide; most of the trees in the east are young, grow fast and reach a size where fertility increases, so any indirect impact of the climate that stimulates their growth also increases seed production “, explains in a note the professor of environmental sciences at Duke University (USA) and study co-author, James S. Clark. “We see the opposite happening with the oldest and largest trees in the west. There are small and large trees in both regions, of course, but they differ enough in their size structure to respond in different ways, ”he details. The problem with this is that the North American forest landscape could change dramatically in the future.
Forest vulnerability to pests has increased more than other traditional enemies of the forest, such as fire or wind
In Europe, a recently published work has studied the impact of climate change on the vulnerability of the forest to the attacks of its three usual enemies (in addition to humans): fire, wind and pests. Based on the use of the satellite of forests from the Urals in Russia to Ireland and databases of the three types of events since 1978, scientists were able to create a vulnerability index of European forests, understood by such as the biomass fraction forest that could be lost if one of these disturbances occurs. Later, they related it to the climate observed at this time.
In absolute terms, the greatest dangers continue to be fires and wind storms, but exposure and vulnerability to pests have grown at a higher rate in these 40 years than the other two threats. The authors estimate that almost 60% of the European forest biomass (which occupies about two million square kilometers, one third of the European soil) is vulnerable to wind blows, fires, insect outbreaks or a combination of these.
“Such increased vulnerability seems to be largely driven by the increase in temperature, which is the dominant factor in 91% of the area,” the researcher from the Joint Research Center (JRC) of the European Commission explained in an email. lead study author Giovanni Forzieri. The general increase in temperature on the one hand and, on the other, the influence of heat on exposure to pests could explain the trend. The process seemed to be accentuated around the year 2000, which corresponds to a thermal anomaly of up to 0.5º compared to the average from 1970 to 1990. “This suggests that, around the year 2000, the temperature reached a turning point that altered substantially the resistance of forests to outbreaks of pests ”, details Forzieri.
The work, published in Nature Communications, also detected an edge effect. “The cold climates forests of Finland, the north of European Russia and the Alps and also to a certain extent those of hot and arid environments in the interior of the Iberian Peninsula emerge as especially fragile ecosystems. They are characterized by a high overall vulnerability and a progressive intensification due to changes in the climate ”, concludes the JRC scientist.
“There will be areas, such as the coldest, that will be favored by climate change,” says the forestry researcher at the Pablo de Olavide University and co-author of the first of the studies Raúl Sánchez Salguero. But in other regions, such as the Mediterranean basin, limiting conditions will accumulate. For him, “there may be other forests, but they will no longer be the ones there are now.”