«In the future we will see massive deaths of trees due to heat»

What these experts are asking is: How hot is too hot and how dry is too dry for the planet’s forests? As they describe, the Earth is made up of more than 60,000 species of trees, which are key to life and structural and economic support for human civilization. However, the health of these forests, vital in the carbon and water cycle, and in the maintenance of biodiversity, is increasingly threatened, as is the case in other ecosystem services.

With increasing temperatures, atmospheric and soil drought increases during warmer periods and vegetation suffers more heat stress. As a consequence, severe physiological responses can be observed in trees, such as reduced growth, collapse of plant tissue and, finally, death. “Our results show that all episodes of forest mortality have occurred in very dry and warm years, which has led to the death of numerous adult trees, which are usually more resilient than young ones,” says Rosana López, ecophysiologist at the Department of Systems and Technology. Natural Resources of the Higher Technical School of Forestry, Forestry and Natural Environment Engineers of the Polytechnic University of Madrid (UPM).

Beyond the limit

It is true that “plants have a certain capacity for acclimatization to heat and drought, but the pace of climate change increasingly exceeds their resistance threshold, and this limit will continue to be exceeded by global warming, which will aggravate the situation and will lead us to see massive deaths in forest populations in the coming years,” López warns.

The expected short-term result will be simplified tree communities, reduced in height and foliage, where the species most tolerant to drought and heat will survive and those less adapted to high temperatures will die. «One might think that the populations of tree species most affected by this thermal increase are those in desert climates, due to their already low availability of water, but what we have seen is that this phenomenon is occurring in all areas of the planet. , even in those that are humid. In the Amazon, for example, since 2015, several mortality episodes have already been reported in areas that, a priori, are humid forests. In fact, we have verified that species from drier areas have more resources to adapt to drought than those from humid areas. What is striking is that the same phenomenon occurs in all biomes, despite having different climates, always as a result of an increase in temperatures and drought in years that exceed the resistance threshold of the species,” says López. .

In particular, historic forests – forest communities established before 1880 that took centuries, and even millennia, to form – face a challenging future, which will suffer the most dramatic changes in extent, composition, age and structure, with negative consequences for planetary scale.

Spain, very affected

Our closest trees are not spared either. «The forests and mountains of the Iberian Peninsula are particularly vulnerable to global change, as witnessed by the increasingly frequent mortality events that we observe in extensive pine forests of Pinus pinaster on the Castilian plateau, Pinus sylvestris in the pre-Pyrenees and Pinus halepensis in the southeast peninsula. Not even the Canary Islands pine (Pinus canariensis), a great survivor of volcanic eruptions, such as the one that occurred last year in La Palma, and which is one of the few species of pine with the capacity to resprout, is foreign,” López laments.

The projection, according to the simulations carried out based on the results of the study, is that with a global increase in temperature of 2ºC, forest mortality will grow by 22%, and up to 140% if the increase is 4ºC. It may seem exaggerated, but the authors state that “these are modest projections.” «At the end of the day these predictions are based on a simulation made from a series of specific parameters, but there are factors that we have not taken into account that could play a fundamental role. For example, we have assumed that the rainfall pattern does not change, but what we are seeing is that it rains less and less, which could worsen the results. On the other hand, we have not included mortality due to fire either, but if we did, the scenario would be much more terrifying,” López clarifies.

Regarding solutions, López highlights that «at a global level, the planet is being deforested, but at the European level the forest area is growing a lot, basically due to the abandonment of agricultural lands. The problem is that forests must be managed, both productively, because we need to consume paper and wood, and ecologically, to conserve them. Adaptive forest management, therefore, is presented as a fundamental tool for the future of our forests,” says the UPM researcher.

Furthermore, researchers say that relying, in part, on trees and other plants to capture and sequester carbon, as some climate proposals suggest, makes it critical to understand the limits of heat and drought that Earth’s forests can tolerate. , in order to know if they will be able to capture the projected amounts of carbon. Especially because the death of trees prevents them from performing their critical function of carbon capture and because plants release carbon in their decomposition process.

Therefore, in order for the research to continue, the database created, hosted on the website of the International Tree Mortality Network and open access, aims to allow other scientists to incorporate additional observations. of forest mortality. “We hope this paper creates some urgency around the need to understand the role of global warming in forest mortality,” the authors note. In the future, the intention is to try to expand the studies to areas of the earth where the vegetation is very little studied, such as central Africa.