“In the future we will see mass deaths of trees from heat”

Decomposing forest mass due to drought.

An international study compiles the first database of climate-induced forest mortality events, from 1970 to 2018, in 675 locations on all continents

Elena Martin Lopez

As living beings that they are, trees die naturally. However, the climatic conditions of heat and drought that have occurred in recent decades have contributed to their mortality skyrocketing. This is how an international team of researchers exposes it in a
worked published in the journal 'Nature Communications'. Scientists have created the first global database that documents climate-induced forest mortality events in all biomes -parts of the planet that share the same climate: Mediterranean, tropical...- based on the review of 154 studies carried out in 675 locations spread across all continents between 1970 and 2018.

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 a structural and economic support for human civilization. However, the health of these ecosystem services, vital in the carbon and water cycle, and in the maintenance of biodiversity, is increasingly threatened.

With rising temperatures, soil and atmospheric 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, eventually, death. "Our results show that all episodes of forest mortality have occurred in very dry and hot years, which has led to deaths even in adult trees, which are usually more resilient than young ones," says Rosana López, ecophysiologist at the Department of Systems and Natural Resources of the Higher Technical School of Forestry, Forestry and Natural Environment Engineers of the Polytechnic University of Madrid (UPM).

Forest with dry trees.

Beyond the limit

It is true that "plants have a certain ability to acclimatize 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," warns López.

The expected result in the short term will be simplified tree communities, reduced in height and leafiness, where the species that are more tolerant to drought and heat will survive and those that are less adapted to high temperatures will die. "One might think that the populations of tree species most affected by this increase in temperature 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 episodes of mortality have already been reported in areas that, a priori, are humid forests. In fact, we have found 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 increased temperatures and drought in years that exceed the resistance threshold of the species," says López. .

Set of dry trees.

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

Spain, very affected

Nor are our closest trees spared. «The forests and mountains of the Iberian Peninsula are particularly vulnerable to global change, as evidenced by the increasingly frequent mortality events that we observe in extensive Pinus pinaster pine forests on the Castilian plateau, Pinus sylvestris in the foothills of the Pyrenees and Pinus halepensis in the peninsular southeast. Not even the Canary Island 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 pine species with the ability to resprout, is foreign," laments López.

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." “After all, 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 regime 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 from
fire, but if he did, the scenario would be much more terrifying, ”says López.

Regarding the solutions, López points out that “at a global level, the planet is deforesting, but at a European level the forest area is growing a lot, basically due to the abandonment of agricultural land. 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.

On the other hand, the 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 sequestration 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, intends that other scientists can incorporate additional observations. of forest mortality. "We hope that this paper will create 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 extend the studies to areas of the earth where the vegetation is very little studied, such as central Africa.

Source link