by Giorgio Vacchiano, Alessandra Bottero, Daniele Castagneri, Ugo Chiavetta, Giorgio Matteucci, Francesco Ripullone and Martina Pollastrini

Think of a forest: a community of trees, large and small animals, grasses, fungi and micro-organisms in the soil, all interacting with one another. A forest is a complex system that establishes a delicate balance with the environment in which it lives, and in particular with the climate. For this reason, drought has a significant impact on forest ecosystems. In every forest, the selective pressure exerted over time by the characteristic features of that environment has favoured the organisms best able to adapt to them. As a result, the plant and animal species that inhabit a forest today are those that evolution has equipped with the form, growth rate, and capacity for tolerance and resistance best suited to living and reproducing in that climate — traits passed down from generation to generation. Thanks to these characteristics, and to the interconnections among all living beings, the forest exists in a state of "dynamic equilibrium": its species are able to respond to unforeseen events in ways that limit negative impacts as much as possible.

But things are changing. The community of climatologists and forest ecologists agrees that the global climate is changing rapidly compared to the past. The cause is the human emission of "greenhouse" gases — carbon dioxide, methane and nitrous oxide. The main manifestations of climate change are the rise in average annual and seasonal temperatures, the alteration of precipitation patterns (with rainfall that is more intense but less frequent), and the increase in extreme events (tornadoes, floods, heatwaves and wildfires). Among these phenomena, one of the most concerning in the Mediterranean basin is the increase in drought. The organisms that live in the forest, and in particular trees, are already facing a major challenge.

There are three types of drought

But what is drought? Scientists distinguish three types of drought. Meteorological drought, which corresponds to a reduction in rainfall compared to long-term climate averages, measured using quantitative indices such as millimetres of accumulated precipitation or the number of rainless days.

The hydrological drought, which, following the absence of rainfall in the water balance, causes a reduction in surface runoff, in the flow of rivers and lakes, and in the amount of water circulating underground. Normally, these effects manifest with a delay of several days relative to meteorological drought.

Finally, agricultural drought, which refers to the impacts of meteorological and hydrological drought on crops and natural vegetation. To be properly defined, agricultural drought must take into account the vulnerability of different crop or vegetation types and their stage of development, from seed germination to maturity.

The thirst of trees

As with all living beings, water is essential for trees too. When they do not have enough of it, plants tend to slow down photosynthesis. The reduction in photosynthetic activity is one of the most evident and well-studied effects of drought, because it leads not only to a reduction in the amount of wood produced, but also in carbon sequestration within forest biomass.

According to a recent study coordinated by the Los Alamos national laboratories in the United States, high-intensity droughts — which will occur with increasing frequency throughout the twenty-first century — will reduce the amount of carbon that trees can absorb through photosynthesis by more than three times as much as has occurred during droughts so far. Particularly intense or prolonged droughts can directly cause the death of trees which, due to the increasingly severe reduction in photosynthesis, enter a state of "carbon starvation". Furthermore, drought also affects the reproductive cycle of trees — that is, flowering, pollination, fruiting, the production of viable seeds and the survival prospects of young seedlings. More frequent and intense droughts will therefore bring about changes in the forest landscape, as observed by two scientific studies coordinated by the Universities of Munich and Wurzburg, in Germany, in the forests of central Europe, where extensive areas of tree decline and tree death were observed following the drought of the summer of 2018.

As anyone who has a vegetable garden or simply some potted plants knows well, the amount of water that plants need also depends on air temperature. The warmer it is, the more water plants need, and the more easily they come under water stress when water is scarce. Certainly, forests located in warm regions with low rainfall — semi-arid and arid regions, such as many Mediterranean woodlands — even if better adapted than others to reduced precipitation, can be subject to drought. It has nonetheless been observed that the increase in droughts, linked to global warming, is causing the decline and death of trees and shrubs in various forests around the world, including tropical forests, which grow in areas with abundant rainfall, and boreal forests, which exist in regions with low temperatures.

In some parts of the world characterised by a dry climate — such as southern mainland Italy and the Italian islands — a worsening of water conditions could lead to the natural replacement of plant species and forest communities, shifting from evergreen woodlands, such as holm oak forests, to Mediterranean maquis formations with numerous shrub species, and even to garrigue formations. In other regions too, we may observe a change in the tree species that populate the forest. One of the greatest problems with these changes is their speed, which may not allow for adaptation as it did in the past.

How plants respond to water stress

Not all tree species are affected by drought in the same way. Some species are more resistant (meaning they are less affected by the immediate effects of drought) and others are more resilient (meaning they are better able to recover after a drought). All these species, in fact, adopt very different strategies in response to drought.

Species known as isohydric, including Mediterranean pines, close their stomata at the first signs of water stress — these are microscopic openings found on leaves through which photosynthesis takes place, absorbing carbon dioxide and releasing water in the form of vapour (transpiration). Closing the stomata makes it possible to reduce water consumption. However, if water vapour can no longer escape, CO2 can no longer enter, so photosynthesis slows down and, at the extreme, stops altogether.

Conversely, anisohydric species, such as poplars and some deciduous oaks, keep their stomata open until drought intensity reaches high levels. This entails a large expenditure of water, but allows the plant to photosynthesise even under moderately dry conditions. Although this second strategy involves higher water consumption, anisohydric species are also found in drought-prone environments.

Species that live in environments characterised by drought may have particular adaptations — in the shape, size and texture of their organs — that help reduce water consumption or make use of it more efficiently. For example, many Mediterranean species cover their leaves with hairs (trichomes) and with a wax that reduces transpiration and therefore water loss. The development of the root system also plays an important role in how plants respond to a lack of water in the soil. A recent study carried out by Italian and Spanish researchers confirmed that species with deeper roots are able to access water resources at depth, which are harder to deplete, enabling them to cope better with periods of drought. Beyond differences between species, it has also been shown that differences in the depth of individual plants' root systems (measured using the deuterium isotope of water to trace the depth from which each plant was absorbing water) play a decisive role in plant survival during intense drought.

The combination of different species within a woodland can therefore influence the overall resistance of the plant community. For example, species with root systems distributed across the vertical profile of the soil can make better use of water and soil nutrients than a woodland made up of plants drawing resources from the same depth, and thus competing with one another.

More generally, plants resist drought through three strategies:

• avoiding adversity (stress escape). Plants that follow this strategy complete their development cycle by avoiding drought. For example, plants with a very short biological cycle that is completed before the arrival of summer drought, such as various species of Mediterranean scrubland and many herbaceous species;
• avoiding stress (stress avoidance). This category includes plants that possess morphological and/or functional adaptations that make it possible to minimise the stress caused by drought. For example, plants such as aloe or other succulent plants, capable of accumulating and storing water in their leaves; or oleander, which has a thick leaf cuticle that limits transpiration; or certain conifers with thin, needle-like leaves;
• tolerating stress (stress tolerance). Plants capable of tolerating drought are characterised by physiological mechanisms that allow them to reduce the effects of water stress. For example, certain desert-dwelling plants that are able to dry out and rehydrate within a matter of hours.

Within the same woodland, in the event of drought, the less resistant species will be the first to suffer and, eventually, to die. For example, in the Alps, Scots pine, larch and silver fir suffer less than Norway spruce, which has shallower roots that are less capable of seeking out water at depth. In the Mediterranean environment, Aleppo pine is one of the species most adapted to drought, and therefore better placed to withstand climate change, being, among other things, a species that is adapted to and favoured by the passage of fire.

The new rules of the climate game

Climate predictive models tell us that by the end of the twenty-first century, if global warming continues at its current rate, southern Europe, Italy included, will have a hot and dry climate in which only typically Mediterranean species, such as evergreen oaks and Mediterranean pines, will be able to survive. Other forest species will find the climatic conditions necessary for their development only at high altitudes in the central-northern Apennines and in the Alps.

In recent decades, a progressive increase in the vulnerability to climate change of forest ecosystems has been observed, with numerous cases of tree decline and mortality across every part of the globe, caused above all by the increasing frequency and intensity of drought. In Italy, numerous cases of decline and die-off of oak species have been reported, such as pedunculate oak in the Ticino Regional Park, as well as conifers, including Scots pine and silver fir, and chestnut at lower altitudes. The most common symptoms of forest decline are defoliation and crown dieback, reduced leaf size and delayed leaf emergence, bark cracking and, in many cases, plant death. Drought-induced decline is therefore widespread in Italy, but at present there is a lack of in-depth knowledge about its causes and mechanisms, and above all the extent and full distribution of the phenomenon have not been determined. Recently, an initiative was launched by researchers at the University of Basilicata and the Sisef working group "Forests, between Mitigation and Adaptation" to carry out a census of all forest decline sites in Italy and create a national database.

Such phenomena, especially in the Mediterranean environment, can rapidly alter the composition, structure and functioning of forest ecosystems, with serious consequences for biodiversity, landscape, carbon sequestration and other ecosystem services. Knowing where and when drought events occur helps to understand what is causing damage to forests and to identify their vulnerability, with a view to preparing management interventions aimed at safeguarding the most susceptible woodlands.

Monitoring water stress in forests

For all the reasons described so far, the scientific community is engaged in activities to monitor the effects of drought on forests, with the aim of developing effective forest management methods to make them less vulnerable to water scarcity.

In recent years, scientists