How can we adapt forests to climate change?

Climate change is having an impact on many tree species and on the very functioning of forest ecosystems. Foresters are wondering how best to adapt their forests to make them more resilient. This is no easy task, as there are no miracle recipes and the factors to be taken into account come from a variety of fields: plant physiology, soil protection, silvicultural strategy, introduction of new species and provenances, etc.

This article sets out some basic points relating to these matters that foresters cannot ignore.

1. Plant physiology^*

Preamble

Water (raw sap¹) rises in the shaft thanks to a tension phenomenon, directly linked to the evaporative demand of the air. This demand
is satisfied by evapotranspiration² leaves. So, under the effect of heat (solar energy), the liquid water contained in the leaves passes from
into the ambient air in a gaseous state. This leads to a water deficit in the leaves, which puts the tree under stress and causes the sap to rise. By a "domino effect", the rise in sap causes tension in the roots, which compensate by absorbing water from the soil (raw sap). Evapotranspiration is therefore the driving force behind the circulation of water in the tree.

When there is a shortage of water in the soil and drought sets in, the tension becomes greater, especially when the evaporative demand from the air is high (dry air). This can lead to ruptures in the water columns of the vessels conducting ascending sap. This rupture phenomenon is known as cavitation. This causes air bubbles to form in the water column.gas embolism. This hinders or even makes it impossible for water to circulate.

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1.1 Strategies of tree species to cope with drought

Among forest species, there are three strategies for adapting to drought.

1.1.1. The avoidance strategy

The avoidance strategy is based in particular on :

• the phenology¹ asynchronous to the dry period that occurs with certain species. In the Mediterranean region, the holm oak, for example, will grow in spring and autumn. In summer, from June to September, it stops growing altogether;
• the early closure of stomata², In other words, closing the stomata stops evapotranspiration and blocks water inside the plant. In other words, closing the stomata stops evapotranspiration and blocks water inside the plant.

This avoidance strategy is not found in temperate hardwoods, which have limited stomatal regulation. In the event of edaphic drought, the stomata will close partially, with the degree of opening varying according to the species. The American red oak, for example, is better at this game than our native oaks.

1.1.2. Tolerance strategy by delaying desiccation

This strategy is inherent to species capable of developing deep roots. They are therefore likely to draw water from deep down and continue photosynthesising as long as water is available, even in the event of a «surface drought» and heatwave.

This strategy is also based on morphological adaptation of sap-conducting vessels (thicker) which increases their resistance to cavitation and therefore reduces the risk of embolism.

1.1.3. The desiccation tolerance strategy

This strategy concerns plants capable of storing water in their tissues. It does not apply to temperate regions. Some species combine several strategies, such as holm oak, which combines avoidance and tolerance by delaying desiccation. This is also the case for certain species of Cupressus and evergreen hardwoods in general. Native oaks and Atlas cedar use the strategy of tolerance by delaying desiccation (deep rooting). Pines, on the other hand, use an avoidance strategy.

Whether the strategy adopted is a single one or a combination, its effectiveness will vary from one species to another. Sessile oak, for example, will be more resistant to drought than pedunculate, which is more sensitive to lack of water and will only be at its best on sites that are constantly supplied with water.

It is absolutely essential to understand these two strategies for adapting to drought in order to make the right choice of species for the site. For example, Atlas cedar is highly resistant to drought, but its root system is very sensitive to soil compactness and anaerobiosis. However, it is its powerful taproot that gives it this resistance. Consequently, it is a silvicultural mistake to plant it on soils with a compacted, waterlogged or shallow horizon. In such circumstances, the Atlas Cedar's root system does not develop, which undermines its strategy for adapting to drought. Note that Atlas cedar rooting is also severely hampered by sudden textural changes in the soil.

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1.2. Limitations of adaptation strategies

The closure of stomata linked to avoidance strategy leads to a drop in photosynthetic activity. Gas exchange between the leaf and the atmosphere is blocked, the tree preserves its water but CO₂ necessary for photosynthesis stops. The tree will therefore draw on its starch reserves to ensure its metabolism. After a one-off drought, the tree will quickly return to its normal function, but there will still be some problems. prolonged and recurrent drought will gradually weaken the tree (it will no longer have enough starch to produce enough new leaves in the spring), which will end up by starve to death (not enough photosynthesis and no more reserves).

In the tolerance strategy by delaying desiccation, As long as water is available at depth, the tree will continue its photosynthetic activity. As soon as there is a shortage of water, embolism sets in and the tree (or branch) ends up by die of thirst. For species that adopt this strategy, it is the intense and prolonged drought that are harmful to them.

1.3 Premature leaf fall

Another possible response to drought is premature leaf fall to limit water loss (as in birch, for example). This survival strategy poses a major problem in terms of the tree's reserves, as it not only reduces photosynthetic potential but also leads to a loss of nutrients. In autumn, as the leaves fall, around 50% of the nutrients they contain are recovered and stored in the twigs for budburst the following spring. When the tree has to «shed» its leaves during the growing season, there is no recovery of nutrients, and the leaves fall green to the ground. This considerably weakens the tree.

1.4. Defence and water stress

The synthesis of trees' defence molecules (polyphenols, alkaloids, etc.) requires an enormous amount of energy (sugars) (see diagram in preamble). In the event of water stress, which leads to a drop in photosynthesis and therefore a reduction in reserves, the tree will give priority to reducing the most energy-intensive activity, i.e. the synthesis of defence molecules, in order to ensure its metabolic survival (respiration and growth).

This principle obviously varies from species to species. Spruce is a typical example of the negative effect of water stress on the species' defences. In the event of water stress, the spruce will stop synthesising its defence molecules. The result, which is unfortunately well known to foresters, is massive attacks by the spruce bark beetle in monospecific stands.

1.5 Indicators of tree decline

Trees that have suffered one or more droughts show the following symptoms:

1. branch or tree mortality, depending on the degree of cavitation within the individuals ;
2. sparse crowns at budburst in the year following stress;
3. a reduction in growth ;
4. sensitivity to pathogens.

1.6. Growth and adaptation strategy

It is logical that droughts should have a different impact on tree growth depending on their adaptation strategy. The avoidance strategy will have a negative impact on growth (reduced photosynthesis). On the other hand, a pedunculate oak can perfectly well maintain its productivity in the middle of a drought if its roots are sufficiently deep and find available water (no drop in photosynthesis).

Furthermore, installation on the best sites from a hydric point of view will not have as positive an impact in terms of growth in species that use the desiccation delay tolerance strategy as in species that rely on the avoidance strategy. The former are genetically «programmed» to «armour» their vessels to resist cavitation, which requires a lot of energy and carbon. They will always «concentrate» on this aspect first. The latter, on the other hand, freed from water constraints, will photosynthesise in favour of their growth. In other words, the allocation of sugars produced by photosynthesis has different priorities depending on the species' adaptation strategy.

Differences in growth gain depending on the site are also marked for species that use the same strategy. For example, a pubescent oak will have a narrower range of potential growth than a sessile oak, because the pubescent oak has a stronger strategy of tolerance by delaying desiccation. In other words, the pubescent oak will not grow as much (or at all) as the sessile oak on the best sites, but will be much more resistant to drought on difficult sites.

1.7 Rooting stresses

It is the roots located at a depth of more than one metre that ensure the absorption of water in dry periods. The graphs below show that 80 % of the roots are located before 60 cm, 12 % between 60 cm and 1 m and only 8 % beyond the metre (study carried out on ash). In spring, most of the «surface» roots absorb the water needed by the tree, but as the growing season progresses (less water available at shallow depths), the deep roots take over and end up absorbing almost all the water.

Consequently, rooting constraints are of major importance and undoubtedly influence the choice of species to be planted on a given site (see photos below).

The main constraints on rooting are waterlogging (anaerobiosis) and soil compactness. Sensitivity to these two factors varies from species to species. Sessile oak, for example, is sensitive to prolonged waterlogging but not very sensitive to soil compactness. Atlas cedar is very sensitive to both.

Good illustrations of the correlation between drought adaptation strategies and rooting constraints. On the left, a 12-year-old Atlas cedar (drought tolerance strategy) on a cracked slab. After a few difficult years, the cedar's roots have found their way into the cracks and are growing deep down to ensure a supply of water. In the centre, an Atlas cedar of the same age on soil with a compact slab at 40 cm. Its growth is almost non-existent, and the annual shoots are drying out. Contrary to popular belief, the cedar does not regulate its evapotranspiration very well and has very poor drought resistance. It makes up for this with its deep roots, provided they can develop. On the right, 12-year-old Arizona cypresses on the same compact slab soil. Arizona cypress uses a mixed strategy. If it cannot develop its roots (delaying strategy), it limits its evapotranspiration by closing its stomata (avoiding strategy). Photos : National Forestry Office (ONF).

2. Soil protection

2.1. Nutrient export

The shorter the rotation, the greater the export of nutrients (nitrogen, phosphorus, potassium) from the forest. In fact, the higher the proportion of juvenile wood in the holdings, the greater the export of nutrients.

The graph below provides essential information about the risk of soil impoverishment as a result of forestry operations. The vast majority of nutrients (50 to 80 % depending on the species) are found in branches with a diameter of less than 7 centimetres.

Consequently, exporting fine branches should be avoided at all costs, especially on poor soils, where harvesting by the whole tree (including the crown) should be avoided.

One of the silvicultural adaptations put forward in the face of climate change is to shorten silvicultural cycles, but beware of exports: shortening revolutions without limiting them will have disastrous effects on our forest soils, our primary production tool. The cumulative effect of shortening rotations and exploiting the total biomass leads to very high exports that the natural mineral balance of an ecosystem cannot compensate for.

Such exports impoverish the soil, which becomes more acidic. This has a negative impact on :

• structural stability: the soil becomes more susceptible to erosion;
• biological activity in the soil ;
• productivity.

2.2. Avoid soil compaction

Foresters should be aware that :

• the porosity of the soil is of biological origin. Earthworms, in particular, aerate the soil. This aeration allows water and air to circulate in the soil;
• Settling is «definitive» (see note), and there is no mechanical way of remedying it;
• settling reduces productivity;
• repeated use of heavy machinery on soil, even dry soil, leads to structural deterioration¹;
• Stands on compacted soils are more rapidly weakened in the event of stress, and dieback is consequently more frequent. The loss of porosity leads to numerous problems such as waterlogging, poor rooting, a reduction in mycorrhizae, a loss of biological activity in the soil, etc.

Consequently, certain recommendations are imperative in order to minimise and circumscribe the passage of heavy machinery:

• Compulsory operating partitions and compliance with these in the specifications;
• encourage cabling of logs (wider partitions) ;
• Work the soil sparingly and occasionally (only when vegetation is blocked). Avoid touching the soil (e.g. by milling) to avoid destructuring it (soil life is mainly present in the first 15-20 centimetres) and mineralisation (leaching into watercourses);

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3. Silvicultural strategy

3.1 How can forestry be adapted to climate change?

As well as introducing new species and provenances, and using the silvicultural levers discussed below, we also need to ensure that we allow the local resource to express its capacity to adapt, and not consider that the «game is up». The intrinsic diversity of trees is very high, at least in certain species, which gives them great capacity for adaptation.

To adapt forests to climate change, foresters need to act on three levels: density, structure and composition. Density refers to the intensity and frequency of thinning. The multi-storey structure refers to a layered forest with trees of multiple ages (irregular high forest). Finally, composition refers to the mixture of species (mixed forest).

Acting on density

A more dynamic thinning regime tends to improve the water balance because, on the one hand, there is less interception of rain by the foliage, which increases the quantity of water that reaches the ground before being evaporated, and on the other hand, there is less competition for the water resource because there are fewer trees (less evapotranspiration). In other words, the denser the stand, the faster the soil water deficit threshold is reached. Thinned stands grow better during a drought¹ and better growth resilience after a dry spell.

However, there are limits to the intensity of intervention, as thinning can harm residual trees. The stronger the thinning, the better the water balance in the plot, but evapotranspiration at tree level increases (development of the crown and in full light).

This is an important fact to bear in mind, particularly in continuous cover mixed silviculture (CCMS). Objective trees that have been cropped will be more exposed to water stress. In CCM, even more than for other silvicultural treatments, care must be taken to ensure that the species/station match is optimal from a hydric point of view, all the more so if we designate shade species (beech, for example) whose leaves evapotranspire abundantly. You should also be aware that the light given to the lower strata encourages their growth, which will have a significant impact on the water balance of the stand as a whole.

Dynamic silviculture is fa￾vorable to the water supply of the stand on condition that it is not :

• nor excessive ;
• or brutal;
• and early (and do not neglect depressions).

The resulting reduction in the number of revolutions makes it possible to :

• limit the ageing of stands, which increases their vulnerability to all types of stress ;
• limit exposure to risks over time ;
• Promote wood production for mitigation (sequestration and substitution).

This is subject to vigilance regarding exports (see point 1 of the previous chapter).

Acting on the structure

On plains and soils regularly subject to drought, it has not been proven that irregular high forest is one of the forest management solutions that can limit water stress. It is not possible to say that one system is better than the other in these conditions.

Natural stands that are regularly subjected to drought are regular, such as forests of Aleppo pine or ponderosa pine.

Can we conclude from this that on soils with a xeric tendency, the irregular structure can be called into question? This question in no way calls into question the use of irregular high forest on less constraining and, a fortiori, optimal sites.

Acting on the composition

Mixing species has positive and negative interactions.
Negative interactions are :

• competition for resources between species in the same ecological niche¹;
• antagonism by allelopathy².

Positive interactions are :

• Complementarity: the available resources will be better shared between species with different ecological niches (tracing/pivoting roots, shade/light species, etc.);
• facilitation: the beneficial effect of one species on another species with which it interacts.
  A well-known example is the Robinia/Poplar association, which is beneficial to the poplar, which grows faster because of the nitrogen fixation by the Robinia. Another example is the alder/pedunculate oak association on temporary groundwater. The alder buds earlier than the pedunculate oak and dries out the soil (evapotranspiration), which makes it easier for the oak to develop deep roots, making it more resistant to summer drought.

In practice, the interactions due to diversity are positive for the level of exposure to drought (lower) for temperate deciduous forests on deep soil.³. The sharing of underground space by roots seems to play an important role in the effect of diversity on drought resistance.⁴.

Composition and growth

In terms of growth, mixed stands are generally more productive (particularly in terms of improving hay). The figures below give us the following information (left figure = overall effect of mixing, right figure = effect of mixing by species):

• in the beech/oak mixture, the effect is slightly positive for the beech, but the oak loses out. The oak will have more vitality in a pure oak forest than in a beech/oak forest.
• the beech/spruce mixture is very favourable to the growth of the beech, but not to that of the spruce;
• the beech/fir mixture is particularly favourable to the growth of the beech;
• the spruce/hemlock mix is favourable to the spruce;
• for the oak/pine mixture, the mixture effect is positive for oak but not for pine;

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4. Introduction de nouvelles essences

Il est utile de rappeler que le continent européen a perdu énormément d’espèces suite aux glaciations, ce qui n’est pas le cas de l’Amérique du Nord¹. Ainsi, sous des latitudes comparables, il n’est pas rare de retrouver un rapport de dix pour un en termes d’espèces au profit du continent américain.

L’introduction de nouvelles essences et provenances a pour objectif de maintenir une sylviculture, notamment de production, avec des essences mieux adaptées à des conditions plus sèches et plus chaudes². Elle s’appuie sur la migration assistée (voir ci-après) et l’introduction d’essences allochtones (voir encadré). Ces introductions exigent une connaissance profonde de l’autécologie³ des essences.

Le projet Trees For Future⁴, mené par la Société Royale Forestière de Belgique, teste différentes provenances et essences d’arbres en forêt au sein d’un réseau de parcelles expérimentales réparties à travers le pays (voir l’article en page 8).

La migration assistée de provenances

Il s’agit d’introduire des arbres d’une espèce déjà présente sur le territoire mais issus du Sud de son aire de répartition. Par exemple, on parlera de migration assistée de provenances lorsqu’on plante en Belgique un hêtre commun issu d’une graine récoltée en Italie ou dans le Sud de la France. L’objectif est ici d’introduire des gènes de résistance à la sécheresse présents dans ces populations du sud et de permettre leur propagation au sein des populations locales. Il s’agit donc d’enrichir la diversité génétique d’une espèce locale pour lui permettre de s’adapter au nouveau climat. Les essences concernées par ce type de migration dans le projet Trees for Future sont le chêne sessile, le hêtre, le tilleul à petites feuilles et dans une moindre mesure, le chêne pubescent parce que bien qu’indigène, il est aujourd’hui très rare chez nous.

La migration assistée d’essences

Dans ce cas, on introduit une espèce qui n’est pas encore présente sur le territoire mais qui vit plus au sud sur le continent dans des conditions climatiques similaires à celles qui sont attendues dans les prochaines décennies en Belgique. Par exemple, on parlera de migration assistée d’essences quand on plante en Belgique des chênes méditerranéens (chêne de Hongrie, chêne chevelu), des sapins méditerranéens (sapin de Céphalonie, sapin de Bornmüller…) ou des pins méditerranéens (pin maritime, pin de Macédoine, pin de Bosnie…).

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