Forest management and silviculture
SuperScience.Foreste.Siccità.Home.mp4Concepts, guidelines and technical advice for improving the resilience of deciduous oak forests to climate change.
Forests are not trapped in a glass case like butterflies nailed to a pin.
Table of contents
Introduction
This section illustrates how forest management and silviculture can enhance the resilience of Mediterranean deciduous oak forests, which are suffering from increasing drought stress.
Resilience is commonly defined as the persistence of a system under changing environmental conditions and disturbances. Here we support the approach that forest resilience can be viewed as the persistence of ecosystem functionality and associated environmental and socio-economic benefits.
In the Mediterranean region, environmental variables and human actions have interacted for centuries to shape forest characteristics. In today's society, a wide range of new benefits (naturalness, biodiversity, carbon sequestration, landscape, etc.) are expected from the forest, in addition to the traditional (timber harvesting, slope protection, hydrological regulation, etc.) and basic needs (firewood, grazing, etc.), which are still alive in marginal inland areas.
Anthropogenic climate change is now challenging the functionality of forests and may jeopardize the benefits they can provide. In the Mediterranean region, where climate change is causing heat waves and droughts of increasing intensity, deciduous oak forests are particularly vulnerable. They often grow on degraded soils with low fertility and have a simplified structure as a result of intense and long-lasting human impacts (mainly grazing and unregulated logging).
Forest management and silviculture can plan actions at different spatial and temporal scales to maintain the functionality of forest ecosystems. These may consist of management directions and silvicultural practices aimed at: keeping in line with natural trends of the ecosystem; exploiting functional diversity; regulating plant competition; promoting forest regeneration; improving soil water content and root water uptake; reducing anthropogenic disturbances.
The vast landscape of deciduous oak forests in inland Basilicata, winter view.
Keep in line with natural trends
Trends observed
Deciduous oak forests in the Mediterranean bio-region cover a range of environmental conditions and show differences in composition and structure. In more drought-prone areas, downy oak (Quercus pubescens) is the dominant species, able to withstand long periods of intense drought. Turkey Oak (Quercus cerris) and Hungarian Oak (Quercus frainetto) are more sensitive to drought and prove to be susceptible to drought-induced decline due to a less conservative water use strategy.
Overall, the following trends can be recognized in response to increasing drought:
Mixed oak forests at the lower boundary of the deciduous forest belt are likely to become increasingly dominated by Quercus pubescens under a drier climate. In some cases, a gradual shift towards more xeric communities can be expected with the entry of tree species with drier bioclimatic optima (e.g. the entry of Aleppo pine into open canopy oak forests could be expected). In these cases, silvicultural practices should support ongoing natural trends and favor tree species that are better able to cope with drought. It is imperative that forest management plans address the long-term impacts of forest trajectories and changes on ecosystem functionality and expected benefits;
Forest communities dominated by Quercus frainetto in drought-prone areas are likely to experience high mortality rates. Given the relatively small range of this species in southern Italy and its importance from a conservation perspective, silvicultural practices (see next paragraphs) can be planned to slow down the process of tree decline and transition to more xeric forest communities;
Forest communities dominated by Quercus cerris in the driest sites of their natural distribution may show symptoms of decline. Depending on the case, a choice can be made between applying silvicultural practices to maintain the current forest physiognomy or to accommodate the ongoing community shifts;
Forest communities dominated by Quercus cerris in mesic sites of the submontane deciduous forest are generally in healthy conditions. However, the possibility of a drier climate cannot be excluded. In this perspective, a better resilience of the forest community can be achieved by silvicultural treatments aimed at creating a more heterogeneous forest.
Even-aged, single-layered and grazed oak forests in Basilicata.
Exploiting functional diversity
Drought-induced hydraulic failure is thought to be the primary mechanism underlying tree decline and mortality in deciduous oak forests. Strategies for tolerating or escaping drought can be quite different between species, and can be more or less effective depending on how the drought develops.
Both isohydric (leaf water potential strongly controlled by stomatal regulation) and anisohydric (variable leaf water potential) plant behaviors can be recognized. Species may also differ in how they cope with post-drought conditions. After soil water content has recovered, some species rapidly replace damaged xylem. This process can lead to depletion of carbon reserves and increased susceptibility to parasitic attack. Other species respond by increasing leaf area, which increases the risk of subsequent droughts.
Indeed, tree species explore a number of solutions in terms of morphological and functional traits to adapt to drought conditions. The variability observed in the field is an effect of genetic diversity (among species and individuals within species) and phenotypic plasticity, i.e. the ability of trees to modify their somatic characteristics to successfully cope with changing environmental conditions.
In heterogeneous and species-rich forest communities, functional diversity can be included in the selection criteria for silvicultural practices. Tools are now available to facilitate this approach and make it realistic within current forest management practices. For example, the existing databases of functional traits of tree species (see e.g. TRY database), remote and proximal sensing technologies applied to precision forestry, etc.
These tools can be used to produce high resolution functional maps to support forest management at the landscape scale and to guide silvicultural treatments at the forest community level. Process-based models can be helpful in this approach by simulating: the probability of tree decline as a function of tree size and canopy social position; post-drought community trajectories; the effects of local-scale topographic variability, etc.
In the Gorgoglione and San Paolo Albanese oak forests, thinning to reduce competition have been applied. Results are currently being evaluated.
Regulating plant competition
Regulating competition among trees through thinning can help reduce tree stress during drought and improve the overall health and vigor of the forest, making it better able to withstand the effects of drought.
However, the effectiveness of thinning depends on the type of thinning applied and on factors such as stand density, soil type, tree species and microclimatic conditions. Under Mediterranean conditions, thinning may in some cases exacerbate drought stress if it leads to increased water loss from understory vegetation or from the soil due to increased solar radiation.
In Mediterranean deciduous oak forests, the following cases can be considered:
sub-Mediterranean oak forests affected by drought-induced decline and mortality (our study cases) are often low-density forests due to lack or scarcity of natural regeneration: very few seedlings survive and grow to the top of the canopy. Recent observations have shown that the trees that survive into droughts are the tallest trees with the highest stem growth rates, which are able to draw water from deep in the soil and from the water table. In this case, little or no benefit is expected from low thinnings that remove only declining trees or trees in the smallest diameter classes. Where feasible, crown thinning is advisable: focusing on mid-canopy trees will benefit larger trees with the highest likelihood of escaping and surviving drought.
submontane Quercus cerris-dominated communities growing on mesic sites are mostly represented by even-aged and single-layer forest stands, generally in good health. However, a more structurally diverse forest may be advisable to increase the potential to respond to future drought conditions. Selection thinning, i.e. the selective removal of co-dominant trees to reduce canopy overcrowding and neighbor competition, will promote stand structural differentiation and improve the health, growth rate, and resilience of the remaining trees.
Forest regeneration
Natural regeneration
It is the very foundation of forest resilience: the self-replacement potential of natural regeneration is the key process for maintaining a forest ecosystem. Natural regeneration allows new trees to grow and replace those lost to disturbances such as drought. This allows the forest cover to be maintained, with the associated environmental benefits.
In addition, the ability of trees to regenerate naturally from seed ensures that the genetic diversity of a forest community is maintained, which is important for resilience to drought disturbances. Natural regeneration is cost-effective, environmentally friendly, and often results in a more diverse and resilient forest ecosystem. Lack of or poor self-replacement potential leads to significant changes in forest community and ecosystem characteristics.
In sub-Mediterranean oak forests affected by drought-induced decline and mortality, a lack of natural regeneration is the rule, with few seedlings surviving and very few plants able to grow to the top of the canopy. In most cases, this is the result of livestock overgrazing (grazing at a higher intensity than the land can support).
Overgrazing is associated with soil compaction and damage to seedlings and saplings. It also increases soil erosion, with loss of nutrients and moisture, which further hinders seedling growth. In some cases, grazing creates large canopy gaps where opportunistic species can establish and outcompete natural tree regeneration.
Soil compaction due to overgrazing can be a significant problem in deciduos oak forests in Italy (here an image of an oak forest in Basilicata), as it reduces the ability of oak trees to access water from the soil and exhacerbates the effects of drought.
Artificial regeneration
In our study cases (Hungarian oak or mixed Hungarian-Turkish oak stands), relying on natural regeneration may be challenging in the short term. Therefore, artificial regeneration may be necessary to kick-start the regeneration process, provide more immediate benefits such as habitat restoration and soil erosion prevention, and ultimately promote ecosystem resilience and conservation.
Artificial regeneration involves planting seedlings or direct seeding in small, scattered areas, and may include some soil preparation and protection from grazing by fencing. Artificial regeneration should be carried out according to the principles of genetic conservation. This means that the choice of reproductive material should always give preference to local material, which guarantees the maintenance of the evolutionary and adaptive characteristics of the population.
This is all the more important given the limited genetic information available on Q. frainetto and the fact that the populations in southern Italy can be considered endangered and marginal.
On a small scale, soil disturbance treatments (e.g. mechanical scarification) can be planned to improve degraded soils prior to planting or to improve soil conditions for seed germination and seedling growth. However, these treatments can be costly and should be used with caution.
Coppicing
Coppicing is a traditional forest management technique in which a tree is cut down to its base and allowed to regrow from the stump. This results in the production of multiple stems that are periodically harvested and mostly used as firewood. Coppicing has been used for centuries to manage deciduous oak forests in the Mediterranean region.
The large root system that develops from a stump provides shoots with access to moisture and nutrients and can be an important factor in plant recovery from periods of drought. In places where deciduous oak forests show clear signs of drought-induced decline, maintaining the coppice system where it is still used may be an option.
Overgrazing
Grazing management in Mediterranean forests is a key issue to promote natural regeneration and resilience of deciduous oak forests. Guidelines for livestock grazing in forest management plans can help to promote forest regeneration and avoid negative impacts on soil properties (see below).
However, this can be challenging as it requires balancing forest dynamics with the needs of local communities. For example, after forest regeneration, a temporary grazing exclusion is required, which can easily lead to conflicts between stakeholders (foresters, farmers, local communities). For these reasons, sharing management objectives with the local community is crucial to ensure natural regeneration and forest resilience in areas where overgrazing is the rule.
The local community can provide important knowledge and their involvement can be critical to the success of any management plan. In addition, involving the local community in decision-making and planning can help build trust and create a sense of ownership and responsibility for the forest ecosystem. This can lead to more sustainable management practices and better long-term outcomes.
Effective communication and collaboration with local communities can also help identify and address conflicts, and ensure that management strategies are socially, economically, and environmentally sustainable.
Overall, the involvement of local communities in forest management planning and implementation is an essential component of sustainable forest management and the resilience of Mediterranean forests to climate change.
Improving soil water availability and root water uptake
Water consumption: the understory
During drought conditions, the amount of water available for uptake by trees can be greatly reduced by increased evaporation from the soil and increased competition from understory plants (grasses and shrubs) that make it more difficult for trees to access soil water resources. Controlling understory vegetation can help increase water availability for tree root uptake during drought.
Managing stand density and thinning to maintain a lower layer of suppressed trees (hornbeam (Carpinus orientalis L.) is often found growing in the lower layer of oak woodlands) could be effective in controlling the development of grasses and shrubs in the forest understory.
Understory vegetation can be controlled by prescribed burning, which can reduce competition, improve soil moisture, and provide space and suitable soil conditions for oak seedling establishment.
Livestock grazing (sheep and goats) can be used to reduce the density of grasses and shrubs, which can help improve water availability for tree root uptake. Grazing should be managed to avoid soil compaction and unsuitable soil conditions for forest regeneration (see below).
Factors interact in a complex way, so all management actions and techniques should be carefully considered to ensure they do not have unintended negative consequences.
Root water uptake and soil compaction
Rooting depth and distribution are factors which largely affect the amount of water trees can access during drought. In some cases, the roots may be shallow, which means that they can only access a small amount of soil water. In other cases, the roots may be limited to a particular soil layer, which can restrict water uptake. Water uptake from deep in the soil and the water table has been found as a decisive condition to secure the xylem water transport and maintain the healthy tree status.
Thinning that selects candidate trees for deep roots (large trees with high growth rates) can improve tree water uptake.
In Mediterranean oak forests, soil conditions that make root development and water uptake more difficult are often due to grazing. The constant movement of grazing animals can lead to soil compaction through trampling: the repeated pressure of the animals' hooves can crush the soil, creating a dense layer that can make it difficult for water to penetrate and for roots to grow. Grazing can lead to changes in plant cover that can affect soil compaction. For example, if grazing animals selectively feed on certain plant species, this can lead to changes in the distribution of plant roots in the soil. This can affect the structure of the soil, making it more susceptible to compaction. Grazing can also lead to soil erosion, which can result in the loss of topsoil, making it harder for plants to establish and roots to penetrate the soil.
Grazing management practices can help reduce soil compaction. These include: rotational grazing, a technique in which livestock are moved between different grazing areas over time, allowing each area to recover between grazing periods; rest periods, which are periods when grazing is prohibited in certain areas of the forest to allow vegetation and soil to recover. This can be particularly important during dry periods when vegetation may be more vulnerable due to reduced access to soil water.
As mentioned above, involving the local community in the planning and implementation of grazing is crucial for sustainable forest management and improved forest resilience.
Different types of grazing (cattle, sheep, goats) can have different impacts. Both cattle and sheep grazing can cause soil compaction. However, because cattle are larger and heavier than sheep, cattle grazing can result in deeper and more severe soil compaction, especially in areas where the soil has a high clay content. Sheep are known for their selective grazing behavior, resulting in a more patchy distribution of grazing pressure, potentially leading to more localized soil compaction. Sheep and goats are similar in size and weight, but have different grazing behaviors that can affect the amount and distribution of pressure on the soil.
In general, goats are more agile and tend to climb and browse trees and shrubs, while sheep graze more on grasses. This means that goats are more likely to cause soil compaction in steeper terrain or rocky areas, while sheep may cause more damage in flatter areas or in places with high grass densities. Overall, the effects of cattle, sheep and goat grazing on soil compaction depend on many factors, including the intensity and frequency of grazing, the type of soil and vegetation, and the topography of the area.
Grazing management practices that consider all of these factors can help minimize the risk of soil compaction and its associated negative effects.
Foodplain oak forests in protected areas under increasing drought
Floodplain hardwood forests are important relic ecosystems, often found within protected areas. These ecosystems are at risk from climate change and increasing drought because they are adapted to a hydrological regime that includes consistent soil moisture, periodic flooding, and a shallow water table.
Changes in the hydrological cycle can alter the frequency and intensity of flooding events, which can affect nutrient cycling and soil development. As a result of changes in precipitation and human activities such as groundwater extraction and dam construction, water tables can be lowered. This exacerbates drought conditions, which have a range of effects, including reduced growth, increased susceptibility to pests and diseases, and tree mortality.
Ongoing research is investigating: what are the most critical stressors affecting these ecosystems and the associated risks; interspecific differences in tree vulnerability; the link between phenotypic and genetic traits; and the role of genetic variability in determining tree resilience to drought. The results will help plan actions to maintain the health and improve the resilience of these relic ecosystems.
Two alluvial forests were selected as case studies with the aim of identifying silvicultural and management actions useful for the conservation of these ecosystems: one alluvial forest in southern Italy (Bosco Pantano, Policoro, Matera) and one in northern Italy (Parco del Ticino, Milano, Lombardia).
Bosco Pantano is a hygrophilous forest that grows along the Sinni River (Basilicata), close to where the river flows into the sea. While the dominant tree species is the ash (Fraxinus angustifolia), the importance of this forest is mainly due to the presence of a small relic population of the pedunculate oak (Quercus robur).
Recent research has shown that the oak population in Bosco Pantano is genetically distinct from other populations in the species' natural range and is genetically depleted. This highlights its ecological and evolutionary importance and the need for conservation efforts to protect the unique genetic makeup of this small population. This is all the more necessary as there is very little natural regeneration in the forest.
An action to conserve this population was then initiated, which could serve as an example for further actions. In the fall, acorns were harvested from selected trees in the population and sown in the nursery. Here, the seedlings were provided with optimal growing conditions (proper light, water and nutrients). Once the seedlings had grown to an appropriate size, they were transplanted into the forest and adjacent restoration areas. Regular monitoring and maintenance activities are now carried out to ensure the survival and growth of the seedlings.
A comprehensive management of the Bosco Pantano reserve could also include actions to restore a proper hydrological balance of the site. In particular, actions aimed at: increasing the water level downstream of the dam on the Sinni River; reducing agricultural groundwater extraction, which can help prevent saltwater intrusion; improving the regulation of irrigation canals to recharge the water table.
Dissemination
Foreste.Ripariali.Intervista Ripullone.mp4Regional TV news interview (in Italian) about researches anmd management perspectives for floodplain forests. Lenght: 3'26'.