Forest regeneration offers a direct and immediate opportunity to manipulate species or stand composition. A highly recommended option to secure the adaptive response of established regeneration is to raise the level of genetic diversity within the seedling population [23,30-32], either by natural or artificial means. The more diverse and larger the seedling population is, the more potential there is for populations to adapt to environmental changes. Because the genetic composition of populations needs to be shifted to cope with environmental changes, present diversity levels should be increased compared to local regeneration conditions [33].

Under natural regeneration regimes, increased genetic diversity can be achieved by benefiting successive fruiting years, thereby increasing gene flow from external male parents to different seed trees. Enrichment sowing and planting in naturally regenerated stands is another option to raise the level of diversity. Natural selection works on the gene pool (local and introduced trees) and over time a population better adapted to the changing conditions develops [34,35].

The size of the seedling population is far lower in the case of artificial than in natural regeneration, unless sowing is used. Generally, only selected certified tree seed can be used in Europe. To raise levels of genetic diversity, seedlings coming from different seed stands can be mixed at the nursery stage. In addition, local material can be supplemented with introduced material from other seed sources. Given that the fitness distribution in the population is unknown, best adapted genotypes to climate change may either originate from local sources or from external sources. This is why supplemental regeneration is suggested. Supplemental regeneration also enhances evolutionary responses to new selection pressures by increasing local diversity [36].

Several national seed transfer regulations have defined provenance regions where only local populations of European tree species are recommended for establishment, but elsewhere there are no rules preventing the use of non-local material except in natural parks or reserves. Preadapted seed transfer should be based on results of provenance tests and integrate knowledge on genetic variation among populations and predicted changes of bioclimatic envelopes. Examples of climate preadapted seed transfers were recently developed for lodgepole pine (Pinus contorta) in British Columbia, either based on response functions [37], or combining ecological and genetic data [38]. In Europe, similar approaches were used for Scots pine (P. sylvestris) by Reich and Oleksyn [39], who concluded that transfer of populations would need to be made over increasingly larger distances from the south of Europe.

Tree breeding could support the development of genetic material better adapted to future climate conditions, but current expectations are rather modest. Most breeding programs of tree species in Europe focused on improving trees for production and quality traits, whereas selection criteria related to adaptation to higher CO₂, higher temperatures, or longer water stress periods have been mostly neglected, or have only been introduced during the last decade [40]. Compared to traditional methods more sophisticated approaches based on biotechnology (in vitro propagation, somatic embryogenesis, gene transfer) may significantly shorten the time required to produce new commercial varieties, once genes of significance for adaption have been identified [41,42]. Research efforts are being conducted to identify candidate genes related to traits that will respond to climate change: (i) bud burst [43-46]; (ii) bud set [47,48]; and (iii) drought resistance and water use efficiency [49-51]. However, the achievements obtained so far are not at the stage of introducing these genes (by crossing or other means) in commercial varieties yet. Therefore, to date the only way through which breeding activities may take into account climate change would be to raise diversity within the varieties produced by seed orchards. This can be done by supplementing seed lots harvested in seed orchards with seeds stemming from other origins or other seed orchards.

Regenerating species best adapted for current conditions may result in a higher risk of dieback in the future [24]. The promotion of specific tree species includes the risk of unanticipated losses caused by “new” plant diseases that emerge and affect particular species or species groups, such as experienced in the case of Dutch elm disease or most recently with Ash dieback [52]. On the other hand, selecting new plant material adapted to the projected future climatic conditions includes considerable risk of “prognostic error” given the high uncertainties in projections of future climate at local and regional scales. Furthermore, the stands may be adapted in a sub-optimal way to current conditions [31].

Besides adaptation options aiming at reproductive material, technical options can be applied in the regeneration phase. Planting in drought prone areas can be adapted via wider initial spacing of trees in combination with rigorous weed control to reduce competition for water resources from weeds [23]. Furthermore, a shift in planting season from spring to autumn with additional site preparation could enhance drought resistance of freshly planted trees due to improved initial rooting. Planting of container seedlings might also reduce the risk of immediate drought effects [30,53]. Furthermore stand establishment with tree species that have good sprouting potential may increase future flexibility in stand regeneration as sprouts are more drought resistant compared to seedlings. Indeed, tree sprouts generally have higher rates of survival than seedlings under water stress as they benefit from resources stored in parent plants and a more extensive root system [54].

Based on the ECHOES database, climate change is already taken into account in decisions related to regeneration of forest stands. In all bioclimatic regions, species and provenances better adapted to projected future conditions are being considered to select and species that perform well across different site conditions are favored. Also tree species diversity is widely considered. In temperate regions, natural regeneration is used for the main tree species and sometimes enrichment planting of species which are robust against climate change is used in combination with natural regeneration. At nursery stage, moist-preserving chemicals are utilized in seedling production, transport and planting in Slovakia, and in Sweden nurseries have taken measures in the selection of a more plastic genetic material. In Spain nursery culture was improved to increase survival of planting stock in plantations [55,56] and the number of species planted in afforestation programs has been increased to promote diversity [57].

While species choice in the regeneration phase has more long-term impact, practices taking place after stand establishment to promote target species composition, stand stability, quality, and structure as well as enhancing growth of crop trees have effects on shorter timescales. Proposed adaptation measures aim mainly at modifying frequency or intensity of tending and thinning activities, but may also include altering tree species composition by targeted stand tending operations.

Adaptation measures in tending and precommercial thinning should support mixed stands of well-adapted tree species, inter alia to distribute risk via diversification [23]. A recent simulation study in Austria corroborated the positive adaptation effect of such measures for a wide range of stand and site conditions [58]. In the boreal zone and in high altitudes, thinning schedules should be adapted to the increased growth rates [59,60]. At drought prone sites thinning improves the recovery of growth in subsequent years after dry periods [23,61,62]. Both experimental data (e.g., [63,64]) and simulation studies (e.g., [65]) support the positive effect of thinnings on traits of individual tree stability. However, since canopy openings affect stand integrity and increase surface roughness, intensified thinning might be limited by increased short-term susceptibility to abiotic and biotic damages such as windthrow (e.g., [66]) or insect infestation [67]. Improved individual tree stability increases flexibility in the stand regeneration phase, thus balancing the negative effects of increased surface roughness through higher stability in partial regeneration cuts.

To counteract negative impacts of climate change in over-mature, structurally uniform forests, standing stocks have to be reduced by adapted thinning regimes focusing on increasing and maintaining structural diversity [58]. Tending and thinning can also help to manage increasingly mal-adapted stands in a changing environment [68].

Small-scale harvesting interventions and promoting harvesting systems which support natural regeneration of suitable species were recommended to increase spatial heterogeneity and biodiversity [23]. However, small-scale cutting may induce loss of light-demanding tree species, which are often also more tolerant to water stress and high temperatures [69] because they are typically pioneer species. Yet, this might be balanced by intensifying disturbance regimes under climate change (see below). Recently, continuous cover forestry and variable retention systems have been proposed as robust strategies under climate change [70], but their overall benefits in this context remain poorly investigated [22,71].

Late stand development stages are generally vulnerable to major disturbance factors (e.g., windthrow, bark beetles, fungal diseases), and harvesting activities may increase stand susceptibility [67,72]. Adaptation should thus aim at reducing predisposing stand characteristics, e.g., via minimizing stand edges exposed to prevailing winds [73] or intense direct sun exposure.

In the Boreal region, technological development in harvesting machinery will be crucial to adapt harvesting operations to warmer winters with shortened frost periods. The wintertime increase in soil moisture and reduced periods of frozen soil may cause reduced carrying capacity of soil for timber harvesting [74].

ECHOES data show that adaptation of thinning and harvesting practices is already ongoing throughout European forestry. Many countries report on modifying thinnings to reach stand structures more robust to projected conditions and extreme events. That is done by either modifying the frequency and intensity of thinning, by promoting species mixtures and/or uneven-aged/structured stands. In the temperate region, small scale regeneration methods like gaps, irregular shelterwood systems and group selection systems are preferred to increase spatial heterogeneity in forest structures. In the boreal region, harvesting methods are already adapted to better suit wet sites. In Cyprus, the observed dieback of trees has lead to the felling and export of dead trees for insect outbreak control and to decrease fire risk [75].

Forest management strategies include response options beyond the level of individual stands. In forest planning, the shortening of rotation periods can be an appropriate management response to accelerated growth in mountainous or boreal environments [58], but trade-offs with in situ carbon storage [76] and a lower nutrient budget due to increased biomass extraction should be considered. Diversification of tree species mixtures and management approaches between neighboring forest stands or within a forest landscape increases adaptive capacity, hedges risks and improves the overall resilience of forest ecosystems [24,77,78]. As there is uncertainty about the best timing of changing species or provenances in forest regeneration, both reactive and proactive adaptation strategies can be applied simultaneously, for instance in different forest stands of a management unit. Adaptive forest management planning [79] systematically integrates results of previous interventions to iteratively improve and accommodate change, by learning from the outcomes of applied silvicultural practices under the changing external conditions [80,81].

At the stand level, individual response strategies can be mutually exclusive. For example, preferring natural regeneration in mixed forests with long rotation cycles is incompatible with planting productive genotypes managed in short rotation cycles. Over larger geographical scales of management units and forest landscapes, however, such strategies can also be combined. In addressing the complexities of landscape-scale planning forest ecosystem models and multi-criteria decision analyses have considerable potential to support forest management under changing environmental conditions [20,26,82]. As part of decision support systems these tools can facilitate a holistic uncertainty management [83,84].

Based on the ECHOES database, ongoing adaptation measures in management planning concern mainly modifying the timing of the final harvest. Generally, rotation lengths are reduced due to increased productivity or to speed up the stand replacement in vulnerable stands. However, in Pinus nigra forests in Mediterranean mountains of Spain, rotation periods are lengthened due to observed difficulties in natural regeneration and mortality due to summer drought. In temperate regions and in Spain, forest growth models and Decision Support Systems (DSS) to evaluate impacts of climate change and to identify suitable management options are already developed.

Disturbance agents are expected to gain importance under climate change. Occurrence and performance of biotic agents are in flux when environmental conditions change, and there is evidence that there will be higher probabilities of damage by insect pests and fungal diseases at higher temperatures [85-87]. The observed continental scale increases in damage from disturbances is linked to ongoing climatic changes, yet changes in forest vegetation are of equal importance in explaining recent unprecedented disturbance levels [88]. Intensive forestry practice over decades has promoted stands of non-autochthonous tree species and of high structural uniformity in many parts of Europe—e.g., plantations of Norway spruce (Picea abies) in central Europe—with the unintentional consequence of increasing susceptibility to disturbances. Consequently, there is a high demand for comprehensive planning systems which incorporate pest risk assessment and aim to improve forest health and stability [89,90]. Stands mixed with species not equally susceptible to specific pest species remain less affected than monocultures [67], because the limited food resources keep pest population levels lower. On the other hand, the selection of tolerant or resistant families and clones may also be an adequate measure to reduce the risk of damage by pests and diseases in pure stands [67]. To understand the complex reasons why different diseases and pests may become a problem, comprehensive knowledge on the susceptibility of specific forest communities to disturbance by specific pathogen or insect pest species is needed. However, such knowledge is mostly lacking at the forest management level, and is also under-represented in the scientific literature. Expert models such as for the Eurasian spruce bark beetle (Ips typographus) [91,92] help foresters to identify risks and to gain awareness of the scope of potential activity especially with regard to climate change.

Increasing temperature with decreasing summer precipitation (at least in central and southern Europe) and more intense singular precipitation events [93] may lead to increasingly dry periods and increased fire risk. Current fire prevention policies need to be adjusted to cope with longer and more severe fire seasons, increasing fire frequency, and larger areas exposed to fire risk, especially in the Mediterranean region [94]. Adaptation measures include: (i) modification of forest structure (e.g., tree spacing and density, regulation of age class structure); (ii) fuel management (prescribed burning, thinning, pruning and biomass removals, grazing), with priority given to forest types with the ability to regenerate after fire (e.g., young Pinus halepensis and P. pinaster forests); (iii) creation of a landscape mosaic of forest types including species with reduced flammability, (iv) infrastructure planning for direct fire attack in relation to the specific behavior for each fuel model [95]; and (v) implementation of policies to limit the abandonment of burned areas and actions to prevent the spread of invasive species in burned areas.

Forest protection measures should be integrated in the landscape level forest planning to balance stand level management goals and risk mitigation. A reduced rotation length will lower the risk of financial losses in stands susceptible to windthrow, bark beetle attacks or other disturbances predominantly affecting later development stages [96]. Increasing tree species diversity distributes risks and is likely to mitigate large-scale damage in the case of extreme events, but alternative species usually produce less timber than the currently favored fast growing conifers.

Integrating risk into forest management decision making is facilitated by applying a risk management process comprising three major steps: (i) risk analysis or risk assessment; (ii) risk handling; and (iii) risk control. For the first step different kind of models are used [97]. Risk handling in forestry can be promoted by educational efforts, e.g., through training courses focusing on identification and prevention of risks and furthermore on mitigation of occurred damages [98]. The second step involves determining expected costs of the disturbance with and without control or adaptation measures. Adaptive disturbance management would benefit from improved fire, storm and pest risk simulation [99] and projections of the effectiveness of alternative management options to reduce disturbance calamities. Furthermore, as the existing coping strategies may not be sufficient under climate change, new insurance concepts should be developed to distribute risks [100].

The preparedness to respond to increased pest and disease risks on the European level is only moderate; there are only a few ongoing measures recorded in the ECHOES database. In UK, the incidence of the Red band needle blight (Dothistroma septosporum) has increased dramatically particularly on Corsican pine (Pinus nigra ssp. laricio), and due to the extent and severity of the disease on this species, a temporary planting moratorium has been set on the Forestry Commission estate [101]. Despite the high demand for monitoring of pest species and diseases, Croatia, Hungary, Italy and Serbia are the only countries in the ECHOES database reporting monitoring of new and existing pest species and diseases as an ongoing adaptation measure. In addition, integrated control schemes for biotic risk agents are explored in selected forests in Austria. Shorter rotation length is used to reduce the risk of storm damage in several countries. In Germany, storm damage models and site mapping are applied to improve risk assessment. In France emergency plans for the handling of large-scale wind-blown wood have been developed in response to large recent damages. Design and promotion of fire-smart management at management unit/landscape level is an ongoing measure in the Mediterranean.

A dense forest road network is a prerequisite for the small-scale, structurally diverse thinning and harvesting practices recommended to adapt harvesting systems. Vital forest functions depend on such small-scale measures particularly in complex terrain such as mountain forests [102,103]. Access is furthermore a key element in coping with disturbance events (e.g., insect pest outbreaks, forest fires) as well as in proactive forest protection routines. Moreover, infrastructure supporting the mitigation of large-scale disturbance impacts should be expanded. Timber storage capacities—wet storage where the moisture level of timber is maintained at a high level by artificial irrigation, and foil storage where timber is stored by wrapping it in foil—could be expanded in order to muster both the phytosanitary necessity of an immediate salvage as well as a buffered release of timber into the timber market to prevent price volatility [104]. Forest road maintenance needs to be adapted to increasing peak precipitation and runoff [15] and shorter periods of frozen ground during the winter as well as thawing permafrost require changes in management schedules and/or machinery to respond to reduced accessibility to forest stands in the Boreal zone [105].

Forest infrastructure can also directly affect forest susceptibility to climate change. To adapt to increasingly dry conditions Cuculeanu et al. [106] proposed an augmentation of storage lakes and irrigation canals in Romania. Such measures might, however, only be feasible for highly valuable crop species and where irrigation is not in conflict with other land uses. Zebisch et al. [98] suggest for Germany that infrastructure measures should prevent a decreasing ground water table (e.g., via deactivating drainage systems) and that the natural water regime in floodplain forests should be restored.

So far, only few adaptation measures have been taken to adapt infrastructure and transport to climate change; only Finland, Cyprus, Croatia and Spain have reported ongoing adaptation measures in this category of the ECHOES database. In Finland, the proportion of high-capacity transportation by railroads has increased and a reduced impact harvesting technology has developed to better protect wet soils. As water availability is perceived as one of the most important factors in relation to oak decline in Croatia, a network of stations for the monitoring of groundwater levels was installed across the distribution range of lowland and floodplain forests of Pedunculate oak. This measure aims at collecting a sufficient amount of data on groundwater level dynamics to provide the basis for more in-depth understanding of the oak decline process. Furthermore, in Cyprus, Spain and Croatia infrastructure is improved for fire detection, control and suppression to meet increasing fire risks under climate change.