To assess the nature conservation value of a forest conservation object, we considered the initial value (before conservation measures were implemented) and the conservation value achieved after application of a measure over varying time periods. According to Frenz and Müggenborg (2016), worth of preservation alone is not enough for an object to justify a legal priority protection setting – conservation objects must also be (potentially) threatened. Thus, we differentiated between the two components ‘worthy’ (contributing to the preservation of characteristic species and gene pools in natural or semi-natural landscapes or ecosystems) and ‘need’ or ‘urgency’ (degree of threat as a result of adverse effects of land-use and environmental changes) to assess the conservation value of the objects (Fig. 1).

Figure 1.

Assessment of the nature conservation value of forest conservation objects.

We based the assessment of the need for protection on the national and the European Red List status categories (Janssen et al. 2016; Finck et al. 2017) translated into an ordinal scale (Table 1). The Red List status categories encompass long-term threat (assessed at national and regional levels), the current trend (stable, increasing, decreasing), rarity, and the ability to regenerate (Finck et al. 2017). Threats are “human activities or processes that have impacted, are impacting, or may impact the status of the taxon being assessed” (IUCN 2013).

The forest structures and processes that we assessed have a high urgency for protection. For example, the retention of deadwood and a natural forest development are commonly in conflict with the economic goals of forest management.

Based on an assumption that the maintenance of core ecosystem functions was of high value we selected forest conservation objects, whether they represent structures, processes, or habitat types, as worthy of preservation if they are integral parts of natural self-sustaining, or semi-natural, managed forest ecosystems (Frenz and Müggenborg 2016). We also assumed that higher value would be placed on objects with a greater importance for a region’s natural and cultural heritage. The longer the habitat continuity, i.e. the period in which a conservation object has evolved its typical biodiversity, the more important it is to be preserved (Nordén et al. 2014). As the habitat continuity increases, so, too, does the responsibility of preserving the conservation object to meet “the needs of the present without compromising the ability of future generations to meet their own needs” (WCED 1987). Wood-pastures, for example, have a centuries-long habitat continuity (Bergmeier et al. 2010; Plieninger et al. 2015), and are regarded as being part of the European cultural-natural heritage (Leuschner and Ellenberg 2017).

Apart from habitat continuity, other factors determining the worth of a conservation object were the quantitative (absolute number of species) and qualitative (relative to a desired reference state) contribution of a conservation object to the species pool of a natural landscape. For example, intact peat bog woodlands may have a relatively low absolute number of species, but a high qualitative contribution to the typical diversity of the natural landscape. We based our assessment of the worthiness on expert valuations and distinguished six levels in a qualitatively ranked ordinal scale (Table 2).

For example, dry oak-hornbeam forests (Galio-Carpinetum) have a Red List status of 1–2 (Endangered to Critically endangered; Finck et al. 2017) which means their need for protection was high (4). Furthermore, they have a very long habitat continuity (HC = 5), a high quantitative (Q1 = 4), and a very high qualitative (Q2 = 5) contribution to the diversity of the natural landscape. Their worthiness of preservation resulted in ‘very high’ ([5+ ((4+5)/2)]/2 = 4.75).

We assert that structures and processes, as essential components of natural forests, are highly worthy insofar as they allow maintenance of key ecosystem functions (Walentowski and Winter 2007). The final nature conservation value resulted from the calculation of the mean values of the two protection criteria, worthiness and need, with the classes 0 = no, 1 = very low, 2 = low, 3 = moderate, 4 = high, and 5 = very high conservation value. In the example above the final conservation value is high ((4.75+4)/2 = 4.375).

The nature conservation value assessment was carried out for eight forest structural elements, four processes, and 55 forest-related habitat types (Finck et al. 2017; see Suppl. material 1: Table S1). In the main body of the present paper, representative assessments for 8 out of 67 forest conservation objects were made, characterised in Table 3.

We assessed the suitability of contract-based funding for forest conservation objects by comparison of the initial and final conservation value (Fig. 2). The initial conservation value of the conservation object was scaled between very low (0) and high (5). After projecting the expected development and outcomes over a contract length, we calculated a final conservation value, again scaled between very low and high (Fig. 2). As relevant development periods differ greatly among conservation objects, we considered three potential contractual periods: short-term (< 10 years), mid-term (10–30 years), and long-term (> 30 years).

Figure 2.

Development pathways of the initial nature conservation value.

The assessment of the worthiness of, and need for, protection of forest conservation objects resulted in a single nature conservation value, although each individual variable may have different values (Suppl. material 2: Table S2). Conservation objects may achieve a high value when preservation measures have been implemented and have produced positive results, when degraded objects have been restored successfully (restoration measures), or when the objects have been newly created. A high conservation value towards the end of a contractual period indicates an improvement of an initially lower conservation value, or the prevention of value loss of an initially high value.

Contract-based funding would be particularly suitable for conservation objects with high initial conservation value that would suffer value loss in the absence of conservation measures, or for objects with rather low initial value but considerable restoration potential to achieve a higher final value. If the conservation value of a newly created conservation object (initial value = 0) was likely to increase over a given contract period, contract-based funding of conservation measures was also considered reasonable. If both initial value and restoration potential were low, contract-based conservation was deemed inappropriate. The suitability assessment is depicted as a four-level colour scheme, reflecting the final value (Table 4).

More than 82% of all conservation objects were assessed as being highly or very highly worthy of preservation. However, only 39% had a high to very high need for protection and these were found exclusively within the group of objects of high to very high worthiness. Thus, some conservation objects can be regarded as very valuable, but are not seriously threatened, such as mesic beech forests or riparian alluvial forests (Suppl. material 1: Table S1). Forest structures and processes made up only a small proportion of all conservation objects. For forest structures, the proportion of low-value and non-threatened objects was higher than that of highly valuable and threatened ones, since many structures are being developed or newly implemented (e.g., the active supply of deadwood, or the designation of potential habitat trees).

One quarter of all forest conservation objects were assessed as having a high to very high initial nature conservation value (Fig. 3). The conservation objects coppice-with-standards, wood pastures, intact bog and fen woodlands, continuation of natural forest development, natural succession after large-scale disturbance, deadwood retention, eyrie tree protection (nesting sites for birds of prey) and protection of habitat trees were assessed as having very high conservation value. About three quarters of all conservation objects were ascribed a moderate to very high initial conservation value. Habitat types, comprising 55 out of the 67 identified forest conservation objects, made a major contribution to high conservation-value objects (initial value higher than 3; Table 5).

Figure 3.

Initial nature conservation value (NCV) of all 67 forest conservation objects analysed.

As many as 42 out of 67 forest conservation objects proved suitable for contract-based conservation measures (Suppl. material 1: Table S1). Most of the assessed forest structures and processes were considered suitable or very suitable for contract-based conservation, irrespective of the contract period. For forest habitat types, accounting for the largest part of all assessed conservation objects, the findings are more nuanced. Short-term contracts (<10 years) were found to be very suitable for 15 out of 67 forest conservation objects (3 process-related, 4 structural and 8 habitat types; Table 6 and Suppl. material 1: Table S1). The conversion of forest stands of non-native trees, the continuation of traditional forest management (wood pastures, coppice-with-standards), and the retention of deadwood belong in this category. Mid-term contracts (10–30 years) were found to be very suitable for 31% of all conservation objects, including the resumption and continuation of traditional forest management, the restoration of degraded habitat types, the active creation of habitat trees, micro-habitats, as well as the conservation management of high-valued habitat types (Suppl. material 1: Table S1). Long-term contracts (>30 years) were assessed as being very suitable for about 33% of all conservation objects, mostly the same as for mid-term contractual periods, though with a few exceptions, such as the continuation of a natural forest development, or the retention of potential habitat trees. Contract-based agreements were rated not suitable for 34% of all conservation objects, regardless of the contractual period. This category includes almost exclusively habitat types, chiefly because they are either legally protected habitats (Box 1) or low-valued pioneers.

For most suitable conservation objects (82%) contract duration was considered of little relevance. Nevertheless, longer funding durations are to be preferred. This would not apply, however, to wind-throws or other large-scale disturbances left to natural succession, because here, the early succession stages are the intended objective.

Measures to actively supply deadwood were assumed to have a positive short- to long-term effect on the richness of saproxylic (depending on dead or decaying wood) organisms (Table 7). Therefore, short-term contracts were considered suitable. When contracting for mid-term periods, it should be considered that, due to decay, deadwood needs to be replenished to ensure continuous provisioning of different deadwood qualities (see deadwood estimation tool, Meyer et al. 2009). With further contractual period extension, the conservation value is expected to increase, provided that a continuous deadwood supply is guaranteed. Natural deadwood, or silvicultural routine deadwood, has a very high initial conservation value, making even short-term contracts very suitable. Mid- to long-term contracts to secure continuous deadwood supply would result in a very high conservation value.

We considered trees with trunk diameter far beyond the typical harvest size (DBH > 80 cm for deciduous trees on normal sites, for oaks > 90 cm), and/or the site-specific harvesting age (e.g., beech > 200 a, oak > 300 a), as well as trees rich in microhabitats and/or with very large crowns or low crown bases, to be particularly qualified to become habitat trees (Table 7). As the natural formation of tree microhabitats was assumed to take >50 years at minimum (Larrieu et al. 2012), only long-term contracts qualify. Trees with microhabitats created through management measures have no initial object-specific conservation value (Table 7), but this may increase soon, making even short-term contracts reasonable. Mid- to long-term contracts were considered very suitable to achieve very high conservation value.

Forests with long habitat continuity, where forestry ceased many decades ago, contribute considerably to the biodiversity of the natural landscape. Therefore, their worthiness was rated very high (Table 7). Due to their low presence in German forests (only 2.8% of the total forest area; Engel et al. 2019), their need for protection is also very high. The continued protection of forests with a long-lasting natural development was recommended for all contractual periods. Semi-mature forests that have been recently decommissioned have a moderate need for protection. Positive effects on biodiversity of such forests may only be measurable after many years or decades. Therefore, contract-based decommissioning of forests was assumed to be suitable for mid- to long-term periods only. Follow-up contracts were recommended for prolonged natural development.

Natural forest succession after major disturbance events requires silviculturists to refrain from salvage logging, deadwood removal and replanting. Untouched early-successional stages are rarely found in privately-owned forests and are thus regarded as highly vulnerable (Table 7). As such pioneer habitats support numerous warmth- and light-dependent species, they are worthy and, consequently, of high initial conservation value. As disturbed areas decrease in object-specific conservation value over time, mid-term contracts were considered particularly suitable. Long-term contracts would only be meaningful if non-disturbed, surrounding stands are simultaneously targeted beyond the given conservation object.

Traditional coppice-with-standards woodlands can be protected from being transformed into high forests by continuing their specific management. As coppice-with-standards contribute much to the biodiversity of the natural landscape, they were granted a very high worthiness (Table 7). Due to their extreme rarity (less than 0.4% of the forest area in Germany; Albert and Ammer 2012) and susceptibility to management change, they were also assessed as having a very high need for protection and risk of value loss. Therefore, all contract terms were considered suitable, with long-terms preferred.

Abandoned and ‘overstood’ coppice-with-standards may be restored by resuming the former management. As a moderate loss of habitat continuity and species richness was assumed, their worth of, and need for, protection were given medium ratings (Table 7). Since one rotation cycle usually takes 20–30 years, short-term contracts do little to increase the conservation value of ‘overstood’ coppice-with-standards. More suitable contract periods are mid- to long-term.

As part of the landscape’s natural vegetation, intact bog and fen woodlands have a very long habitat continuity and, consequently, very high worthiness. Due to their high level of endangerment, they also have an urgent need for protection (Table 7). Intact bog and fen woodlands have been protected under the Federal Nature Conservation Act. As mere preservation is not compensable (Box 1), contract-based conservation was considered unsuitable, unless combined with additional measures. As remnant or slightly degraded bog and fen woodlands may still contribute to the biodiversity of the natural landscape, they have been assigned medium to high worthiness and medium need for protection (Table 7). Because the restoration of slightly degraded bog and fen woodlands promptly leads to a value increase, even short-term contracts were deemed to be adequate.

The qualitative contribution of lichen-rich dry sand pine forests to the biodiversity of the natural landscape was top-rated and, consequently, their worthiness was also high (Table 7). Being endangered, they have a very high need for protection. However, as a legally protected habitat type, forest owners cannot be compensated for its mere preservation (Box 1). Contract-based maintenance was therefore considered unsuitable unless combined with extra measures, such as rotational litter and topsoil removal.

For degraded forms, if still restorable and credited with medium conservation value, financial compensation for measures to initiate recolonisation of characteristic lichen species was recommended. Short-term contracts were considered suitable, although long-term contracts rendered higher conservation value.

A long habitat continuity and high relevance for the biodiversity of the natural landscape were assumed to result in very high worthiness (Table 7). Our assessment is that financial compensation for preservation-friendly management of dry and mesic beech forest complexes is highly recommendable, whatever the contractual period, if it clearly extends beyond regular forestry practice.

By means of various indicators or criteria, evaluating conservation objects may be understood as the transfer of factual knowledge to a valuation scheme (Plachter 1991; Schultze et al. 2016). This valuation approach has formed the basis of many studies that have applied scoring techniques (Usher 1994; Gastauer et al. 2013; Capmourteres and Anand 2016; IUCN 2016), and we used it to develop our framework of reasonable and operational measures to assess the nature conservation value of forest conservation objects.

Our conservation valuation comprises different attributes, with single summarised scores, to allow for its country-wide application. With contextual modifications such as other Red List levels to specify the need for protection, the approach may be applicable in yet other regions. By including forest structures, processes, and habitat types, we tried to cover relevant attributes of forest biodiversity. The selected conservation objects are representative for forest conservation management and include those in urgent need of conservation actions. They are particularly relevant in times of climate change, as they encompass short-term objects (e.g., wind-throw sites), climax habitat types (e.g., beech forests), habitats of carbon sink relevance (bog and fen woodlands), habitats with climate-sensitive species (e.g., dry pine forests), and habitats with considerable economic potential for financial risk spreading (coppice-with-standards).

We showed that contractual agreements can be appropriate to support conservation measures in forests. The evaluation of 67 forest conservation objects showed that contract-based conservation agreements prove suitable for 42 objects, albeit with different contract durations. Short-term contracts are less suitable for the retention of habitat trees and for decommissioning semi-mature forests, while long-term contracts are not recommended for funding natural succession after large-scale disturbance. Contract-based conservation is particularly suitable for high-valued objects, such as coppice-with-standards, that depend on active conservation measures to prevent deterioration. Even short-term contracts may be adequate in cases of objects with low to medium initial conservation value if a prompt value increase is to be expected, e.g., newly created habitat trees. In contrast, short-term contracts are less meaningful for conservation objects with low initial conservation value and slow value improvement.

Permanent compensation and long-term agreements would be required for private owners of forests under permanent statutory use restriction (e.g., in bog and fen woodlands). A short contract duration, covering only initial investment expenses but no further maintenance measures, would fail to produce a return on landowner’s investment. However, if there is a general willingness of forest owners to accept follow-up contracts, and if suitable funding resources are available, short-term contracts are better than no agreement.

As far as forest habitat types are concerned, our conservation objects are in line with the EU Habitats Directive (Natura 2000) and the European Nature Information System (EUNIS) classification (Suppl. material 3: Table S3) and our approach may help to improve the mandatory assessment of the conservation status. In the EU Natura 2000 network, the preservation of diverse forest structures (e.g., deadwood, habitat trees) is a necessary element for a particular forest habitat type to achieve favourable conservation status (Winkel et al. 2015; Alberdi et al. 2019). Since a high proportion of European forest habitat types have been assigned an unfavourable conservation status (European Commission 2015), enhancing these forest structures helps to improve their conservation status.

Our suitability assessment revealed that the conservation or restoration of forest conservation objects may have synergetic effects and simultaneously result in the protection and improvement of other objects. These synergies should be given special consideration (Margules and Pressey 2000; Cimon-Morin et al. 2013). Potential trade-offs and competing objectives across conservation objects should be weighed in the light of the conservation objectives, site conditions and the expected value development. For instance, natural forest development and coppice-with-standards management cannot be implemented in the same site. In general, forest owners cannot meet all possible conservation objectives in a single stand. A contract usually covers a single conservation object and the necessary measures (setting, extent, feasibility, financial framework), but several contracts may be concluded for different objectives in the same forest stand.

Given an underlying value structure that aims to protect typical regional forest biodiversity, the responsibility to protect can only be justified for native species appropriate to the site and location, long-term natural and semi-natural processes and structures, and the cultural development history (Meyer 2013). Consequently, management in privately-owned non-cultural types of forest should be committed to close-to-nature forestry (extension of rotation periods, deadwood provisioning, and tree retention). Since this paradigm shift may cause additional costs for forest owners, suitable compensation structures are needed.

However, financial incentive systems in privately-owned forests are as yet lacking in Germany (Seintsch et al. 2018). Other countries successfully developed their own subsidy programmes, such as the ‘English Woodland Grant Scheme’ introduced in 2005 (Forestry Commission 2010; Fuentes-Montemayor et al. 2015), replaced in 2015 by ‘Countryside Stewardship’ grants. Such a country-wide system can lead to more transparency and acceptance among forest owners to support forest biodiversity conservation. Although some German federal states have developed their own incentive instruments, there is substantial variability in requirements and capacity for funding across states. For instance, the Bavarian contract-based forest conservation programme supports the conservation of coppice-with-standards woodlands, the preservation of habitat trees and deadwood. In Hesse, forest conservation measures are funded by the Natura 2000 Foundation, but only within the Natura 2000 network. Additional funding options with differing requirements and payment amounts exist in Germany, yet none of these have nation-wide applicability (BMEL 2019; European Commission 2020). Unfortunately, the operational implementation of these general systems has by no means reached the individual private forest owner. Franz et al. (2018a) pointed out that there is an urgent need for action and to create the prerequisites for contract-based conservation in privately-owned forests, such as a solid foundation of trust, the involvement of committed intermediaries, result-oriented payments, success bonuses, as well as the identification of suitable indicators. Our comprehensive catalogue of forest conservation objects and measures eligible for contract-based funding is valid throughout Germany and in line with the Federal Compensation Directive (BMU 2020) just published. It does not, however, explain the possible trajectories between initial and final conservation values of objects. Forest owners are encouraged to use our catalogue for their conservation intentions. Given that they know the tree species composition and structural characteristics of their forest stands, they can easily identify conservation objects such as potential habitat trees, and choose a reasonable contract duration. The biggest challenge yet for contract-based nature conservation is to find suitable funding options, which vary between the German federal states. Authorities, nature conservation agencies, or NGOs might assist on this point. Therefore, while this paper provides a rationale and an objective-related design for contract-based nature conservation on forests, it can’t guide private forest owners towards an operational implementation. Such a guidance, generalised at the level of administrative units or federal states, remains yet to be elaborated.