Suitability of contract-based nature conservation in privately-owned forests in Germany

The successful implementation of contract-based nature conservation in privately-owned forests requires a framework of reasonable operational measures. Our study aimed at developing such a framework by; 1) defining forest conservation objects including structures, processes, and habitat types, 2) assessing their conservation value based on the need for, and worthiness of, protection, 3) reviewing the suitability of contract-based measures for conservation. Overall, we defined 67 conservation objects, with 8 of them used as case studies: deadwood, habitat trees, natural succession after large-scale disturbance, coppicewith-standards, bog and fen woodlands, dry sand pine forests, and beech forests. We considered contractbased conservation suitable if, within the contract period, outcomes of measures resulted in ecological upgrading or avoidance of value loss. We identified contract-based conservation suitable for 42 combinations of objects and measures. Our approach of assessing the potential of contract-based measures for forest conservation is novel with regards to its broad range of objects, defined criteria, and various contract periods. It can help to progress conservation and improve outcomes of measures, especially in privatelyowned forests in Germany. Further prerequisites are sufficient financial resources, effective administration, consultancy and the midto long-term stability of funding programmes.


Introduction
In the European Union , about 60% of the forested area is privately owned, with huge differences among the member states (Eurostat 2018). Germany, which lies slightly below the EU-28 average with about half of its forest area privately-owned (Polley et al. 2016), may serve as an example to highlight the problems and opportunities for nature conservation in private forests. Implementing conservation measures in private forests may cause additional costs or expenses for forest owners (Sotirov 2017). At present, forest conservation measures in private forests are implemented in Germany primarily through regulations, rather than through subsidies. In stark contrast to agriculture, contractual agreements and funding instruments to compensate for economic losses caused by the implementation of nature conservation measures are rarely used in German forestry (Güthler et al. 2005;Franz et al. 2018b). However, German legislation indicates that the country grants voluntary agreements preference over legal regulations and constitutes in § 3(3) of the German Federal Nature Conservation Act that "… priority shall be given to reviewing whether the intended purpose could also be achieved via contractual agreements". Contract-based agreements are assumed to have a higher acceptance among private forest owners than purely regulatory measures (Franz et al. 2017). The National Strategy on Biological Diversity seeks to "promote contract-based nature conservation in 10% of privately-owned forest land" (BMU 2007), but this target is still far from being achieved, not least because the conditions for contract-based forest conservation have not yet been met (Franz et al. 2018a). Furthermore, overall funding frameworks, for instance for the implementation of Natura 2000, are lacking (Geitzenauer et al. 2017;Sotirov 2017). In contrast to regulations, contract-based nature conservation strives to achieve a consensual, bilateral agreement. In Germany, such voluntary agreements are usually contracted between private forest owners and funding bodies such as the country, federal states, foundations, or private investors. Context-specific conservation measures, referring to specific conservation objects, funding periods and amounts, as well as possible monitoring to verify success, are contractually agreed upon. A broad consensus among different stakeholders in Germany with respect to conservation objectives ) may further promote the implementation of contract-based conservation in private forests.
A prerequisite for the implementation of nature conservation measures in forests is the identification of an operational catalogue of forest conservation objects covering all aspects of forest habitat and biodiversity conservation. An approach using conservation objects accounts for temporal context-specificity and spatial variability, if there is a broad selection of widely accepted and properly defined objects and consensus about suitable preservation measures. At present, the most commonly addressed conservation objects in private forests are habitat trees, deadwood, and historical types of forestry use, such as coppicing or wood pasture (Franz et al. 2018b). However, numerous further objects may be taken into consideration in order to fully tap the potential of private and other forests for the restoration and preservation of biodiversity.
The aim of our study was to develop a comprehensive catalogue of forest conservation objects and measures eligible for contract-based funding. We built on the framework of conservation objectives suggested by Demant et al. (2019) and focussed on forest habitat types, structural elements, and developmental processes as the most relevant conservation objects. We identified the conservation value of the objects by assessing the need for protection (owing to threat, endangerment) and the worthiness of preservation. The guiding questions for our study were: (1) How can forest conservation objects be assessed in a way that reflects their nature conservation value, particularly in terms of their need for, and worthiness of, preservation? (2) Which forest conservation objects are suitable for effective contract-based conservation measures and over which contractual periods should measures reasonably be funded? (3) What consequences for nature conservation practitioners and forest owners can be derived?

Assessment of the nature conservation value of forest conservation objects
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). 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 centurieslong 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).

Forest conservation objects
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.

Suitability assessment scheme
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).
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  Table 3. Characteristics of 8 case study forest conservation objects.

Characteristics
Possible conservation measure during contract period

References
Deadwood Key structure in forest ecosystems, variable in terms of amount, decay stages, size classes, wood diameters, microclimatic conditions, and tree species. Retention of dead trees or logging residues; supply ringbarking, crown cutting, felling or knocking-over of trees. Harmon et al. 1986;Davies et al. 2008;Lassauce et al. 2011;Lindenmayer et al. 2012;Agnew and Rao 2014;Seibold et al. 2015; Habitat trees Characterised by various tree-related microhabitats (e.g., hollows or dead branches), indicating habitat continuity; important for countless species supported by dieback structures of old-growth forest stages.
Protection of existing habitat trees and retention of potential once; creation of structures by breakingoff branches, making bark injuries or bark-removal, constructing cavities, dendrothelms (water-filled tree hollows). Winter and Möller 2008;Fedrowitz et al. 2014;Kraus et al. 2016;Larrieu et al. 2018;Asbeck et al. 2019;Gustafsson et al. 2019;Mölder et al. 2020 Natural forest development Characterised by typical regional and local-scale old-growth forest structures and associated biodiversity. With ongoing cessation of forestry interventions, typical developmental and structural features gradually develop over long periods of time.
Continuation of natural forest development initiated several decades ago, recent decommissioning of nearnatural commercial forests. Minimum standards as defined by Engel et al. (2016, p. 38) apply. Meyer and Schmidt 2008;Vandekerkhove et al. 2011;Kraus and Krumm 2013;Paillet et al. 2015 Natural succession after large-scale disturbance Natural disturbances (e.g., by wildfires, windstorms, or insect infestations; intensity and frequency are expected to increase under climate change) are important drivers of forest dynamics and associated biodiversity. They contribute to maintaining pioneer species and habitats, enhance structural heterogeneity, and make forests more resilient to future disturbances.
Allowing and supporting natural development in earlysuccessional stages. Runkle 1989;Franklin et al. 2002;Lindenmayer et al. 2008;Swanson et al. 2011;Seidl et al. 2017;Thorn et al. 2018; AK Waldökologie GfÖ 2019; Müller et al. 2019 Coppice-with-standards Two-layered stands with an overstorey consisting of mature trees (standards) used for timber and fruit setting. Even-aged understorey regrowth (coppice) consists of multi-stemmed trees cut at a 20-30-year rotation cycle. Offer a mosaic of habitats and structures favourable for light-demanding and thermophilic species due to conditions of alternating shade and light. Abandoned coppice-with-standards with all trees left uncut ('overstood', stems having the size of mature forest stands) are commonly converted to high forests (even-aged forest stands).
Continuation and resumption of coppice-with-standard management. Barnthöl 2003; Groß and Konold 2009;Kirby et al. 2017;Meyer et al. 2018;Unrau et al. 2018 Bog and fen woodlands Ecosystems of coniferous or broadleaved trees and shrubs on low-productive peaty soils with high water level. When intact, they contribute to climate protection, if drained, they emit greenhouse gases at high rates. Habitats for many specialised, rare and endangered species, and highly threatened by hydrological changes caused by forest management and drainage.
Restoration of degraded bog and fen woodlands by raising the water level, regeneration of the acrotelm, the active peat zone containing living plants, removal of nonnative tree species and renouncement of peat extraction. Moore and Knowles 1989;Joosten 2012;EEA 2013;Joosten et al. 2015;EEA 2019 Dry sand pine forests Lichen-rich dry pine forests on nutrient-poor, acidic sands with low shrub, herb, and litter cover. Being the result of historical land use (mainly litter raking and sod cutting) they depend on nutrient removal to accommodate typical epigeous (growing on the soil surface) lichen species. They are highly endangered, mainly due to discontinuation of litter raking, and by nitrogen deposition caused by agriculture and traffic emissions and have both high historic-cultural and biodiversity significance.
Prolonging of rotation cycles beyond conventional harvesting age, thus preserving old-growth-associated biodiversity, and enhancing natural regeneration. Kroiher and Bolte 2015;Meyer et al. 2015; 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).  Table 4. Description and assignment of the final nature conservation value (NCV) to the suitability assessment of conservation measures and the corresponding colour in Table 7 and Supplement S1.

Initial nature conservation value of forest conservation objects
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 nonthreatened 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-withstandards, 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).

Suitability of contract-based forest conservation
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. Shortterm 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.  Table 6. Suitability assessment proportions of forest conservation objects for different contract terms (years).

Suitability Contract duration (years) Forest conservation object group Structures (8)
Processes (  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.

Deadwood
Measures to actively supply deadwood were assumed to have a positive short-to longterm 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.

Habitat trees
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 objectspecific conservation value (Table 7), but this may increase soon, making even shortterm contracts reasonable. Mid-to long-term contracts were considered very suitable to achieve very high conservation value.

Natural forest development
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 succession after large-scale disturbance
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.

Coppice-with-standards
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.

Bog and fen woodlands
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.

Dry sand pine forests
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.

Beech forests
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.

Discussion
Assessing the nature conservation value of forest conservation objects 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 Box 1. Legally protected habitat types.

Special case: Legally protected habitat types
Some German forest habitat types are legally protected according to § 30 BNatSchG. These are primarily natural and self-sustaining habitat types that do not require management, and include among others fen and bog woodlands, riparian forests, forests of ravines, slopes and screes, and xerothermic forests and shrub lands. Destruction or actions with significant adverse effects are prohibited by law. Forest owners are obliged to protect and maintain these habitats and to refrain from destruction or considerable impairment. Private land owners cannot be compensated for fulfilling these legal obligations. In contrast, for habitat types that rely on active conservation measures, such as mixed oak forests derived from coppicing, financial compensation appears reasonable.
Likewise, for restoration of degraded habitat types, such as drained swamp forests, financial compensation is possible. The successful restoration of degraded habitats may result in permanent restriction of the forest owner's right of disposal once the status of a legally protected habitat is reached. Franz et al. (2018a) argued that, for reasons of fairness, this permanent use restriction should be permanently compensated. 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).

Contract duration to safeguard forest conservation objects
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. Contractbased conservation is particularly suitable for high-valued objects, such as coppicewith-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.

Consequences for nature conservation and forestry practice
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 (EU-NIS) 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 ele-ment 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 contractbased 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.

Conclusions
The nature conservation value assessment of forest conservation objects provided in this paper enables forest owners to assess the conservation value of objects in their forest stands and to consider options for contract-based nature conservation, specifically in privately-owned forests in Germany. We also touch upon the much-discussed topic of conservation responsibility. We believe that the comprehensive catalogue of forest conservation objects and measures may be applicable in a wider Central European context. Furthermore, the nature conservation value assessment can help to improve the conservation status of Natura 2000 forest habitat types. We showed the suitability of many conservation objects to financial incentives and advocate conservation object-dependent variation in contract duration. We noticed a particular need for action in the case of conservation objects susceptible to an imminent loss of value in the absence of conservation measures.
Currently, however, a general framework for successful implementation of contract-based forest conservation, including factors such as legal security, fairness, continuity, and flexibility, is not available. The reference framework presented here and the considerable number of combinations of objects and measures found suitable for contract-based conservation, together with the recommendations for a forest conservation funding system given by WBW and WBBGR (2020), may help to enhance this implementation process. For the sake of diversified nature conservation in forests, politicians and stakeholders at all governmental levels should rethink and revise benefit payment programmes towards mid-to long-term contracts (Gemeinholzer et al. 2019), and thus encourage private forest owners to acknowledge biodiversity-related funding.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.