Pinus cembra is one of the most characteristic woody species of the alpine treeline (Körner 2012; Tranquillini 2012). It is considered an emblematic species of the European tree flora and has attracted the attention of local people, foresters and researchers for centuries (Kasthofer 1828; Caudullo and De Rigo 2016). As a consequence, there is a large amount of research on various aspects of the biology, ecology, distribution, biogeography and genetics of P. cembra on a large geographical scale (Gugerli et al. 2001; Höhn et al. 2009; Casalegno et al. 2010; Neuschulz et al. 2018; Dauphin et al. 2020, 2021; Zięba et al. 2020). Far fewer studies have been performed at the local level, especially on the peripheral populations of this species (Lendvay et al. 2014; Tóth et al. 2019). The information on P. cembra from the Western Prealps, for example, is either very old (Rikli 1909) or based on only a few individuals collected exclusively from the largest and best known populations (Gugerli et al. 2009). Our work is the first detailed synthesis of the distribution, population sizes, site topography and sylvicultural history of P. cembra, covering the entire territory of the canton of Fribourg in Switzerland.
Our results confirm the peripheral situation of P. cembra in the canton of Fribourg, attaining its northwestern distribution and climatic limits in this region (Dauphin et al. 2020). Other than in the Central Alps, the species rarely forms monospecific populations in this region and grows exclusively in very specific and spatially restricted microhabitats (Doutaz et al. 2006). According to our detailed estimations, there were fewer than 8000 individuals of P. cembra in the study area. The largest concentration of trees and populations occurs along the chain of Gastlosen, on the border with the cantons of Vaud and Bern, with approximately 6850 individuals (86%). Furthermore, much smaller populations occur in the Vallon des Morteys, Hochmatt and Ziebegg. The smallest and most isolated natural population occurs on a very steep part of the Dent de Broc (ca. 10 individuals). Some of these populations or their fragments are growing in very steep and inaccessible cliffs and are certainly primary forest patches. There is no doubt that those populations could not be planted, and their structure, with a mix of ages, also confirms this assumption (Tranquillini 2012; Nussbaumer 2016). Although many trees in these natural forests may be very old (P. cembra can live up to 1000 years, Caudullo and De Rigo 2016), the trees in the canton of Fribourg are rather small (mainly 8–12 m high), and only an extremely small proportion attain a diameter > 80 cm (Fig. 5).
According to Gugerli et al. (2009), P. cembra migrated into the Western Prealps during postglacial colonization, similar to all other alpine regions, from peripheral refugia. Although the species reached our study area many thousands years ago (Burga et al. 1998), it was not able to colonize other neighboring high mountain chains of the canton of Fribourg, which are situated further north or west, such as the Moléson or Kaiseregg region (Rikli 1909; Info Flora 2021). The mutualistic seed disperser of P. cembra, the Eurasian nutcracker, is very common in the canton of Fribourg. The distances between the main concentration of P. cembra along the Gastlosen to the other uncolonized high-altitude mountain chains are very short. Thus, the distance was certainly not an obstacle for the nutcracker, which is able to cover distances up to 15 km (Caudullo and De Rigo 2016).
We decided to use the term afforestation for P. cembra in our study region because the species was probably not present before in the planted area, and we do not know when the forest became absent from these areas before plantations. With only 0.15% of the remaining P. cembra trees planted (out of ca. 450,000), and with no known wood harvest, one century after the afforestation, we can conclude that this impressive campaign in the canton of Fribourg was a failure. Moreover, P. cembra was unable to regenerate where it was artificially introduced since no seedlings and no juvenile trees were found in the remaining planted sites, and thus it will probably completely disappear from these locations in the near future. In comparison, in natural populations, also at very high altitudes, the natural regeneration and recruitment is very common (Tranquillini 2012). In the natural P. cembra forests in our study area, the proportion of juvenile trees is often higher than 30% (Doutaz et al. 2006), attaining ca. 10 juvenile trees per hectare (Nussbaumer 2016).
The number of planted trees (448,025) must be considered as a minimum estimate. First, we did not find all of the information for every afforestation project in the archives. We discovered several planted stands without any historical records and, thus, without numbers of seedlings used in these sites, for example, for Moléson (MOL) and Combe d’Allière (ALL; Figs. 1 and 2). Second, some information was contradictory between various sources, mainly for the large afforestation projects in Höllbach-Gérine (Online Resource 1). In such cases, we always selected the most prudent numbers. Independent of the exact numbers, the magnitude of this past afforestation campaign is enormous in terms of both time and costs.
Without having studied it in detail, we also observed the poor success of afforestation of some other species planted together with P. cembra (in particular P. mugo, P. strobus and Larix decidua).
Possible causes of afforestation failure
There are several possible reasons why this huge afforestation effort in the canton of Fribourg turned out to fail in the mid-term and we discuss some of them bellow. Unfortunately, historical data for such causes does not exist, thus we can only formulate hypotheses according to the present state and available knowledge. We miss information about the historical afforested area, where no tree is remaining today. As these locations are not precisely known, we could only consider in our analysis places where planted trees are remaining. We also miss information about early twentieth century silvicultural practices and if management activities where undertaken following the plantations.
Unsuitable climatic conditions and topography. As explained above, P. cembra reaches the margin of its natural distribution in the canton of Fribourg. The climate of this region is more oceanic than in the Central Alps, i.e., in the core distribution area of the species. Under less continental conditions, P. cembra is generally outcompeted by other species, mainly Picea abies (Ettlinger 1975; Höhn et al. 2009). Thus, P. cembra is able to occur naturally only in microrefugia with harsh continental conditions: on cliffs and cliff edges, at the edges of large blocks and sometimes on crests. These conditions are present in the canton of Fribourg mainly along the chain of Gastlosen and especially in Forêt du Lapé, the largest population, with a large number of huge blocks. The unique topography of this area results in site characteristics that reduce competition (mainly of spruce) and help the establishment of P. cembra: (1) temporarily strong aridity; (2) low accumulation of snow and early melting, which exposes the soil, young trees and vegetation layer to extreme cold in winter conditions (Schönenberger 2001; Doutaz et al. 2006). For example, Schönenberger (1975) showed that planted Picea abies in locations with early snow melting were more frequently damaged by frost than P. cembra. The latter is one of the most cold-hardy trees known and is well adapted to frost and supporting winter temperatures below -43 °C without any damage (Caudullo and De Rigo 2016); (3) high wind exposure and air dryness; and (4) shallow soils with low fertility. Therefore, according to our results, the rugged topography and microtopography seem to be key factors for P. cembra survival in this region. Thus, this could also be one of the main problems in artificially planted stands outside of the natural distribution area of P. cembra. The afforested sites where the species still persist possess microtopography that is too homogeneous (see Fig. 4), favorable to other more competitive species.
We did not find any important difference between the remaining planted stands and natural populations (see Fig. 5) in regard to the elevational range or slope aspect. Therefore, it seems that neither altitude nor exposure of sites selected for afforestation was a relevant cause of the failure. However, it cannot be completely ruled out that some planted stands where P. cembra completely disappeared (see Fig. 2) were located at altitudes too low and/or on unsuitable south-exposed slopes. Unfortunately, the lack of information on the exact location of the historically afforested areas do not allow us to carry out any comparative analysis.
Competition. The vegetation that exists around planted seedlings can either be considered as competition and cause mortality, or as facilitation and protect seedlings from stressful environmental conditions, thereby improve their establishment. The effect of herbaceous alpine vegetation on P. cembra seedling emergence and first-year performance were studied in a field experiment in the French Alps (2100 m a.s.l.). Total emergence and locally-germinated seedling survival were not affected, but for seedlings planted at 2 months of age, negative vegetation impacts dominated for all response parameters: first-year survival, growth and carbohydrate accumulation (Loranger et al. 2017). The possible absence of management of vegetation following plantations could be an important factor of failure. Pinus cembra is a light-demanding species that can tolerate only moderate shadows (Fourchy 1968). Today, more than a hundred years after plantations, the access to light in planted stands is low, with well-established and competitive vegetation, which probably hinder the natural regeneration of P. cembra. In contrast in natural populations, a rugged topography and microtopography allow more light to enter the populations, especially at the edges of cliffs and large blocks. The vegetation is also less competitive, lower and sparse in these places.
Pathogens. It has long been demonstrated that pathogens can cause high mortality among planted seedlings of P. cembra (Rikli 1909; Fourchy 1968), mainly due to snow blight (Phacidium infestans) and brunchorstia disease (Gremmeniella abietina). In an afforestation project that took place in Davos (Switzerland) in the 1950s and 1960s, approximately 47.6% of all P. cembra trees were infected by P. infestans approximately 20 years later (Schönenberger 1975). In the same region, for another plantation performed in 1975, only 5% of P. cembra trees survived 30 years later due to pathogen attack (Barbeito et al. 2013). How newly planted stands, sometimes isolated, can be infected by pathogens is difficult to know, but long-distance dispersal seems to be possible, for example for Gremmeniella abietina. It occurs through wind-borne ascospores (but reported as absent in the European strain, EPPO 2021), or conidia rain-splash dispersal could also play a role for long-distance dispersal with the help of wind (Petäistö and Heinonen 2003). Gremmeniella abietina is more successful in trees that are stressed by adverse environmental conditions (Senn 1999).
It was demonstrated that in planted stands, persistent snow cover and low wind speed were two factors favoring the development of pathogens, and P. cembra survived mostly at sites where the snow melted early (Schönenberger 1975; Senn and Schönenberger 2001). This corroborates our results since natural populations occur where snow does not persist for a long time due to topography, thus reducing the impact of pathogens on seedlings and young trees. Accordingly, the microrefugia of P. cembra in this region may be considered as not only climatic and topographic refugia but also as “refugia from pathogens”.
Wildlife damage. Browsing can generate important damage in forest plantations and P. cembra can be impacted (Ulber et al. 2004; Barbeito et al. 2013; Oberhuber et al. 2019). Here, again, the topography and microtopography of natural populations may largely protect juvenile individuals of P. cembra from browsing by ungulate herbivores. This impact of browsing could be larger in planted stands, although we have no data to support this assumption. Moreover, practically all large herbivores were either regionally extinct or present at very low density in the nineteenth century due to intensive hunting (Breitenmoser 1998). Thus, wildlife damage is certainly very marginal as an explanation of afforestation failure.
Planting practices. It is well documented, that the afforestation at high altitudes, and especially at timberline, is an extremely difficult endeavor (Schönenberger 2001; Senn and Schönenberger 2001). In the treeline ecotone, the environmental conditions are very harsh (Körner 2012). Kronfuss and Havranek (1999) for example, studying artificial afforestation of P. cembra in Tyrol (Austria), demonstrated that the height growth of young artificially planted trees decreased with altitude by about 5% per 100 m. The ecological conditions encountered near the tree line should be given the greatest consideration for successful afforestation (Mullenbach 1982, 2000). However, the context of the afforestation activities in the canton of Fribourg is different. Plantations were carried out in our study area mainly between 1500 and 1900 m a.s.l. (probably even lower). It means that the main plantations were not close to the timberline but in much lower altitudes, and thus in better conditions for afforestation.
However, even at these lower altitudes, laborious methods of cultivation and planting are generally advised, e.g. to use potted rather than bare-rooted plants. Cluster arrangement must be preferred to a regular planting pattern, since it allows consideration of micro-site conditions, the establishment of a good surface structure, and the preclusion of uniform stands. The growth conditions in mountainous regions impose appropriate methods of tending for young growth and thickets (Schönenberger et al. 1990, 1995). It is advised that for plantation near the tree limit, the planting material should lie within a 100 m altitude range relative to the planting site. It is also suggested to breed the plants in montane altitudes and to transplant them into a nursery located above 1500 m to ensure acclimatization (Ulber et al. 2004).
The planting practices used for P. cembra afforestation in the canton of Fribourg are largely unknown. Considering the period of plantations and some historical pictures, seedlings used were almost certainly bare-root. Pinus cembra often has difficulties with rooting in this case (Felix Gugerli, personal communication). In high-altitude forestry, potted plants are preferred to bare rooted plants in order to reduce the stress to which trees are exposed when transferred from the nursery to a high altitude afforestation site (Senn 1999). Moreover, P. cembra trees are usually strongly mycorrhized. Three basidiomycete species of the genus Suillus form a strictly host-specific symbiosis with it, as well as a number of other generalist ectomycorrhizae (Rainer et al. 2015). It has been demonstrated that ectomycorrhizal fungi are favorable or even necessary for its establishment (Mullenbach 2000). Failure of afforestation in the Austrian Alps have been reported and linked to the absence of ectomycorrhizal fungi (Moser 1967). Inoculation of the mycelium in the nursery and cultivation of the plants in large buckets with inoculated substrate seems to be a factor of success for planting trees on unforested areas (Mullenbach 2000).
Inappropriate methods for plantings could be one of the main problem for historical afforestation. However, plantations occurred during many years. If high mortality occurred right after plantation, it would have probably been detected and avoided. Forestry was already a well-developed discipline at this time. Secondly, plantation occurred in the canton of Fribourg mainly between 1500 and 1900 m a.s.l., probably even lower, but no trees are remaining. It means that the main plantations are not close to the tree line, where the condition are less difficult for afforestation.
Seed production and dispersal. According to our surveys, trees in planted stands produce cones with seeds. Seed cones appear in P. cembra after 40–60 years (Caudullo and De Rigo 2016); thus, all artificial plantations in our study area are potentially able to reproduce via seeds for more than half a century. A sound cone yield an average of about 40 seeds, but it was estimated that a high proportion (> 80%) are aborted or attacked by pathogens (Dormont et al. 1996). Large wingless seeds of P. cembra are dispersed mainly by the corvid N. caryocatactes, which caches them as food reserves over winter but does not retrieve all seeds (Fourchy 1968). This bird is present more or less everywhere in the Prealps where P. cembra occurs, notably also where planted stands are located. Thus, neither seed production nor dispersal seem to be limiting factors that could prevent planted stands from regenerating. However, further study should be conducted to determine whether the seeds were viable, e.g., due to possible inbreeding effects (Salzer and Gugerli 2012).
Ecotypes. We did not find any information about the origin of the seeds used in the afforestation with P. cembra. We only know that plant nurseries were mainly regional. According to the selected ecotype and the region of origin, it may be possible that certain plants were not well adapted to the local conditions, which additionally decreased the chances of success. It was demonstrated experimentally for P. cembra that significant differences (e.g. height growth, diameter at root-collar and number of buds) can be observed between provenances (Blada 1997). Preliminary results of our parallel genetic investigation, performed with plant material collected in both natural populations and planted stands of the canton of Fribourg, indicate significant genetic differentiation between natural and planted stands (Sonnenwyl 2021). Thus, the seeds used for the production of seedlings were probably not collected from the natural populations in the canton of Fribourg. However, the use of seeds of very remote origin for local afforestation was not exceptional in the nineteenth century (Lendvay et al. 2014; Raffl et al. 2018). Because of the dangerous topography of natural populations in the canton of Fribourg, seed collection was likely easier in better accessible regions.