Nyttan av naturhänsyn för marksvampar: resultat fem år efter avverkning

Everyday consideration (i.e. retention forestry) is an important tool in forestry that can help many species survive the felling phase. This applies not least to the species-rich group of ectomycorrhizal fungi with around 2000 species in Sweden, which for their energy supply depend on symbiosis with living trees. Felling therefore inevitably results in that the majority of mycorrhizal fungi become extinct locally and have to re-establish in the planted forest. Just under 200 of Sweden's mycorrhizal fungi are red-listed because they mainly occur in older forests or are unusual and with special environmental requirements. They have difficulties with re-establishment and are disadvantaged by clear-cutting. At the same time, the mycelia of mycorrhizal fungi are long-lived and can persist for many decades and probably much longer, provided a continuity of trees on the site. Individual, or groups of, living retention trees are a way to enable red-listed and other mycorrhizal fungi to survive the felling phase and be included in the new community. Recent research also indicates that soil eutrophication following the felling phase selects for other mycorrhizal fungi than those that normally dominate in older forests. For example, some corticoid fungi, with poor ability to mobilize soil nutrients from organic material, seem to benefit at the expense of mainly web caps (Cortinarius), which are particularly effective at releasing organically bound nutrients under poor conditions.The purpose of the study was to investigate the importance of leaving different number of retention trees when felling in older pine forests, to promote survival of mycorrhizal fungi, with or without supplementary prescribed burning. This is a nature conservation issue of great practical importance, and it is important to quantify what effect different levels and performance of retention can have on the quantity and species composition of ectomycorrhizal fungi in general both in the short and long term, especially for species of special interest in nature conservation.The study was conducted within a site called Effaråsen, a large-scale long-term experiment located 30 km west of Mora. The site includes entire forest stands that have been burned and / or felled, where the total level of retention varies with a gradient of management methods that extend from a generally practiced level to a high level of environmental considerations. The retention design includes both green tree retention and the creation of dead wood. In this study only the living trees were of interest.In the survey, we collected soil samples along transects from each sample area. We sampled in 2012, before felling with different levels of retention and prescribed burning was implemented. Five years later, in 2017, we conducted a follow-up survey. Fungal DNA-markers in soil samples were mass-sequenced, identified, and analyzed with a focus on mycorrhizal fungi. We examined the species composition, the proportion of mycorrhizal fungi in the total fungal community and the number of species in relation to the distance to the nearest tree. We also included a supplementary study of occurrences of fungal fruiting bodies, which was carried out in the autumn of 2021. This was done to better investigate the hypothesis that mycelium of red-listed mycorrhizal fungi and other species of nature conservation interest may survive in the vicinity of retention trees, just like more common species of mycorrhizal fungi.In the soil sample survey, we identified 141 different species of mycorrhizal fungi, of which four are red-listed. There was a clear correlation between the occurrence of mycorrhiza and distance to retention trees. In parallel with that the average distance to retention trees increased from an average of 3 meters on the control surfaces to 14 meters at the lowest retention level, the proportion of mycorrhizal fungi gradually decreased from 11 percent to 2 - 4 percent in relation to the total fungal community in the soil and the number of mycorrhizal fungal species halved. At the same time, the species composition changed; occurrences of web caps and and the velvet bolete (Suillus variegatus) gradually decreased with decreasing level of retention, while a special corticoid fungus (Piloderma sphaerosporum), which is already known to dominate fungal communities in young forests that have undergone a felling phase, was less affected.The survival and occurrence of mycorrhizal fungi after felling is directly linked to the amountof roots in the soil and the spatial distribution of living root systems. More green tree retention resulted in higher survival. Prescribed burning in combination with felling gave the biggest change, i.e. had the greatest impact on the occurrence, species richness and species composition of the mycorrhizal fungi. The main reason was high mortality of trees due to the relatively intense burning. The inventory of fruiting bodies was able to detect the presence of five additional red-listed fungi and one other species of nature conservation interest at retention trees and along the edges adjacent to other tree-bearing stands or riparian forest buffer zones.The mycorrhizal community in the felling with the highest level of retention (the NS treatment), which corresponded to 50 percent left retention trees (including girdled trees), did not differ clearly from the undisturbed control forest areas. Here, the presence of the predominant mycorrhizal fungi was more or less intact. It is these dominant species that are responsible for most of the functionality of mycorrhizal fungi and drive essential processes in the soil. With lower density of retention trees, the occurrence of most species gradually decreased, with the clearest negative impact on velvet bolete and web caps, which are believed to play an important role in tree uptake of nutrients from organic matter in the mor layer, and less clear effect on corticoid fungi, which are believed to have lower ability to mobilize organically bound nutrients.It is unclear what significance this shift in dominant species may have over time for the functioning of mycorrhizal fungi. It is also unclear to what extent re-establishment of different species takes place in managed forest and whether the species composition has time to approach the composition that characterizes older forests during a rotation period. Previous studies have shown that major changes in fungal diversity persist at least 50 years after felling of pine forests. Higher levels of retention trees are a way to reduce the effects of felling on the diversity of mycorrhizal fungi and probably also speed up the return to a more species-rich community with preserved functionality.Probably the vast majority of red-listed and other mycorrhizal fungi of nature conservation interest can survive a felling with the help of retention trees. The challenge is to select retention trees where the interesting and care demanding fungal species grow. Their occurrences are usually not known when a forest is felled and if they are found in the stand, their distribution is very local, usually only with a few square meters of mycelium.We assess that the benefit will be greater, for both care demanding species and the functionally more significant and more common species, if retention trees are spread over the felling area rather than being aggregated in groups. In this way, the probability that root systems of the retention trees overlap with mycelia of more unusual species should increase, while functionally important species, such as web caps, can more effectively survive and spread in the growing forest stand.More retention trees unequivocally result in that a larger part of the mycorrhizal fungi surviving felling and persist in the new growing stand. The continuation of the study aims to quantify the effects of different levels of consideration as a basis for more well informed decisions about trade-offs between production interests and different environmental considerations.