Effects of forest restoration by fire on polypores depend strongly on time since disturbance – A case study from Finland based on a 23-year monitoring period
Introduction
Fire is one of the most important disturbance factors in boreal forests under natural conditions. Wildfires create landscape mosaics consisting of stands at different successional stages, affect stand structure and dynamics, and produce large amounts of dead wood by causing tree mortality (Johnson et al., 1998, Jonsson and Siitonen, 2012, Niklasson and Granström, 2000, Shorohova et al., 2009). A large group of organisms in boreal forests, mainly consisting of invertebrates and fungi, are dependent on or favored by fire (Dahlberg, 2002, Saint-Germain et al., 2004, Wikars, 1992). These so-called pyrophilous species occur mainly in recently burnt sites and at site types where natural fire frequency is the highest such as dry pine forests. Studies on the effects of fire on post-fire fungal succession have concentrated on ground-dwelling fungi (Horikoshi et al., 1986, Moser, 1949, Petersen, 1970, Vásquez Gassibe et al., 2011, Zak and Wicklow, 1980). According to these studies, there are several strictly fire-dependent species, especially in Ascomycetes, which usually emerge very soon after fire. Most fire-dependent macrofungi seem to be soil and litter saprophytes. For example, in Sweden a total of 40 macrofungal species have been reported as fire-dependent, and of these 32 are soil and litter saprophytes (Dahlberg, 2002).
During the last century, modern forestry and other land-use forms coupled with efficient fire suppression have replaced fire as the main disturbance factor in large regions in the boreal and temperate zones. This change from natural to human-caused disturbance dynamics has greatly affected forest composition and structure and, consequently, also biodiversity. In some regions, numerous organisms dependent on fire have declined and become threatened because of lack of fires (Kouki et al., 2012, Wikars, 2001). For instance, reduction of burnt forest areas (including other young stages of natural succession) has been identified as the main cause of threat to eight regionally extinct and 68 threatened species in Finland (Rassi et al., 2010). In other regions, such as in several types of coniferous and mixed forests in North America, the fire regime has shifted to the other direction, so that fires are less frequent but larger and more intensive than in the past (Allen et al., 2002, Fulé et al., 2012). The altered fire regime may have severe negative effects on biodiversity (Driscoll et al., 2010). A general, widely applicable objective of fire management is to avoid population extinctions within a defined management area due to the effect of an adverse fire regime. Prescribed burning can be used to reduce fire loads and risk of uncontrolled wildfires, but also to restore habitats for fire-dependent and dead-wood inhabiting species.
Polypores are principle decomposers of dead wood in boreal and temperate forests (Boddy, 2001). Besides providing the basic ecosystem services, wood decomposition and nutrient cycling, they provide habitats for many other saproxylic forest species (Siitonen, 2012, Stokland and Siitonen, 2012). Many polypore species have declined owing to intensive forest management which has caused loss of old-growth forests and reduced the amounts of decaying wood (Lonsdale et al., 2008, Junninen and Komonen, 2011a).
Several recent studies have explored the effects of wildfire or prescribed burning on wood-inhabiting species, including saproxylic beetles (Hyvärinen et al., 2009, Saint-Germain et al., 2004, Toivanen and Kotiaho, 2007a) and wood-decaying fungi (Junninen et al., 2008, Olsson and Jonsson, 2010, Penttilä and Kotiranta, 1996). However, all these studies have only followed the short-term effects (1–5 years) of fire on species assemblages. According to these studies, the short-term effects on saproxylic beetles are generally positive, i.e. burnt sites have higher species richness and higher numbers of rare and red-listed species, whereas the short-term effects on wood-decomposing fungi appear to be negative. The only studies in which the more long-term (29 and 16 years, respectively) changes in saproxylic beetle assemblages were investigated are not experimental follow-up studies but retrospective chronosequence studies (Boulanger and Sirois, 2007, Toivanen and Kotiaho, 2007b). Knowledge on the long-term effects of fire on wood-decaying fungi seems to be almost totally lacking. A recent study by Kurth et al. (2013) explored long-term changes in communities of wood-inhabiting fungi in a 32-year chronosequence study based on wood samples and molecular identification of fungi. However, there were very few polypores and other species belonging to Basidiomycota in the species assemblages they found.
The aim of this study was to describe both the short-term effects of fire on wood-decaying fungi, and the long-term (>20 years) changes in species composition, and particularly in the occurrence of red-listed species. In addition, the aim was to explore how the amount and quality of dead wood affect the post-fire fungal succession.
Section snippets
Study area and study sites
The study area is situated in Patvinsuo National Park in eastern Finland close to the Russian border. The area lies in the middle boreal zone, just next to the border between southern and middle boreal zones. Climate in the area is slightly continental. The extent of the national park is about 100 km2 and it is characterized by large peatlands and pine-dominated forests. Before the park was established in 1982, part of the area had been managed for forestry purposes. Consequently, at the moment
Dead wood
In 2002, thirteen years after the burning, the volume of dead wood in the seminatural stand was 125 m3/ha, of which 71 m3/ha was pine, 39 m3/ha spruce and 15 m3/ha birch. In 2011, twenty-two years after the burning, the volume had declined to 95 m3/ha, of which 64 m3/ha was pine, 25 m3/ha spruce and 5 m3/ha birch. In 2002, the volume of dead wood in the managed stand was 32 m3/ha, of which 25 m3/ha was pine, 5.5 m3/ha birch and 1.5 m3/ha grey alder. In 2011, the volume had declined to 28 m3/ha, of which 23 m3
Discussion
According to our results, the short-term effects (<5 years) of fire on polypore communities were negative. This result is similar to previous studies in which short-term effects of fire on wood-decomposing fungi have been investigated (Junninen et al., 2008, Kurth et al., 2013, Olsson and Jonsson, 2010, Penttilä and Kotiranta, 1996). Intensive fire destroys the mycelia and decreases the inoculum potential of many fungi by consuming dead wood and by creating extreme environmental conditions (Pugh
Acknowledgements
We thank the organizations that funded our study: Metsähallitus, Ministry of the Environment, Finnish Environment Institute (project: Restoration of forests for species recovery), Maj and Tor Nessling Foundation and Finnish Cultural Foundation. We thank Kaija Eisto, Raimo Heikkilä, Heikki Kotiranta, Tuomo Niemelä, Jorma Pennanen, Seppo Piirainen and Raimo Virkkala for their valuable help and co-operation during this project and Juha Heikkinen for advice and help in the statistical treatment of
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