Growth sites of polypores from quantitative expert evaluation: Late-stage decayers and saprotrophs fruit closer to ground
Introduction
The functional characteristics, or traits, of organisms are central to the big questions in ecology, such as why there is such a variety of species, why they occur where they do, and how they give rise to ecosystem-level properties and processes. While this is by no means a new insight, conceptual advances have only relatively recently provided a common framework for the ecological approaches that explicitly take traits into account (McGill et al., 2006, Webb et al., 2010). The emergence of the field of trait-based ecology has been accompanied by increasing efforts to systematically gather trait information for a large number of species, most famously exemplified by the TRY database of global plant traits (Kattge et al., 2011). The first comprehensive analyses of the TRY data are now reshaping our understanding of plant diversity (Díaz et al., 2016). Initiatives of similar scale are so far absent for the Fungal Kingdom, but the momentum is growing: for instance, Aguilar-Trigueros et al. (2015) recently reviewed and called for trait-based approaches in fungal ecology.
The obvious hurdle obstructing the way towards a more trait-based fungal ecology is the scarcity of trait data – understandably, given the remaining gaps in the knowledge of basic ecology and taxonomy of fungal species that are prerequisites to the identification and robust definition of relevant traits. While carefully measured trait data is unquestionably pivotal, its accumulation is slow especially given the urgent nature of many of the applied questions we are facing, such as predicting and preventing the further decline of threatened species. We argue that in relatively well-known groups such as the polyporoid wood-inhabiting fungi, the active body of professional and non-professional experts presents a large reservoir of qualitative but also semi-quantitative information on species traits that could be tapped to complement traditional scientific measurements.
Dispersal is one of the key life history processes that shape population dynamics and hence is at least indirectly relevant for most, if not all, ecological phenomena. Dispersal-related traits are thus highly valuable for any trait-based approaches. Following the movement ecology framework of Nathan (2008), dispersal-related traits include those affecting movement capacity and those affecting navigation capacity. In passive airborne dispersers such as many fungi, the main dispersal-related traits are the characteristics of the diaspores that determine their aerodynamic behaviour and longevity in the atmosphere, the reproductive organs that determine the manner in which they are released, as well as the site and timing of diaspore release (Gregory, 1973, Soons et al., 2004, Damschen et al., 2008, Nathan et al., 2011, Norros, 2013). These traits are involved in both making movement possible (movement capacity) and determining where the diaspores are likely to land (navigation capacity).
In airborne dispersal, the site of diaspore release is important primarily due to the different exposure of different sites to turbulent airflows. In general, both average wind speed and turbulence increase with height from the ground (Stull, 1988, Seinfeld and Pandis, 2006). This vertical turbulence gradient is prominent also within plant canopies such as forests (Massman and Weil, 1999). Thus, fruit body height from the ground is the most obvious indicator of the dispersal potential provided by different sites of diaspore release. Indeed, greater release height has been shown to enable longer dispersal distances in plants (Soons et al., 2004, Gomez, 2007) and has become a standard parameter included in mechanistic models of airborne dispersal (Nathan et al., 2011). For forest fungi, mechanistic model simulations predict that spores do reach longer dispersal distances when released higher in the canopy, even though their dispersal is less sensitive to release height than that of larger propagules such as plant seeds (Norros et al., 2014).
Dispersal-related traits are particularly relevant when trying to understand and predict the responses of different species to habitat loss and fragmentation. Habitat fragmentation affects the balance of the costs and benefits of dispersal (Comins et al., 1980, Gilpin and Soulé, 1986, Bonte et al., 2012), and the optimal dispersal strategy in a fragmented landscape depends on the specific conditions (Travis et al., 2010, North et al., 2011, Poethke et al., 2011). Thus, it is plausible to hypothesize that the populations of different dispersers respond differently to fragmentation over ecological timescales, although it is not easy to predict whether high dispersal capacity should make species more or less vulnerable to fragmentation. There is some empirical evidence for a link – either positive or negative – between dispersal capacity and vulnerability to fragmentation from plants and animals (e.g. Henle et al., 2004, Pereira et al., 2004, Ockinger et al., 2010), but the question has very rarely been addressed in fungi.
By occupying a spatially limited and ephemeral, ever changing substratum, wood-inhabiting fungi are confronted with a need to be efficient colonisers, reproducers and recolonisers (Jönsson et al., 2008, Lonsdale et al., 2008). The need for efficient colonisation is even more evident taking into account that wood in different decay stages host clearly different fungal assemblages (Heilmann-Clausen, 2001), and thus the window of opportunity for an individual species is likely to be short. Moreover, in most human-dominated areas of the world, forest habitats have been dramatically altered by forestry and fragmented by other land use practices (Foley et al., 2005, Woodcock et al., 2015). Wood-inhabiting fungi have shown an inevitable response to the changes: for example in Finland and Sweden almost half of all polypore species are currently red-listed (Gärdenfors, 2010, Rassi et al., 2010) and, for a number of species, there is evidence for a negative effect of fragmentation beyond the linear effect of decreasing resources (Nordén et al., 2013). On the other hand, some species have not been affected and others even seem to benefit from fragmentation (Nordén et al., 2013). Attempts to relate the different fragmentation responses of wood-inhabiting fungi to species traits have so far revealed that fragmentation vulnerability is higher in specialist than generalist species, in species that decay primarily spruce, in species in which fruit bodies appear at intermediate to late stages of decay, in species preferring large, naturally fallen trunks, in annual species, in rare species, and in species with small spores (Berglund and Jonsson, 2008, Berglund et al., 2011, Stokland and Larsson, 2011, Nordén et al., 2013). Although there is some evidence suggesting that rare wood-inhabiting fungi are limited by dispersal (Norros et al., 2012), a direct link between dispersal capacity and vulnerability to habitat fragmentation has so far been elusive (Norros, 2013).
Here, we present and analyse expert-based data on the sites of spore release in polyporoid wood-inhabiting fungi. Using a quantitatively formulated query form, we collected evaluations from Finnish experts of the typical heights at which 140 polypore species produce their fruit bodies in Finland. Based on the expert evaluations, we assessed (1) how consistent and species-specific the growth heights of different polypore species are. Further, we studied (2) how the preference for different growth heights is related to species-specific spore morphology, fruit body longevity, substratum preferences, commonness in Finland and vulnerability to habitat fragmentation.
Section snippets
Fruiting height estimates
To obtain fruit body height estimates for polypore species we utilised the extensive knowledge of Finnish polypore specialists. Finland is globally probably the leading country considering ecological research activity focusing on polypores (Junninen and Komonen, 2011), using polypores as conservation tools (Halme et al., 2009) and hence also the number of experienced field mycologists specialised on polypores. We prepared a questionnaire and gave these experts an opportunity to answer to it by
Results
Our analyses of variance demonstrated that there were significant differences in the fruiting height estimates for different species that were consistent across the different experts. The species effect was significant for both versions of the analysis (Analysis 1: 10 experts, 10 species; Analysis 2: 6 experts, 87 species, Table 1), and species explained about two thirds of the variation in mean fruiting height (68% for Analysis 1, 70% for Analysis 2). The significance of an expert effect could
Discussion
Our results clearly showed that we were able to obtain consistent estimates of fruit body fruiting height from quantitative expert evaluations. The three species with adequate data but inconsistent estimates are all examples of species which, according to our experience, tend to grow on different substrata in different areas or differently managed sites. For example, A. macra seems to fruit on partially erect dead Salix branches in certain biotopes, but also on the lower side of downed Aspen
Conclusions
Fungi are a megadiverse group and laborious to study in field conditions, for example due to poor taxonomic knowledge, low detectability and limited availability of skilled field personnel (Dahlberg et al., 2010, Halme et al., 2012, Heilmann-Clausen et al., 2015). Our study demonstrates that experience-based expert knowledge is one source of valuable information that can be tapped to achieve a better understanding of certain fungal traits and their interrelationships. At least some of the
Acknowledgments
We are indebted to all of the experts who provided the fruiting site evaluations that made this work possible, and to Noora Kaunisto for drawing Fig. 1. We would also like to thank Claus Bässler and three anonymous reviewers for insightful comments which greatly improved the article. This study was funded by LUOVA graduate school and Ella and Georg Ehrnrooth Foundation through grants to VN and Maj and Tor Nessling foundation through a grant to PH.
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