Crustose lichens associated with dead wood in northwestern North America: a review Review Draft, 25 March 2005 Toby Spribille1, Curtis Björk2 and Trevor Goward2 Abstract. Substrate affinities were reviewed for 506 species of epiphytic crustose lichens in northwestern North America based on literature and recent field research. A total of 241 species have been recorded as growing on wood, of which 50, or 10% of all epiphytic crustose lichens, have been found only on this substrate. The list of apparently obligate lignicoles includes both well-known species frequently encountered on wood as well as several that have been only recently reported or are as yet undescribed. More work is needed to determine the occurrence, richness and ecological amplitudes of lignicolous species in the various forest ecosystems of northwest North America. Introduction The role of dead wood in forested ecosystems has been the topic of increasing discussion and research in recent years. This interest in dead wood has been propelled by several factors: 1) the observation that levels of dead wood in many forested ecosystems have fallen precipitously from what they were historically (Linder & Ostlund 1998); 2) recognition of the importance of dead wood for various plants and animals; and 3) forestry-driven (and in part, regulation-mandated) research to identify minimum levels of dead wood for the maintenance of nutrient cycling function and viability of dead-wood-dependent species. In natural forests, dead wood occurs in a large variety of forms. Perhaps in its most unobvious manifestation, dead, decorticate lower branches of old living trees may provide the first habitat for lignicoles in a developing forest. Tree death and eventual loss of branches and bark creates a snag, with a more appreciable surface area for lignicole colonization. The fall of a snag may be sudden, creating a prostrate log on the forest floor, or the falling snag may become lodged in other trees, slowing its fall to the ground to a process of years. Logs in turn exhibit many characteristic stages of decay and weathering as the dead wood comes into contact with the ground and is buried under snow in winter. The relative amount and decomposition behaviour of dead wood is furthermore ecosystem- and climate-specific, with higher rates of rot in moist than in dry ecosystems (Brown et al. 1996). The life histories of many species are closely linked to the various types of dead wood. In British Columbia, for instance, approximately one-third of all forest-dwelling bird and mammal species use snags as sites for foraging, roosting or nesting (Bunnell & Kremsater 1990). Both coarse and fine woody debris have also been demonstrated to be important for entire communities of less conspicuous organisms, such as saproxylic insects (Bouget & Duelli 2004) and wood-inhabiting fungi (Kruys & Jonsson 1999, Norden et al. 2004) 1 Department of Vegetation Analysis, University of Göttingen, 37073 Göttingen, Germany; [email protected] 2 Edgewood Blue, Box 131, Clearwater, BC V0E 1N0, Canada 1 The importance of dead wood for lichens has long been appreciated, particularly in Europe (e.g., Chlebicki et al. 1996, Lõhmus & Lõhmus 2001, Humphrey et al. 2002). Even in Europe, however, no systematic surveys have been made of the contribution of lignicolous lichens to overall biodiversity and it is difficult to estimate what percentage of the overall flora they constitute. In northwest North America, there have been no published surveys focused specifically on lignicolous lichens, even at a local scale. Nevertheless, scores of floristic and taxonomic surveys from different parts of northwest North America over the past 25 years have provided a rich, until now untapped source of anecdotal evidence on substrate affinities for hundreds of lichen species. Here, we present the first systematic, literature-based survey of substrate affinities of a subset of the overall lichen flora of northwestern North America, namely the bark- and wood-dwelling crustose lichens. Materials and Methods We used a pre-existing database of 569 epiphytic crustose lichens of northwestern North America, defined here as including British Columbia, Washington, Oregon, California (north of 37 degrees), Idaho and Montana; individual lichen records from southeastern Alaska were also used in some instances. Our decision to use crustose lichens was based mainly on two practical considerations: 1) crustose lichens have been repeatedly identified as one of the main lichen groups displaying particular affinities for dead wood, both in northern Europe and in preliminary studies in British Columbia (Goward et al., in prep., Spribille, unpubl.); and 2) we were able to access a literature database developed for work on an upcoming crustose lichen flora of the same study area (Spribille & Goward, in prep.). Our first step was to filter out taxonomic problems, synonyms and questionable records from our database. This provided us with a list of ‘accepted’ names (Appendix 1). Next, we checked every known recent literature reference for each species in the database for information on substrate. Substrates were recorded as follows: wood, bark, moss/plant detritus, leaves, or resin. In addition to available literature, we also took into consideration recent unpublished studies (Goward et al., in prep., Spribille, unpubl.) as well as verified herbarium specimens in the collections of the authors. A discriminating approach was used to ensure data quality: most of the literature used was published after 1980, and special emphasis was given to recent systematic and floristic treatments. A list of references used for the review is in Appendix 2. Finally, substrate affinities were summarized in a master table and analyzed with an emphasis on wood-dwelling crustose lichen species. For analysis, crustose lichens were divided into two categories: non-calicioid crusts, encompassing four fifths of the lichen flora and customarily referred to simply as crustose lichens; and calicioid crusts (sensu Goward 1999), i.e. those bearing stalked apothecia, including numerous non-lichenized species of the order Mycocaliciales traditionally studied by lichenologists (Tibell & Wedin 2000). Results Of the 569 species included in the initial database, substrate data could be found for 506 species (Table 1; Appendix 2). The remaining 63 species fell into four categories: records available but substrate not indicated (2 species), records based on unverified, unpublished 2 specimens not seen by us (12 species), species previously assumed to occur in the flora but for which no supporting evidence could be found (14 species) and species for which literature was not available at the time of this writing (35 species). Of the 506 species for which data could be found, 241 have been recorded as occurring on wood and 434 on bark (Table 1). The ratios of species on bark vs. wood are quite different for non-calicioid and calicioid species groups: a similar number of calicioids occurs on wood (63 species) and bark (73 species), whereas among non-calicioid crusts, 363 species are found on bark, more than twice as many as the number found on wood (178 species; see Table 1). Of particular interest are the species that occur only on a single substrate type. When all crustose lichens are taken together, fifty species, or 10% of all species for which we had data, were recorded only on wood (obligate lignicoles; Tables 2 and 3). A large number of species are found only on bark, with 206 non-calicioid crustose (51%) and 34 calicioid species (32%) apparently bound to certain bark substrates and not known to go over onto wood, moss or other substrates. A smaller relative percentage of non-calicioid crusts are found only on wood (28 species, or 7%) compared to calicioids (22 species, or 21%). Discussion The available data suggest that a significant proportion of crustose lichens occur on dead wood and a large portion of these are strictly limited to this substrate. In addition, of the numerous species that occur both on wood and bark, many occur primarily on wood and only rarely on bark and vice versa. More detailed constancy or frequency data would be useful to highlight this pattern in more detail, but such detail will remain unavailable without more extensive and systematic exploration of the epiphytic crust lichen flora of the region. While it is true that the incompleteness of substrate data for some species may erroneously give the impression that some species are strict lignicoles or corticoles, most of the species included on the list have been collected multiple times and reported in more than one study. The increased level of collecting since the 1980s has permitted the sketching of ecological profiles for many species. Examples for such species include Lecanora orae- frigidae and Lecanora xylophila, two crust species found on driftwood, logs and fence rails in coastal areas (Brodo & Vänskä 1984). Calicioid lichens are another example of a group about which our knowledge has continuously grown in recent years. Many of the obligately lignicolous calicioids have been surveyed systematically in independent ecological studies in both Oregon by Rikkinen (2003a) and British Columbia (Goward et al., in prep.); ecological profiles for many species are thus becoming apparent. Furthermore, a review of literature from Europe, where many of the same lignicolous species also occur, reveals that, with few exceptions, species reported to be dead-wood obligates in the present review are also found primarily on this substrate across the Atlantic. It is in fact quite likely that the numbers of obligate lignicoles will rise. It is worthy of note that calicioids, which are proportionately much better represented on wood than non- calicioid crusts, have also been surveyed in far more detail in recent years than non-calicioid crusts. In fact, nearly all of the obligately lignicolous calicioids were discovered since 1990. Moreover, of the obligate lignicoles, three of the non-calicioid and five of the calicioid species are new to science, and thus the products of the most recent taxonomic research.