Ecography 31: 741750, 2008
doi: 10.1111/j.1600-0587.2008.05503.x
# 2008 The Authors. Journal compilation # 2008 Ecography
Subject Editor: Francisco Pugnaire. Accepted 18 June 2008
Lichens on dead wood: species-substrate relationships in the
epiphytic lichen floras of the Pacific Northwest and Fennoscandia
Toby Spribille, Göran Thor, Fred L. Bunnell, Trevor Goward and Curtis R. Björk
T. Spribille (tspribi@uni-goettingen.de), Dept of Vegetation Analysis, Albrecht von Haller Inst. of Plant Sciences, Univ. of Go¨ttingen, Untere
Karspu¨le 2, DE37073 Göttingen, Germany. G. Thor, Dept of Ecology, Swedish Univ. of Agricultural Sciences, P.O. Box 7044, SE750 07
Uppsala, Sweden. F. L. Bunnell, Dept of Forest Sciences Dept, Univ. of British Columbia, 3041-2424 Main Mall, Vancouver, BC, V6T
1Z4, Canada. T. Goward, Edgewood Blue, Box 131, Clearwater, BC, V0E 1N0, Canada (mailing address of T. G.: Herbarium, Dept of
Botany, Univ. of British Columbia, Vancouver, BC, V6T 1Z4, Canada). C. R. Björk, Stillinger Herbarium, Univ. of Idaho, Moscow, ID
83843, USA.
Dead wood is an important habitat feature for lichens in forest ecosystems, but little is known about how many and
which lichens are dependent on dead wood. We reviewed substrate use by epiphytic lichens in the combined floras of
Fennoscandia and the Pacific Northwest of North America based on literature and herbarium data and analyzed substrate
affinity relative to life form, reproductive mode and major phylogenetic group within the floras. A total of 550 (43%) of
the 1271 epiphytic species in the combined floras use wood, and 132 species (10%) are obligately associated with dead
wood in one or both regions. Obligate and facultative wood-dwelling guilds in the two floras were strongly similar in
terms of internal guild structure in each region, but differ somewhat in species composition, while the bark-dwelling guild
differs strongly in both. Most obligate dead wood users are sexually reproducing crustose lichens. The largest numbers of
species are associated with forest structural features such as logs and snags that have been greatly reduced by forest
practices. Conservation of lichens inhabiting wood requires greater attention to crustose lichen species and the
development of conservation strategies that look beyond numbers and volumes of dead wood and consider biologically
meaningful dead wood structure types.
Dead wood is a primary habitat for many organisms,
including invertebrates (Berg et al. 1994, Grove 2002),
macrofungi (Bader et al. 1995, Ódor et al. 2006),
myxomycetes (Ing 1994), bryophytes (Söderström 1988,
Anderson and Hytteborn 1991) and lichens (Lõhmus and
Lõhmus 2001). Forest practices have significantly reduced
amounts of dead wood over large areas of nemoral and
boreal forests (Angelstam 1997, Linder and Östlund 1998,
Wilhere 2003). Many researchers have found a link between
reduction in dead wood and declines in wood-dependent
species, particularly those dependent on wood of certain size
or decay classes (Warren and Key 1991, Amaranthus et al.
1994, Siitonen 2001). More recently, there has been
increased investment in developing forestry methods that
take habitat structure of these organisms into consideration
(Rosenvald and Lõhmus 2008), especially for snag-dependent birds (Walter and Maguire 2005, Hutto 2006) and
invertebrates (Hyvärinen et al. 2006, Tero and Kotiaho
2007, Davies et al. 2008). There have been few such studies
for lichens, concentrating mainly on logs (Bunnell et al.
2008) and stumps (Caruso et al. 2008). Concerns about
impacts of forest practices on lichens have instead focused
more on the value of old-growth forests (Rose 1992, Tibell
1992, Goward 1994). As the pressure increases to extract
dead wood for salvage or as a source of energy, all organisms
relying on it will be impacted. The number of lichen species
potentially affected is unknown.
Lignicolous (wood-dwelling or saproxylic) lichens are
often overlooked in reviews of dead wood as habitat (e.g.
Harmon et al. 1986). Dependence of some lichen species
on dead wood is well established (Darbishire 1914, Räsänen
1927), but the number of species and types of specific
habitat affinities are unknown. Data on lignicolous lichens
must be gleaned from anecdotal accounts scattered throughout the taxonomic and floristic literature, or from broader,
forest stand-level ecological studies. In most, lignicolous
lichens are treated among other epiphytic species. Most
ecological research specific to lignicolous lichens and their
substrate has been limited to regional case studies, almost all
from northern Europe. These studies have documented the
richness of lichens on dead wood in forest environments
(Forsslund and Koffman 1998, Kruys et al. 1999, Jansová
and Soldán 2006, Nascimbene et al. 2008) or on secondary,
anthropogenic wood substrates such as barns (Svensson
et al. 2005) as well as differences between various types of
dead wood (Lõhmus and Lõhmus 2001). To date, however,
741
a comprehensive review of dead wood-dependent lichens
has not been published for any larger geographic area.
Our intent is to provide an overview of lignicolous
lichens as an aid in circumscribing them as a group for
conservation, research and management. Using two well
studied northern epiphytic lichen floras, that of the Pacific
Northwest of North America (hereafter PNW) and Fennoscandia (FS), we a) assess how many species are
facultatively and obligately dependent on wood; b) characterize the biology of these species based on life form,
reproductive strategy and phylogenetic group; and c)
catalog what is known about different dead wood microsites
they use. Finally we examine the relevance of our findings in
light of current conservation and management approaches.
Materials and methods
Study areas and species included
Our decision to base analysis of species on the PNW and
Fennoscandia was based on two considerations: 1) the
knowledge of the lichen flora of both regions is relatively
good in terms of both species and substrates used; and 2)
both regions are large enough to provide a comprehensive
overview. The PNW as defined here includes British
Columbia, Washington, Oregon, California north of
378N, Idaho and western Montana, an area of ca 1.89
million km2. Fennoscandia has a total area of ca 1.17
million km2 and is defined here as including Norway,
Sweden and Finland. Both regions occupy large areas on the
eastern side of large oceans and are mountainous, but also
have broad differences. The PNW includes large areas of
nemoral vegetation and Mediterranean climate, with a
humidity gradient from arid to rain forest. Fennoscandia
is arctic-boreal with only minor areas of nemoral vegetation
that has been heavily impacted by humans and no arid land.
Nearly all of Fennoscandia was glaciated in the last glacial
period, whereas areas of the PNW south of about 478N
were not. Partly because of their different glacial histories,
the number of tree species is much higher in the PNW. We
included all lichenized fungi as well as non-lichenized
calicioid fungi traditionally studied by lichenologists (hereafter ‘‘lichens’’). Non-calicioid lichenicolous fungi are not
included.
Data set
We initially assessed two parallel databases of lichensubstrate affinities, one for the PNW and one for
Fennoscandia. Lacking a consolidated regional checklist
for the PNW, we assembled our database from a variety of
sources: for macrolichens Goward et al. (1994), McCune
and Geiser (1997), and Goward (1999) and recent articles;
for calicioid fungi Goward (1999) and Rikkinen (2003a, b,
c); and for crustose species Spribille (2006) and subsequent
publications (Spribille and Björk 2008, Spribille et al.
2009). The PNW database contained 866 species. For
Fennoscandia, the database was derived from Santesson
et al. (2004) and recent updates (Svensson 2007); Xylographa corrugans will soon be reported as occurring in
Fennoscandia and is therefore included. A total of 900
742
species were included in the latter database. The two
databases were subsequently combined.
Substrate data specific to each of the two regions were
derived from a review of the literature and herbarium
specimens. In the course of developing lichen floras for the
PNW, three of us (TS, CB and TG) had a large amount of
published and unpublished substrate data at our disposal.
For Fennoscandian species, literature data on substrate were
derived from Foucard (2001), Santesson et al. (2004) and
unpublished data (GT). All epiphytic species were noted as
occurring on one or more of five substrates: wood, bark,
resin, conifer needles or other lichens; rock- and soildwelling species with B1% of their occurrences on any one
of those substrates were excluded. Using these assignments,
we classified epiphytic lichens into three substrate guilds for
each region: 1) ‘‘obligate lignicoles’’ (species with 99% of
their occurrences on wood in at least one region); 2)
‘‘facultative lignicoles’’ (defined as all other species found
on wood plus other substrates such as rock and/or soil and/
or bark); and 3) ‘‘corticoles’’ (defined as all remaining
epiphyte species, including both obligate and facultative
corticoles). To take into account species common to both
regions but with different guild assignments in each, guild
numbers and ratios were calculated individually for each
region or, when considering ratios of the combined floras,
guild assignments were calculated sequentially as all species
obligately lignicolous in one or both regions; then all species
facultatively lignicolous in one or both regions. All
remaining species are thus corticolous in one or both
regions.
For obligate lignicoles, we also identified specific habitat
microsite types wherever possible based on available
literature, herbarium data and our own field experience
(microsite types with B1% of the occurrences are not
included). All obligately lignicolous species were furthermore assigned as occurring on conifers (C) and/or
angiosperms (A) using a minimum 1% occurrence threshold.
Data analysis
To examine patterns underlying lignicole biology, we
analyzed guild assemblage structure based on three species
characteristics. First, we examined the relative proportions
of three different lichen life forms: 1) macrolichens (species
with three-dimensional aerial thalli, including all foliose,
fruticose and squamulose species; Goward et al. 1994,
Goward 1999, except for Hypocenomyce, which is treated as
a microlichen in accordance with Fennoscandian traditions); and 2) microlichens (species with thalli so closely
attached that their removal requires lifting of the substrate),
subdivided for the purposes of analysis into a) calicioid
crusts (hereafter ‘‘calicioids’’; species bearing either stalked
fruiting structures and/or loose spore masses; Tibell and
Wedin 2000); and b) non-calicioid crustose lichens (hereafter ‘‘crusts’’). Macrolichens, calicioids and crusts each
have a distinctly different gross morphology, and thus can
be thought of as functional groups. None, however,
represents a natural monophyletic group.
Our second analysis examined whether certain reproductive modes occur more often among lignicoles than in
other guilds. We recognized four reproductive modes: 1)
sexual, reproducing via spores (Cheiromycina and Szczawinskia, two small genera reproducing by conidia, are
lumped in here for convenience); 2) asexual via soredia or
goniocysts (e.g. Micarea hedlundii) (hereafter ‘‘soredia’’); 3)
asexual via isidia; and 4) asexual via coarse thallus
fragments. Species that reproduce both by spores and
asexual reproductive structures were assigned to the species’
main reproductive stategy.
The third analysis examined substrate affinity by higher
level taxonomic grouping. Each species was assigned to its
taxonomic order, where known. Although 15 orders are
represented, the large majority of epiphytic lichens belong
to a single order, Lecanorales. To improve the resolution of
our analysis we analyzed members of Lecanorales additionally at the level of family. All taxonomic assignments follow
Lumbsch and Huhndorf (2007) and subsequent phylogenetic revisions (Lumbsch et al. 2008).
Statistical analyses were restricted to chi-squared tests of
relative distributions of features of the lichen flora in the
two areas. Tests employed numbers of species, not
percentages.
Results
Species pool
Combining the epiphytic floras of the Pacific Northwest
and Fennoscandia yields a pool of 1271 species, including
531 species (42%) found in both regions (Table 1). Of the
combined floras, a total of 550 species (43%) are found on
wood, 132 (10%) of them as obligate lignicoles in at least
one of the two regions and 418 as facultative lignicoles. The
remaining 721 species (57%) of the pooled flora are
corticoles (Table 1). The high percentage of species on
dead wood (facultative and obligate lignicoles) is reflected
in each flora individually: a total of 349 species have been
recorded on dead wood in the PNW and 378 in
Fennoscandia. There are 67 species of obligate lignicoles
in the PNW of which 18 are unique to the PNW.
Fennoscandia has 97 species of obligate lignicoles; 49 of
these are unique to Fennoscandia (Table 1 and Supplementary material, Appendix 1). In terms of the relative
numbers of obligate lignicoles, facultative lignicoles and
corticoles, the two floras do not differ significantly (x2
4.86, 2 DF, p 0.088).
The lignicolous lichen floras of the PNW and Fennoscandia are more similar to each other than are the corticole
floras. Of the 132 species identified as obligate lignicoles in
the pooled data set, a total of 65 (50%) are shared by both
floras: 33 as obligate lignicoles in only one of the two
regions, 32 as obligate lignicoles in both (Supplementary
material, Appendix 1). Among facultative lignicoles, the
percentage of shared species is similar (62%, 261 species);
the combined total of facultative and obligate lignicoles
common to the two floras is 326 species, or 59%. This
stands in contrast to the corticole flora: of the 721 corticole
species in the pooled floras, only 205 (28%) are found in
both regions (Table 1).
Guild assemblage structure
Although relative numbers of obligate and facultative
lignicoles and corticoles do not differ overall between the
two regions, life forms are represented differently. The
majority of obligate lignicoles are microlichens, including
crusts (PNW 61%; FS 77% of lignicole species) and
calicioids (PNW 33%; FS 16%; Fig. 1). There are only
few obligately lignicolous macrolichens, all of them from
the genus Cladonia. In the two other guilds, by contrast,
macrolichens constitute a greater proportion of species:
25% (PNW) and 30% (FS) of facultative lignicoles are
macrolichens, as well as 25% of corticoles in Fennoscandia
and 50% in the PNW. Crusts are likewise well represented
with 62% (PNW) and 58% (FS) of facultative lignicoles
and 45% (PNW) and 71% (FS) of corticoles. Calicioids
comprise a similar number of species in each guild, but they
are proportionally better represented among obligate
lignicoles (see above) than among facultative lignicoles
(PNW 13%; FS 12%) and corticoles (PNW 5%; FS 4%;
Fig. 1). The comparative macrolichen/crust/calicioid ratios
differ significantly when the two epiphytic floras are
compared as a whole (x2 44.1, DF 2, p 0.001).
Within guilds there is a significant difference in the ratios
of the three life forms between regions for corticoles (x2
71.4, 2 DF, p B0.001) but not for facultative lignicoles
(p 0.32) and marginally for obligate lignicoles (x2 6.0,
DF 2, p 0.05).
Table 1. Numbers of lichen species by guild in the Pacific Northwest (PNW) and Fennoscandia (FS) and their percentages of each line
category (in parentheses).
A Obligate B Facultative C Total wood D Corticoles
lignicoles
lignicoles
users (AB)
Individual floras
1) Total PNW flora
2) Total FS flora
Combined floras
3) Species found only in PNW
4) Species found only in Fennoscandia
5) Species found in both regions: combined
guild assignments*
6) Total combined floras [(3)(4)(5)]
E Total number of
epiphytes (ABD)
67 (7.7)
97 (10.8)
282 (32.6)
281 (31.2)
349 (40.3)
378 (42.0)
517 (59.7)
522 (58.0)
866
900
18 (5.2)
49 (12.4)
65 (12.2)
81 (23.5)
76 (19.2)
261 (49.2)
99 (28.8)
125 (31.6)
326 (61.4)
245 (71.2)
271 (68.4)
205 (38.6)
344
396
531
132 (10.4)
418 (32.9)
550 (43.3)
721 (56.7)
1271
*guild assignments are calculated sequentially as all species obligately lignicolous in one or both regions, then all facultatively lignicolous
species in one or both regions; all remaining species are corticolous in one or both regions.
743
Figure 1. Proportions of life forms and reproductive modes by guild. Numbers above bars represent absolute species numbers.
Reproductive mode abbreviations: sexual spore dispersal; sor sorediate; isid isidiate; frag fragmenting.
Sexually reproducing species account for over 75% of
the obligate lignicole flora (Fig. 1). This is substantially
more than among facultative lignicoles and corticoles, in
which proportions of sexually reproducing species vary from
57 to 66%. It follows that species reproducing asexually by
means of soredia, isidia or fragmentation are poorly
represented among obligate lignicoles (sorediate spp.
PNW 18%, FS 21%; isidiate spp. PNW 3%, FS 4%) but
proportionally greater among facultative lignicoles (sorediate spp. PNW 31%, FS 32%, isidiate spp. PNW 4%, FS
5%) and corticoles (sorediate spp. PNW 28%, FS 27%,
isidiate spp. PNW 8%, FS 5%). Of the three modes of
asexual reproduction within the pooled floras of the two
regions, by far the most disperse by soredia; species that
reproduce by isidia constitute only just under 6% of the
pooled floras (75 species) and are most numerous among
corticoles. An even smaller percentage (4%, 46 species)
reproduce by fragmentation, mainly macrolichen species of
the genera Bryoria, Cladonia and Usnea. The ratios of the
four reproductive modes differ between the two regions
(x2 12.3, DF 3, p 0.006), with Fennoscandia having
more sexual and fewer asexually reproducing species.
Within obligate and facultative lignicoles there is no
significant difference in the relative proportions of repro744
ductive modes between the two regions (p 0.83 and 0.84,
respectively). Within corticoles, sexual reproduction is more
strongly represented in Fennoscandia (x2 20.1, DF 3,
pB0.001).
The obligate lignicole flora includes species from eight
taxonomic orders as well as numerous calicioid lichen taxa of
uncertain taxonomic position (Supplementary material,
Appendix 1). Of the taxonomic groups analyzed, the
families Cladoniaceae and Pilocarpaceae and the order
Baeomycetales are more species-rich in the obligate lignicole
and facultative lignicole guilds than in the corticole guild.
The families Parmeliaceae and Ramalinaceae and orders
Arthoniales, Peltigerales and Pertusariales, by contrast, are
species-rich among corticoles and facultative lignicoles and
nearly absent among obligate lignicoles (Fig. 2). The order
Teloschistales and family Lecanoraceae are well represented
in all guilds. When Lecanorales (including smaller families
not shown in Fig. 2) are analyzed as one group alongside the
seven other major taxonomic orders, there are no significant
differences between regions in ratios of taxonomic groups for
obligate or facultative lignicoles (p 0.41 and 0.98, respectively), corticoles (p 0.49) or the two floras taken as a
whole (p 0.53). However, if major families of the
Lecanoraceae are analyzed alone there are significant
Figure 2. Proportions of selected taxonomic groups by guild. Numbers above bars represent absolute species numbers. Major families of
Lecanorales: Cla Cladoniaceae; Lec Lecanoraceae; Par Parmeliaceae; Pil Pilocarpaceae; Ram Ramalinaceae. Other orders:
Art Arthoniales; Bae Baeomycetales; Myc Mycocaliciales; Ost Ostropales; Pel Peltigerales; Per Pertusariales; Tel Teloschistales.
differences between the two regions for corticoles (x2
24.6, DF 4, p B0.001) and the combined flora (x2
22.4, DF 4, p B0.001), primarily because of the greater
representation of Parmeliaceae in the PNW. In the same
analysis, there are no significant differences for obligate or
facultative lignicoles (p 0.55 and 0.95, respectively).
745
Our analysis shows some groups notably more speciesrich in the PNW than Fennoscandia (e.g. Parmeliaceae,
Mycocaliciales, Teloschistales) and some vice versa (e.g.
Lecanoraceae, Arthoniales). The largest genera of obligate
lignicoles are Lecanora (15 spp.), Lecidea s.lat. (13 spp.),
Micarea (13 spp.) and Xylographa (6 spp.) (Supplementary
material, Appendix 1). Five genera (Brucea, Elixia, Lignoscripta, Mycocalicium, and Xylographa) are completely
restricted to wood.
Microsite types
Current data are sufficient to broadly characterize the type
of dead wood used by 121 of the 132 obligate lignicoles in
our two study areas (Supplementary material, Appendix 1).
A total of 109 species were identified as occurring on
conifer wood, while only 24 obligate lignicoles have been
recorded on angiosperm wood. We furthermore differentiated ten microsite types used by obligate lignicoles.
According to present data, the largest number of species
were found on stumps (53), logs (48), and snags (47),
followed by anthropogenic wood substrates such as fenceposts and old barns (32), decorticated branches and twigs
(21), burned wood (7) and dead wood scars of living trees
(6 species). Microsite types used by fewer species were
maritime driftwood (5), freshwater driftwood (2) and
beaver scars (1). Microsite type could not be determined
for 20 species.
Discussion
Patterns in the lignicole floras
The epiphytic lichen floras of the Pacific Northwest and
Fennoscandia count among the best known within the
circumboreal coniferous forest zone. The histories and
scientific traditions in lichenology differ, however, in the
two regions. In Fennoscandia, nearly three centuries of
floristic tradition have made its lichen flora one of the most
intensely researched in the world. Notwithstanding the
similarity in the number of recorded epiphyte species, the
PNW is less thoroughly explored, and the knowledge of its
flora is highly dynamic: a large portion of the epiphytic
lichen flora was only first reported in the last 20 yr
(Spribille 2006). We acknowledge that epiphytic lichen
numbers will continue to increase in the PNW and
Fennoscandia with the exploration of remote areas. Future
research will show what effect this may have on ratios
reported here.
The cause of obligate dead wood affinity in lichens is
poorly understood. For a minority of obligate lignicoles,
mostly members of the order Mycocaliciales, close
association with wood can be attributed to saprophytic
as opposed to lichenized nutritional mode (Tibell and
Wedin 2000). Facultative saprophytism, in which a single
fungal species may occur as either a saprophyte or a lichen
in different settings, has been proposed for Chaenotheca
(Tibell 1997) and Stictis (Wedin et al. 2004) and should
be explored in the Baeomycetales. However, saprophytism
is not currently believed to play a role in the biology of
most lignicolous lichens. For these species, other forms of
746
niche specialization may be involved. Our data suggest
close affinity for dead wood is aligned with both
functional traits and phylogeny. As noted by Forsslund
and Koffman (1998), most obligately lignicolous lichens
are crustose and reproduce sexually, although these traits
are by no means unique to lignicoles. Macrolichens and
asexually reproducing species, by contrast, are largely
absent from the obligate lignicole guild, though they are
common among facultative lignicoles and corticoles. It is
possible that low biomass and presumably rapid sexual
reproduction among obligate lignicoles are an adaptation
to their relatively ephemeral substrate and place species at
a competitive disadvantage when establishing on more
stable substrates. Discrimination against macrolichens
would eliminate most Parmeliaceae and Peltigerales from
the pool of obligate lignicoles, but does not explain the
lack of obligately lignicolous members of the Pertusariales.
The highly similar proportions of life forms, reproductive modes and taxonomic groups among obligate and
facultative lignicoles between the two floras could plausibly
be attributed to substantial species overlap in these guilds
(59%). This however implies that a species found in both
regions maintains a fixed ecological envelope. Although this
is clearly the case for some species (e.g. Xylographa spp.),
numerous others do not espouse the same microsite
adaptations in both study areas (Supplementary material,
Appendix 1). This apparent niche drift may be related to
genetic variability. Evidence is increasing for divergence of
Old and New World lichen populations in pre-Pleistocene
times and low or non-existent gene flow since then
(Printzen et al. 2003, Palice and Printzen 2004). If the
low gene flow trend holds, the ecological envelopes of
disjunct, long-isolated species could make for an interesting
case study in phylogenetic niche conservatism (Wiens
2004).
Even if every species found in both floras were to have a
fixed ecology across its range, it remains that almost 40% of
the combined lignicole species pool is not shared. That the
proportional make-up of lichen life forms, reproductive
strategies and taxonomic groups is nonetheless largely
congruent in the two regions is particularly striking. There
would theoretically have been plenty of opportunity for
proportions to diverge given disjunct area with distinct
species composition and hundreds of thousands of years to
do it. The fact that this did not happen suggests the
existence of common underlying mechanisms that lead to
similar functional group assemblages occupying homologous microsites in disjunct regions. The pattern bears
resemblance to taxonomic and functional group proportionality in other organism groups, e.g. fish and corals
(Bellwood and Hughes 2001), leading some to suggest the
existence of regional level assembly rules related to function
and taxonomic group. Assessing whether or not assembly
rules are at play in lignicolous lichens will require data on
species composition from other regions and the parsing out
of the species overlap effect. Within the circumboreal realm,
regional-scale data of sufficient resolution are currently
available only for the two regions treated here.
An intriguing by-product of our analysis is the observation that the corticolous lichen floras of the two regions are
substantially more dissimilar than the lignicole floras.
Arguably the greater latitudinal spread of the PNW
compared to Fennoscandia could mean more temperate
species are included in the PNW flora. However, even in
Parmeliaceae, the group with the largest single difference
between the two floras, only 27 of 149 species in the PNW
are restricted to areas south of 508N, suggesting that
differences hold up even at higher latitudes. The strong
endemism among PNW corticolous macrolichens was
already recognized by Goward and Ahti (1992), who
suggested evolutionary isolation of the PNW as an
explanation. We additionally found that the two corticole
floras differ significantly in their internal life form,
reproductive mode and taxonomic group proportions.
Much as the congruencies discussed for lignicoles might
be ascribed to high species overlap, incongruencies among
corticoles could be attributed to low overlap. However, here
too we suspect that the root causes are more nuanced. Bark
generally contains higher concentrations of secondary
metabolites than wood (Obst 1998). We hypothesize that
the richness of tree species and bark chemistries offered a
more varied substrate landscape over evolutionary time than
the chemically more neutral wood substrate.
Inventory and monitoring of lignicolous lichens
Our analysis of substrate use shows that a similar percentage
of epiphytic lichen species in both floras use dead wood,
and approximately one in ten species are completely
dependent on it. The latter, listed in their entirety in
Supplementary material, Appendix 1, include several species
of epiphytic lichens under threat, not only on account of
the worldwide decline of dead wood in managed forests, but
also because in many regions these species and their habitats
have not yet been flagged for conservation concern.
A large number of obligate lignicoles are poorly known
in terms of their taxonomy, ecology and distribution. This
is especially true of genera such as Absconditella, Lecanora,
Lecidea, Micarea and Verrucaria. The genera Lecanora
and Lecidea s.lat. contain six species Lecanora apochroeoides, L. dovrensis, L. pseudohypopta, Lecidea consimilis
and L. subhumida from Fennoscandia and Lecidea pullula
from the Pacific Northwest that have not been recorded
since their description in the late 1800s (1934 in the case of
L. subhumida). Another species, Lecidea scabridula, was only
recently rediscovered after having not been recorded since
the 19th century. The state of baseline knowledge is
exemplified by the genus Xylographa, which despite being
one of the most common and easily recognized of obligate
lignicole genera in the PNW, has only been reported a few
times (Spribille 2006).
One reason for the inventory deficit in lignicolous
lichens is the common emphasis by ecologists on macrolichens. It follows from life form ratios among facultative
and obligate lignicoles that studies of dead wood in forest
ecosystems based on macrolichens will miss the majority of
species and thus fine-scale assemblage structure. This has
been borne out by substrate inventories that included crusts
(Forsslund and Koffman 1998, Bunnell et al. 2008, see also
Ellis and Coppins 2006). Although macrolichens have been
advanced as a stand-level surrogate for overall biodiversity
(Negi and Gadgil 2002) or overall lichen richness (Bergamini et al. 2007), there are no data to suggest that
macrolichens are indicative of richness patterns in different
wood microsites. Indeed, given the absence of macrolichens
in many dead wood microsites, this is unlikely.
Emphasis on macrolichens has been particularly strong
in the PNW. Red-listing and ‘‘Survey and Manage’’
tracking of lichens in the PNW focuses on macrolichens
(USDA 2001, COSEWIC 2005); only one obligately
lignicolous microlichen, Calicium abietinum, is currently
tracked in the PNW (Supplementary material, Appendix 1).
The lack of region-specific taxonomic treatments and keys
for crustose lichens has meant that complete species
inventories are difficult and often necessarily digress into
systematic studies. The launching of an epiphytic crustose
lichen flora project for British Columbia will improve the
availability of identification literature in the PNW in future
(Spribille 2006). In Fennoscandia the situation is better
owing to availability of a checklist and floras (Foucard
2001, Santesson et al. 2004). Within the project ‘‘Signal
species’’ (Nitare 2000) initiated by the Swedish Forest
Agency, and now also in use in Finland, Norway and the
Baltic states, the majority of lichens used are crustose. In
Europe in general, crustose lichens are assuming increasing
importance in studies of old forest and landscape habitat
structures (Rose 1992, Kruys and Jonsson 1997, Berglund
and Jonsson 2001, 2005, Ellis and Coppins 2006).
Lignicolous lichens and forest management
Our review of microsite affinities (Supplementary material,
Appendix 1) shows that the largest numbers of obligate
lignicoles occur on stumps, logs and snags. Although these
are the most commonly studied structural types in dead
wood studies (Berg et al. 1994, Forsslund and Koffman
1998, Lõhmus and Lõhmus 2001, Jansová and Soldán
2006, Bunnell et al. 2008), it is not clear that they translate
to biologically meaningful environmental envelopes. Bunnell et al. (2008) found significant differences in the lichen
flora between hard and soft logs assigned to the same decay
class using regionally standard down wood classification
protocols. Similarly, Goward et al. (unpubl.) and Svensson
et al. (2005) found significant differences in the lichen flora
between the exposed and shaded sides of snags and/or
between hard/erect and soft/leaning snags. Stumps are a
common result of forest practices, but are infrequently
sampled. In general, wherever an effort is made to further
discriminate lichen habitats, numerous microsites have been
recognized (Räsänen 1927, Rikkinen 2003b), corroborating
what is known for basidiomycetes and saproxylic insects.
Improved methods are needed for randomized sampling of
lignicoles that reassess current notions of sampling unit and
plot size and discriminate structural types more closely
fitting to the ecological behaviour of wood-dependent
species.
Down wood generally is recognized as being one of the
critical habitat elements in the maintenance of forest
biodiversity (Berg et al. 1994, Humphrey et al. 2002).
Forest practices have long discriminated against coarse dead
wood. While modern silvicultural approaches increasingly
recognize its value (Bunnell et al. 2002, Beese et al. 2003),
overall levels of coarse dead wood in forests have fallen to a
fraction of what they once were. Managed forests of today
747
in Fennoscandia contain only 210% of the coarse woody
debris (diameter 10 cm) once found in natural forests
(Linder and Östlund 1998, Fridman and Walheim 2000,
Jonsson et al. 2005). Dramatic rates of decline also have
been projected for parts of the PNW (Maser and Trappe
1984, Spies et al. 1988). We cannot rule out that many
lignicolous lichens have experienced concomitant declines.
In Fennoscandia, Cyphelium notarisii (Areskoug and Thor
2005) and C. trachylioides (Arup 1999), both associated
with dry, weathered wood, may be such species.
The obligate relationship of some lignicoles to their
substrate means they are directly threatened by activities
that reduce amounts of coarse dead wood. Many bryophytes, unlichenized fungi and invertebrates reliant on dead
wood are likewise threatened. Indiscriminate conservation
of dead wood by volume may not be sufficient to meet the
objectives of maintaining viable populations of microsite
specialists. Dead wood habitats often have been treated as
inextricable components of unmanaged forests, but recent
studies suggest that structural features retained at timber
harvest (slash, fallen snags, pre-existing fallen logs) help to
sustain the flora otherwise found in unmanaged stands
(Hazell and Gustafsson 1999, Bunnell et al. 2007, 2008,
Caruso et al. 2008). Development of conservation strategies
that look beyond numbers and volumes and consider
biologically meaningful dead wood structure types is
necessary to ensure that specific habitats are accounted for
in forest management practices (Jonsson et al. 2005).
Concluding remarks
Lichen-substrate relationships are still a little-researched
topic, and there is accordingly much work to do. At a
regional level, it will be important for lichenologists to work
together with conservation planners to identify flagship and
indicator species for survey and monitoring. Regional
identification guides such as Foucard (2001) play an
important role in enabling inventory and can be developed
outside Fennoscandia. At the level of ecosystem management, stand-level planning and reserve design approaches
need to be refined to take into account habitat distribution
at smaller spatial scales. A biologically informed approach to
classifying dead wood types will be instrumental to the
development of forest management approaches such as
retention silviculture and ensure better results for securing
populations of rare and/or declining species. On a macroecological scale, species-substrate relationships show promise for shedding light on long-term lichen diversification
patterns as more insight is gained into the role of function
and phylogeny in niche adaptation.
Acknowledgements This project was funded by the British
Columbia Forest Sciences Program and the British Columbia
Ministry of Water, Land and Air Protection (now Ministry of
Environment). S. Abrahamczyk and V. Wagner helped with the
databases. We thank A. Botnen, C. Printzen, J. Sheard, T.
Tønsberg and C. Williams for providing unpublished substrate
data, and A. Dahlberg, C. Printzen, J. Rikkinen and three
748
anonymous reviewers for helpful comments on an earlier draft
of the manuscript.
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Fred Bunnell