habitat selection and ecology of xanthoria elegans (link) th. fr. in glacier forefields:...

Journal of Biogeography (1997) 24, 363–373

Habitat selection and ecology of Xanthoria elegans (Link) Th. Fr.in glacier forefields: implications for lichenometry

D P. MC Department of Geography, Brock University, St. Catharines, Ontario, L2S 3A1, Canada

Abstract. Habitats occupied by the largest Xanthoria may reflect the importance of microscale factors (e.g.reflected rather than direct solar input). Closure of X. eleganselegans (Link) Th. Fr. thalli at seven glacier forefieldscommunities and coalescence of thalli was only found atin the Canadian Rockies were studied to investigate thesites that were naturally fertilized with dung. It is concludedlichenometric assumption that large thalli occupy ideal sitesthat all clasts do not afford homogeneous or idealfor growth. The largest thalli were found on steep orenvironments for lichen growth and do not have an equaloverhanging facets at the base of grey limestone clasts thatchance of being colonized. This raises doubts concerning thewere embedded in moraines. These thalli werevalidity of statistical normality assumptions in lichenometryunfragmented, had nearly circular outlines, were borderedand the use of grids to assess closure in lichen communities.by barren rock and had SSE to S orientations. This is

consistent with the general expectation that south-facing Key words. Lichen ecology, Xanthoria elegans,sites offer high solar input and a long snow-free season. lichenometry, primary succession, glacier forefields,

Canadian Rockies.Orientations other than south could result by chance or

is known about habitat selection by this species. However,INTRODUCTION

the ease with which the species can be positively identifiedand its tendency to form sparse, single-species communitiesUsers of lichenometry often assume that the largest lichenmake it a good candidate for studies that involvethalli in a community are among the first to have becomemicrohabitat description.established and that these thalli occupy the ideal sites for

The research reported here describes the ecology andgrowth (e.g. Beschel, 1961a). Unfortunately, fewmicrohabitats occupied by X. elegans thalli that have beeninvestigators have attempted to test these assumptions orused to estimate lichenometric ages and growth-rate datadescribe the characteristics of ‘ideal’ sites. A knowledgein related studies (McCarthy, 1992, 1993; Smith, McCarthyof the microenvironmental preferences of lichenometrically& Colenutt, 1995; McCarthy & Smith, 1995). Readersuseful species would assist users of lichenometry inshould consult those works for a description of the regioninterpreting thallus-size data. It might, for example, allowand the study sites.objective assessment of the assumption that all clasts in a

search area provide homogeneous growing environmentsand have an equal opportunity of being colonized. This

AN INTRODUCTION TO THE ECOLOGY OFassumption is critical in studies that use statistical normality

X. ELEGANSassumptions in lichenometry (e.g. McCarroll, 1994) orattempt to define size–density relationships and closure in Xanthoria elegans has a broad ecological amplitude. Fahselt

and Krol (1989:135) indicate that it thrives in areas havinglichen communities (e.g. Innes, 1986a; Haines-Young, 1988).Xanthoria elegans (Link) Th. Fr. is a yellowish-orangish less than 6 cm annual precipitation and can survive

submerged in streams for much of the growing season. Itthrough orange to reddish, crustose to foliose lichen. It hasan extremely broad circumpolar and alpine distribution is uncertain how thalli of this species are influenced by

various environmental factors (e.g. aspect, moisture,(Thomson, 1984; Clauzade & Roux, 1985). However, it isnot restricted to alpine or polar sites and in North America, temperature). In the Canadian Arctic, it has been reported

as colonizing all aspects (Fahselt et al., 1988), but wasfor example, it can be found on man-made and naturalsubstrates from the sea-water spray zone to the boreal forest most common on aspects other than north and on warmer

microsites of large rocks. In Colorado the species has beenand in the grasslands of the continental interior (e.g. Hale,1955; Rudolph, 1955; Hinds & Hinds, 1993; Hinds, 1995). described as being most common on sites shaded by grass

or trees (Carrara & Andrews, 1973). This distributionIt has long been used in lichenometry (e.g. Beschel, 1954)and has been the focus of a wide variety of lichenological presumably reflects the increased humidity of shaded sites.

Although it is ocasionally described as being a ‘limestoneinvestigations (e.g. Hooker, 1980a, 1980b; Fahselt, Maycock& Svoboda, 1988; Fahselt & Krol, 1989). Remarkably little species’ (e.g. Hinds, 1995:408) it inhabits both calcareous

1997 Blackwell Science Ltd 363

364 Daniel P. McCarthy

TABLE 1. Distribution of Xanthoria elegans (Link) Th. Fr. thalli on the various rock types.

Site Limestone

Black Brown Dark Light CalciteGrey Grey

E L E L E L E L E L

Beatty 12 1 3 0 74 0 0 0 12 5Elk 0 0 2 2 31 13 32 13 2 0Foch 1 3 2 7 3 5 3 40 10 14 7

2 0 0 1 0 0 0 16 2 3 03 9 2 8 0 4 2 34 5 10 55 0 0 4 1 2 0 46 0 0 06 9 6 2 0 3 3 23 20 3 1

Foch all 22 10 22 4 14 8 159 36 30 13Haig E. 0 1 2 14 0 1 3 8 4 0NW North 48 2 48 5 47 1 8 0 2 0Petain 1 16 1 0 0 3 1 17 3 5 0

2 16 2 7 3 18 0 12 0 0 03 20 0 1 0 3 0 7 0 0 0

Petain all 52 3 8 3 24 1 36 3 5 0Putnik 1 0 15 9 18 5 24 0 0 2N 208 30 130 44 246 38 457 100 90 33% of N 87.4 12.6 74.7 25.3 86.6 13.4 82.0 18.0 73.2 26.8

Total number of thalli on embedded (E) clasts: 1131.Total number of thalli on loose (L) clasts: 245.

and siliceous rocks (e.g. Osborn & Taylor, 1975; Wong & random sampling or statistical normality. Thus, theapproach used here is to present intersite comparisons thatBrodo, 1992; Hinds & Hinds, 1993; Hinds, 1995). The

growth and local distribution of X. elegans is also known use subsets of the largest individuals at each site.to be sensitive to naturally occurring fertilizers (i.e. it isknown to be ornithocoprophilous) and it is not unusual tosee ‘orange zones’ at bird or rodent perches (e.g. Vitt et al., SUBSTRATE CHARACTERISTICS AND1988). LICHEN GROWTH

Various morphological growth forms have been described(e.g. Filson, 1984; Fahselt & Krol, 1989), but the species is Table 1 shows the distribution of X. elegans thalli on various

lithologies at the study sites. The data show that c. fiveeasily identified by its bright pigmentation. It has beendescribed as possessing swollen, orange-yellow thalli (in times more thalli were found on embedded than on loose

rocks. Light grey limestones had the most thalli (c. 40% ofstreams), compact orange thalli (on boulders) or darkorange-red thalli on the driest rock faces (Fahselt & Krol, the clasts) while calcite-rich and brown limestones had few

(c. 22% of the total). Although X. elegans is capable of1989:136). Fahselt & Krol (1989) suggest that these formsare a flexible response to environmental stimuli not colonizing other substrates, few thalli were seen on chert

and none was found on shales, logs, bone or soils. Whilelichenologically distinct populations.the data in Table 1 suggest that stable, grey limestone clastsare preferred by this species, it has yet to be determined if this

APPROACHsimply reflects the availability, instability and/or differentsurface areas and micro-environmental conditions (e.g.In this study, a complete search was used to identify all

relatively intact circular to oblong thalli on lichenometrically surface roughness, water and heat retention) of the variouslithologies.dated moraines (Smith et al., 1995) and on control surfaces

that were used to compile a growth curve (McCarthy & At least half the thalli studied were found within 20 cmof the ground and were at the base or on the lowest quarterSmith, 1995). At all sites, the Largest Inscribed Circle

(L.I.C.) and the longest axis of all thalli (≥10 mm diameter) of the rock. Lesser amounts were found on the crest (i.e.uppermost face of the rock) and sides. In the Foch Glacierfound on clasts shared by the largest twenty thalli in the

population were measured to the nearest millimeter using forefield, 52% of X. elegans thalli were at the base of rocks,16% were on the crest and 32% were on the sides of rocksa flexible transparent ruler. However, on moraines near

Foch Glacier, all thalli≥10 mm diameter (L.I.C. and longest (Fig. 1). Few thalli were found on smooth rock surfaces;most were in concavities and cracks or were wrapped aroundaxis) were inventoried. Consequently, the dataset is

inappropriate for statistical tests based on assumptions of the edges (‘corners’) of rocks. Presumably, cracks,

Blackwell Science Ltd 1997, Journal of Biogeography, 24, 363–373

Implications of Xanthoria elegans (Link) Th. Fr. for lichenometry 365

X. elegans thalli may be an infrequent occurrence on sitesthat are not fertilized with animal droppings.

ORIENTATION OF X. ELEGANS THALLI

Thallus orientations were measured to the nearest degreeusing a Suunto compass and clinometer accurate to within2°. Since lichen covered rocks have irregular surfaces theorientation data were collected so as to represent the slopeof most of the thallus surface. Summary statistics for theorientation data have been calculated using the vectoralgebra and trigonometric functions detailed by Mardia(1972) and Batschelet (1981).

Summary data for the orientation (aspect) of X. elegansthalli at the study sites are provided in Table 2 and Fig. 3and facet data are presented in Table 3 and Fig. 4. Themean vector and mean angular deviation will be used assummary statistics. Since these values are meaningful onlyFIG. 1. Histogram showing the frequency of Xanthoria elegansfor unimodal samples, frequency distributions were assessed(Link) Th. Fr. thalli in various size classes that were found at the

base, crest and sides of rocks in the Foch Glacier forefield. This is using histograms (not reported). They revealed that thea complete sample (N=318) of X. elegans ≥10 mm diameter in majority of sample sets are approximately unimodal, butthis forefield. show wide dispersion about the mean. None of the

multimodal distributions was clearly bimodal orquadrimodal, so none could be transformed or subdividedinto unimodal sets (Batschelet, 1981:21).

depressions and sites near the base of rocks are preferred The mean aspect data (Table 2, Fig. 4) show the generalbecause they are relatively moist and sheltered. populations at eleven of fifteen sites had mean vectors in

In the Foch Glacier forefield none of the thalli was shaded the SSE to S quadrat. Data from the Putnik, Foch 2 andby plants and 90% of the thalli were clearly bordered by Foch 6 sites show most thalli there have a northerly aspect;barren rock (Fig. 2). This suggests that the community may thalli at NW Northover have a SW orientation. The widebe open to colonization and demonstrates that the species dispersion of points is indicated by the mean angularhas a scattered distribution. However, in some forefields deviations which include about 67% of the points in eachlong, narrow bands of overlapping X. elegans thalli were sample. The mean vector lengths (0=dispered; 1=all thallifound near rodent burrows (‘orange zones’). This suggests have the same orientation) indicate dispersion is similarthat closure of X. elegans communities and coalescence of around the mean northerly and south easterly orientations.

The mean aspects of the largest thalli at these sites (Table 2)are in most cases similar to those of the general population.

The mean facet data (Table 3) indicate that steep sites(60–90°) and overhangs (>90°) tend to be occupied by thelargest thalli. Moderate slopes (45–70°) are colonized bysmaller thalli. However, in all cases there is considerablescatter around the mean and no clear trends are apparent.

The orientation data show that a high proportion of thalliin these populations have a SE aspect and occupy moderatefacets. This is consistent with the general expectation thatsouth-facing thalli would receive greater solar insolationand have a longer snow-free season than might be expectedat north-facing sites. Orientations other than SE could resultby chance or may reflect the importance of microscalefactors (e.g. reflected rather than direct solar input).

CIRCULARITY OF X. ELEGANS THALLI

The tendency of certain species to form circular thalli is ofinterest to lichen ecologists and users of lichenometry.

FIG. 2. Histogram showing the number of marginal contacts of Lichen ecologists have questioned the possible ecologicalXanthoria elegans (Link) Th. Fr. thalli in various size classes fromadvantage of circularity and have described mechanisms thata complete sample (N=318) of X. elegans ≥10 mm diameter inregulate thallus shape (e.g. Hill, 1984). Thallus circularity isthe Foch Glacier forefield.important in lichenometry because it is used to distinguish



TABLE 2. Mean orientation data (aspect) for the largest 10, 20 and all Xanthoria elegans (Link) Th. Fr.sampled at the study sites.

Site N Mean aspect±mean angular Length of meandeviation (degrees) vector

1 Beatty 107 179.5±79.2 0.04410 163.1±62.3 0.41020 130.1±73.5 0.176

2 Elk 95 159.5±77.4 0.08810 166.6±72.5 0.19920 165.0±77.8 0.079

3 Foch 1 94 126.8±70.7 0.23810 172.2±79.3 0.04220 114.4±72.9 0.190

4 Foch 2 22 41.7±77.5 0.08410 46.3±59.2 0.46720 24.3±76.0 0.121

5 Foch 3 79 146.0±73.2 0.18410 103.3±60.3 0.44720 123.5±76.1 0.118

6 Foch 5 53 111.5±67.3 0.31010 128.1±69.1 0.27220 54.3±71.9 0.212

7 Foch 6 70 1.3±73.6 0.17410 33.8±59.4 0.46320 25.7±69.1 0.273

8 Foch 318 113.0±74.9 0.145(all sites) 10 132.0±69.6 0.262

20 143.5±75.2 0.1389 Haig 33 153.4±63.4 0.388

10 142.3±69.6 0.26120 159.0±68.3 0.289

10 NW 161 234.0±79.1 0.047Northover 10 264.0±71.6 0.219

20 238.4±67.9 0.29711 Petain 1 46 172.4±74.0 0.165

10 142.7±73.0 0.18720 169.3±80.3 0.018

12 Petain 2 58 169.9±78.1 0.07110 161.7±72.7 0.19520 156.1±75.7 0.126

13 Petain 3 31 114.7±74.1 0.16410 117.7±76.7 0.10420 135.9±80.9 0.004

14 Petain 135 163.6±76.3 0.114(all sites) 10 113.6±76.3 0.114

20 122.1±75.8 0.12515 Putnik 74 12.5±69.1 0.273

10 27.6±65.8 0.34120 8.4±56.6 0.512

between single and composite thalli and has generated the flow of carbohydrates from the algae to the growingregion at the periphery of the thallus (Hill, 1981:268).debate regarding the usefulness of various measures of radial

growth. Accordingly, the following sections will review the Others (e.g. Rhizocarpon spp.) have an algal layer in areolesscattered throughout the thallus. In placodioidal lichenscontrols of thallus circularity and will present data that

describe the circularity of X. elegans thalli at the study sites. (e.g. X. elegans), radial growth occurs by the formation andbranching of radial lobes that can expand without cracking(Hill, 1981:268).

BackgroundThere is, however, little evidence to show that these

structural adaptations guarantee thallus circularity. AplinLichens have a variety of structural adaptations that allowthem to maintain their form while they swell and shrink in & Hill (1979), Hooker (1980b) and Hill (1981, 1984) have

suggested that lobe division and competition between lobesresponse to changes in moisture content. Some crustoselichens (e.g. Lecanora spp.) develop tangential dessication in placodioidal species promote thallus circularity. However,

this conclusion is based on observations of small X. eleganscracks near their margins that are thought to block or slow



Innes (1986b:256) used this assumption to argue that thalluscircularity should decrease over time. He reasoned thatthere was a greater likelihood that older/larger thalli wouldencounter growth inhibiting factors or find increasedcompetition for space. There is, however, little data to showhow thalli respond to imbalances in environmental factors(e.g. spatial imbalance in moisture, wind, fertilizers, mineralbands in a rock). Presumably, if certain factors inhibitgrowth, other factors have the opposite effect. Thus, itwould seem reasonable to suggest that a combination ofenvironmental and genetic factors rather than simply thallussize and/or age must ultimately determine thallus shape.

Results

In this study, the most complete sample of a X. eleganspopulation (≥10 mm diameter) comes from the FochGlacier forefield. Consequently, these data will be used toexamine the relationship between circularity and thallus sizein a relatively uniform environment.

Fig. 5 gives two views of X. elegans size data from theFIG. 3. Plot of Xanthoria elegans (Link) Th. Fr. orientation data(aspect). This plot shows the mean orientation (aspect) and angular Foch Glacier forefield. The graphs show that thalli weredeviation data for X. elegans thalli. circular to oval in most size classes. The very slight trend of

increasing circularity with increasing thallus size is consistentwith what was suggested by Hill (1984), but is contrary towhat Innes (1986b) described for yellow-green Rhizocarponthalli (c. 20 mm diameter) and it may be incorrect to

extrapolate the results to larger thalli. spp.Data from this study (Fig. 5) suggest that large X. elegansIn contrast, Innes (1986b) presented data to show that

the circularity of some Rhizocarpon thalli (non-placodiodial thalli tend to have circular outlines. However, it is possiblethat the apparent trend may be an artifact of a samplinglichens) decreased with an increase in substrate age and

thallus size. Consequently, Innes (1986b) argued that growth strategy that rejected non-circular thalli. Although this couldbe tested by doing an analysis of variance on a randomcurves based on the L.I.C. will underestimate maximum

growth rates of lichens and should not be used in sample of all thalli (all shapes) at this site, it seems unlikelythat this would produce very different results. Only ca.lichenometry. While this conclusion may be reasonable for

some Rhizocarpons, his generality is not wholly 2–5% of thalli in this forefield were rejected because theywere in complex mosaics (e.g. at bird and rodent perches)substantiated by data from other lichenometrically useful

species. Further, Innes (1986b:258) recognized that his data or had a L.I.C. to long-axis ratio of <0.5. In addition, it ispossible that sampling and measurement of grossly oblongshow the L.I.C. is an adequate measure for certain yellow-

green Rhizocarpon spp. on young substrates, but might be thalli would lead to spurious results due to the inclusion ofcomposite thalli.inappropriate for use on older substrates. Innes (1986b:255)

also found there was less variation between L.I.C. values Data from the Foch Glacier forefield show that theproportion of circular X. elegans thalli may increase withfor various thalli than there was between the L.I.C. values

reported by various operators. He concluded that this increasing thallus size. However, the apparent trend doesnot mean that the largest thallus will be circular or thatindicated the L.I.C. may be a less reproducible measure than

is the longest axis. However, it might be more reasonable to there should be little difference between growth curves/ratesthat are calibrated using the L.I.C. or the long axis of thisconclude that measurements (and growth curves?) are more

dependable if done by one rather than several operators. species.Clearly the issue of which diameter to measure is of littleconcern if thalli with grossly irregular outlines are rejected

FRAGMENTATION OF X. ELEGANS THALLIas composite.

In order to understand why some thalli are more circular Some species tend to lose portions of the thallus by a processknown as central fragmentation. This loss can affect thallusthan others it is ultimately necessary to know how thalli

respond to stimuli/obstacles at the thallus margin. shape (relative circularity) and may lead to problems inlichenometry where circularity is used to distinguish singleUnfortunately, there is little information available (cf. Hill,

1981). Several workers have suggested that deviations from from coalesced thalli. Since most users of lichenometry havestudied species that are not prone to fragmentation (e.g.circularity occur due to reduced rather than increased

growth along one or more radii (e.g. Lock[e], Andrews & yellow-green-black Rhizocarpon spp.) the topic has receivedlittle attention. Beschel (1961a) suggested that thallus ringsWebber 1979; Innes, 1986b). Although radial growth can

be blocked, it has yet to be shown that this is the primary resulting from the loss of the central portion of a thalluscontinue to show radial growth and are, therefore, usefulor sole cause of non-circularity in any species. Nevertheless,



TABLE 3. Mean orientation data (facet) for the largest 10, 20 and all Xanthoria elegans (Link) Th. Fr.sampled at the study sites.

Site N Mean aspect±mean angular Length of meandeviation (degrees) vector

1 Beatty 107 53.6±71.7 0.21710 58.6±45.9 0.67920 59.5±71.3 0.226

2 Elk 95 74.3±78.1 0.07210 87.3±73.8 0.171

20 67.6±76.3 0.1133 Foch 1 94 46.0±78.2 0.068

10 129.1±77.8 0.07920 121.2±67.3 0.310

4 Foch 2 22 48.1±55.9 0.52510 72.5±59.0 0.47020 48.6±56.9 0.507

5 Foch 3 79 83.2±77.6 0.08310 57.7±62.9 0.39720 106.7±69.0 0.274

6 Foch 5 53 51.9±79.8 0.03110 131.4±74.0 0.16520 115.2±72.2 0.206

7 Foch 6 70 54.3±69.1 0.27310 77.8±53.3 0.56720 84.3±62.8 0.399

8 Foch 318 78.9±79.6 0.035(all sites) 10 76.4±68.0 0.296

20 112.2±71.0 0.2339 Haig 33 51.9±71.8 0.215

10 122.8±59.3 0.46520 131.8±67.0 0.315

10 NW 161 67.6±78.4 0.063Northover 10 80.2±69.7 0.260

20 85.0±70.1 0.25111 Petain 1 46 85.7±77.3 0.090

10 50.0±60.5 0.44320 64.1±71.0 0.233

12 Petain 2 62 67.6±79.6 0.03410 66.1±76.1 0.10420 65.2±76.2 0.117

13 Petain 3 31 88.0±76.6 0.10610 99.4±66.8 0.32120 73.2±76.0 0.119

14 Petain 141 58.7±79.2 0.044(all sites) 10 85.7±74.7 0.15

20 57.9±75.8 0.12515 Putnik 74 68.7±70.6 0.240

10 64.2±71.0 0.23120 72.3±65.8 0.340

in lichenometry. However, workers who measured species winds, air pollution, Gilbert, 1971; Seaward, 1976), thethickening and eventual fragmentation of many crustosethat are prone to fragmentation have either ignored its

potential significance (e.g. Miller & Andrews, 1972; Miller, and foliose thalli is often described as a probable indicatorof senescence (e.g. Beschel, 1961b; Armstrong, 1973). In1973) or measured only relatively intact thalli (e.g. Osborn

& Taylor, 1975). Obviously, subjective decisions are made fact, little is known about when and how fragmentationproceeds. Pentecost (1980:141) has suggested that thallusin lichenometry regarding what constitutes a relatively intact

thallus and whether large, near-circular thallus rings or arcs detachment and fragmentation first occurs at the center ofa thallus and the area affected enlarges radially at the radialof severely weathered thalli must be rejected.growth rate until the thallus is removed. There are, however,no known data to link fragmentation rate and thallus age

Backgroundor radial growth rate. Observation of the complete lossof thalli by the fragmentation process (McCarthy, 1989)Although thallus fragmentation may result from abrasion

and non-biological factors (e.g. invertebrate grazing, strong suggests it is incorrect to assume that older lichen



Hypogymnia physodes (L.) Nyl. thalli can be washed orotherwise removed and transported to safe micro-sites wherethey can become established. Thus, while some lichen species‘use’ fragmentation as a means of dispersal it is not knownif this is a common adaptation in many crustose or foliosespecies.

Results

Fragmentation data for X. elegans have been summarizedas cumulative curves (Fig. 6). The curves show that at leasthalf of the thalli had 0–5% fragmentation and that maximumvalues in excess of c. 60% fragmentation (i.e. thallus ‘rings’)were rare. In total, <30% of the thalli were reported asmoderately to severely fragmented. However, arcs ofseverely fragmented thalli were found at several sites. Sinceit is unknown whether these arcs are remnants of single orcomposite thalli they are not represented in the data.

Investigation of fragmentation rates within various thallussize classes in the Foch Glacier data set shows fragmentationincreased with thallus size in the <50 mm thallus size classes

FIG. 4. Plot of Xanthoria elegans (Link) Th. Fr. orientation data (Fig. 7). This trend is reversed in the largest thallus size(facet). This plot shows the mean orientation (facet) and angular

class (i.e. 50–59 mm). The increase in fragmentation withdeviation data for X. elegans thalli. Although the mean vector isincreasing thallus size is consistent with the expectation thatalways the mid-point of the mean angular deviations, the meanfragmentation should be encountered in older/larger lichens.angular deviations in this figure have been truncated at zero degrees.

This was done because none of the thalli had a facet between 270 The apparent reversal of the fragmentation trend in theand 360 degrees. largest thalli suggests that large thalli are brittle, but remain

intact because they occupy sheltered sites. A different andmore detailed approach (e.g. repeat photography as usedcommunities must become closed to recruitment. Thereby McCarthy, 1989) would be required to better investigateis, therefore, concensus that the fragmentation process isthe possible link between thallus size, age, environmentalimportant in maintaining open and diverse licheninfluences and fragmentation rate.communities (Pentecost, 1980; Woolhouse, Harmsen &

Fahrig, 1985).DISCUSSIONAlthough there have been few studies on the

fragmentation process (McCarthy, 1989), Armstrong (1990) Xanthoria elegans thalli were found on a wide variety ofaspects, facets and lithologies; however, there is no evidencehas presented evidence to show that fragments of

FIG. 5. Scatterplots and histograms showing long axis and L.I.C. values for Xanthoria elegans (Link) Th. Fr. from the Foch Glacierforefield. A: Scatterplot showing L.I.C. versus long axis of X. elegans. B: Histogram showing the frequency of X. elegans thalli that haveregular (circular and nearly-circular) outlines (L.I.C.: long axis ratios of 0.9–1.0).



to suggest that larger thalli should be found on sites thatare clearly different from those occupied by most thalli.Nevertheless, without controlled experimentation it is notpossible to assess the degree to which the various siteattributes have interacted to influence lichen establishmentand growth. Presumably, investigations of lichenenvironment at the microscopic scale might reveal moresubstantial differences between sites occupied by the averageand/or largest thalli.

Evidence of thallus circularity and the observation of aslight increase in central fragmentation with increasingthallus size suggests that it may be reasonable to estimatemaximum thallus size (and lichenometric ages) from thearcs of severely fragmented X. elegans thalli. However, anapparent tendency to form circular thalli does not meanthat the largest individuals will be circular or that growthcurves compiled by various measures of thallus diameterwill be identical.

Aside from the general observation that X. elegans wasnearly always established on brown limestone clasts if theywere present, there is no available evidence to show howthe radial growth of X. elegans is affected by substratecolour or type. However, general observations at bird androdent perches indicate that the thalli tend to coalesce wheremoisture and nutrients are in ample supply (e.g. at cracksand concavities). Although it is unknown if radial growthis faster at these sites, thalli at perches were invariablythicker and smaller than those on adjacent rocks. However,thalli at perches had marginal contacts with other thalli.Consequently, they are unacceptable for use in lichenometry(cf. Beschel, 1961a:1045) and are an unlikely source oflichenometric error.

While the data provide a preliminary description ofhabitats occupied by X. elegans, the controls on the localdistribution of this species are enigmatic. The species doesnot form dense communities in the glacier forefields of theCanadian Rockies and is not found on similar depositsin nearby valleys (cf. Osborn & Taylor, 1975). Furtherinvestigation is needed to better establish the biotic andabiotic factors that control its distribution, establishmentand growth.

IMPLICATIONS FOR LICHENOMETRY ANDSTUDIES OF LICHEN COMPETITION

Lichenologists and users of lichenometry have longrecognized that lichen colonization patterns and growthrates are a function of the complex interplay ofmicroenvironmental and biogeographical factors (e.g.thallus age, species, genetics). Unfortunately, users oflichenometry have focused on thallus size and have used

FIG. 6. Cumulative curves for Xanthoria elegans (Link) Th. Fr.fragmentation data. These graphs use a probability scale as the y-axis. This is standard practice in the construction of cumulativecurves and is not intended to imply that the populations shouldhave a statistically normal distribution. The y-axis shows thepercentage of thalli in the population that have a lowerfragmentation than is indicated on the x-axis intercept.



FIG. 7. Scatterplot (A) and histogram (B) showing fragmentation data for Xanthoria elegans (Link) Th. Fr. in the Foch Glacier forefield.A: Scatterplot of fragmentation (%) versus diameter (L.I.C.). B: Histogram showing the frequency of intact and slightly fragmented X.elegans thalli in various size classes.

‘judicious’ approaches to sampling in an attempt to minimize range of non-uniform habitats. Since species do not surviveon sites which are outside their range of tolerance, closurethe potential effect of microenvironmental variables on their

data. Indeed, it it the subjective nature of lichenometry and must be defined on the basis of potentially inhabitable andsurvivable space. Further, if conditions for establishmentthe apparent willingness of users to recognize, but not

directly measure micro-environmental gradients that has differ from those needed for later growth, inhabitable spaceshould be defined as space which can be colonized, but mayled to debate (e.g. lichen-kill zones, Koerner, 1980; or

green zones: Haines-Young, 1983, 1985; Mahaney & Spence, not prove suitable for long-term survival. Barren space maybe: (i) uninhabitable, or (ii) suited for establishment and1985). This continued acceptance of subjectivity in

lichenometry does little to address the criticisms that have juvenile growth, but either (iia) inadequate for growth atlater stages, or (iib) perfect for establishment and growthbeen levelled at the technique (e.g. Jochimsen, 1966; Worsley,

1981). For example, approaches that use inferential statistics at all stages. Measures of community closure (e.g. percentageof lichen coverage: Haines-Young, 1988) must be sensitivebased on an assumed statistical normality in thallus-size

distributions recognize that they use convenient, but to the fact that a community may be closed to therecruitment of certain species, but open to others. That is,simplistic assumptions about lichen ecology and micro-

environment (e.g. McCarroll, 1994:395). Indeed, the closure may be taxon-specific, time-specific and habitat-specific. Data presented in this study, for example,statistical approach assumes that lichens have a normal

distribution of thallus sizes and that any temporal or spatial demonstrate that the tendency for central fragmentation inX. elegans may ensure that closure is temporary or mayvariability in lichen establishment, growth and micro-

environmental gradients can be averaged-out (e.g. never be reached. Thus, without better understanding oftaxon and habitat-specific needs, researchers may be unableMcCarroll, 1994:395). Clearly, the habitat data presented

in this work show that a large scatter in thallus sizes may to recognize closure when it occurs. This will lead toacceptance of the false null-hypothesis (Type II error).reflect a large variability in micro-environmental conditions.

It is tempting to reason that because large thalli of this The potential for Type II error is high in lichenometricwork because it is difficult to formulate appropriate andspecies tend to occupy sheltered, south-facing microsites on

stable rocks, their behaviour is consistent with the safe-site testable null-hypotheses concerning lichen populationdynamics without knowing more about lichen biology andmodel that has been used to explain seed germination and

establishment by higher plants (e.g. Harper, Williams & life history. The situation is not unique to lichen studies,and has long been debated in community ecology (e.g.Sagar, 1965). However, there may be significant dangers in

assuming that the laws that describe seed plants (e.g. self- Connor & Simberloff, 1979; Gilpin & Diamond, 1984;Pleasants, 1990).thinning rule or 3/2 law, Yoda et al., 1963) and the sampling

designs that are typically used to assess these laws (e.g. grid Clearly, the use of sampling methodologies that assumeuniformity in microenvironment are based on the accuracyor quadrat sampling, Haines-Young, 1988) can provide

meaningful measures of intraspecific competition or closure of assumptions made regarding the spatial and temporalheterogeneity and/or importance of unmeasured variables.in lichen communities.

Lichen colonized surfaces are irregular and provide a More research into microscale processes will eventually lead



Haines-Young, R.H. (1983) Size variation of Rhizocarpon onto a better understanding of the role of microenvironmentmoraine slopes in southern Norway. Arctic Alpine Res. 15,on lichen establishment and growth. Biogeographers, lichen295–305.ecologists and users of lichenometry should therefore be

Haines-Young, R.H. (1985) Discussion of ‘Size variation ofencouraged to provide more descriptive data with which toRhizocarpon on moraine slopes in Southern Norway’: a reply.assess the taxon and habitat-specific ecology of lichens.Arctic Alpine Res. 17, 212–216.

Haines-Young, R.H. (1988) Size–frequency and size–densityrelationships in populations from the Rhizocarpon sub-genusACKNOWLEDGMENTSCern. on moraine slopes in southern Norway. J. Biogeogr. 15,

This research has benefited greatly from the shared insights 863–878.and encouragements of Professors Dan Smith and John Hale, M.E. Jr (1955) Studies on the chemistry and distribution of

North American lichens (1–5). Bryologist, 58, 242–246.Sheard. I wish to thank the authorities at Peter LougheedHarper, J.L., Williams, J.T. & Sagar, G.R. (1965) The behaviourand Elk Lakes Provincial Parks for allowing me to work

of seeds in soil. J. Ecol. 53, 273–286.at the study sites and thank Kelly Skuse and Glen BlahutHill, D.J. (1981) The growth of lichens with special reference tofor their cheerful assistance in the field. I also wish to thank

the modelling of circular thalli. Lichenologist, 13, 265–287.Loris Gasporotto for working wonders with my originalHill, D.J. (1984) Studies on the growth of lichens I. Lobe formation

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habitat selection and ecology of xanthoria elegans (link) th. fr. in glacier forefields:...

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