Water and substrate control of cliff communities: patterns among species and phyla

Journal of Vegetation Science && (2014)

Water and substrate control of cliff communities:patterns among species and phyla

Ken Aho, T. Weaver & Sharon Eversman

Keywords

Cliff ecology; Community ecology; Natural

experiment; Substrate gradient; Taxonomic

scale; Water gradient

Abbreviation

NMDS = Non-metric multidimensional scaling

Nomenclature

Anthophyta and Pterophyta Dorn (1992);

Bryophyta Lawton (1971) and Anderson et al.

(1990); Marchantiophyta Stotler & Crandall-

Stotler (1977); Lichens Esslinger (2013)

Received 3 October 2013

Accepted 28March 2014

Co-ordinating Editor: Rune Halvorsen

Aho, K. (correspoding author,

[email protected]): Department of Biological

Sciences, Idaho State University, Pocatello, ID

83209-8007, USA

Weaver, T. ([email protected]) &

Eversman, S. ([email protected]):

Department of Ecology, Montana State

University, P.O. Box 173460, Bozeman, MT

59717-3460, USA

Abstract

Aims: Cliff communities tend to be compositionally simple but taxonomically

diverse at the phylum/division level. As a result, they provide a setting for exam-

ining the responses of distinct phyla (e.g. Anthophyta, Bryophyta and lichens),

species within these phyla and communities comprised of these phyla, to the

same environmental gradients. Our aims were both descriptive and hypothesis-

driven. We sought to identify major community shifts across water–substrate

gradients in a poorly studied system (inland North American cliffs), while con-

sidering extant hypotheses concerning the relationships of organisms to envi-

ronments at different taxonomic scales.

Location: Sub-alpine cliffs in northeastern Yellowstone National Park.

Methods: Cliff faces on twomountains were blocked at nine waterfalls. At each

block, community composition was recorded from random samples obtained for

three levels of water availability (xeric, mesic and hygric) on two substrates

(limestone and andesite). Patterns of cover and richness were compared using

Friedman’s Rank F-tests. Community patterns were discerned using permuta-

tionMANOVAs, ordinations, and cluster analyses.

Results: Phyletic composition varied in response to water availability. In gen-

eral, lichens dominated dry sites, while vascular plants dominated wet sites. An-

thophyta was the most narrowly specialized phylum, and Bryophyta was the

most generalized. Beta diversity was higher at drier sites, due to the dominance

of the diverse lichen group. As a result, communities on lime and andesitic sub-

strates were indistinguishable in hygric environments, but significantly different

at drier sites. This may have occurred because, while water is limiting for neither

substrate in hygric environments, andesites are more likely to absorb and store

atmospheric water in drier environments. Indicator species analyses identified

27 distinct species distinguishing water and substrate conditions, or their inter-

actions.

Conclusions:Our experimental design uniquely accounted for extraneous vari-

ables in its examination of water and substrate effects on cliff communities.

Water strongly controlled community composition both among and within

phyla through both direct effects and interactions with substrate.

Introduction

Cliff communities are of interest to plant ecologists

because of their high levels of endemism (Ursic et al.

1997; Graham & Knight 2004), specialized adaptations

(Larson et al. 1999; Aidar et al. 2010) and unique com-

positional forms that are often species-poor, but diverse

at the phyla/division level (Larson et al. 2000). On a cliff

face, vegetation varies dramatically with the features of

its microenvironments (Armstrong 1988). Controlling

factors may include water availability and substrate char-

acteristics, including mineralogy, stability, porosity and

albedo (Pentecost 1980). These variables often interact in

subtle ways to influence the availability of resources,

with concomitant effects on cliff community composition.

For instance, regions where different substrates meet will

often produce water discharge zones, favouring hydro-

philic species (Larson et al. 2000), and non-porous

1Journal of Vegetation ScienceDoi: 10.1111/jvs.12205© 2014 International Association for Vegetation Science

substrates may store water poorly, favouring xeric species

(Aho & Weaver 2006).

The effect of environmental factors may (or may not)

differ strongly among distinct phyla (e.g. lichens, bryo-

phytes, pteridophytes, anthophytes, etc.). Several authors

have predicted that the richness and abundance of plant

phyla will change in parallel ways across the same envi-

ronmental gradients (McCune & Antos 1981; Cox & Lar-

son 1993), particularly in light-limited environments (del

Moral & Watson 1978; Tilman 1988). Others researchers,

however, predict that phyla will respond differently to gra-

dients because they are likely to be the by-product of dif-

ferent evolutionary pressures (Grime 1979; Austin 1990;

Vittoz et al. 2010). The consideration of environmental

controls of the distribution of phyla also prompts questions

concerning patterns of adaptation of species within phyla.

In particular, do the species in these groups collectively or

singly demonstrate relatively broad patterns of adaptation,

or do they tend to have narrowly focused niches across

environmental gradients?

In this paper we describe the communities and ecology

of an unstudied cliff system in the sub-alpine zone of Abs-

aroka Range in the northern Rocky Mountains. We have

two goals:

1 To quantify the importance of substrate type and water

availability to cliff communities in an experimental frame-

work that controls for extraneous variables.

2 To consider the responses of organisms at two taxo-

nomic scales: at the level of phyla to assess phyletic con-

straints to adaptation, and at the level of species to deduce

intra-phylum patterns, provide concise inter-phyla com-

munity descriptions and identify indicator species of

water–substrate conditions.

Methods

Study area

Cliff formations in northeast Yellowstone National Park

(45° N, 110° W) provide a unique setting for examining

the water–substrate control of cliff communities. In this

region, a distinct horizontal layer of limestone/dolomite is

overlain with thick volcanic deposits, producing near-ver-

tical cliff systems of 300–650 m. Perennial waterfalls fall

across both substrates at many locations. The cross-juxta-

position of waterfalls and substrate layers provide replicate

natural blocks containing both moisture and substrate

levels.

The calcareous sedimentary layers consist of mostly

Cambrian Pilgrim limestones deposited 500–570 MYA

(Metesh et al. 1999). This formation produces a consistent

stratum, 30–55-m high, of limestone and limestone-

conglomerate cliffs on the lower slopes (2200–2300 m

a.s.l.) of mountains (Metesh et al. 1999). Volcanic layers

100–600-m thick directly overlay the limestone layer.

These are andesitic deposits from the Eocene Lamar River

Formation (47–49 MYA), a subunit of the Absaroka Volca-

nic Supergroup (Smedes & Prostka 1972).

To consider aspect effects, cliffs on twomountains, Barr-

onette and Abiathar Peak, were selected for study. The

mountains lie directly east–west of each other, and are sep-

arated by Soda Butte Creek, a major tributary of the upper

Lamar River drainage system in Yellowstone National Park

(Figs 1 and 2).

At the elevations (2100–3200 m) studied, the climax

vegetation types are Abies lasiocarpa/Vaccinium scoparium

and Pinus albicaulis/V. scoparium, although the study area is

mostly occupied by seral Pinus contorta (Pfister et al. 1977).

Precipitation is relatively low (629 � 15 mm yr�1; mean

� SE, n = 34; NE Entrance snotel site) compared to

higher-altitude sites in the region; and the growing season

is short (� 3 mo). Detailed descriptions of the local and

regional climate are provided in Aho (2006) and Weaver

(1980, 1990), respectively.

We note that the vegetation ecology of cliffs of North

America, including higher-altitude regions of the Rocky

Mountains and Coastal Cordillera, is at present virtually

unknown. Previous studies of North American cliffs have

been mostly limited to low-altitude areas in the eastern

part of the continent (e.g. Wiser 1998), primarily along the

Niagara Escarpment (Larson et al. 1999; Kuntz & Larson

2006; many others).

Field sampling

Sample sites were established near the junction of the

andesitic and limestone layers at nine waterfalls. Five

waterfalls were sampled on the east face of Barronette

Peak (Figs 1, 2a), and four on the west face of Abiathar

Peak (Figs 1, 2b). Samples from both substrates were

acquired at each waterfall in three zones: hygric (areas

within the spray zone of waterfalls), mesic (areas 10–30 m

from the spray zone) and xeric (in the spray-free zone 30–

100 m from the waterfall). Thus, six substrate/moisture

combinations were sampled at each waterfall: (1)

andesite–xeric, (2) andesite–mesic, (3) andesite–hygric, (4)

limestone–xeric, (5) limestone–mesic and (6) limestone–

hygric. The resulting experimental format was a nine-

times replicated two-way (substrate andmoisture) blocked

design. While the natural experiment remained mensura-

tive (non-manipulative), the blocking of replicates allowed

for stronger statements of causality because extraneous,

potentially confounding factors (e.g. aspect, biotic interac-

tions) were being controlled (held constant) within blocks.

Two randomly placed 1-m2 point intercept grids were

sampled in each ‘treatment’ type at each waterfall. Each

grid contained 100 uniformly distributed points in a

Journal of Vegetation Science2 Doi: 10.1111/jvs.12205© 2014 International Association for Vegetation Science

Controls of cliff community species and phyla K. Aho et al.

10 9 10 configuration. At eachXY intersection of gridlines

the species present at that point was recorded. Data from

the two grids were averaged to create a single observation

for each species–’treatment’ combination at each waterfall.

The resulting frequencies were surrogates of percentage

cover because individual measures for each species could

range from zero to 100, and the total frequency for the grid

could not exceed 100.

Vouchers for vascular plants, ferns, liverworts, mosses

and lichens were deposited in the Yellowstone National

Park herbarium, Gardiner, MT (YELLO). Nomenclature

follows Dorn (1992) for vascular plants and ferns, Lawton

(1971) and Anderson et al. (1990) for mosses, Stotler &

Crandall-Stotler (1977) for liverworts and Esslinger (2013)

for lichens.

Access to the sample sites was gained using rock

climbing equipment and techniques. Sampling of cliff

faces was accomplished with rappelling and ascending

fixed ropes. Trees and large boulders were used as climb-

ing anchors.

Analysis

The cover of phyla on cliffs [i.e. flowering plants (Antho-

phyta), ferns (Pterophyta), mosses (Bryophyta), liverworts

(Marchantiophyta) and lichens] was compared using

Friedman’s Rank F-test, a rank-based permutation ana-

logue of a randomized block-design ANOVA. A simulta-

neous inference procedure specific to this test was used to

adjust P-values (Hollander &Wolfe 1999).

Tests for community-level differences were conducted

using the NP-MANOVA procedure of Anderson (2001).

Bray–Curtis dissimilarity served as the underlying resem-

blance metric (Bray & Curtis 1957). At least 10 000

permutations were used for calculation of each P-value.

P-values from multivariate pair-wise tests among

Fig. 1. Location of the study area and the sites sampled. The inset map locates the area in Yellowstone National Park (polygon in NW Wyoming) and the

northern Rocky Mountains. Replicate sites are located at waterfalls on Barronette (1–5) and Abiathar (6–9). The Park highway lies between Barronette and

Abiathar, east of Soda Butte Creek, and near the centre of the orthophoto.


K. Aho et al. Controls of cliff community species and phyla

‘treatments’ were adjusted for simultaneous inference

using Holm’s sequential Bonferroni procedure (Holm

1979).

To graphically depict community relationships, we ordi-

nated site/species data using non-metric multidimensional

scaling (NMDS; Kruskal 1964). Bray–Curtis dissimilarity

was used as the resemblance metric. To account for extre-

mely high levels of beta diversity (av. Bray–Curtis dissimi-

larity = 0.904), the step-across method of De’ath (1999)

was again used to redefine and extend dissimilarities

among sites with no taxa in common. That is, unit dissimi-

larities were replaced by sums of dissimilarities between

sites and shared stepping-stone sites, that both had non-

unit dissimilarities. A three-dimensional NMDS solution

was obtained to account for the level of complexity in the

data, as measured with Kruskal’s stress (Kruskal 1964).

Stress was reduced considerably when the number of

NMDS dimensions was increased from two to three

(results not shown).

To find the optimal three-dimensional NMDS configu-

ration, the best (lowest stress) solution resulting from each

of two of 100 random starting configurations was obtained.

These two solutions were found to be identical (correlation

from Procrustes rotation = 1) with low stress (12.7), inti-

mating a strong solution, unlikely to be trapped in local

minima. Strengthening conviction in the model, a congru-

ent projection was obtained through detrended correspon-

dence analysis (DCA; Hill 1979) (Procrustes correlation =0.86; P = 0.001). To enhance interpretability, NMDS axes

were centred and rotated, a posteriori, to principal compo-

nents and rescaled in half-change units (Hill 1973).

Indicator species analysis (ISA; Dufrene & Legendre

1997) and tabling were used to identify species that were

indicative of particular environments. P-values for ISA

indicator values were calculated with Monte Carlo proce-

dures using 10 000 iterations.

A total of 104 species were considered in quantitative

analyses: 42 lichens, 22 bryophytes, 36 anthophytes, two

pteridophytes and two marchantiophytes. To avoid group-

ing errors, specimens keyed only to genus were deleted

from analyses if other specimens in the genus were suc-

cessfully keyed to species. It should be noted that our sam-

pling effort was not intended to provide a comprehensive

species list, but to allow detection of important community

shifts across water–substrate gradients. Preliminary analy-

ses indicated that our sampling scheme was probably suffi-

cient to achieve this goal. In particular, first order jackknife

estimates of species richness were 56.6 � 6.8 anthophytes,

29.9 � 3.1 bryophytes and 49.9 � 3.4 lichens (margins

are SEs), suggesting that ca. 36/57 = 63% of flowering

plant species, 22/30 = 73% of bryophytes and 42/

50 = 84% of lichens were found in our samples.

All analyses were run using the R computational envi-

ronment (R Foundation for Statistical Computing, Vienna,

AT) with heavy reliance on the packages asbio (Aho 2014)

and vegan (Oksanen et al. 2014).

Results

The most dominant phylum at the study site (mean � SE)

was the lichen group [cover = 28.9 � 3.59%; richness

(species 2 m�2) = 4.39 � 0.54], followed by bryophytes

(cover = 13.4 � 2.73%; richness = 1.6 � 0.17) and antho-

phytes (cover = 4.1 � 0.99%; richness = 1.76 � 0.31).

The most dominant species at the study site were the

lichens Staurothele drummondii (cover = 3.85 � 1.18%),

Aspicilia caesiocinerea (3.71 � 1.15%), Placynthium nigrum

(3.54 � 1.46%) and Staurothele fissa (2.15 � 0.87%), the

moss Philonotis fontana (4.85 � 1.29%) and the vascular

plant Mimulus guttatus (1.69 � 0.11%). Species specific to

“treatment” types are listed in Table A1.2.

To quantify the distribution of phyla across gradients,

we compared their cover and richness among the six

water–substrate combinations. Neither water nor substrate

main effects were examined separately in these analyses

because of the presence of confounding interaction effects

(a)

(b)

Fig. 2. Horizontal views of the study area. (a) Barronette looking west

from Abiathar. (b) Abiathar looking east from Barronette. Circles denote

locations of waterfalls (blocks) descending from andesite (A) above, to

limestone (L) below, the dotted line.



(see below). After adjustment for simultaneous inference,

significant cover differences between water–substrate lev-

els were apparent for lichens (v25 = 40.58, P = 1 9 10�7),

bryophytes (v25 = 34.98, P = 1.5 9 10�6) and anthophytes

(v25 = 38.29, P = 3.3 9 10�7; Fig. 3a). Significant richness

differences were evident for lichens (v25 = 36.68,

P = 6.9 9 10�7) and anthophytes (v25 = 32.32, P = 5.1 9

10�6), but not bryophytes (Fig. 3b). Results of 4 9 (62 –

6)/2 = 60 pair-wise comparisons for phyla are shown for

percent cover and richness in Figs 3a and b, respectively.

These results confirm that lichens were most important at

xeric sites, whereas bryophytes and anthophytes were

dominant at hygric sites. Cover differences were less dis-

tinct along the water gradient on limestone, compared to

volcanic substrates (Fig. 3). For instance, the cover of

bryophytes was not significantly different among water

levels on limestone, but was distinguishable among water

levels on andesite.

We also considered the simultaneous variation of phyla

(phyletic composition) and species (community composi-

tion) with respect to water, substrate and their interaction

using blocked NP-MANOVA analyses. We found that the

(a) (b)

Fig. 3. Comparisons of (a) cover and (b) richness of phyla found in six environments on cliffs. Water–substrate combinations listed across the abscissa:

X = xeric, M = Mesic, H = Hygric. Medians are given by the height of bars. Error bars are 95% confidence intervals for the true median using the method of

the binomial inverse cumulative distribution function (Aho 2013). Bars with different letters are significantly (a = 0.05) different using Friedman’s F-rank test

after adjustment for family-wise inference using the asymptotic method of Hollander &Wolfe (1999).



phyla of site blocks were indistinguishable at a = 0.1 (F8,40= 1.5, P = 0.13), that significant phyla differences existed

with respect to bothwater (F2,40 = 60.5, P ≤ 0.001) and the

interaction of substrate and water (F2,40 = 3.3, P = 0.014)

and that phyla differences among substrates were signifi-

cant at a = 0.1 (F2,40 = 2.5, P = 0.091). At the species

level, communities varied among blocks (F8,40 = 1.8,

P ≤ 0.001), as well as water levels (F2,40 = 8.4, P ≤ 0.001),

substrate types (F1,40 = 4.0, P ≤ 0.001) and the interaction

of water and substrate (F2,40 = 4.0, P = 0.02). The statisti-

cal significance of interactions required that the effect of

moisture and substrate on communities be considered in

combination in post-hoc multivariate pair-wise tests (Aho

2013; Table 1). In these comparisons, community differ-

ences between substrates at mesic and xeric sites were sig-

nificant after adjustment for family-wise type I error,

whereas hygric-andesite and hygric-lime communities

were indistinguishable (Table 1).

Ordination representations of variation in community

composition supported NP-MANOVA results. Specifically,

wet and dry sites were distinctly dissimilar in community

space, while intermediate and dry communities were

rather similar. Axis 1, and to a lesser degree Axis 2, sepa-

rated sites along a water gradient. Note that hygric sites

had higher scores along Axis 1 than xeric sites (Fig. 4).

Axis 2 largely separated sites by macro-aspect although, as

noted above, this factor was correlated with water avail-

ability (Fig. 4a). Specifically, Barronette (drier, east-facing)

sites had higher scores along Axis 2 than Abiathar (wetter,

west-facing) sites. Ordination Axes 2 and 3 separated sites

by substrate (Fig. 4b). In particular, a gradient running

from the lower left to the upper right of Fig. 4b separated

andesite sites (bottom left) from lime sites (top right).

Indicator species analysis identified 16 species signifi-

cantly (a = 0.05) indicative of water categories, and 17

species significantly indicative of water–substrate combi-

nations, but only five significant indicators of substrate

(Table 2). Note that the strength of indicators ranges

from rather weak (indicator value <30) to extremely

strong (indicator value > 70). Significant ISA species are

overlaid in Figs 4a and 4b by plotting, for each species,

the average NMDS scores of sites in which the species

occurred, weighted by species cover (i.e. wascsores).

Thus, species locations (indicated with ‘+’) indicate the

sites (and categorical assignments) with which they were

most strongly associated. With respect to water (across

both levels of substrate), particularly strong indicators of

xeric conditions were the lichens Aspicilia caesiocineria,

Staurothele drummondii and Xanthoria elegans, while strong

indictors of hygric conditions were the moss Philonotis

fontana and the vascular plant Epilobium clavatum. With

respect to substrate (across all levels of water), relatively

strong indicators were the lichen Aspicilia caesiocineria for

andesite, and the lichen and moss genera Collema (i.e.

C. polycarpon, C. tenax and C. undulatum) and Schistidium

(probably mostly S. acoparum) for limestone. No species

were significantly indicative of mesic environments, or

water–substrate combinations involving mesic conditions.

In general, significant taxa are widespread throughout

North America and the circumboreal zone, with observed

indicator characteristics supported by the extant literature

(Table 2).

Discussion

Phyla on gradients

The primary aim of this study was to assess the relative

importance of lichens, bryophytes and vascular plants

along water supply and substrate gradients. We found dis-

tinct responses of these groups to these factors, particularly

along the water gradient. Thus, our study supports the

hypothesis that the same environmental constraints will

produce disparate responses for distinct vegetation phyla

(cf. Grime 1979).

Lichens were dominant with respect to both cover and

richness on dry cliff faces (Fig. 3). This accords with the

common knowledge that lichen species dominate xeric

lithic surfaces, due (assuredly) to their general properties

for desiccation resistance (e.g. non-reducing sugars that

maintain structural membranes in the absence of water,

and antioxidants that reduce oxidative stress; Kranner

et al. 2008). Likewise, lichens were rare at hygric sites. In

these environments excess water is likely to stimulate fun-

gal respiration, causing mycobionts to become parasitic,

leading to destruction of the lichen symbiosis (Ahmadjian

1993). Interference competition for space and light with

relatively productive bryophytes and anthophytes will also

tend to be higher in hygric environments (Grime 1979; cf.

Connell 1961).

Bryophyte cover was highest at hygric sites, particularly

on andesite (Fig. 3a). The number of bryophyte species,

however, was sub-equal across ‘treatments’ (Fig. 3b),

Table 1. Results of NP-MANOVA tests for the null hypothesis of identical

communities (Anderson 2001). A = andesite, L = limestone, X = xeric,

M = mesic, H = hygric. P-values adjusted for simultaneous inference using

the method of Holm (1979). Results sorted by F-statistic magnitude.

Test F1,8 Adj. P-val. Test F1,8 Adj. P-val.

AX vs AH 8.53 0.0002 LX vs AX 4.05 0.0098

LH vs AX 7.95 0.0002 LH vs AM 3.97 0.0002

LX vs AH 7.03 0.0004 LX vs LM 3.93 0.0198

LM vs AX 6.74 0.0002 LM vs AM 2.78 0.0235

LX vs LH 6.42 0.0002 AX vs AM 2.41 0.0198

LM vs AH 5.68 0.0002 LX vs AM 2.2 0.0235

LM vs LH 5.18 0.0002 AH vs LH 0.97 0.4729

AM vs AH 4.16 0.0002



suggesting an overall generalist adaptation strategy relative

to other cliff groups. The generalist mien of Bryophyta was

due to both species specialized for hygric conditions (e.g.,

Pohlia wahlenberghii and Philonotis fontana) and species

broadly adapted to drier conditions, particularly on lime-

stone (e.g. Schistidium; probably mostly S. apocarpum;

McIntosh 2013; Fig. 4).

Anthophyta was the most specialized group, as its mem-

bers generally occupied only the wettest locations (Figs 3,

4), particularly those in which structural discontinuities

and weathering by waterfalls provided sites for soil accu-

mulation (cf. Ursic et al. 1997). Cliff anthophytes

undoubtedly contribute to soil-building feedback loops

(Aho & Weaver 2010) that, through deposition and

decomposition of detritus, may facilitate – over the short

term, their own survival, and over the long term, coloniza-

tion of new individuals and species – leading to succession

(Connell & Slatyer 1977).

(a)

(b)

Fig. 4. Relation of cliff communities to each other, their environments and indicators of those environments. Three-dimensional NMDS (a) Dimensions 1

and 2 and (b) Dimension 2 and 3. Spiders connect sites with the same water/substrate categorical assignment in (a) and the same substrate in (b). Ellipses

are 95% confidence intervals for the true factor-level centroid. Centroids labels: A = Andesite, L = Limestone, X = Xeric, M = Mesic, H = Hygric. Numbers

in (a) and (b) indicate significant (a = 0.05) indicator species of water/substrate interactions and substrate, respectively (Table 2). Indicator species name

locations are wascores. 1 = Aspicilia caesiocinerea, 2 = Caloplaca saxicola, 3 = Lecidea atrobrunnea, 4 = Lecidea stigmatea, 5 = Lobothallia alphoplaca,

6 = Physcia dubia, 7 = Rhizoplaca melanopthalma, 8 = Rhizocarpon geminatum, 9 = Rhizocarpon geographicum, 10 = Xanthoria elegans,

11 = Philonotis fontana, 12 = Epilobium clavatum, 13 = Mimulus guttatus, 14 = Saxifraga odontoloma, 15 = Saxifraga rivularis, 16 = Staurothele fissa

17 = Collema spp., 18 = Lecidea atrobrunnea.



Table 2. Species that indicate water, substrate and water–substrate combinations at a = 0.05, their indicator values [values range from 0 (no indication)

to 100 (perfect indication)] and their ISA P-values. Also included is information concerning citationsa, life formb, species coverc and constancyd within speci-

fied groups, and geographic rangee.

Indicators (supporting citations)a LFb Ind. val. P-val. Coverc (Mean%� SE) Cond(%) Geog. Rangee (citation)a

WATER

XERIC

Aspicilia caesiocinerea (1) L 52.3 0.002 9.5 � 1.7 61 Entire (1)

Candelariella spp. (1*) L 74.5 0.001 3.8 � 0.5 89 Entire (1*)

Lecidea stigmatea (1) L 35.6 0.004 3.7 � 0.9 39 Entire (1)

Lobothallia alphoplaca (1) L 22.2 0.029 0.2 � 0.1 22 Disjunct, mostly southern (1)

Physcia dubia (1) L 22.2 0.028 0.5 � 0.2 22 Entire (1)

Rhizocarpon geographicum (1) L 22.2 0.031 0.3 � 0.1 17 Entire (1)

Schistidium spp. (2†) B 46.9 0.005 3.0 � 0.7 61 Entire (2†)

Staurothele drummondii (1) L 62.8 0.001 10.1 � 1.7 72 Entire (1)

Xanthoria elegans (1) L 67.1 0.003 4.4 � 0.9 72 Entire (1)

HYGRIC

Cratoneuron filicinum (17) B 22.2 0.030 2.7 � 1.0 22 Probably entire (16, 17)

Cystopteris fragilis (2,10) P 25.0 0.026 0.3 � 0.1 28 Entire (6)

Epilobium clavatum (10) A 77.8 0.001 2.0 � 0.4 78 Entire (8)

Mimulus guttatus (2,10) A 66.3 0.001 5.0 � 1.3 67 Entire (1)

Philonotis fontana (9) B 85.0 0.001 13.9 � 1.7 89 Entire (9)

Saxifraga cespitosa (2,10) A 22.2 0.030 0.6 � 0.2 22 Entire (5)

Saxifraga odontoloma (2,10) A 50.0 0.001 2.0 � 0.4 50 Southern (8)

SUBSTRATE

ANDESITE

Aspicilia caesiocineria (11-13) L 46.6 0.001 7.2 � 1.5 48 Entire (1)

Lecidea atrobrunnea (11-13) L 18.5 0.049 0.61 � 0.23 19 Entire (1)

LIMESTONE

Collema spp. (1‡) L 35.2 0.046 8.9 � 2.2 41 Entire (1‡)

Staurothele fissa (1) L 29.6 0.006 4.30 � 1.16 Mostly southern (1)

Schistidium spp. (2†) B 51.6 0.002 2.4 � 0.6 56 Entire (2†,15)

WATER-SUBSTRATE

ANDESITE-XERIC

Aspicilia caesiocinerea (11–13) L 73.2 0.001 18.3 � 1.7 89 Entire (1)

Caloplaca saxicola L 33.3 0.022 1.1 � 0.3 33 Southern (1)

Lecidea atrobrunnea (11–13) L 43.1 0.004 1.8 � 0.4 44 Entire (1)

Lecidea stigmatea (11,15) L 31.2 0.025 5.6 � 1.2 44 Entire (1)

Lobothallia alphoplaca L 27.8 0.037 0.1 � 0.1 11 Entire (1)

Physcia dubia L 31.5 0.039 0.94 � 0.24 33 Entire (1)

Rhizocarpon geographicum (1) L 30.6 0.022 0.6 � 0.1 33 Entire (1)

Rhizocarpon geminatum (1) L 28.7 0.040 1.7 � 0.5 33 Entire (1)

Rhizoplacamelanopthalma (11–13) L 30.6 0.020 0.6 � 0.1 33 Entire (1)

LIMESTONE-XERIC

Collema spp. (1‡) L 31.6 0.042 8.9 � 0.5 41 Entire (1‡)

Schistidium spp. (2†) B 65.1 0.001 5.8 � 0.9 67 Entire (2†)

Xanthoria elegans (14) L 57.2 0.001 6.9 � 1.1 78 Entire (1)

ANDESITE-HYGRIC

Philonotis fontana B 55 0.001 18.0 � 2.0 89 Northern (4,7)

Saxifraga rivularis A 33.3 0.020 0.2 � 0.1 33 Entire (9)

LIMESTONE-HYGRIC

Epilobium clavatum A 58.4 0.001 1.4 � 0.3 89 Southern (3)

Mimulus guttatus A 34.8 0.038 6.33 � 1.75 56 Mostly southern (3)

Saxifraga odontoloma A 37 0.007 2.66 � 0.54 56 Entire (3)

aCitation numbers refer to both columns 2 and 7. 1 = Brodo et al. (2001), 2 = Kershaw et al. (1998), 3 = Hitchcock & Cronquist (1973), 4 = Grout & Dole

(1932), 5 = Hitchcock et al. (1961), 6 = Haufler et al. (1993), 7 = Rydberg (1921), 8 = NatureServe 2013, 9; = Lawton (1971), 10 = Dorn 1992, 11; =

Eversman (1995), 12 = Eversman (1998), 13 = Eversman et al. (2002), 14 = Thompson (1984), 15 = McIntosh (2013), 16 = Harpel (1980), 17 = Crum &

Anderson (1981).bLF = life form: A = Anthophyte, M = Marchantiophyte, P = Pteridophyte, B = Bryophyte, L = lichen.cAverage cover in designated type (e.g. xeric).dConstancy in indicated type (e.g. xeric).eRefers to approximate North American latitudinal range: Southern = 30°–50° N, Northern = 40°–70° N, Entire = 30°–70° N.

*C. vitellina and C. aurella, both dominant at the study area.†S. apocarpum.‡C. polycarpon, C. tenax and C. undulatum.



Community composition along gradients

Cliff community composition varied among sites with

respect to water, substrate and combinations of water–sub-

strate levels (Table 1, Fig. 4). Relative to phyla-level analy-

ses, an increasing distinctiveness of ‘treatments’ was

evident at the level of species. This outcome illustrates the

importance of taxonomic scale to all ecological studies (cf.

Dodd et al. 1995).

Four separate analyses indicate that water is more

important in distinguishing/controlling species distribu-

tions at the spatial scale and range of conditions under con-

sideration. First, in NP-MANOVAs, water is a stronger

predictor than substrate for both phylum and species com-

position at sites. Second, ISA results reveal a larger number

of significant water indicators, compared to substrate indi-

cators (Table 2). Third, NMDS axes (and those of confir-

matory ordinations using other methods) show a clear

community gradient along a well-defined water gradient

(Dim. 1; Fig. 4), whereas substrate-specific communities

are relatively poorly distinguished (Dims 2 and 3; Fig. 4b).

Fourth, in supplementary cluster analyses (flexible-b link-

age; b = -0.25; Lance & Williams 1967; App. 2), higher

level divisions distinguish sites by levels of water availabil-

ity, whereas substrates were separated, if at all, only at

finer (less dissimilar) divisions, requiring many more clus-

ters (Fig. A2.1).

The importance of water–substrate interactions to com-

munity composition is demonstrated with NP-MANOVAs,

which show significant interaction effects and largely sig-

nificant differences among the communities of crossed fac-

tor levels (Table 1); ordinations, which distinguish site

communities based on particular combinations of water

(Fig. 4a) and substrate (Fig. 4b); and indicator species

analyses, which find more significant (a = 0.05) indicators

of crossed conditions than indicators of main effects

(Table 2). That interactions among microclimatic factors

control the distribution of biota is well supported by other

cliff studies involving lichens (Armstrong 1988; Buschbom

& Kappen 1998), bryophytes (Alpert 1985) and antho-

phytes (Ursic et al. 1997; Larson et al. 2000). Accordingly,

our post-hoc factor level comparisons emphasize interac-

tions, not main effects. Despite this, indicator species

analyses are made with respect to water and substrate

main effects as well as their interaction because of the

potential usefulness of these additional classifications

(Table 2).

The importance of water–substrate interactions to com-

munity composition is likely due to the effect of cliff sub-

strates on water availability (Pentecost 1980; Larson et al.

2000). Andesite rocks at the study site were more porous

than limestones, allowing the former to both absorb more

atmospheric water and retain more water when wetted

(Aho & Weaver 2006). Thus, andesites may provide local

water reservoirs for extended periods of time, particularly

for xerophytic/poikilohydric species, which may persist

and even transpire in environments with water potentials

of �10 MPa for vascular plants (Sperry 2003) and around

�38.5 MPa for lichens (Kranner et al. 2008). The effect of

the interaction of water and substrate on cliff community

composition is prominent in the NMDS ordination, and

emphasizes the ‘wetter’ character of andesitic substrates.

Specifically, along the water gradient (diagonal to Axis 1

and 2), limestone sites and their centroids lie further to the

xeric (upper-left) end of the gradient than sites represent-

ing andesite within hygric, mesic and xeric categories

(Fig. 4).

Examples of substrate control of available water at a

broader scale, and resultant broad-scale control of plant

communities, are found throughout the literature. For

instance, xeric Pinus species often dominate calcareous

substrates because of their aridity (Whittaker & Niering

1968) and limestone ‘dry’ and siliceous ‘wet’ alpine plant

communities have been distinguished on the Beartooth

Plateau in Montana (Bamberg & Major 1968) and in the

French Alps (Michalet et al. 2002).

While hygric communities of andesite and limestone are

statistically indistinguishable, communities of andesite-

xeric and limestone-xeric groups are significantly different

(Table 1) due largely to differences in lichen species com-

position (Fig. 4). Thus, substrate only differentiates com-

munity types at xeric environments. We posit that this

outcome is largely due to the covariance of substrate water

availability differences and community composition. Spe-

cifically, waterfalls at the study site provide continual

access to water and suitable conditions for hygric species

across both substrates. It contrast, in water-limited envi-

ronments subtle differences in the water availability of

andesite and limestone are likely to become more physio-

logically important, resulting in differing communities (cf.

Karlin & Bliss 1984). An increase in beta diversity with

increasing aridity is clearly demonstrated by inflated scat-

ter down the water gradient of the ordination (Fig. 4).

From overlays of statistically significant ISA species it is

also apparent that phyla-level differences largely distin-

guish communities along the water gradient (Fig. 4a),

while species-level differences in communities (primarily

among lichens) largely distinguish substrates (Fig. 4b).

Substrate water storage/absorption effects on communi-

ties may be amplified in the general climate of the study

area where summer rainfall is infrequent (Weaver 1985),

but the average early morning (05:00 h) relative humidity

exceeds 80% every month at lower altitudes, and is likely

to be still higher in the cliff/canyon areas studied (cf. Fink-

lin 1983). These conditions are likely to produce substrate-

specific wetting and drying cycles, a primary control for



lichen community composition in most environments

(Buschbom&Kappen 1998).

Substrates may also affect cliff communities by influenc-

ing both physical (e.g. stability and texture for coloniza-

tion) and chemical (nutrient and toxin availability)

environments (Pentecost 1981). First, with respect to

physical properties, continually exfoliating cliffs may influ-

ence community composition by selecting for species that

establish rapidly, resulting in lower levels of diversity (Lar-

son & Kelly 1991). We note that at our study site andesitic

surfaces are qualitatively more friable than lime surfaces

(Aho & Weaver 2006). Second, with respect to chemical

properties, differences in substrate pH could influence the

availability of minerals and toxins (e.g. Al, P, Fe, Ca) affect-

ing cliff synecology, particularly among lichens (Brodo

1973; Armstrong 1990). At the study site limestones are, of

course, more basic than andesitic substrates (pH 8.7 vs 6.1,

respectively; Aho & Weaver 2006). We, however, tenta-

tively reject the importance of pH (or other chemical influ-

ences) on community composition because, while a

correlation between pH and community composition

exists on dry sites, none is present on wet sites. We note

that stronger pH influences on communities can result

from larger disparities in substrate pH, e.g. acidic-granite vs

limestone (cf. Aho 2006, p. 79).

Still other factors undoubtedly affect community

constituency. Among topographic properties, aspect is

significantly associated with community composition in

NP-MANOVA analyses (F1,52 = 2.75, P = 0.002) and

distinguishes sites along the second NMDS axis (Fig. 4).

West-facing aspects are likely to be wetter than east-facing

aspects due to local rain shadow effects, and southwestern

aspects are warmer than southeastern aspects due to ambi-

ent heat storage and warming in the afternoon (Geiger

et al. 2003). These effects, however, are held constant by

blocking, allowing an unalloyed examination of substrate

and water effects on between-phyla patterns of cover and

richness in analyses. For instance, while mean� SE eleva-

tions of water categories are almost identical (xeric =2383.7 � 1.4 m, mesic = 2374. 6 � 1.3 m, hygric =2380.2 � 1.3 m), sampled sites are situated at slightly

higher elevations on Abiathar, the west-facing and taller

mountain (E-facing sites = 2355.5 � 0.6 m, W-facing sites

= 2409.5 � 1.7 m). Nonetheless, elevations (and other

aspect-associated effects) are held constant as substrate

and water effects are considered within each block. The

unavoidable exception is the confounding of elevation and

substrate. This occurs because the andesite layer geologi-

cally overlays the limestone layer and thus is situated at a

higher elevation in each block. We reduce the importance

of this effect by noting that the elevations for andesite and

limestone categories are similar, 2401.8 � 1.5 m,

2357.1 � 0.9 m, respectively, and that this difference

likely has negligible effects on communities (cf. Virtanen &

Crawley 2010).

While the ranking of mesic and xeric categories is surely

consistentwithin blocks, the positioning of sites in the ordi-

nation (Fig. 4) indicates that several sites defined as mesic

are probably very dry. Note that along the NMDS water

gradient, xeric and mesic sites are mixed due to the poorly

defined (broad confidence ellipse) mesic groups, and that

several mesic sites occur at the far left (driest) part of the

gradient (Fig. 4a). This conclusion is also supported

through the ordering of sites and species in supplementary

cluster analyses (Fig. A2.1, App. 2). Specifically, when

sorting the relev�e dimensions using classification results,

one mesic-limestone site and five mesic-andesite sites are

strongly associated with xeric cliff species, e.g. Aspicilia cae-

siocineria, Collema spp., Lecidella stigmatea and Staurothele

drummondii (Fig. 4a). This mixing of general mesic and

xeric designations among blocks undoubtedly explains the

inability of our study to detect statistically significant indi-

cators of mesic conditions.

Sorted relev�e tables (App. 1, Tables A1.1, A1.2) and clus-

ter analyses (App. 2) allow reconsideration of mesic types

and refinement and extension of a priori water supply des-

ignations. Four distinct classification evaluators indicated

that a four-cluster pruning of the dendrogram was optimal

(App. 2). The resulting clusters can reasonably designated

a posteriori as ‘xeric’, ‘hygric’, ‘xeric/mesic’ and ‘mesic’

types (Fig. A2.1). The ‘mesic’ cluster is predominantly a

subsample of west-facing a priori mesic limestone sites

(LI7, LI8, LI9) dominated by the lichens Placynthum nigrum

and Gyalidea hylinescens (Fig. A2.1). The ‘xeric/mesic’ clus-

ter is strongly indicated by Schistidium spp., which is most

dominant on dry limestone (e.g. LX4 and LX6), and Stau-

rothele fissa, which is dominant on xeric and mesic lime-

stone sites (e.g. LM6, LM2, LM5, LX5, LX2). Issues with

mesic designations are not evident for hygric and xeric

sites, as these types – as identified in cluster and ordination

analyses – strongly correspond to a priori designations.

Acknowledgements

This study was made possible in part by a grant from Yel-

lowstone National Park. (YNP-NPS #YELL-05116) and as-

sistantships from Montana State University. Help with

identifying difficult specimens was provided by Dr. W.

Hong (University of Great Falls) for liverworts, Dr. Bruce

McCune (Oregon State University) for lichens, and Judith

Harpel (University of Washington) for mosses. Finally,

thanks to Tim Seipel for invaluable field assistance, and to

Rune Halvorson and two anonymous reviewers whose

careful review and edits have improved themanuscript.



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Supporting Information

Additional Supporting Information may be found in the

online version of this article:

Appendix S1.Relev�e Tables.

Appendix S2.Cluster analyses.



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