Botanical Journal tif the Linnean Sociery (1990), /04: 35-59. With 15 figures
Growth and production dyna~nics of boreal
1nosses over cli~natic, chemical and
topographic gradients
DALE H. VITT
Department
of Botany, University of Alberta, Edmonton, Alberta, T6G 2E9, Canada
VITT, D. H., 1990. Growth and production dynanti.cs of boreal mosses over climatic,
chemical and topographic gradients. Three basic questions arc addressed in this paper. Each of
them considers a separate aspect of moss growth and produc-tion. A common theme throughout is
that moss populations are dynamic, highly active entities. The study of these dynamics can tell us
not only about the mosses themselves but also about the systems in which they live. The following
conclusions are reached. (I) Moss populations are organized into complex canopies, some having at
least 6000 leaves em--'. Drought-tolerant mosses may achieve high rates of growth when moist, but
generally are dry and inactive a large percentage of time. (2) For the ectohydric, drought-tolerant
moss, Hylocomium splendens, growth is highly variable over its North American boreal range and can
be related to precipitation and continentality. However, for the endohydric, less drought-tolerant
moss, Polytrichum strictum, growth is more constant over its North American range and is not as
distinctly related to broad macroclimatic patterns. It may have nutrient limited growth, whereas
Hylocomium splendens may have climatically limited growth. (3) (a) In mires, calcium, magnesium,
sodium and hydrogen ions are important chemical factors that are dosely correlated to mire type.
Available amounts of nitrate, ammonium, phosphorus and total organic nitrogen in surface waters
do not correlate with mire type; likewise, they do not increase across the bog-rich fen gradient.
Extreme-rich fens and bogs have about the same concentrations of these components, but water-flow
differences may modify the total nutrient input during a season in fens. (b) Moss produ(·tion in
extreme-rich fens is similar to, or somewhat less than, in bogs and poor !Cns. However,
decomposition is much greater in extreme-rich fens. Peat accumulation may, therefore, be greater in
bogs and poor fens than in extreme-rich fens. (c) Net produrtion on bog hummocks is about half
that of hollows, whereas in poor fens and rich fens, production on hummocks is greater than or equal
to that in the hollows. Decomposition in poor fens and bogs is much less on hummocks, while there
are few differences between height extremes in rich fens. Hummocks appear to be maintained in
bogs due to low decomposition rates, while in rich fens they are maintained by relatively high
production. (d) Production rates of Sphagnum species are generally similar to or higher than those of
brown mosses. Different species of Sphagnum dominate different parts of the chemical and
topographic gradients in bogs while different brown moss species dominate parts of comparable
gradients in rich fens.
ADDITIONAL KEY WORDS:-Biogeochemistry - brown moss - fen - hummock - Hylocomium
splendens - mires - Sphagnum.
CONTENTS
Introduction .
Background
Bryophyte importance in natural ecosystems
Population stability
Water relations
Material and methods .
0024-4074/90/090035 + 25 $03.00/0
35
36
36
36
37
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39
© 1990 The Linncan Society of London
36
D. H.VITT
M iter chemistry . . . . . . . . . . . . .
Production . . . . . . . . . . . . . .
Decomposition
. . . . . . . . . . . . .
Results and discussion . . . . . . . . . . . . .
Growth and population structure . . . . . . . . .
Growth and climatic gradients. . . . . . . . . .
Chemical and production gradients in mires . . . . . .
Production and decomposition along topographic gradients of mires.
Acknowledgements
. . . . . . . . . . . . .
References
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. 41
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. 46
. 51
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. 43
. . . . . 56
. . . .
57
INI'RODUC'I'ION
This paper is a synthesis of considerable data gathered across boreal Canada
over the past five years. It poses as many questions as answers, but it does this in
an attempt to examine the growth and production of boreal mosses from a
dynamic point of view. Indeed, the tentative answers that can be suggested are
possible chiefly because of greater understanding of the ecology of boreal systems
and the population dynamics of mosses in these systems, rather than to advances
in cryptogamic botany. The purpose of this paper is not to review previous
studies of moss production, but to examine the growth of mosses from a
somewhat different perspective. The basic questions addressed here are: (1) are
there overlooked attributes of moss populations that could be important in
understanding the growth of these plants; (2) how important is macroclimate in
affecting moss growth; and (3) in mires, an ecosystem often dominated by
mosses, what are some of the important physical and chemical gradients that
affect moss production? More specifically with respect to the latter, is there more
production in rich or poor fens; do hummock species really produce more
biomass than those of the hollows, and do brown mosses (mostly
Amblystegiaceae) grow faster than peat mosses (Sphagnaceae)?
BACKGROUND
An understanding of moss growth and production patterns should involve an
appreciation of at least the factors discussed in this section. Unfortunately, until
recently we have known little about some of these, although since the late 1960s,
a considerable amount of data has led to somewhat different ideas on the ecology
of moss populations.
Btyophyte importance in natural ecosystems
Most boreal landscapes are characterized by an abundance of mosses. This
moss component is particularly well developed in areas of organic soils and on
inorganic soils where coniferous trees dominate. In the cool temperate rain
forests of the North American west coast, epiphytic mosses occur in abundance.
Similarly, in polar regions, mosses can be a conspicuous component of the
vegetation, yet few ecologists have considered these plants in any context other
than as an inert carpet in which vascular plants grow (Savile, 1972). Likewise,
until recently little regard was given to the role of mosses in boreal systems.
The view that ascribes little importance to the moss component of vegetation
is still prevalent in the literature, but a number of recent studies suggest a very
BRYOPHYIX PRODUCTION
37
important role for these organisms. Skre, Oechel & Miller (1983), Weber & Van
Cleve (1984) and Bayley et al. (1987) all have demonstrated the importance of
cryptogams in nutrient interception and retention. The potential for nutrient
release from lichens owing to repeated wetting/drying cycles has been shown to
be of significance by Dudley & Lechowicz (1987) and is currently being studied
in tropical epiphytic bryophyte systems (Coxson, personal communication).
These studies, along with those of P6cs (1976, 1980) in a tropical system, Vitt &
Pakarinen (1977) in an arctic system, and Damman (1978) and Malmer (1962)
in peatland systems, provide the basis for suggestions that mosses can play a
critical role in the interception of nutrients, and their subsequent release for use
by vascular plants. In some ways the system can be viewed as a dual one: the
first of vascular plant-herbivore interactions and the second of moss-decomposer
interactions. The two systems are joined when vascular plant roots receive
nutrients via decomposition of the moss layer.
Population stability
The view of early ecosystem ecologists that mosses are inert has also been (and
still is) prevalent in the bryophyte literature in regard to evolution, speciation,
migration and population stability. A frequent axiom of the bryologist is the
great age of the taxa with which he works. Extensions of this in the minds of
some are made in two directions. First, it implies low speciation rates and old,
stenotypic species. However, in recent years the suggestion that mosses are
evolving at slow rates and have remained static for a long period of time has
been challenged on the basis of a number of different lines of evidence. Foremost,
there is no real evidence for this in the fossil record (Krassilov & Schuster, 1984;
Vitt, 1984). These authors all report that Mesozoic mosses cannot be placed in
modern families and that modern genera are only present in the Cenozoic (Early
Tertiary). Also inchded here are studies of genecology by Longton (1976),
cytology by Newton (1981), structure by Vitt (1981) and phenology by Clarke
& Greene (1970, 1971). All these suggest that there is much more genetic
variation within bryophyte populations than had previously been thought.
Furthermore, enzyme polymorphisms among a variety of bryophytes suggest
that genetic variability may be as high as that for flowering plants (de Vries,
Zanten & van Dijk, 1983).
Secondly, the concept of antiquity in bryophyte taxa implies stable
populations, limited seasonal growth, and little effect of disturbance. Again,
recent studies of population ecology of mosses tend to dispute these earlier ideas.
Reviews of population structure and population strategies by Slack (1982) and
During & van Tooren (1987) all point to many mosses being colonist, shuttle or
fugitive species, with highly dynamic life histories. Epiphytic mosses of tropical
forest canopies are limited in age by the longevity of the substrate, while
successional sequences on rotting logs proceed through distinct stages (Muhle &
Leblanc, 1975). These data all suggest that many mosses grow rapidly, and form
relatively large populations in habitats subjected to sizeable amounts of change.
This scenario requires rapid growth and high rates of production early in the life
history of a population. Unfortunately, most of us study mature populations
wherein the rapid initial growth has stagnated. Local colonization, young
population growth, and extinction need to be studied, perhaps following such a
38
D. H. VITT
model as that proposed by MacArthur & Wilson (1967) in their Equilibrium
Theory of Biogeography.
However, it is dangerous to assume that all mosses follow the patterns
described above. In the boreal forest, feather mosses dominate the inorganic soil
terrain, while brown mosses or Sphagna often dominate organic terrain. These
habitats, especially the terrestrial and emergent ones, seem relatively stable.
Competition between species superficially seems evident, yet no data exist that
document this supposed stability. In western boreal Canada few upland, Picea
glaucu (Moench) Voss or Pinus banksiana Lamb. forests reach 100 years in age;
most reach less than 50 years before being burned. Likewise, most North
American peatland stratigraphies reveal highly dynamic histories with drought,
fire or flooding all recognizable disturbances during the past 5000 years.
Thus, evolutionary rates of taxa, as well as growth rates of populations
probably should be regarded as variable from group to group. Many moss
species are young, others are perhaps old and senescent. Likewise, many moss
species have highly dynamic life histories; others are more conservative and
occupy habitats with greater stability, and hence with stronger competitive
interactions. All these strategies require the ability to grow rapidly during some
part of the population life history.
Water relations
The view that mosses all occur in moist, shaded habitats has also, I believe,
been shown to be untrue. Certainly some mosses are truly aquatic, but others
tolerate periods of drought alternating with periods of moisture; still others are
emergent species having a very different set of adaptations (Vitt & Glime, 1984).
The ability of most mosses to withstand periods of drought is largely a
physiological response. In contrast, drought is tolerated by almost all vascular
plants by morphological adaptations. Thus, vascular plants maintain turgor
pressure by a series of complicated morphological features such as welldeveloped conducting systems, leaf stomata and cuticle and roots. Mosses have
these features mostly poorly developed, and have adapted to drought by
reducing photosynthesis and respiration, and withstanding tissue dehydration
(Proctor, 1984). The physical factors that control growth in vascular plants
revolve not so much around water, but around a variety of other factors.
However, in drought-tolerant mosses, growth is directly relatable to the amount
of time the plants are wet and physiologically active (Busby, Bliss 8z Hamilton,
1978). Similarly, Damman (1979) related peat production and the
differentiation of bog types to the length of growing time of Sphagnum species,
that is, the amount of time the moss is wet.
For our purposes here, the following points are relevant: (i) many mosses, if
not most, have the physiological ability to tolerate drought; (ii) during dry
periods growth does not occur; (iii) growth is restricted to periods when the moss
is wet, and (iv) in boreal and temperate systems, the growth periods may be
restricted to intervals of a few hours or at most several days, separated by
generally longer periods of inactivity when dry. No data are available on actual
growing (wet) time, even though to calculate actual growth rates these data are
critical.
BRYOPHYTE PRODUCTION
39
MATERIAL AND METHODS
Population attributes of Drurnmondia frorefens (Hedw.) Britt. were determined
from epiphytic populations collected 3 km north-west of Marquand in Madison
County, Missouri (U.S.A.), in July 1983. Growth was studied from 20
populations of Racomitrium microcarpon (Hedw.) Brid. from granitic rock outcrops
in Pinus bunksiana forest, at the Experimental Lakes Area of Kenora, Ontario,
Canada during May, July and October 1987, and in May 1988.
Single annual increments of Hylocomiurn splendens (Hedw.) B.S.G. from 54
populations throughout its non-arctic North American range were measured and
weighed. Annual increments of 32 populations of Polytrichum stricturn Brid. were
measured from the year previous to collection, all specimens being collected from
Sphagnum fuscum (Schimp.) Klinggr. hummocks only. Specimens from these
analyses are deposited in ALTA.
Water chemist9
Surface waters were sampled from natural pools in Alberta and Ontario. pH
was measured in the field (Alberta), or in the laboratory within 4 hours
(Ontario). Ammonium and NO; were analysed within 24 hours using a
Technicon Autoanalyser as described in Stainton, Cape1 & Armstrong (1977).
Elements were analysed by inductively coupled argon plasma
spectrophotometry. Total organic nitrogen is presented as Kjeldahl nitrogen
minus NHZ. Kjeldahl nitrogen was determined using a Bausch and Lomb
Spectronic 710 spectrophotometer. Further details of analyses are in Bayley et ul.
(1987), Rochefort & Vitt (1988), Chee (1988) and Rochefort (1987). Alberta
study sites are described in detail in Kubiw (1987), Nicholson (1987), Rochefort
(1987), Chee (1988) and Miller & Vitt (unpublished data).
The terms eutrophic and oligotrophic are used in their original limnological
sense (Naumann, 1919; Hutchinson, 1967).
Production
Production of individual species as used in this paper refers to the amount of
plant mass produced in a particular area per year and is numerically equivalent
to net productivity of many authors. Bryophyte production measurements for
each of the mire types were obtained by multiplying production of each species
as determined in small plots by its percentage cover over the mire. In all sites,
bryophyte cover was 100%. Units are g dry weight m-* yr-'. Two study areas
were used (Fig. 1). Bog and poor fen mire segments were studied at Mire 239
located in the Experimental Lakes Area (ELA) of north-western Ontario (Lat.
49"40'N, Long. 93"43'W). The vegetation and chemistry have been described by
Vitt & Bayley (1984). A rich fen was studied intensively at Wagner Natural
Area located 18 km west of Edmonton in central Alberta (Lat. 53"35'N, Long.
113'49'W). The vegetation and chemistry have been described by Miller & Vitt
(unpublished data). Four additional rich fen sites in Alberta were also utilized.
The five Alberta sites are divided into boreal and montane categories. The
boreal category consists of Wagner plus the second study area at Heatherdown,
Alberta. The montane category includes three sites at Nordegg, Crimson Lake,
D. H.
40
vrrr
60N.L.
1
.,
Mi. 0
Km 0
50
100
100
,
' !
.,
Figure 1. A. Map of North America. 0 , Location of Drummondiu prorepens collections;
Experimental Lakes Area, Ontario-site of poor fen-bog study area; A,Wagner Natural Area,
Alberta--site of extreme-rich fen intensive site. B. Alberta, showing loration of 30 peatlands used for
surface water chemistry analyses.
Bog;
poor fen; A,moderate-rich fen; 0 , extreme-rich fen;
6, six sites in close proximity.
+,
and Entrance, Alberta; these sites are higher in elevation, have significantly
lower temperatures and a shorter growing season.
The long term climate (1951-80) at the Ontario and boreal Alberta sites is
quite similar. Yearly mean temperature is 2.0"C at ELA and 2.4"C in central
Alberta; 4-month summer mean temperature is 15.6"C at ELA and 14.6"C in
central Alberta; annual total rainfall is 456 mm at ELA and 391 mm in central
Alberta; days of measurable rainfall are 76 at ELA and 74 in central Alberta.
The montane Alberta sites have yearly mean temperatures of 1.8"C, 4-month
summer mean temperature of 11.6"C, annual total rainfall of 373 mm and 61
days of measurable rainfall. Snowfall is greater at ELA (200 mm) than either
boreal (133 mm) or montane (149 mm) Alberta. Growing season differences do
not appear significant, but the ELA area does receive 65 mm more precipitation
under similar temperature conditions.
Linear growth of Sphagnum was determined by the cranked wire method
(Clymo, 1970), and that of brown mosses by tied threads. Data for Sphagnum are
41
BRYOPHYTE PRODUCTION
from the control area of Mire 239 (Bayley et al., 1987). No differences can be
demonstrated between production estimates from this area and those from a
second mire-(mire 661-Rochefort, 1987). Primary data, statistical design and
methods for calculation of linear growth, surface area, production and moss
increment are given in Rochefort (1987) for Sphagnum and in Miller (1989) for
brown mosses.
Decomposition
Decomposition bags (approximately 2.5 x 2.5 em) were made using 0.3 mm
nylon mesh. Samples of individual species were autoclaved, placed in
monospecific bags, and positioned 10 em below the peat surface. In Ontario,
(Sphagnum species) bags were placed in August 1983, while in Alberta (brown
mosses) they were placed in the mire in May 1983. Further details and the
primary data are in Rochefort (1987) and Miller (personal communication).
RESULTS AND DISCUSSION
Growth and population structure
Population attributes of Drummondia prorepens
Complexity of gametophytes has long been underestimated. Gametophores
produce a number of specialized structures including leaves, paraphyllia,
pseudoparaphyllia, rhizoids, axillary hairs, morphologically different branches
and stems, and of course, sex organs and their associated structures. However,
complexity should not be judged by the number of different types of structures
alone, but also in terms of the number of different individual structures that
must be differentiated.
The population of gametophytes is in reality a canopy of leaves attached to
stems. Each leaf normally has its upper 60-90% green and photosynthetic, and
the remaining lower portion reddish to hyaline, without significant chlorophyll,
the latter portion perhaps serving as a storage area for carbohydrates produced
above by photosynthesis.
My purpose here is to illustrate the complexity of moss populations. Table I
gives some attributes of populations of Drummondia prorepens, a rather small moss
with relatively large leaves. There are few data of this sort, but the numbers are
surprisingly high, with these populations having about 90 stems cm- 2,
approximately 65 leaves per stem and about 6000 leaves cm- 2• In D. prorepens,
the photosynthetic area of each leaf is about 75% of the leaf surface (the lower
25% being hyaline). Thus for each square em of moss, the photosynthetic area is
TABLE
l. Population attributes of Drummondia prorepens ( ± 1 s.o.)
Number of
stems cm- 2
Number of
leaves stem- 1
Number of
leaves cm- 2
Area leaf
(mm2)
Leaf area em-~
(mm 2)
Population I
85 (±10)
N= 15
59 (±19)
5015
0.392 (0.100)
N= 30
1966
Population 2
94 (±17)
N= 15
70 (±29)
6580
0.297 ( ± 0.098)
N=30
1955
42
D.H.VITT
nearly 15 cm'. Simon (1987) estimated 2030 leaves cm-' in Tortula ruralis and
27 966 leaves cm-' in Ceratodon purpureus, and leaf area indices of 44 and 129 for
these two species, respectively.
A model of growth for Drummondia might be as follows, given the assumptions
that photosynthesis occurs only in daylight and when the moss is wet. In southeastern Missouri about 100 days per year have rainfall. Estimating that on the
average the moss is wet only on these days, 12 hours of each occur with light.
This totals to 1200 hours of growth time. Drummondia prorepens has prostrate stems
that produce numerous, erect, closely spaced branches. The Missouri
populations have about 18 branches per cm of stem length (.N = 30 from three
populations) and individual branches each have about 65 leaves. Based on these
data, there are then 1170 leaves cm growth-' yr-I produced over 1200 hours
growing time or 1.0 leaf h-I.
This last figure may be too high, and I present this calculation in order to
stimulate an effort to determine if and where it is incorrect. However, even if out
by a considerable magnitude, the large number of leaves present in a Drummondia
population seems worthwhile considering when determining growth patterns of
mosses. I suggest here that seemingly static, long-term moss populations are not
that at all. For example, Pyluisiellu polyuntha (Hedw.) Grout, a species that forms
stockings on nearly every Populus tremuloides Michx tree across boreal Canada,
has a highly dynamic life history. Large aspens are generally no more than 40-50
years of age. They may have a basal diameter of 50 cm, with a stocking of
P. polyantha about 30-40 cm high. These figures indicate an average increase of
edge growth of about 0.6-0.8 cm yr-* or about 110 cm2 yr-' in total.
Considering that P . polyantha leaves are much smaller than those of Drummondia
prorepens, an immensely active scenario is suggested. This needs further study.
Population growth of Racomitrium microcarpon
Population expansion of the xerophytic moss, Racomitrium microcarpon was
measured in north-western Ontario. At this site on the Canadian Shield, the
moss occupies small depressions on dry, exposed granitic rock shelves (Figs 2, 3),
receiving water from runoff only during and immediately after rainfall. Climatic
characteristics of this area include long, cold winters with abundant snowfall and
relatively short, warm summers. The climate is continental, with only 76 days
receiving measurable rainfall. For this drought-tolerant moss, growth is
restricted to these days of rainfall plus perhaps an additional 10-20 days of
snowmelt.
Growth pattern of R. microcarpon is similar to that of D . prorepens, with
prostrate stems giving rise to short, erect branches. Most populations are elliptic
to circular. Growth and expansion occur along the circumference of the
population.
Table 2 presents the 1987-88 annual growth of seven populations. Expansion
(Figs 4, 5) averages 2.7 cm, with variation ranging between 1.9 and 3.8 cm. If
the populations are idealized as circular or elliptical, then population expansion
varies between about 0.4 and 0.8 cm in diameter. At the Ontario study site, few
populations exceed 20 cm in diameter. These data suggest that populations of
R. microcarpon are less than 40 years old, probably many much less, and have life
histories consisting of repeated local colonization and extinction events.
BRYOPHYTE PRODUCTION
43
Figures 2-5. Habitat and populations of Racomitrium heterostichum. Fig. 2. Habitat. Fig. 3. Population
microhabitat. Fig. 4. Population size on May 4 1987. Fig. 5. Population size on May 4 1988. Area of
ring is 145 cm2.
Growth and climatic gradients
Growth dynamics of Hylocomium splendens
Hylocomium splendens is one of the dominant bryophytes of the upland boreal
forest of North America. It is a pleurocarpous, long-lived perennial that receives
most, if not all, of its nutrients and essential elements from precipitation and
litter fall (Tamm, 1953). Moisture is also largely received from precipitation and
dew (Busby et al., 1978). Its growth, like that of most terrestrial upland and
epiphytic mosses, is dependent on the amount of time the plants are wet
(Callaghan, Collins & Callaghan, 1978). This is, in turn, largely dependent on
rainfall and rates of evapotranspiration.
Hylocomium splendens is also common in coastal rain forests and tundra habitats.
Tundra forms lack the characteristic annual fronds and have been recognized as
H. splendens var. obtusifolium (Geh.) Par. The large coastal form has been named
H. splendens subsp. giganteum Pers. ex Vitt; neither is sufficiently distinct to
TABLE
2. Lateral expansion of seven populations of Racomitrium microcarpon between May 4 1987
and May 4 1988. Data calculated as mean difference in one year's growth along approximately
10 cm of growing front
Population No.
Lateral expansion (in cm)
1
2
3
4
5
6
7
3.8
2.3
3.6
1.9
2.0
2.7
2.8
Mean
2.7
D. H. vITr
44
n
-... .
4
..
I
2
3 4 5
Length (cm)
6
7
-50
I
..'
211
I
,;
I
I I
;
I.,
I I I
,
,;
I
I I
450 950 1450 IS 0 -50 450 950 1450 IS 0
Distance from coast (km)
Distance from coast (km)
Hylocomium splendens
Figure 6. Annual growth relationships of Hylocomium splendens. A. Relationship between annual frond
length and mass. B, C. Relationship between distance from the west coast of North America and B,
annual frond mass, and C, annual frond length.
warrant specific status. The var. obtusfolium has been excluded from the
following analysis.
Length of annual fronds of H. splendens is highly significantly correlated
( r = 0.74) with dry mass of the annual increment (Fig. 6A). Stronger correlation
is shown between some habitat factors and mass rather than with length
(Fig. 6B, C, 7A, B). No significant correlations are present between growth
(mass) and latitude (Fig. 7E) or longitude (Fig. 7F). Populations from eastern
Canada (from 50"W. Long.) to about 118"W. Long. are very similar in amount
of annual growth, but some populations west of 118" have markedly greater
growth (Fig. 7F). Highly significant correlations (at P < 0.01) are present
between growth and precipitation (Fig. 7D, r = 0.71), Tuhkanen's (1980) index
of continentality (Fig. 7B, r = 0.62), and growing season measured as degree
days above 5°C (Fig. 7C, r = 0.54). Degree days and precipitation are also
highly significantly correlated ( r = 0.46). Although it appears that annual
growth in H. splendens is related to both length of growing season and
continentality, the most significant factor is precipitation. Additional variation
in growth can be accounted for by local microhabitat variability.
A smaller data set (Fig. 6B, C) from western North America reveals that
annual growth does not increase evenly across the western boreal zone, but is
constant until the maritime influence becomes pronounced. This pattern of
increased growth along the coast is paralleled by increased precipitation ( r =
0.78); reduced continentality ( r = 0.76), increased mean temperature ( r = 0.71),
but not longer growing season as measured by degree days ( r = 0.37). Thus, I
conclude that several interconnected factors affect annual growth of H. splendens,
with total precipitation and continentality
the most significant.
Evapotranspiration is closely correlated with temperature (Thornthwaite, 1948),
the latter being a major factor in Tuhkanen's index of continentality. These
relationships suggest that evapotranspiration is also a significant factor
controlling seasonal growth of Hylocomium.
Growth dynamics of Polytrichum stricturn
Polytrichum stricturn is an acrocarpous, long-lived perennial almost exclusively
confined to Sphagnum hummocks in ombrotrophic peatlands or on S.jiuscum or
BRYOPHYTE PRODUCTION
7
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g6
.. .
..
.
... : . ... ..·.:..
... ... ·: ...
,' :,. ~
I •
60
0
Precipitation (em)
80
100
120
West longitude
Hylocomium sp/endens
Figure 7. Relationship between annual growth of Hylocomium splendens and various macro-climatic
and geographic parameters. A. Annual frond length and Tuhkanen's index (1980) of continentality.
B. Annual frond mass and Tuhkanen's index of continentality. C. Annual frond mass and degree
days above 5°C. D. Annual frond mass and total annual precipitation. E. Annual frond mass and
North latitude. F. Annual frond mass and West longitude.
S. nemoreum Scop. hummocks. Growth of P. strictum is erect, with the stem apex
maintained about 1 em above the hummock surface. The lower stem is covered
by a dense rhizoidal tomentum. In the genus Polytrichum, moisture is largely
transported to the stem apex by internal conduction from below (Zacher!, 1956;
Scheirer & Goldklang, 1980; Schofield, 1985). Water balance is maintained by
absorption of water from the constantly moist Sphagnum hummock. Growth is
dependent on the water supply present below the moss, yet nutrients and
essential elements are probably obtained mostly from precipitation and litter fall,
as the Sphagnum hummocks in which P. strictum grows are ombrotrophic or nearly
so.
Since growth is linear, it is assumed that the annual length increment is a good
estimate of annual mass increment. Annual length increment of Polytrichum
strictum is not significantly correlated with precipitation (Fig. 8A, r = 0.24),
Tuhkanen's index of continentality (Fig. 8C, r = 0.06), or growing season as
indicated by degree days above 5°C (Fig. 8B, r = 0.24). Little correlation is
present between annual growth and longitude (Fig. 8E, r = 0.00) or latitude
(r = 0.26), although Fig. 8D indicates a tendency towards reduced growth
farther north. Mean annual temperature is correlated (r = 0.31) to annual
length increment at a level of P < 0.1. Longton has published extensively on the
population structure and growth of P. alpestre Hoppe ( = P. strictum-Longton,
1970, 1972, 1974, 1979). He reported variation of the annual growth segments
140
D. H. V I T T
46
I.5
I .o
-f
0.5
..
.. ....
I .o
I.
9
.. .. .
E
0
P
?
0
2.5
50 100 150200250300350°'0-10
0
10
Precipitation ( c m )
Precip./(Mean temperature
20 O%O
+ 10)
60
1.5
I .o
0.5
0
. : *..
.
I .o
....
.-.
0.5
500 1000 150020002508%
Degree days >5"C
50
60
70
North latitude
80
120
140
........
...
I.o
= .
100
80
West longitude
0.0
-20
9 .
-10
-
0
1
10
Mean temperature
("c)
Po& frichum sfricfum
Figure 8. Relationship between annual growth of PoIytrichum strictum and various macroclimatic and
geographic parameters. Annual length increment. A. Total annual precipitation. B. Degree days
above 5°C. C. Tuhkanen's index (1980) ofcontinentality. D. North latitude. E. West longitude. F.
Mean annual temperature.
from 2-5 mm on Signy Island in the Maritime Antarctic to 15-55 mm at North
Temperate Pinawa, Manitoba (Longton, 1974, 1979); this compares to data
presented here that ranges between 2 and 21 mm. Northern sites (65-75"N)
have annual increments between 5 and 10 mm, while southern sites (45-55"N)
have a greater range of values ranging between 4 and 21 mm.
The contrast in correlation patterns between H. splendens and P. strictum is
evident. The annual growth of Hylocomium, an ectohydric, poikilohydric moss, is
strongly affected by macroclimatic patterns of precipitation and continentality,
while Polytrichum strictum, an endohydric moss of stable water regimes, has annual
growth that is not much affected by major macroclimatic pattern; only
temperature can be somewhat related.
It is thus tempting to suggest other limiting factors for growth of P. strictum.
Among these, nutrients such as nitrogen and phosphorus might be suggested.
Rochefort & Vitt (1988) have shown nitrogen to be limiting in growth studies of
Tomentbpnum nitens (Hedw.) Loeske, and Bayley et al. (1987) have demonstrated
increased Sphagnum growth with increased nitrate levels.
Chemical and production gradients in mires
Nutrients and elemental chemistry along the bog-fen gradient
Mosses often form the major component of peat in both ombrotrophic bogs
and minerotrophic fens. Sphagnum is dominant in bogs and poor fens, while
brown mosses, mostly of the families Amblystegiaceae and Brachytheciaceae,
20
BRYOPHYTE PRODUCTION
47
dominate moderate- and extreme-rich fens. The latter are dominated by
Scorpidium scorpioides (Hedw.) Limpr. and Drepanocladus revolvens (Sw.) Warnst.
whereas in western Canada, moderate-rich fens are dominated by Drepanocladus
aduncus (Hedw.) Warnst., D. vernicosus (C. Hartm.) Warnst., D. lapponicus
(Norrl.) Warnst., Brachythecium mildeanum (Schimp.) Milde, B. turgidum (C. J.
Hartm.) Kindb., and Calliergonella cuspidata (Hedw.) Loeske (Chee, 1988).
The vegetation differences of the ground layer are paralleled by chemical
differences in both surface water (Chee & Vitt, 1990) and peat chemistry (Zoltai
& Johnson, 1985; Nicholson, 1988). Differences in surface water chemistry are
well documented for several elements, particularly calcium, sodium and
magnesium, all of which are highest in rich fens and lowest in bogs (Fig. lOA).
Hydrogen ion concentration is highest in bogs and lowest in extreme-rich fens
(shown here as pH-Fig. lOC). The relationship ofpH to calcium in continental
mires normally follows that shown in Fig. lOE. However, few data are available
on either anions (HCO:J, co~-, Cl-, so~-) or for Nand P. This is largely due
to the difficulties in collection and measurement of highly mobile molecules,
especially NO:J, NHt and So~-.
Some authors have considered that the amounts of available nitrogen and
phosphorus follow the same gradient as those for calcium, magnesium and
sodium. If this is so, then rich fens should have higher productivity than do bogs,
while poor fens should be intermediate. Opposed to this view would be one that
advocates a bog to rich fen gradient related to calcium, magnesium, alkalinity
and pH, and a second gradient that is correlated with nitrogen and phosphorus
from oligotrophic bogs and fens at one end to eutrophic marshes and swamps on
the other. Two additional factors are important. First, is the amount of water
flow, for fen systems are influenced by both surface and ground water (plus
precipitation) while bogs are influenced by precipitation only. The result is that
fens, at least potentially, may receive more nutrients over a season due to water
flow, but at any one time the nutrients may not be in higher concentrations than
those of bogs. Schwintzer & Tomberlin (1982) showed that fens in northern
Michigan have lower concentrations of such nutrients as nitrogen and
phosphorus than either bogs or swamps. They suggested that fens have more
tightly biologically cycled nutrients than do bogs. Also, Waughman ( 1980)
presented data from southern German fens showing that NHt and NO:J are
present in lower concentrations in fen peat than in bog peat. Second, seasonal
fluctuation in peat and water chemistry may be significant and has not been
much studied. In North America, seasonal data on some elements have been
presented by Boelter & Verry (1977), Wieder (1985), and Damman (1986).
In earlier literature, some authors have used pH as a highly indicative
measure of overall mire chemistry, comparing data from a variety of sites taken
throughout the field season. Figure 9 compares seasonal pH of a bog, poor fen,
and extreme-rich fen. Two conclusions are clear: ( 1) pH differentiates these
three mire types throughout the season, and (2) bog pH is quite constant
throughout the year and poor fen pH less so, while extreme-rich fen pH is the
most variable. In the latter case, pH is highest in spring and fall during higher
water levels, and lowest due to concentration effects during drawdown of surface
water.
Using data from approximately 100 sites in Alberta, relationships of the major
ions and elements can be seen in Table 3. Of importance are the following. Date
D. H. VITT
48
9-0r-------------------------------------------------------------~
8.0
1985
·······1!1-······
Rich fen W
Rich fen H
7.0
-·-···+·-·-·
Rich fen N
............... Poor fen
•••••••.,........
Bog
1986
Poor fen
Bog
..........................
120
Seasonal pH (days numbered from May I)
Figure 9. Seasonal pH for a bog (ELA, Ontario), poor fen lagg (ELA, Ontario), and three extremerich fens from Alberta. Rich fen W~Wagner; H~Heatherdown; N-Nordegg. Ontario data
extracted from Rochefort (1987); Alberta data from Miller (personal communication).
of water sample collection does not seem to be an important factor. In Alberta,
ground wa~er is highly calcareous near the Rocky Mountain foothills and less so
in the eastern part of the province. Local upland areas in central and eastern
Alberta form areas of discharge and have waters with lower concentrations of
mineral cations. Thus, pH and to a lesser extent calcium are correlated with
distance from the mountains. Owing to this pattern of ground water chemistry,
latitude is somewhat more important than longitude. Mire segment ( = bog,
poor fen, moderate-rich fen, extreme-rich fen-DuRietz, 1949) is highly
correlated with pH and calcium, and also somewhat less so with elemental
sulphur (Fig. I OA, C, F). But of most importance, no correlations are significant
TABLE
3. Pearson Correlation Coefficients for selected surface water chemistry data from 30
localities in Alberta. See Methods for collection and analyses techniques
Date (.N= 13)
Locality (.N = 30)
Latitude
Longitude
Distance from Mountains
(.N= 15)
Mire segment type
*P = 0.001.
Total
NO:! organic N
p
s
pH
Ca
NHJ
0.20
0.53*
-0.76*
-0.60*
0.74*
0.27
0.25
-0.69*
0.19
0.60*
0.05
0.09
-0.07
0.15
0.11
0.09
0.12
-0.11
-0.02
0.08
0.11
-0.10
0.15
-0.17
-0.17
-0,07
-0.24
0.22
-0.10
-0.20
0.06
-0.12
-0.25
-0.02
0.16
0.92*
0.76*
0.07
0.09
-0.07
-0.25
-0.35*
49
BRYOPHYTE PRODUCTION
200
9
A N=IIO
9
C N=94
8
!....
0>
.§ 100
:I:
Q.
6
5
0
u
I
0
2
I
3
4
5
2
0
120
I
4
0
I
2
I i
3
Mire segment
6
.. . .........:· ... ..
:~lit~
... ..
I
I
I
••
Co (mg
r- 1)
~
5
120
D N=94
100
100
:::::80
-reo
:
1/)40
i .
Q.
3
!so
0>
I
:I:
Mire segment
28 N=I06
.
7
E N=93
4
Mire segment
.§
a.
I
I
7
8
4
.
EGO
~40
20
20
5
3
F N=IIO
4
5
6
pH
7
8
9
0
2
3
4
5
Mire segment
Figure 10. Relationships of mire surface water chemistry. A. Calcium and mire segment (I, bog; 2,
poor fen; 3, moderate-rich fen; 4, extreme-rich fen). B. Total phosphorus and mire segment. C. pH
and mire segment. D. Total sulphur and pH. E. pH and calcium. F. Total sulphur and mire
segment.
(even at P < 0.05) between mire segment and N03, NH:, total organic
nitrogen, or phosphorus, and amounts of the nitrogen and phosphorus available
in surface waters are not related to mire segment type. Phosphorus (Fig. lOB) is
present in concentrations usually less than 300 /lg 1- I, and does not vary over the
spectrum of mire segments. These extremely low concentrations of phosphorus
suggest that at least the aquatic portion of these ecosystems may be phosphorus
limited, as are most freshwater aquatic systems. Ammonium (Fig. I lA-C) can
occasionally have higher concentrations in rich fens, but generally occurs at
levels less than l 00 /lg 1- 1• Nitrate (Fig. 11 D-F) is even less abundant, occurring
usually at less than 25 flg l- 1, with a few extreme-rich fens having higher
seasonal values.
Total organic nitrogen (Fig. llG-1) is present in much higher amounts,
generally ranging between 200 and 3000 jlg l- 1• It is here that the most
important trends are evident. Bogs, poor fens and extreme-rich fens have similar,
relatively low concentrations of organic nitrogen, but moderate-rich fens have
higher organic concentrations. In this data set, all extreme-rich fens (having
calcium concentrations over 80 mg l- 1), have low total organic nitrogen ( <
1000 /lg I-'), whereas bogs have values that range between 500 and 2200 /lg l- 1
(Fig. 111). Further data on seasonal changes for this data set are presented in
Chee ( 1988). Rochefort & Vitt ( 1988) presented data that showed increased
growth of Tomenthypnum nitens when fertilized by N03. These results suggest that
this hummock species is limited by nitrogen. They did not study phosphorus,
and the relationships of these two nutrients need to be examined under natural
conditions. Elemental sulphur (S as analysed by plasma spectrophotometry) is
present in very small amounts ( < I mg l- 1) in all mire segments except
extreme-rich fens at pHs above 7.2 (Fig. lOD, F). In these rich fens, sulphur can
occur in very high concentrations (up to 120 mg l- 1).
D. H. VITT
49
c
B
A
...
I
•
.
....,,··.y.•i·
: ,...,_ .:....
~.
G
H
..
.. . ..:,.,.......
I
I
Ill
I
•• I
. . . . ,Ia
I
I
.
Ill,.
~·. : _..
0
2
3
Mire segment
4
F
E
D
•
• .,f~
..... .. .
3.0 4.0 5.0 6.0 7.0 8.0 9.0
pH
.,·~..
.........
=-- : •·:~.... ,. .. .
·~
. ..
40
90
..
140
190
Ca (mg 1- 1)
Figure I I. Relationships of mire surface water chemistry. A-C. Ammonium (NH.t), D-F, nitrate
(NO:l) and G-1, total organic nitrogen plotted against mire segment (1, bog; 2, poor fen; 3,
moderate-rich fen; 4, extreme-rich fen); pH, and calcium (Ca). Mire segment determined by
vegetation criteria alone . .N = 100 in all cases, with vertical bars formed by overlapping points.
In summary, the available forms of nitrogen (NO:J, NH: and total organic
nitrogen) and elemental phosphorus are not significantly correlated with mire
segment, calcium or pH. There is not more nitrogen available in surface waters
of extreme-rich fens than in bogs and poor fens. Moderate-rich fens (sensu Chee,
1988) are characterized by increased total organic nitrogen, while many
extreme-rich fens have large amounts of sulphur.
Production comparisons tif bogs, poor fens and rich fens
The bog used here (at ELA, Ontario) probably is not completely isolated from
ground water, but it does differ significantly in its surface water chemistry
(Fig. 9) and vegetation from the marginal poor fen lagg (Vitt & Bayley, 1984).
For the purposes of this paper it is considered to be a bog, with comparisons
made to the poor fen lagg. Rich fens for the remainder of this paper are extremerich fens characterized by pH greater than 7, marl deposition and dominance of
Scorpidium scorpioides.
Estimates of net moss production in bog and poor fen (Ontario) varied
considerably over the four years measurements were made (between 70 and
225 g m- 2 yr- 1), but were constant between the two areas within a year
(Fig. 12). These estimates are low when compared to those of Grigal ( 1985) who
reported 32D-380 g m -z yr- 1 for a forested bog in northern Minnesota, and to
those of Elling & Knighton (1984) who reported 390 g m- 2 yr- 1 in an open bog
BRYOPHYTE PRODUCTION
51
1987
1986
1985
1984
~
0
250
~
Poor fen
Bog
200
250
Boreal rich fen
Montone rich fen
Total production (g m- 2)
Figure 12. A-D. Total moss production in a bog (ELA, NW Ontario); poor fen lagg (ELA, NW
Ontario), three montane extreme-rich fens (western Alberta) and two boreal extreme-rich fens
(central Alberta). Data take into account vertically projected surface area (Grigal, 1985). Ontario
data extracted from Rochefort (1987); Alberta data from Miller (personal communication).
in Minnesota. However, at Moor House, in England, Forrest & Smith (1975)
estimated Sphagnum productivity at 213 g m- 2 yr- 1 for a site 80% covered by
Sphagnum.
Rich fen production (Alberta) varied from 47 and 93 g m- 2 yr- 1 in higher
elevation, montane sites to 125-131 g m- 2 yr- 1 in lower elevation, boreal sites
(Fig. 12). Low values for montane sites in 1985 appear to be associated with
seasonally low surface water levels (Miller, personal communication). Also, the
large differences in yearly production in Ontario seem associated with
fluctuations in regional climatic variation (data in preparation, Bayley & Vitt).
In summary, annual net bryophyte production in extreme-rich fens averaged
about 101 g m- 2 yr- 1 in Alberta fens. Over four variable years, Ontario poor
fen and bog sites averaged about 132 and 141 g m- 2 yr- 1, respectively. It seems
clear from these data that bryophyte productivity in extreme-rich fens is not
greater than that of bogs and poor fens. This reflects concentrations of available
N and P which, in surface waters do not differ appreciably between these three
mire segment types.
Production and decomposition along topographic gradients
if mires
Hummock-hollow comparisons
The vertical distribution of Sphagnum species along hummock-hollow gradients
has been studied by several authors, including Clymo (1963), Spearing (1972),
D. H. VITT
52
Vitt, Crum & Snider (1975), Horton, Vitt & Slack (1979), Andrus, Wagner &
Titus (1982), Vitt & Slack (1984), Gignac (1987) and others. Vitt & Slack
( 1984) documented niche preferences for species in northern Minnesota,
including the three considered here. Sphagnum angustifolium forms carpets (and
lawns), S. magellanicum occurs on drier lawns and on the sides of larger
hummocks, while S. fuscum occupies the tops of the highest hummocks. These
three species are the dominant Sphagna in both the poor fen lagg and bog
portions of the Ontario study site. Additional indicator species of the lagg fen
that are present in small amounts include S. nemoreum, S. russowii Warnst.,
S. riparium Aongstr. and S. fa/lax (Klinggr.) Klinggr. Of these, only S. fa/lax
occurs in the central portion of the bog, surrounding a small pool not in the
present study area. Vertical zonation of extreme-rich fen bryophytes has been
less well studied, but is descriptively well documented in the literature by Sjors
( 1963), Slack et al. ( 1980), Maimer ( 1986) and Miller & Vitt (unpublished
data). In western Canada, the typical sequence is from Scorpidium scorpioides
submerged in, and emergent from, marl pools and flarks; Drepanocladus revolvens
emergent in somewhat higher carpets; Campylium stellatum (Hedw.) C. Jens. on
lawns and low hummocks; and Tomenthypnum nitens forming higher, often more
isolated hummocks. Other characteristic mosses include Catoscopium nigritum
(Hedw.) Brid., Cinclidium stygium Sw., Calliergon trifarium (Web. & Mohr) Kindb.,
and Meseia triquetra (Richt.) Aongstr.
The vertical zonation of Sphagna in the poor fen and bog is from extensive
carpets through well-developed lawns and mid hummocks, to increasingly welldifferentiated hummocks. It is not strictly comparable in topography to the rich
fen sequence where the emergent (or submerged in high water) carpets ofmosses
in pools containing marl grade to extensive lawns. No clearly differentiated
species specific, mid hummock zone is present. Instead, large clearly defined,
high hummocks are present. For the data presented here, emergent carpets of
S. angustifolium in bog and poor fen and the submerged and emergent carpets of
Scorpidium scorpioides in rich fen are considered equivalent: the mid hummocks
and drier lawns of S. magellanicum, and the wetter lawns of D. revolvens and
C. stellatum are compared as lawns: the mounds of S. fuscum and those of T. nitens
are compared as hummocks.
Comparisons of net production along these topographic gradients are shown
in Fig. 13 and the data are summarized in Table 4. Net production of carpet
TABLE 4. Comparison of production between topographic levels
in bog, poor fen, boreal rich fen, and montane rich fen. Means
calculated from data graphed in Fig. 13 (±s.E.)
Topographic
level
Bog
Poor fen
Boreal
rich fen
Montane
rich fen
Hummock
99
(10)
219
(29)
122
(7)
59
(I)
Lawn
135
(38)
115
(39)
97
(I)
73
(13)
Carpet
160
(19)
14-7
(24)
161
(3)
82
(17)
BRYOPHYTE PRODUCTION
53
Figure 13. Comparison of annual net production along topographic gradients carpet-lawn-hurnmock in a bog, poor fen, boreal rich fen, and montane rich fen. Mean values based on 1984-1987
data in the bog and poor fen and on 1983-1985 in the two rich fen categories.
species is from 11 to 40% higher than those of the lawn zone. Dry lawns in bogs
and poor fens have about 20% less production than the carpets, while
production of rich fen carpets is more variable, having between 0 and 36% more
than the lawns. I n rich fens, the wet lawn zone has more production than the dry
lawn, varying from 24 to 40% more. Thus, in rich fens, the carpet species
(3'. scorfiioides) has about double the production of the dry lawn species
(C. stellatum). Wet lawns of D . revolvens are intermediate. In bogs and poor fen
laggs, the production of the lawn species, S. magellanicum, is about 80% of that of
the carpet producer, S. angustiflium. In northern Quebec, production of
hummock species of Sphagna in poor fens was 58-73 g m-2 yr-', while on lawns
it was 9-19 g rn-' yr-' (Bartsch & Moore, 1985). Hummocks of S.fuscurn in the
bog have about 75% the production of the lawn. These data show that bog
hummocks have less production (about 407") than carpets. However, the poor
fen shows the reverse, with hummock production about double that of the lawn.
Net production of S. fuscum in the poor-fen lagg is also about double that in the
bog. This needs further study, but could be the result of increased nutrients from
minerotrophic sources, more moisture or in our study area a result of complex
successional changes. I n boreal rich fens, production of T. nitens is 25% higher
than that of the lawn, while in montane rich fens it is somewhat less (80%) than
the lawn. The ability of T. nitens to maintain a positive water balance at
increased distances from the water table may explain its relatively large
production values. I n the rich fen system, the drier lawn habitat appears to be
poorly developed.
Decomposition along these same topographic gradients is shown in Fig. 14.
For each of the three Sphagna, no significant differences are present between bog
and poor fen (Rochefort, 1987), likewise there are no significant differences
between boreal and montane rich fens (Miller, personal communication). The
pooled data show distinct differences between sites and between species in bog
and poor fen. Decomposition decreases markedly from carpet to hummock in
54
D.H. VITT
Linear decoy constants
Figure 14. Comparison of decomposition rates, plotted as linear decay constants, in extreme-rich fen,
and bog and poor fen mire systems. Data extracted from Rochefort (1987) and Miller (personal
communication). Rich fen data are means from five sites, poor fen-bog data are means from two
study areas within a single site.
poor fen and bog, with S. fuscum decrease in mass about half that of
S. angustiflium after three years. All differences between species are significant
(Rochefort, 1987). In rich fens, decomposition is similar or slightly less on
hummocks than in carpets, but no significant differences are present. When
compared to bog and poor fen decomposition, rich fen decay constants are about
30% higher in carpets and 60% higher on hummocks.
In summary, net production is similar along the hummock-hollow gradient in
bogs, poor fens and rich fens. In all sites, the carpet level has higher production
than the lawn level. In bogs, hummocks have less production than lawns, whilst
in both poor and rich fens, hummocks have equal or greater production.
Decomposition rates in bogs and poor fens differ from those in rich fens at all
topographic levels. Rich fens show few differences along the topographic
gradient but bogs and poor fens do. The data suggest that hummocks in bogs are
maintained largely by slower decomposition rates relative to carpets, whereas
rich fen hummocks are maintained due to relatively greater amounts of
production. Data for our poor fen site, with high hummock production and low
decomposition rate, are probably explained by the occurrence of rapid
successional changes occurring at the present time. These data also suggest that
BRYOPHYTE PRODUCIION
55
peat accumulation rates of the bog and poor-fen site are higher than that of the
extreme-rich fens.
The controlling factor in accumulation rates is decomposition not production.
Moderate-rich fens, overall probably the most common mire type across boreal
Canada, have higher nutrient levels relative to those of other mire segments, and
have intermediate ranges of pH and basic cations. These systems badly need to
be studied in comparison with those reviewed here. In moderate-rich fen
systems, decomposition should remain similar to that of extreme-rich fens, but
production should be higher owing to higher nutrient levels, giving these fens
greater peat accumulation rates.
Sphagnum-brown moss comparisons
Morphological differences between Sphagna and true mosses (here
represented by brown mosses) are considerable. Sphagnum species, all with a
capitulum, fascicles of branches, and differentiated branches and stems as well as
branch leaves and stem leaves and unique areolation, are very different from
brown mosses with a much simpler stem, irregular lateral branches, and one leaf
type, The complicated capitulum and branch fascicles are lacking.
Comparisons of production between dominant mosses of the bog and poor fen
(Sphagnum dominated) site in Ontario and the extreme-rich fen (brown mossdominated) Albertan sites are shown in Fig. 15. Except for the montane site in
1985, with very dry seasonal conditions, S. scorpioides had production values
ranging between 108-170 g m-2 yr-I. During moist years, its microhabitat
equivalent in the bog and poor fen sites, S. angustifolium, had production of
186-198 g m-' yr-I. However, in the drier years its production dropped to
97-157 g m V 2yr-I. The same pattern is evident for the wet lawn former
Drepanocladus revoluens (91-109 g m-' yr-l excluding the montane site in 1985)
and the dry lawn former Campylium stellatum (58-78 g m-' yr-I) compared to
Sphagnum magellanicum during moist years (103-246 g m-' yr-') as opposed to
drier years (52-73 g m-2 yr-I). Thus, species of Sphagnum dominated systems
produce more during favourable years, but less during time of stress. If the rich
fen data for 1985 (montane) are indicative of rich fen species production during
dry years, then the pattern shown here suggests that for equivalent
microhabitats, Sphagna have higher productivity than brown mosses in carpet
and lawn microhabitats.
Tomenthypnum nitens production values range from 55-64 g m-' yr-' in
montane fens to 104-131 g m-' yr-' in boreal fens; no change in production is
evident for this species in 1985 at the dry montane site. During 1985, T. nitens
contributed much more to the overall site production compared to the other
brown mosses. Sphagnum fuscum values ranged from 156-303 g m-' yr-' in the
poor fen to 69-119 g m-' yr-I in the bog. Production of this species increased
consistently during the four-year period (with increasing drought) in the bog,
but generally decreased in the poor fen during the same period. Thus, in the bog,
as conditions became drier between 1984-1987, S. fuscum contributed more to
relative production. Although S. fuscum production decreased somewhat under
poor fen conditions, its relative contribution still increased during this four-year
period.
In summary, Sphagnum species over a hummock-hollow gradient have similar
or somewhat higher production values than brown mosses over the same
56
D. H. VITT
Rich fen (g m-')
M-Montone B-Boreal
Figure 15. Comparison of production between Sphagnum and brown mosses along topographic
gradients from a bog (ELA, NW Ontario), three montane extreme rich fens (western Alberta), and
two boreal extreme rich fens (central Alberta)). Data not corrected for per cent cover of each
species, but represent the total production of a given species rnv2 yr-'.
gradient. Sphagnum growth was more variable between years in the poor fen,
while rich fen production in boreal sites had greater similarity with that in bogs
and poor fens, than with montane rich fens (Table 4). Production in drier lawns
of rich fens is relatively low, and may be one of the reasons why this zone is not
as well developed as it is in Sphagnum-dominated systems.
ACKNOWLEDGEMENTS
The research was supported by grant A6390 from the Natural Sciences and
Engineering Research Council of Canada (NSERC) for which I am grateful.
Work in Ontario was also supported by the Canadian Department of Fisheries
and Oceans, while work in Alberta was partially supported by the Boreal
Institute of Northern Studies and the Central Research Fund of the University of
A1berta.
I am grateful to Diana Horton for critical comments, to Suzanne Bayley for
use of chemical data from ELA, and to Wai-Lin Chee, Helen Kubiw, Chris
Miller, Barbara Nicholson, and Line Rochefort, whose chemical, production and
decomposition data I have made liberal use. Stephen Zoltai (Canadian Forestry
Service) provided many of our element analyses, while Ellie Prepas (Department
BRYOPHYTE PRODUCTION
57
of Zoology, University of Alberta) provided equipment for nutrient analyses.
Arne Frisvoll (Trondheim, Norway) determined Racomitrium microcarpon. This
paper has benefited much from discussions with C. Miller, L. Rochefort, and
W.-L. Chee; for all of their ideas and support, I am appreciative.
The original analyses of Hylocomium and Polytrichum were completed by George
Fabi and Audrey Boyle; I am pleased to have been able to include updated
versions of their analyses in this paper.
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