Nucleation during primary succession in the Canadian Rockies
D.J. BLUNDON,'D.A. MACISAAC,
A N D M.R.T. DALE?
Botany Department, University of Alberta, Edmonton, AB T6G 1E9, Canada
Received February 17, 1993
D.A., and DALE,M.R.T. 1993. Nucleation during primary succession in the Canadian Rockies.
BLUNDON,
D.J., MACISAAC,
Can. J. Bot. 71: 1093- 1096.
A study of nucleation during primary succession was carried out on age sequences of communities at two sites in the Canadian Rocky Mountains: one at the Mount Robson moraines, British Columbia, the other at Southeast Lyell Glacier, Alberta.
The study concentrated on the associations of species with the nitrogen-fixing plants Hedysarum boreale var. rnackenzii at
Mount Robson moraines and Dryas drummondii at Southeast Lyell Glacier because those plants might serve as nuclei for
colonization by other species, thus facilitating succession. The data show that recruitment of later successional species is
greater in patches of the two pioneer species, but the fact that recruitment takes place away from the plants also suggests
that although there is nucleation, it is not necessary for succession at these sites.
Key words: colonization, nitrogen fixation, nucleation, succession
D.A., et DALE,M.R.T. 1993. Nucleation during primary succession in the Canadian Rockies.
BLUNDON,
D.J., MACISAAC,
Can. J. Bot. 71 : 1093- 1096.
Les auteurs ont conduit une Ctude sur la nuclCation au cours de la succession primaire en suivant les chronosCquences de
communautCs IocalisCes sur deux sites dans les Rocheuses canadiennes : le premier est situC i mont Robson Morraines en
Colombie-Britannique, et le second i Southeast Lyell Glacier en Alberta. L'essentiel de I'Ctude i port6 sur les associations
des espbces avec les plantes fixatrices d'azote telles que I'Hedysarutn boreale var. mackenzii au mont Robson et le Dryas
drumrnondii i Southeast Lyell Glacier, parce que ces plantes pourraient servir de noyau de depart pour la colonisation par
les autres espbces, amorCant ainsi la succession. Les rCsultats indiquent que l'implantation des espbces propres aux Ctapes
ultCrieures de la succession est plus importante dans les colonies des deux plantes pionnibres, mais le fait que l'implantation
se fasse i une certaine distance de ces plantes suggbre que bien qu'il y ait nuclCation, ce ne serait pas, dans ces sites, un
situation de succession.
Mots elks : colonisation, fixation de l'azote, nuclCation, succession.
[Traduit par la rCdaction]
Introduction
In the development of a plant community, one feature that
may be important to an understanding of its dynamics is its
phytosociological structure, the network of associations between
species. These associations result from the biological factors
that cause the plants of different species to grow close together
or far apart (Dale 1982). A particular kind of positive association that may be important during succession is known as
nucleation. The term nucleation, first applied to succession by
Yarranton and Morrison (1974), refers to the phenomenon of
the plants of one species forming centres of establishment and
"nuclei" for the subsequent growth of other colonizers. This
kind of positive influence may be necessary for succession to
proceed or it may merely increase the rate of subsequent
colonization, thus facilitating succession.
Most of the literature on nucleation has focussed on secondary succession. For example, nucleation has been cited as a
major mechanism by which tropical or subtropical forest or
woodland species become established on adjacent rangeland.
In these situations, pioneer trees or remnant trees in logged
areas often are recruitment foci for bird-disseminated seeds
(Debussche et al. 1982; Guevera et al. 1986). Enhanced soilnutrient, moisture, and litter conditions under these trees allow
establishment of the forest species (Koechlin et al. 1986;
Campbell et al. 1990; Manders et al. 1992).
In looking for plants that might serve as such nuclei for
colonization during primary succession, plants that fix nitrogen should be considered. Aarssen and Turkington (1985a,
'Present address: Camosun College, Victoria, BC V8P 552, Canada.
?Author to whom all correspondence should be addressed.
Prinrcd in Canada 1 ImprimC au Canada
19856) showed that very specific grass-legume associations
can develop in mixed pastures that are partially influenced by
nitrogen enrichment by the clover Trifolium repens. Aarssen
et al. (1979) proposed that the legume was a major force in
pasture community change. Another nitrogen fixer, Myrica
faya, controls early succession at volcanic sites in Hawaii
(Vitousek and Walter 1989).
In the chronosequence on the Mount Robson moraines, the
legume Hedysarum boreale var. mackenzii is an important
colonizer and also primary contributor of biological nitrogen
(Blundon and Dale 1990). Hedysarum is one of the pioneer
species on the Mount Robson moraines (Tisdale et al. 1966;
Sondheim and Standish 1983). At the Mount Robson site,
Dryas drummondii was not found to be nodulated and contributed little to the nitrogen input (Blundon and Dale 1990),
unlike at other sites such as Glacier Bay (Crocker and Major
1955). In contrast, at the Southeast Lyell Glacier site, Hedysarum was rare and Dryas was abundant. W e therefore concentrated our attention on Hedysarum at the Mount Robson
moraines and on Dryas at Southeast Lyell Glacier.
The objectives of this study were to investigate the possible
role of Hedysarum and Dryas plants as nuclei for colonization
during succession by investigating the associations of other
species with them.
Methods
Site descriptions
The Mount Robson site in British Columbia is described in more
detail elsewhere (Dale and Blundon 1990; Blundon and Dale 1990),
but we provide a summary here. The Mount Robson glacier has
deposited a terminal moraine and approximately 10 recessional
moraines during the past 200 years. These moraines (53.loN,
1094
CAN. 1.
BOT. VOL. 71,
TABLE1. Species associated with Hedysarurn boreale on moraines of
different ages at Mount Robson, based on the comparison of frequencies in plant-centred and random quadrats
TABLE2. Results of goodness of fit tests for nucleation at Southeast
Lyell Glacier, showing whether the number of stems was significantly greater in Dryas than on the unvegetated surface
Association
Moraine
Age
(years)
8
46
7
52
5
73
3
94
1
184
Positive
Negative
Salix glauca,
Castilleja occidentalis,
Dryas integrifolia,
Salix seedlings
Picea erzgelmanii,
Bractlytheciurn sp.
Dryas drurnmondii,
Dryas octopetala
Campyliurn sp.
1993
Area
*>
Approx. age
(years)
Live Dryas
Sign.*
Dead Dryas
n
Sign.*
n
indicates significantly more stems per unit area in Dryas at the I % level.
Dryas drurnrnondii
Pyrola asarifolia,
Brachytheciuni sp.,
Tortula norvegica,
Ditrichum flexicaule,
Cladonia sp.
Dryas octopetala,
Dryas integrifolia
1 19.1 OW) are at elevations of about 1650 m. The moraines under
study are 1.49-0.66 km from the terminus of the glacier and are
referred to as moraines 1, 3, 5, 7, and 8 (Heusser 1956). The
moraines were formed in approximately 1801, 1891, 1912, 1933, and
1939. Soil development was described by Tisdale et al. (1966) and
more recently by Sondheim and Standish (1983). The development of
vegetation on these moraines can be divided into three phases. The
pioneer Hedysarum phase on moraine 8 was dominated by the herb
Hedysarurn boreale var. mackenzii Nutt. ( R i ~ h . ) ,the
~ low shrubs
( < 1 m in height) Salix vestita Pursh and Salix glauca L., and the
dwarf shrub ( < 0 . 1 m in height) Dryas drumrnondii Richards.
on recessional moraines 7 and 5 was
The Dryas transitional
dominated by D. drurnmorldii, H. boreale, and the low shrub
S. glauca. Picea engelnzarznii Parry ex Englem. was beginning to
emerge above the Salix on moraine 5. The oldest successional plant
community was found on moraines 3 and 1 and on the terminal. It
was dominated by P. engelrnannii, the dwarf shrub Arctostaphylos
ricbra (Rehder & Wils.) Fern., H. boreale, the bryophyte Bractiytheciurn sp., and the lichens Cladonia cariosa (Ach.) Spreng and
Cladonia pyxidata (L.) Hoffn.
The second site is the proglacial morainal deposits adjacent to the
Southeast Lyell Glacier in Alberta at about 1600 m altitude (5 1 "54'N,
116"58'W) uncovered by the retreat of the Southeast Lyell and Mons
glaciers. The development of the vegetation begins with the dwarf
shrub D. drurnrnondii that forms an almost continuous carpet within
20 years. There was almost no other herbs on the young surfaces, but
tree species such as P. engelmarlt~iibegin to establish soon after
deglaciation. Shrubs, primarily Shepherdia canadensis (L.) Nutt. and
Salix spp., appear at age 40 years and the tree canopy starts to close
around 90 years. At that point, the Dryas begins to senesce and dead
areas in the Dryas mat are colonized by mosses. Patches of dead
Dryas may remain intact for many years. The terminal moraine
(130 years) is vegetated by a Picea forest with a dense shrub stratum
and almost no Dryas. There is a ground cover of various mosses.
Sarnpling
Hedysarurn boreale grows in concentric clumps and is thus convenient for a test of nucleation because the patches are all approximately the same shape. The method used was a simplified version of
the sampling method used by Dale (1977) to examine the incidence
of asymmetric relationships (influence) in a mixed-forest community.
3Nomenclature follows Ireland et al. (1980), Moss (1983), and
Egan (1987).
'The test was carried out on recessional moraines 8, 7, 5, 3, and 1 by
comparing the frequency of species in 200 circular quadrats centred
on randomly chosen Hedysarum plants with their frequency in 200
randomly placed circular quadrats of the same size. The diameter of
the quadrats was determined by the average diameter of another 100
randomly selected Hedysarum plants on the moraine being sampled
so that their size varied from moraine to moraine. By basing the size
of the sampling unit on the sizes of the plants under study, the
problems of detecting associations at a scale other than that of the
plants themselves are reduced. Sampling to study nucleation by
Hedysarum was carried out in early July 1985.
At the Southeast Lyell Glacier site, a different sampling technique
was used. Here six areas of Dryas were sampled, aged 25 -45 years.
Each area was sampled with between 6 6 and 250 randomly placed
1-m2 quadrats. The cover of each quadrat was divided up into the
classes of live Dryas, dead Dryas, and bare surface and the proportion of each type recorded. The number of conifer and shrub plants
recruiting were counted for each of the three cover classes. The sampling was carried out in the summer of 1984.
Statistical analysis
In the comparison of the species frequency in Hedysarum-centred
samples with frequency in random centred samples, the original data
were arranged in 2 x 2 contingency tables and the G-test was applied
with William's correction (Sokal and Rohlf 1981). Here the four
entries in the table were the frequencies of presence in plant-centred
samples, absence in plant-centred samples, presence in random samples, and absence in random samples. Because of potential problems
arising from low expected values, tables with expected values less
than one were discarded. In the case of a significant test result ( a =
0.05), if a species is found more often than expected in Hedysarurn
clumps, we say that it is positively associated; if less often, it is negatively associated.
A similar method was used to analyse the Dryas data, by testing
the null hypothesis that the number of tree and shrub stems found in
each ground cover type was proportional to the occurrence of that
cover type. Since we are dealing with a small range of surface ages
and thus concentrating on the establishment of woody species Salix
and Picea, we did not test the individual species associations with
Dryas separately. We again used the G statistic as a goodness of fit
test for a single classification with William's correction (cf. Sokal and
Rohlf 1981, p. 704). We compared the frequency of stems in live
Dryas with their frequency on unvegetated surface, and also the frequency of stems in dead Dryas with that on the unvegetated surface.
Results
The mean diameter ($ standard error) of H. boreale clumps
on moraines 8, 7, 5 , 3, and 1 is 23.68 $ 1.37, 22.84 $- 0.83,
28.47 $- 1.23, 29.13 $- 1.61, and 14.95 $ 0.63 cm, respectively. The results of the association analyses are shown in
Table 1. On the youngest recessional moraine 8, several
BLUNDON ET AL.
species including Salix seedlings were found significantly
more often in neighbourhoods of Hedysarum. On moraine 7,
P. engelmannii and the moss Brachythecium sp. were significantly associated with Hedysarum. On the oldest recessional
moraine 1, Pyrola asarifolia Michx., three mosses and the
lichen Cladonia sp. were significantly associated with Hedysarum. Most of the cases in which there was negative association with Hedysarum were with species of Dryas.
The analysis of the data from the Southeast Lyell Glacier
site provides a very clear result. The density of tree and shrub
stems is not equal in the three cover types: there are consistently more stems than expected in dead Dryas and significantly fewer in unvegetated areas (Table 2). In four of the
areas, living Dryas had significantly more stems than did the
unvegetated surface, indicating enhanced recruitment.
Discussion
At Mount Robson, evidence for nucleation is found on the
early recessional moraines 8 and 7, with S. glauca and
P. engelmannii being associated with H. boreale plants. These
two species eventually become dominant on the intermediate
and late recessional moraines, respectively. On the oldest
moraine (moraine l), Tortula nowegica, Brachythecium sp.,
and Cladonia sp., which are important understory species, are
positively associated with Hedysarum. On gravel outwash of
the Muldrow Glacier, Alaska (63"24'N, 150°36'W), another
moss, Ditrichum jlexicaule, was observed growing within
clumps of Hedysarum (Viereck 1966). During the intermediate phase of succession (moraines 3 and 5) when Hedysarum
patches are coalescing, no species are positively associated
with Hedysarum. The negative association of species like
Dryas on the older moraines may be the result of competition
or may indicate different ecological requirements, but this
aspect was not pursued.
During the early phase of community development, the
Hedysarum clumps form centres of establishment for Salix
seedlings and for the growth of S. glauca and P. engelmannii.
Hedysarum may provide more hospitable sites for the germination of these species and eventual enrichment of the local
substrate with nitrogen (through symbiotic Rhizobium), which
would enhance their growth (cf. Blundon and Dale 1990). During the late phase of community development, the Hedysarum
patches are breaking into smaller clumps and ground-cover
plants like P. asarifolia, mosses, and the lichen Cladonia sp.
are occupying sites within and between these clumps.
Turner (1983) points out that in the few studies that have
suggested that facilitation is the mechanism of community
change, nonobligate facilitation is the most common: late
colonists can establish in areas devoid of earlv colonists but
establish fastei in their presence. Many of the species that
dominate the late phase of community development on the
Mount Robson moraines are present in the early phase of community development; for instance, Picea seedlings were found
on the youngest moraine studied. Facilitation is occurring, but
it is nonobligate, probably affecting only the rate of community development.
At the Southeast Lyell Glacier site, Dryas may have a less
important role in nitrogen input compared with Hedysarum at
the Mount Robson moraines. There is some evidence that rate
of N accumulation under Dryas is much lower than for
legumes and other species common in the early successional
environments (Crocker and Major 1955; Fitter and Parsons
1987; Blundon and Dale 1990). It may be that the Dryas-
1095
centred nucleation is primarily due to modification of physical
environment conditions. The effect of Dryas on microsite stability and soil development (including organic matter accumulation, trapping of fine material, and moisture retention) is
well documented (e.g., Anderson 1967; Watson et al. 1980),
suggesting that a well-developed Dryas mat creates seedbed
conditions favourable to establishment of trees such as Abies
lasiocarpa and P. engelmannii. Seedling mortality of these
conifer species is greatly increased by needle ice, frost heave,
and soil moisture deficits (Noble and Alexander 1977; U.S.
Forest Service 1990), whereas germination is greatly enhanced
on moisture-retaining seedbeds (Day 1964). Since both tree
species can establish on duff substrates (U.S. Forest Service
1990), such as decaying Dryas mats, it seems logical that
Dryas would increase the rate of establishment. Dryas may
also act as nucleation foci by trapping seeds of colonizing
plants, a phenomenon reported in other examples of primary
succession (Day and Wright 1989).
Recruitment of woody species seems to be more strongly
associated with dead rather than live Dryas patches. This may
be because clones that had dead patches were older than those
with no dead cover, and in the former there was a longer
period for favourable microsite moderation to occur. Alternatively, the dead material may be releasing nutrients more
quickly as it decays or the living Dryas affects the other species by competing for resources.
Although there is good evidence that the density of tree and
shrub recruits is correlated with the presence of Dryas,
recruitment occurs even on unvegetated surface, so subsequent stages will occur even without Dryas. Therefore,
although nucleation is occurring, it is not essential for succession.
When the two sites are compared, one sees similar
phenomena. In each, the presenci of an early colonizer is
related to higher rates of recruitment of later woody species,
whether driven by nutrient or physical factors. In both
instances. the overall effect seems to be to facilitate succession
without &ing necessary for succession to occur. What is especially interesting about the comparison between the two sites
is that although the same kinds of processes are occurring, two
different species can be shown to be involved in nucleation.
Acknowledgements
This research has been supported by the Natural Sciences
and Engineering Research Council of Canada and by the
Boreal Institute for Northern Studies. We thank Parks Canada
and the British Columbia Parks Service for permission to carry
out our field studies in the parks. A large number of people
deserve credit for their assistance with the fieldwork.
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