succession in the southern part of the canadian boreal forest

Vegetatio 79:51-63, 1989 © 1989 Kluwer Academic Publishers. Printed in Belgium. 51

Succession in the southern part of the Canadian boreal forest

Yves Bergeron I & Michelle Dubuc 2 I Groupe de Recherche en J~cologie Forestibre, Ddpartement des Sciences Biologiques, Universit~ du Qudbec ?l Montrdal, CP 8888 Succ A, Montrdal, H3C 3P8, Canada; 2D~partement des Sciences Biologiques, Universitd de Montrdal, CP 6128 Succ A, Montrdal, H3C 3J7, Canada; Present address." Department of Biology, McGill University, 1205 Avenue Dr Penfield, Montreal, Canada, H3A 1B1

Accepted 21.10.1988

Keywords: Abies balsamea, Betula papyrifera, Climax, Fire, Picea glauca, Picea mariana, Pinus banksiana, Pinus resinosa, Pinus strobus, Populus tremuloides, Thuja occidentalis

Abstract

Forest succession following fire in a forest mosaic of northwestern Quebec has been studied in order to: (1) describe the successional pathways using communities of different ages and (2) evaluate convergence of successional pathways and possible effect of fire suppression on the establishment of steady-state communities. As a first step, ordination and classification techniques were used in order to remove changes in forest composition which are related to abiotic conditions. Then, ordinations based on tree diameter distributions were used to study shifts in species composition in relation to time since the last fire.

Even under similar abiotic conditions, successional pathways are numerous. However, regardless of forest composition after fire, most stands show convergence toward dominance of Thuja occidentalis and Picea mariana on xeric sites and dominance of Abies balsamea and Thuja occidentalis on more mesic sites. Stable communities of > 300 yr occur on xeric sites while on mesic sites directional succession still occurs after 224 yr. Nearly all species involved in succession are present in the first 50 yr following fire. Only Abies balsamea and Thuja occidentalis increase significantly in frequency during succession. Following initial establishment, successional processes can generally be explained by species longevity and shade tolerance. Early successional species may be abundant in the canopy for more than 200 yr while the rapid decrease ofPiceaglauca, a late successional species could be related to spruce budworm outbreaks. Considering the short fire rotation observed (about 150 yr), a steady-state forest is unlikely to occur under natural conditions, though it may be possible if fire is controlled.

Nomenclature." Hosie, R.C. 1972. Native trees of Canada. Environment Canada, Forestry Service.

Introduction

Fire is the most important natural factor controlling dynamics of the Canadian boreal forest (Heinsel- man 1981; Johnson 1979; Rowe & Scotter 1973; Wein & Maclean 1983). Tree species have developed numerous adaptations to resist fire or to regenerate

quickly following fire (Ahlgren 1960; Rowe 1983). In the case of frequent fire recurrence, successional processes often involve few or no species replacement, thus engendering a cyclic maintenance of the same species (Horn 1981). Cyclic succession has been reported for arboreal communities dominated by Pinus banksiana (Carleton & Maycock 1978;

52

Clayden & Bouchard 1983; Shaft & Yarranton 1973), Pinus resinosa (Bergeron & Gagnon 1987; Heinsel- man 1973) and Picea mariana (Carleton & Maycock 1978; Foster t985; Viereck 1983). Directional succession involving species replacement has also been documented (Bergeron & Bouchard 1984; Carle- ton & Maycock 1978; Dix & Swan 1971; Foster & King 1986). Directional succession occurs mainly on upland sites where post-fire successional communities converge towards forests dominated by Picea spp. and Abies balsamea. However, although a clear temporal replacement sequence has been reported for herb, moss, and lichen ground layers (Clayden & Bouchard 1983; Tamm 1964; Taylor et al. 1987) different growth rates could make it appear that there are several different stages before a mature arboreal community is reached (Viereck 1983). The short fire cycle characteristic of the boreal forest does not allow enough time to further development of the forest and rarely a steady-state is reached (Heinselman 1981). Recent fire suppression may, however, eventually lead to a further successional development toward steady-state or, as suggested by Heinselman (1973), toward paludification and peat- land formation rather than stable upland vegetation.

The study of arboreal succession in fire prone ecosystems is not easy. Since it is not possible to fol- low single sites long enough to observe species replacement, succession must be deduced from similar sites representing different stages following fire. Changes in species composition related to successional processes must be extracted from variations caused by site characteristics or variations in fire behaviour (Heinselman 1981). These problems may be partially overcome by using ordination techniques that sort out time-related phenomena from site variation. Species succession vectors (Golf & Zedler 1972) and size-class ordination (Enright 1982) are useful tools in this context, especially if used con- jointly with the study of stand ages and fire-scar dates.

In this paper we use size-class ordination and fire- history reconstruction to study arboreal succession of a forest mosaic in northwestern Quebec. Our ob- jectives are to: (t) describe successional pathways using communities of different ages and (2) evaluate the convergence of successional pathways and the

possible effect of fire suppression on the establishment of steady-state communities.

Study area

The study area consists of the shores and islands of Lake Duparquet, situated in the Abitibi region of northwestern Quebec at 79021 ' -79°13 'W and 48o26 ' -48°30 'N. The lake covers approximately 50 km 2 and contains 170 islands ranging in size from a few m 2 to more than 1 k m 2. Lake Duparquet is part of a watershed which drains northwards, through Lake Abitibi, to James Bay.

The surrounding region is known as the Northern Clay Belt of Quebec and Ontario, a large physiographic region created by lacustrine deposits from the maximum post-Wisconsinian extension of the preglacial lakes Barlow and Ojibway (Vincent & Hardy 1977). Within this region, hills with partially reworked or eroded morainic deposits are inter- spersed in lowlands covered by clays from the progla- cial lakes (Bergeron et al. 1982). The study site is characteristic of this pattern as the lakeshore topo- graphy alternates between flat clay plains and steep hills.

The nearest meteorological station is at La Sarre, 35 km north of Lake Duparquet. Mean annual temperature is 0.6°C, mean annual precipitation is 822.7 mm and the mean annual frost-free period is 64 days (air temperature below 0°C at 1.5 m above ground). However, freezing temperatures may occur throughout the year (Anon. 1982).

Lake Duparquet is situated at the southern limit of the boreal forest in the Missinaibi-Cabonga section where an association of Abies balsamea, Picea mariana and Betula papyrifera dominates with Pi- cea glauca and Populus tremuloides (Rowe 1972). The area surrounding the lake is composed by forest of A bies balsamea and Betula papyrifera with Picea glauca on rnesic sites. Bogs and hydric sites are dominated by Picea mariana, Thuja occidentalis and La- rix laricina. Fraxinus nigra and Ulmus americana forests occur on floodplains. Picea mariana and Pi- nus strobus are usually found on xeric sites (Berge- ron & Bouchard 1984).

Forest fires have created numerous successional

communities dominated by Populus tremuloides and Betulapapyrifera on mesic sites, by Populus bal- samifera on hydric sites, and by Pinus banksiana on xeric sites (Bergeron & Bouchard 1984). The shores of the lake have undergone many fires, the most important being around 1760, 1797, 1816, 1823, 1846, 1847, 1870, 1887, 1916 and 1944 (Bergeron & Gagnon 1987 and unpublished data). Each island, however, has its own fire history which is poorly correlated with the shore habitat fire history (Bergeron & Gagnon 1987). Logging has occurred in the area since the early 19th century and may have affected species distribution around the lake. However, the islands have remained largely un- touched.

Methods

Sampling

Sites were assigned to one of the following three physiographic units: lakeshore, large island and small island. Shore areas and two large islands were divided into sections extending 100 m along the shorefront and 50 m deep. Ca 15% of each of the 3 physiographic units were randomly selected for detailed study. In the field, sites with heterogeneous soil conditions or forest composition were split into sub-units. This procedure was relatively common for islands, where xeric exposed bedrock alternates with mesic morainic deposits.

Trees >5 cm DBH were sampled in 100x50 m plots or sub-units using the point-centered quarter method (Cottam & Curtis 1956) with 20 systemati- cally located points. Fire history was documented by dating even-aged cohorts of fire-prone species and by collecting tree slabs bearing fire-scars (Arno & Sneck 1977). In the former case, a minimum time since fire is approximated by the age of the oldest individual of a cohort, whereas in the later case, near exact dating of fire is possible. Deposits at the surface (Bergeron etal. 1982), slope (%), aspect (ranked from north to south), drainage class (CCP, 1972), and rockiness (%) were recorded at each site.

Vegetation analysis

Variation in tree species composition was analyzed

53

using ordina.tion and classification techniques. Clas- sification and ordination are complementary techniques. They permit the classification of forest stands and facilitate the identification of relationships between the defined communities, respectively (Gauch & Whittaker 1982). Reciprocal averaging analysis (RA; Hill 1974) and two-way species indica- tor analysis (TWINSPAN; Hill 1979) were used. Gauch (1982) noted the effectiveness of these techniques for community data analysis. RA was pre- ferred to detrended correspondence analysis (DCA) after initial analysis with the two methods showed that RA preserved more interpretable information on the first two ordination axes. Untransformed frequency values (number of occurrences in the 20 point centered quarters) were ordinated. Four discrete frequency classes (1: 1-25%; 2: 26-50%; 3: 51-75%; 4: >75%) were used with the TWIN- SPAN program (Hill 1979). In both analysis, species which occurred in fewer than 10% of the stands were excluded.

The data structure revealed by the ordination analysis was interpreted using Spearman's rank correlations between the position of the stands on the first two axes and the surface deposits, aspect, slope, drainage class and rockiness as well as time since fire. Surface deposits were ranked from coarse to fine textured to perform the rank correlation. In order to lookat oblique correlation of abiotic descriptors with axes a trend surface analysis (Dargie 1984) with multiple linear regression was also performed.

Stand clusters produced by TWINSPAN were interpreted using the same descriptors by means of contingency tables. Prior to this, the quantitative abiotic variables were partitioned into discrete states in such a way as to maximize discrimination between stand clusters. These may be statistically realised in a partition that, among all possible partitions of a variable, maximises the log-likelihood chi 2 statistic (G-statistic, Sokal & Rohlf 1981) for non-homoge- neity of the distribution across the groups of the classification (Legendre & Legendre 1983). The discrimination potential of abiotic variables and time since fire over the classification was investigated by means of information theory (Legendre & Legendre 1983). The basis of this analysis is to consider the global entropy (disorder) of the stand clusters and to

54

discover how this disorder could be reduced by the knowledge of explaining variables. The relative amount of information provided by a variable over the classification can be obtained by computing an asymmetric uncertainty coefficient for each variable. Knowing the most discriminating variable, it becomes possible to combine this with the classification in a new reference variable. Then, a new set of asymmetrical uncertainty coefficients with the other variables was computed in order to extract residual information (Vincent et al. 1986). Results of these correlation and contingency analyses permitted the removal of differences between communities which are related to abiotic conditions. In order to mini- mize this effect in studying succession processes, sites were allocated to homogeneous abiotic subsets for further analyses.

Size-class ordination

Sites belonging to the abiotic subsets were re-ordinated using size-class ordination. The trees were first split according to 5 diameter classes (1: 5 -10 cm; 2: 11-15 cm; 3 :16-20 cm; 4:21-25 cm; 5 :>25 cm). Species frequencies were then considered for each of these size-classes, increasing the number of stands five-fold. On the ordination diagram, points representing diameter classes of the same stand were linked in decreasing order of diameter classes. As- suming that large and small diameter classes represent respectively the past and the future successional stages of the forest, the position and the direction of the vectors provide an interpretation of succession (Horn 1981). This assumption may be questionable in certain cases. Mortality of young stems may be different from one species to another; for this rea- son, individuals of < 5 cm diameter were not used in the analysis. Mortality of small diameter class trees as a result of competition from dominant individuals need not reflect successional change. More- over, vectors may reflect differential growth rates more than species replacement. Despite these limita- tions, size-class ordination has previously been used successfully for the study of succession (Enright 1982; McCune & Allen 1985; Brisson et al. 1988).

Results

Vegetation analysis

The distribution of stands along the first reciprocal averaging ordination axis (Fig. 1) is significantly correlated with surface deposits, drainage, and rockiness, while time since fire is significantly correlated with axis 2 (Table 1). Linear trend surface analysis (Fig. 1) shows a similar pattern of correlation, although it was oblique to the ordination axes for some variables. The TWINSPAN classification produced 9 groups: (1) Fraxinus nigra; (2) Larix laricina; (3)Abies balsamea-Betula papyrifera; (4) Thuja occidentalis-Abies balsamea; (5) Abies balsamea- Betula papyrifera-Picea glauca; (6) Abies balsamea- Betula papyrifera-Picea mariana; (7) Pinus resinosa; (8) Pinus banksiana-Pinus strobus-Picea mariana and (9) Pinus banksiana-Picea mariana. Asymmet- ric uncertainty coefficients (Table 1) show that surface deposits followed by drainage, rockiness, and slope are the most discriminating variables for explaining the classification. If, in a second step, we compute the coefficients for each class of surface deposits, then time since fire becomes the most discriminating variable, although slope remains an important factor.

We conclude from these analyses that abiotic differences between stands mask the relationships be-

Table 1. Spearman correlation coefficients between the first two axes of the RA ordination and explanatory variables, and asymmetric uncertainty coefficients between TWINSPAN classification explanatory variables.

Variables RA TWINSPAN

axis 1 axis 2 1st step 2nd step

Surface deposits 0.61"** -0.29** 0.29 fixed Slope 0.25** -0.19" 0.23 0.53 Aspect -0.13 0.04 0.04 0.34 Drainage 0.45*** 0.19" 0.28 0.42 Rockiness 0.68*** -0.15 0.23 0.42 Time since fire 0.05 -0.37*** 0.17 0.55

* p<O.05. ** p<O.01.

*** p<O.O01.

55

l,,.,,.m

x < e

e e

oe~ • 'N

: 2 oo. m

N

o o

e

o o o

e e

i

! • e

o

ee e

r~nage

. ~ o c k

s~3e oeposit

ge

AXIS I Fig. 1. Plot scores on axes 1 and 2 of RA ordination. Bottom diagram represents intensity (length of the vector) and direction of correlations with abiotic variables and time since fire (age).

tween the composition of forest communities and time elapsed since the last fire. In order to remove these abiotically based differences, we divided our data set into five abiotically homogeneous subsets. The five resulting classes were: (1) silted lowland subject to flooding; (2) boggy habitats; (3) exposed bedrock; (4) morainic deposits with a xeric-mesic moisture regime and (5) clay deposits with a mesic-hydric moisture regime. The first two classes are represent- ed by 4 stands dominated by Fraxinus nigra (153, 186, 191, and 223 yr) and 2 stands dominated by La- rix laricina (111 and 181 yr) respectively. Because these two groups contained few stands representing only limitated age ranges, they were excluded from the following size-class ordinations.

Size-class ordinations

Exposed bedrock The size-class ordination (Fig. 2) shows two groups of long vectors, both converging to a group of short circular vectors at the bottom of the ordination diagram. The position of points in the diagram is related to the specific arboreal composition of a stand for

one of the five diameter classes. Vectors join, in decreasing order, size-classes belonging to the same stands. Vectors with less than five points represent stands where some of the size-classes are missing. The first group of vectors, starting at the upper left of the ordination diagram, is related to a decrease in Pinus banksiana dominance, from large to small diameter classes, while the second group, starting at the right, is related to the dominance of Pinus resinosa in large diameter classes. Pinus strobus is associated with these two pine species. All remaining species are associated with the convergence zone; however, Thuja occidentalis and Picea mariana are the most frequent species on exposed bedrock (Ta- ble 2).

The vector starting position (Fig. 2) is correlated with time since fire (0.26, p<0.1 on axis 1 and -0.33, p<0.05 on axis 2) and so is the vector end point (0.43,p < 0.02 on axis I and - 0.46,p < 0.01 on axis 2). In most cases the time elapsed since fire was determined precisely by fire-scars (Fig. 2). Some stands, particularly on islands, show multiple fire- scars that suggest the occurrence of low intensity surface fires (Van Wagner 1983). In these cases, the time since fire represents the time elapsed since the

56

FIll5" 104"

io3 :." .174"

Fig. 2. Succession vectors ordination for the samples from xeric exposed bedrock. Time elapsed since fire is indicated at the start of each

vector; the symbol * indicates a m i n i m u m age estimate, and underlining indicates the occurrence o f one or more subsequent surface fires.

Aba: Abies balsamea; Bpa: Betula papyrifera; Pgl: Picea glauca; Pma: Picea mariana; Pba: Pinus banksiana; Pre: Pinus resinosa; Pst:

Pinus strobus; Toc: Thuja occidentalis.

last major stand-initiating fire. The vector associated with Pinus banksiana show

a clear pattern of directional succession. Young stands generally have short vectors while older

Table 2. Mean relative frequency and constancy for tree species

present in > 10070 of the sites for each of the abiotic subsets of

sites,

Bedrock Moraine Clay

n = 2 5 n = 4 4 n = 1 8

f (°70) c (070) f (070) c (°70) f (07o) c (%)

Abies balsamea 16.4 64 33.6 98 37.8 94 Betula papyrifera 8.7 72 18.7 98 13.0 89

Picea glauca 8.1 76 10.6 89 12.1 94 Picea mar iana 25.8 84 14.1 77 3.3 28

Pinus banksiana 12.0 53 2.5 16 + +

Pinus resinosa 5.7 36 1.1 11 + + Pinus strobus 2.2 20 1.0 11 - -

Populus tremuloides + + 7.5 61 11.0 78

Thu ja occidentalis 18.1 92 10.0 68 20.1 89

+ present but with < 10°70 constancy.

stands have longer vectors. The oldest stand (183 yr) has a vector which starts at the middle of the ordination diagram suggesting that mortality has already affected large Pinus banksiana trees. Vectors cor- responding to Pinus resinosa stands show the same general pattern of directional succession. Except for the oldest stand (196 yr), however, vectors differ only by the position of intermediate diameter classes which are distributed closer to the start of the vectors for younger stands and to the end of the vectors for the older ones. This phenomenon could be related to the absence of young stands in the sample set and/or to the longevity of Pinus resinosa. Indeed, Pinus resinosa can live up to 300 yr (Heinselman 1981). Moreover, in some stands fire occurrence of less intensity (Fig. 2) may have maintained the habitat open and thus retarded succession towards more shade-tolerant species.

The vectors associated with the dominance of Thuja occidentalis and Picea mariana (Fig. 2) suggest self-replacement by these species. Some of these stands are over 300 yr and could be considered climax forests. However, a number of Picea mariana- Thuja occidentalis stands are much younger than

57

the Pinus banksiana or Pinus resinosa stands, thus excluding the possibility of their constant derivation from the latter two. It appears that in the absence of pine, Thuja occidentalis and Picea mariana domi- nate immediately following fire.

Morainic surface deposits An initial ordination isolated six stands dominated in large diameter classes by Pinus resinosa (255 and 79 yr), Pinus banksiana (97, 139, and 79 yr) and Pi- nus strobus (115 yr), all converging towards stands dominated by Abies balsamea. Pine species occur occasionally on this habitat type, although Abies balsamea, Betulapapyrifera and Picea spp. are more common (Table 2). A second ordination, excluding these six stands, shows three groups of vectors (Fig. 3a, b, c). The first group (Fig. 3a) is character-

Aim

Fig. 3. Succession vectors ordination for the samples from xeric-

mesic morainic surface deposits. Time elapsed since fire is indicated at the start o f each vector; the symbol * indicates a min imum age estimate. Parts a, b and c represent different groups o f vectors

belonging to the same ordination diagram. Aba: A bies balsamea; Bpa: Betula papyrifera; Pgl: Picea glauca; Pma: Picea mariana; Ptr: Populus tremuloides; Toc: Thuja occidentalis.

163

157 12B °

7" 163 21 138

~r

225

,~~22518 190

58

ized by Populus tremuioides dominance in the largest diameter class. Some vectors show dominance shared by Betula papyrifera, Picea glauca, or Picea mariana at the intermediate diameter classes. All vectors converge, more or less, towards A bies balsamea and Thuja occidentalis stands. The second group of vectors (Fig. 3b) is characterized by dominance of Picea glauca at large diameter classes,

Betula papyrifera at intermediate classes and A bies balsamea and Thuja occidentalis dominance in the small classes. The remaining vectors (Fig. 3c) are shorter but still directional, shifting dominance from Betula papyrifera or Picea mariana towards Abies balsamea and Thuja occidentalis.

Time since fire is correlated with the origin ( - 0.36, p < 0.02) and the end point ( - 0.36, p < 0.02) of the succession vectors on axis 1, but is not significantly correlated with axis 2. However, axis 2 does show a correlation with slope (--0.38, p<0.02) that suggests some abiotic effects on the ordination. Excessive slope could be responsible for the presence of two young stands (41 and 99 yr) of Picea mariana at the lower left of the ordination diagram (Fig. 3c).

128"

t28

139

\ "85 P~ 103

Fig. 4. Succession vectors ordination for the samples from mesic- hydric clayey surface deposits. Time elapsed since fire is indicated at the start of each vector; the symbol * indicates a minimum age estimate. Aba: Abies balsamea; Bpa: Betula papyrifera; Pgl: Picea glauca; Pma: Picea mariana; Ptr: Populus tremuloides; Toe: Thuja occidentalis.

gests that directional succession is still occurring.

Clayey surface deposits The pattern of the succession vector ordination (Fig. 4) is quite similar to the pattern observed for morainic deposits (Fig. 3). Larger diameter classes are dominated by Populus tremuloides, Picea glauca or Betula papyrifera, and the vectors converge toward Abies balsamea and Thuja occidentalis dominance. Picea mariana and Pinus species are only oc- casional species in this habitat type (Table 2). Time since fire is not significantly correlated with either of the ordination axes. The lack of correlation of axis 1 is related to the presence of two stands at the left of the ordination diagram with a minimum age of 85 yr. The low frequencies of Picea glauca and Populus tremuloides in these stands and the shape of the vectors suggest that they are older. With the removal of these two stands, a significant correlation ( - 0.68, p < 0.01) is attained between time since fire and the position of the end of the vectors on axis 1. Despite the age of the older forests (exceeding 200 yr), the absence of vectors showing the characteristic circular shape of self-replacing forests sug-

Species replacement

All species are present in the first 50 yr interval after fire (Fig. 5). The only exception of Pinus resinosa is clearly attributable to a lack of data. Species are

sorted by the length of time they can remain in the forest. Pinus banksiana decreases in frequency after 150 yr and disappears completely after 200 yr while Populus tremuloides can live somewhat longer. Betulapapyrifera is still abundant after 200 yr while Picea glauca shows a decrease by the end of this period. Pinus resinosa and P. strobus can persist more than 200 yr moreover Pinus resinosa does not

decrease in frequency on xeric sites. Picea mariana begins to decrease in frequency

only after 300 yr. On xeric sites its frequency in- creases from 0 to 300 yr while it decreases on more mesic sites. Abies balsamea and Thuja occidentalis both show an increasing pattern in frequency from 0 to 300 yr. After 300 yr, the frequency of Thuja occidentalis continues to increase while that of Abies balsamea decreases.

59

u~

eo Bpa

2

2

Xeric

• : , o o o ' +

Yea r5

Xeric-mesic

Years

Pgl

Toc

&

100

80

ac-

40

20

Mesic- hydric

Bpa Ptr Pg I

Years

Pba

Total

Pfr

Y e a r s

Fig. 5. Mean species frequency for all stands and the three abiotic subsets, in relation to time interval since fire succession gradient. Note that mean frequency is computed using only stands where the species is present. Aba: Abies balsamea; Bpa: Betulapapyrifera; Pgl: Picea glauca; Pma: Picea mariana; Pba: Pinus banksiana; Pre: Pinus resinosa; Pst: Pinus strobus; Ptr: Populus tremuloides; Toc: Thuja occidentalis.

Discussion

Successional pathways

The results presented here show that, even given constant abiotic conditions, there are numerous successional pathways following fire. On xeric sites, communities dominated by Pinus resinosa, Pinus banksiana, Pinus strobus and Picea maHana can be observed immediately following fire disturbance. On mesic or hydric sites, Populus tremuloides, Betu- la papyrifera, Picea mariana or P. glauca, and some- times pine species may be the dominant species fol-

lowing fire. Moreover, size-class ordinations show a continuous range of species composition instead of discrete successional patterns.

These diversity of arboreal composition may be a function of intensity of the last fire. For example, a regime of less intense surface fires on islands may be responsible for the abundance of Pinus resinosa stands (Bergeron & Gagnon 1987) instead of Pinus banksiana or Picea mariana stands which are fa- voured by intense crown fires (Heinselman 1981). However, similarity of the current forest composition to the preburnt composition is usually relatively high: Many species resprout following fire (Betula

60

papyrifera, Populus tremuloides), while others store seed banks in serotinous (Pinus banksiana) or semi- serotinous cones (Picea mariana) (Fowells 1965; Rowe 1983). The first colonisation, which can origi- nate from a very old event, may be more important than fire behaviour per se. Thus, it appears that in the boreal forest a prediction model of forest composition, even with uniform abiotic conditions, must include more than the recent temporal dimension.

Succession processes

The results presented here are consistent with Egler's (1954) concept of initial floristic composition and the tolerance model of Connell & Slatyer (1977). Each successional stage can be explained by the suc- cessive disappearance of early successional species that are unable to reproduce under a closed canopy. As succession proceeds, intolerant species disappear and shade tolerant species become progressively more abundant. Along this successional gradient, species are sorted in relation to the time they are able to persist in the forest. As stated by Noble & Slatyer (1980), succession is led by the processes acting at the population level and is predictable by vital attributes of the species such as longevity and shade tolerance. This statement appears to be rather ap- propriate for the processes that drive succession in the boreal forest.

Species in the forests studied may be ranked from the shortest to the longest time period that they are expected to persist in the canopy: Pinus banksiana < Populus tremuloides = Picea glauca < Betu- lapapyrifera = Pinus resinosa = P. strobus < Picea mariana < Abies balsamea < Thuja occidentalis; Behind the general pattern, which agrees with longevity and shade tolerance characteristics, this rank- ing shows several interesting anomalies.

First, it appears that early successional species such as Betula papyrifera and Populus tremuloides may be abundant in the canopy for more than 200 yr, this despite their mean longevities reported are considerably lower (Heinselman 1981). Since we did not core any Populus tremuloides, we do not know whether regeneration by root suckers, and not longevity of original colonists, may be responsible

for the long period this species is present in the canopy. However, in the case of Betula papyrifera, while vegetative regeneration is possible, we cored and aged many trees over 200 yr old. Such results empha- size the importance of regional variations in species attributes and their consequences on forest succession.

More interesting is the decrease of Picea glauca 200 yr after fire while Betula payrifera remains abundant. Picea glauca is known as a long-lived (up to 300 yr) shade-tolerant species while Betula papyrifera is generally considered shorter lived and shade intolerant (Fowells 1965; Heinselman 1981). The rapid decrease of Picea glauca from the forest could be related to spruce budworm (Choristoneura fumi- ferana) outbreaks which have affected this part of the boreal forest several times (Blais 1983). Despite the fact that Picea glauca is less affected by the spruce budworm than Abies balsamea, it appears that the budworm-caused mortality of Picea glauca is nonetheless important and that the post-outbreak forest environment does not favour its regeneration.

The frequency patterns o f Picea mariana are related to habitat characteristics and suggest an interac- tion between abiotic factors and recruitment of new individuals. On mesic sites, Picea mariana appears to be displaced by shade-tolerant species while on more open xeric habitats, it can remain longer. De- spite the shade tolerance (Heinselman 1981), it appears that establishment of seedlings is not possible on mesic sites and that closed forest conditions in- hibit layering (Fowells 1965). However, it is possible that the aging of the forest could lead to a dynamic pattern of gap formation favouring successful seed- ling establishment in the future.

The increase of both Abies balsamea and Thuja occidentalis as succession proceeds may be related to some kind of facilitation process (sensu Connell & Slatyer 1977). On the one hand, Abies balsamea produces large seeds that germinate easily on moist humus (Bakuzis & Hansen 1965; Fowells 1965). On the other hand, germination of Thuja occidentalis seeds is enhanced by the presence of dead and rotten wood on the forest floor (Scott & Murphy 1987). As a forest ages, both moist humus and rotten wood become more abundant and may enhance the invasion of these species. Delay of invasion appears more im-

portant for Thuja occidentalis than Abies balsamea which had saplings of < 5 cm, abundant in stands of < 50 yr (Y. Bergeron, pers. obs.).

Convergence and stability

Within specific, uniform abiotic conditions all of the observed communities, regardless of the dominating species immediately following fire, converge. This pattern is closely related to the increase in abundance of shade tolerant species, particularly Abies balsamea and Thuja occidentalis. Even in the youngest forest observed (< 50 yr), convergence appears to be occurring because late successional species are already present. Such a directional succession has been described for other parts of the boreal forest (Carleton & Maycock 1978. Dix & Swan 1971; Foster & King 1986; Furyaev et al. 1983).

Non-converging patterns may be observed when seeds of shade tolerant trees are not available. This is unlikely to occur in our study area as the lake landscape provides numerous fire breaks which in turn allow for an abundance of unburnt forests at any particular time. Seeds of shade-tolerant species can easily be dispersed from these forests into recent- ly burned-over areas. The situation appears to be quite different on the mainland where large fires may leave fewer seed sources. Such a phenomenon likely produced the extensive, monospecific mainland forests that have a less convergent pattern, described by Bergeron & Bouchard (1984).

Similarly, the abundance of Thuja occidentalis in the lake landscape and its near-absence in most of the mature communities on the mainland (Berge- ron & Bouchard 1984) appears also to be related to the smaller extension of unburnt forests on the mainland as compared with the lake landscape. Heinselman (1973) suggested large recurrent fires may be responsible for the exclusion of Thuja occidentalis from the mainland and its restriction to more protected lake landscapes. Moreover, the low intensity of the fire regime associated with lake landscapes appears to favour Pinus resinosa (Bergeron & Gagnon 1987; Van Wagner 1971) and Juniperus communis (Diotte & Bergeron 1989), which both suffer from regeneration difficulties following the

61

high intensity fires occurring in the mainland landscape.

The applicability of the concept of self replacement or climax for the boreal forest is questionable (Rowe 1961). In the context of a recurrent disturbance such as fire, the equilibrium concept of climax is inapplicable at the community level. Equilibrium, if it does exist, can only occur at the landscape level where a stable pattern of patches of different ages are observed. Our results corroborate this point of view. The mean stand age of the study area is less than 150 yr and succession continues after 224 yr (Figs. 3 and 4). Self-replacement was observed only on two sites supporting communities of over 300 yr (Fig. 2). These xeric habitats are found on small islands protected from fire.

If we use mean stand age as an approximation of fire rotation (Heinselman 1981), it is unlikely that self-replacement of forests occurs in the interval between fires. However, recent fire control by man (since about 1930), may lead to self-replacing forests. Our results suggest that in the long run, the only remaining dominant species will be Abies balsamea and Thuja occidentalis on mesic and hydric sites, and Thuja occidentalis and Picea mariana on xeric sites. Even in the absence of fire, however, gap dynamics (Foster & Reiners 1986; Hytteborn et al. 1987) or larger natural disturbances such as wind- throw and insect outbreaks (Furyaev et al. 1983) could be alternate mechanisms that generate forest succession.

Acknowledgements

This research was funded by the Natural Sciences and Engineering Research Council of Canada and the Quebec Department of Education (FCAR). We particularly wish to thank Daniel Lemieux who did a large part of the field work and dendrochrono- logical analyses. Dr Pierre Legendre of Montreal University and Alain Leduc provided useful advice on numerical methods. We are grateful to Sylvain Archambault, Pierre R. Dansereau, Daniel Gagnon, Brian Harvey, Danielle Lalonde, Marie Saint- Arnaud, and Francois Tetrault. We also thank two reviewers for their useful comments.

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