Populus tremuloides Michx. postfire stand dynamics in the northern boreal-cordilleran ecoclimatic region of central Yukon Territory, Canada

Forest Ecology and Management 258 (2009) 1110–1120

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Populus tremuloides Michx. postfire stand dynamics in the northern borealcordilleran ecoclimatic region of central Yukon Territory, Canada W.L Strong * Arctic Institute of North America and University of Calgary, Faculty of Environmental Design, 2500 University Drive N.W., Calgary, Alberta T2N 1N4, Canada

A R T I C L E I N F O

A B S T R A C T

Article history: Received 6 January 2009 Received in revised form 22 May 2009 Accepted 30 May 2009

Postfire vegetation development among 8–185-year-old stand was assessed based on 100 releve´s from the northern boreal-cordilleran ecoclimatic region (61–638N) in the central Yukon Territory, Canada. Vegetation sampling included only stands thought to have originated from postfire Populus tremuloides Michx. regeneration that occurred on well drained and low gradient sites. Seven vegetation types were recognized based on cluster analysis and Kruskal–Wallis testing. Releve´ ordination using Detrended Correspondence Analysis (70% explained variance) indicated six of the vegetation types represented a secondary successional chronosequence, based on their juxtaposition and a strong correlation of the primary axis with stand age (r = 0.89, P < 0.001). No correlation (P > 0.05) occurred between stand location and age. The youngest vegetation (8–11 years) had a moderate cover of P. tremuloides and Salix spp. up to 5 m tall, with a ground cover of Ceratodon purpureus (Hedw.) Brid. and Bryum caespiticium Hedw. This vegetation was expected to result in P. tremuloides, mixed P. tremuloides and Picea glauca (Moench) Voss, and P. glauca/Hylocomium splendens forest stands with increasing age, respectively. P. tremuloides//Calamagrostis purpurascens—Arctostaphylos uva-ursi stands formed the mid-seral vegetation. Along the chronosequence, total tree, P. tremuloides, shrub, and herb cover peaked 50–70 years after stand initiation; P. glauca cover, total and nonvascular species richness, and dominance concentration gradually increased (P < 0.001); vascular plant richness decreased; bryophytes had a U-shaped abundance pattern; and total plant cover was constant through time (125%). Richness totalled 113 species with averages of 13–18 per releve´. Coarse woody debris was most abundant (maxima 100– 223 m3/ha) during the first 20 years of stand development then declined to <50 m3/ha. Successionally, a stem exclusion stage occurred (years 8–18), but with a delayed peak of 2–4 years and reduced densities (1.47 stems/m2) relative to southern boreal stands. No understory suppression, and therefore, no reinitiation stage occurred. Following stem exclusion, an accelerated canopy transition stage occurred relative to southern boreal forests due to early establishment rather than better height-growth rate of P. glauca relative to P. tremuloides. P. glauca tended to equal the cover of P. tremuloides 95–100 years after stand initiation. The oldest vegetation type in the chronosequence more closely resembled old-growth than a gap dynamic stage of development, possibly because of its youthful average age of 125 years. A modification was proposed for the canopy transition stage (Chen–Popadiouk stand development model) to account for the ‘‘forced’’ replacement of P. tremuloides by P. glauca. Differences in stand development were attributed to the cold northern climate. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Boreal forest Ecology Succession Trembling aspen Wildfire

1. Introduction The Yukon Territory contains some of the highest latitude boreal forest in North American with a northern limit near 63.78N latitude, only possibly equalled by forests in adjacent Alaska (Viereck et al., 1983; Ecoregions Working Group, 1989). The

* PO Box 40186 Station Main, Whitehorse, Yukon Territory Y1A 6M9, Canada. Tel.: +1 867 667 2924; fax: +1 867 667 2924. E-mail address: [email protected]. 0378-1127/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2009.05.041

Canadian forests are part of a boreal-cordilleran ecoclimatic region that occurs along the eastern slopes of the Rocky Mountains as far south as southwest Alberta (50.58N). This ecoclimatic region represents a 280,000 km2 ecotone between boreal mixedwood forests to the east and more westerly coniferous cordilleran forests (Rowe, 1972). A unique feature of this ecotone is its diverse upland arboreal assemblage that includes Populus tremuloides Michx., Populus balsamifera L., Picea albertiana S. Brown emend. Strong and Hills (2006) and Picea glauca (Moench) Voss, Pinus contorta Dougl. ex Loud., Picea mariana (P. Mill.) B.S.P., Abies lasiocarpa (Hook.) Nutt., Betula papyrifera Marsh., and occasionally Salix spp.

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Elsewhere boreal mixedwood stands are typically limited to the first four taxa on comparable sites in western Canada. Although much is known about southern and mid boreal-cordilleran forests (e.g., Annas, 1977; Corns, 1983; Archibald et al., 1984, 1996; Beckingham et al., 1996), available literature provides limited insight to the types of plant communities that occur in the more northern forests or their ecology. This information gap probably occurs because of limited ground access, remoteness from major research institutes, and lack of productive commercial forests to promote the demand for regional stand-level information (e.g., Archibald et al., 1996). Of the few regional synecological studies that have been conducted in the boreal portion of the Yukon, they are either extremely general in their classification of communities (e.g., ‘‘Open trembling aspen-spruce forest’’, Zoladeski et al., 1996), lack species composition data beyond floristic listings (Oswald and Brown, 1986; also Zoladeski et al., 1996), or use prominence indices (Reid, 1977) and what appear to be averaged uneven interval cover/abundance class numbers to represent species abundance (Orlo´ci and Stanek, 1979). These limitations make such studies difficult to utilize beyond the original intent from a plant community analysis perspective. More recently, boreal vegetation research in the central and southern Yukon has focused on the establishment and early development of forests after wildfire (e.g., Johnstone et al., 2004; Johnstone, 2006; Johnstone and Chapin, 2006), rather than the analysis of more mature plant communities and their ecology. The lack of fundamental synecological research in these highlatitude forests could potentially limit the success of forestoriented sustainable resource management practices, because the applied ecological principles are largely derived from the study of more southern forests. From a scientific perspective, it limits the assessment of regional ecological and biodiversity patterns stemming from differences in latitude. It is also not known how stand development patterns relate to current models. The most commonly cited North American forest stand dynamics model was proposed by Oliver (1981, stand initiation ! stem exclusion ! understory reinitiation ! old-growth stages), based on information primarily derived from coniferous Pacific northwest and eastern hardwood forests. A lesser known revision of the Oliver model was proposed by Chen and Popadiouk (2002, stand initiation ! stem exclusion ! canopy transition ! gap dynamic stages) for boreal mixedwood stands, which is an extensive and commercially important forest type in Canada. The latter model remains untested, although Redburn and Strong (2008) identified the potential need to recognize mature P. tremuloides stands that persist for an extended period of time in the absence of latesuccessional P. glauca. The objectives of this study were to: (i) characterize the types of P. tremuloides and associated later successional vegetation that occur on relatively level, well to moderately drained sites in the central Yukon Territory; (ii) characterize stand development patterns among the sampled sites; and (iii) compare the identified stand development patterns with trends reported in more southerly forests. 2. Methods and materials 2.1. Study area Sites were sampled along a 223 km north–south section of the Klondike Highway (No. 2) from Little Fox Lake (km 266, 61.3688N) to north of Pelly Crossing (km 489, 63.0248N) in the central Yukon Territory (Fig. 1). This route traverses the Central Yukon Plateau, which is characterized by broad northwest-southeast trending valleys with ridged and strongly rolling terrain that was glaciated. Moraine occurs on lower slopes and outwash deposits are common

Fig. 1. Location of vegetation sampling plots (dots) in the Yukon Territory of northwestern Canada. Numbers in parentheses indicate the year or decade when burning last occurred and lines perpendicular to highway identify their approximate limits (Yukon Forest Management Branch, unpublished map). Bar within Central Yukon Plateau area of inset indicates the general location of study area.

in valley bottoms (Smith et al., 2004). The Central Yukon Plateau occurs within the Northern Boreal-Cordilleran (NCb) ecoclimatic region of northwestern Canada (Ecoregions Working Group, 1989), but also includes Alpine (NCa) and Subalpine (NCs) regions. The

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W.L. Strong / Forest Ecology and Management 258 (2009) 1110–1120

Boreal region, and focus of this study, occurs beneath the Subalpine at elevations below 1200 m (Oswald et al., 1983). The lowest elevations (460 m) along the study traverse occurred about 28 km north Carmacks. The Boreal portion of the region has cool May–August (average 10.8 8C) and cold November–February (18.8 8C) ambient temperatures (Braeburn station, Environment Canada, 2004). An average of 151 mm of the 280 mm of annual precipitation falls during summer. The climatic conditions are equivalent to a Dfc regime under to Ko¨eppen classification system (Henderson-Seller and Robinson, 1986), which has a constant water supply and <4 summer months with mean temperatures of 10 8C. Early and mid-seral upland forests are composed of P. tremuloides, P. tremuloides–P. glauca, or P. contorta, with the latter of limited abundance and reduced frequency with increasing latitude. These forest types typically have low-growing (<70 cm tall), mixed herb and deciduous shrub understories. Grasslands are common on steep southerly aspects. Oswald et al. (1983) considered P. glauca stands with feathermoss understories to be the most common mature forest type in the area. Wildfires are a component of the natural environment and extensive burns occurred in 1998, 1995, and 1958 within the study corridor (Yukon Forest Management Branch, Fire history map, 2008, unpublished). The southern 43 km of the study corridor was burnt in 1958, with a portion again burnt in 1998 (Fig. 1). An extensive wildfire also occurred south of Pelly Crossing in 1995. Earlier burns occurred but are undocumented. Forest stands older than 200 years are rare (Oswald et al., 1983). 2.2. Field sampling Sampling was limited to stands that were dominated by P. tremuloides (POTR) or originated from postfire POTR succession. For late-successional stands dominated by P. glauca (PIGL), their acceptability for inclusion was based on the occurrence of live, standing dead, or remnant downed POTR trees; or in the absence of such evidence, the presence of topography and site conditions consistent with its occurrence. Only sites with 3% gradients and well to moderately drained soils (Luttmerding et al., 1990, p. 43); without atypical levels of disease or insect infestation, or notable recent or historic tree felling; and within 1 km of a road were sampled. Recently burnt sites (<20 years) were excluded if extensive salvage harvesting of snags or downfall had occurred. An attempt was made to distribute samples throughout the study corridor and across as broad an age-range as possible. Vegetation composition and species abundance sampling were based on 20-m  30-m plots that included a centrally located 30-m transect. Along the transect five 2.5-m  2.5-m and five 1-m  1-m quadrats were used to sample vascular plants 1–2.5 m and <1 m tall, respectively. Ground bryophytes and lichens, and epiphytic tree lichens to a height of 2.5 m were also sampled in the smaller quadrats. Within each large quadrat occurred one smaller quadrat and both were placed at 5-m intervals along each transect beginning at the 5-m mark. The 20-m  30-m plot was used to assess the cover of trees and shrubs >2.5 m tall. Ocular percent canopy cover estimates (sensu Daubenmire, 1968, p. 43) were used to assess species abundance. Vascular plant and bryophyte, and lichen nomenclature are based on the Integrated Taxonomic Information System (ITIS Partners, 2008) and Esslinger (2008), respectively, except where noted. Table 1 contains full scientific names for most cited species. Point-centered quarter method (Mueller-Dombois and Ellenberg, 1974, p. 110) was used to estimate tree densities and to sample stem diameters (1.3 m elevation), with sampling points located at 10-m intervals (5-, 5-, 15-, and 25-m marks) along each vegetation sampling transect. The size limits for inclusion in point-

centered quarter sample were based on stand-specific and ageappropriate criteria, so only the dominant cohort was sampled, e.g., saplings were not included in mature forests, but were only measured in early successional stands. Heights and ages were determined for the two tallest POTR, or PIGL if lacking, within each plot; residual trees from the prefire vegetation trees were excluded. The height of trees >4 m tall was determined using a measuring tape and clinometer, whereas shorter trees were measured with a 10-cm incremented 2-m ruler. Stand ageing was based on annual growth-ring counts that were obtained with an increment bore from immediately above the root collar. Small trees (<3 cm diameter) were cut near the root collar and the lower most portion of the stem retained for ageing. This location for ageing was chosen to avoid missing growth increments that would not occur if coring were done at a higher elevation. Occasionally (<5%), it was necessary to core older POTR at 30 or 100 cm above the root collar zone to obtain a complete increment core. In these cases, either 3 or 5 years were added to the ring counts to account for missing growth, respectively. These correction values were based on POTR height growth measurements and field observations from the most recently burnt sites. Increment cores were counted using a binocular dissecting scope, after trimming the cross-section surface with a stiff razor blade. The oldest tree regardless of species was considered to represent the stand age. The length and median diameter of coarse woody debris 5 cm diameter was measured within the boundaries of each 2.5m  2.5-m vegetation sampling quadrat. Diameter was measured midway between the ends of each piece of coarse woody debris within the confines of a quadrat. All woody stems on or within 2.5 m of the forest floor, and at an angle of <458 were included. Woody materials were excluded, if covered with mosses or litter, or substantially decomposed. Soil drainage, moisture and nutrient regimes (Luttmerding et al., 1990, p. 35 and 38), soil subgroup (Soil Classification Working Group, 1998), and humus form (Green et al., 1993) were classified based on a pit dug to a depth of +60 cm on the right-hand side and within the first 5 m of each vegetation sampling transect. Aerial photographs from the late 1950s to 1998 were reviewed to determine the type of overstory vegetation that occurred on the younger sampled sites prior to burning. 2.3. Data summarization and analysis Quantitative analyses of the releve´ data incorporated both species abundance and vertical structure to help elucidate stand development patterns (Franklin et al., 2002; Clark et al., 2003). The recognized strata corresponded to those used for composition sampling: tree, >2.5 m tall; tall shrub, 1–2.5 tall; low shrub (<1 m tall) stratum, which included forest floor bryophytes and lichens (ground stratum); and epiphytic lichens. Cluster analysis based on relative Euclidean distance and Ward’s method (McCune and Mefford, 1999) was used to agglomerate releve´s. This combination of algorithms produced the most distinctive groupings at a relatively low level of amalgamation, with minimal chaining. The resulting groups also occupied relatively discrete Detrended Correspondence Analysis (McCune and Mefford, 1999) ordination spaces. Ordination was used to display the relative botanical relationship among releve´s and vegetation types. The proportion of explained variance within the ordination was based on relative Euclidean distance. Naming of cluster analysis groups was based on stratal dominance, similar to the Scandinavian sociation approach (Mueller-Dombois and Ellenberg, 1974), but are referred to as ‘‘vegetation types’’ or ‘‘types’’ (sensu classification level VI, Strong et al., 1990). When naming vegetation types, a slash between species names indicate a change in stratum and a dash represents

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Table 1 Species composition and selected characteristics of a postfire vegetation chronosequence from central Yukon Territory. Variable

Vegetation typea A

B1

P B2

C

D

E

F

Tree stratum (>2.5 m tall) Populus tremuloides Michx. Picea glauca (Moench) Voss Salix bebbiana Sarg. Salix planifolia Pursh

Species: average percent cover [standard deviation] constancy classb, Scheffe´ rank test results 12[12]9a 17[15]8a 37[9]10ab 60[10]10b 34[7]10ab 5[7]6a +[+]1ac +[+]2ab 22[9]10abc 12[10]8b 38[10]10c 39[10]10c 1[2]2a 1[2]2a +[+]3a 1[3]3a 1[2]2a 0a 0a +[+]2a 0a 1[4]2a +[1]4a 0a

2[2]5 36[5]10 0 0

<0.001 <0.001 0.273 0.013

Tall shrub stratum (1–2.5 m tall) Picea glauca (Moench) Voss Populus balsamifera L. Populus tremuloides Michx. Salix bebbiana Sarg. Salix glauca L. Salix planifolia Pursh

+[+]1a +[+]1a 12[9]10b 2[5]5a 2[3]4a +[1]2a

+[+]2a +[+]3a 7[7]10ab 2[4]5a +[+]3a 5[8]3a

1[1]5a +[1]2a 0a +[+]2a 0a 0a

+[1]3a +[+]1a 1[3]4a +[1]3a +[+]+a +[1]2a

2[3]7a +[+]1a +[+]2a +[+]2a 0a +[1]2a

+[1]4a 0a +[+]2a +[+]+a 0a 0a

1[1]7 0 0 0 0 0

0.057 0.320 <0.001 <0.030 <0.001 0.139

+[+]2 +[+]2a +[+]1a 0a

+[+]3a 1[1]3ab +[+]3a 0a

+[+]2a 3[5]3ab +[+]2a +[+]3a

+[+]2a 14[14]8b +[1]2a +[1]4a

0a 4[7]4ab +[+]6a +[1]4a

0a 0a +[+]2a 2[7]3a

0 +[1]2 0 +[+]2a

0.142 0.001 0.133 0.025

0a 5[7]6a +[+]1a 7[5]9b 0a

+[+]2a 19[7]10b +[+]5a 6[5]10b 0a

0a 35[7]10b +[+]2a 2[3]10ab 0a

+[+]2a 11[9]9ab +[+]1a 4[3]10b +[+]+a

+[+]2a 11[8]9ab +[+]1a 2[2]10ab +[+]1a

0a 3[4]8a 0a +[1]3a 0a

+[+]2a 4[3]10 +[+]7 +[+]2 0a

0.098 <0.001 0.041 <0.001 0.579

+[+]1a +[+]1a +[+]1a 1[1]4a +[1]2a +[+]2a 2[4]7a 5[4]9c 0a 0a 2[3]4a 1[1]6a 3[5]5a 2[4]4a 2[4]5a +[+]2a 0a

+[+]3a 0a 0a +[1]3a 1[3]3a 0a 2[1]7a 9[8]10bc +[+]2a +[+]2a +[1]2a +[1]5a 1[1]5a 2[3]3a 4[6]5a 1[2]2a +[1]2a

+[+]3a +[+]2a +[1]3a +[+]3a +[1]2a 0a +[+]3a 1[1]8abc +[1]2a 0a 0a +[+]3a 0a 0a 0a +[+]2a +[1]2a

+[+]2a 1[3]4a 1[2]2a 2[3]5a 1[2]5 1[1]4a +[1]4a 1[1]6ab +[+]3a 3[6]4a 8[14]6a +[+]1a 1[2]2a +[+]+a +[1]3a 5[9]4a +[+]2a

+[+]1a +[1]3a +[+]1a 3[3]9a 2[2]6a 1[1]2a 1[3]3a +[+]6 +[+]1a 2[3]4a 7[8]7a 0a +[+]1a 0a +[1]3a 3[6]3a +[+]3a

+[+]1a 1[2]5a +[1]1a +[1]4a +[1]2a +[+]+a 1[2]5a +[+]3 +[+]3a +[1]3a 1[2]4 0a +[+]+a 0a +[+]+a +[1]4a +[+]+a

0a 0a 0 0 0 0 2[2]10 +[+]5 +[+]2 +[+]2 0 0 0 0 0 1[3]2 0

0.391 0.028 0.317 0.040 0.079 0.015 0.132 <0.001 0.215 0.013 0.010 <0.001 0.004 <0.01 0.008 0.330 0.245

45[12]10b 11[3]10c 0a 0a 0a 0a +[+]1a 0a

9[11]7ab 10[16]10bc +[+]2a 0a 0a 0a 0a 0a

1[3]2a 1[3]2ab +[+]2a +[+]2a 0a +[+]2a +[+]8a 0a

+[2]1a +[1]+a +[+]2a +[+]1a +[+]1a +[+]+a +[+]2a +[+]1a

+[+]1a +[+]1ab 0a +[+]1a +[+]1a +[+]1a +[+]2a +[+]1a

0a 0a +[+]4a +[2]4a +[+]3a +[+]2a +[+]2a 2[2]9b

0 0 +[+]10 1[2]7 +[+]5 4[4]10 +[+]2 6[6]10

<0.001 <0.001 0.029 0.002 0.078 0.093 0.004 <0.001

0a

0a

0a

+[+]+a

0a

2[5]5a

22[8]10

<0.001

0a

0a

+[1]3a

1[2]5a

3[4]9ab

57[23]10b

9[11]7

<0.001

0a +[+]1a 0a 2[4]4a 2[4]5a 2[4]7a

0a +[+]2a 0a 1[2]3a +[+]5a 9[13]5a

+[+]3a +[1]2ab +[1]5a +[+]3a +[+]3a 0a

+[+]1a +[+]2a +[+]2a +[1]1a +[+]2a +[+]2a

+[+]3a +[+]4ab +[+]2a +[+]2a +[1]2a +[+]1a

1[2]3a 2[2]9b 2[2]7ab +[+]+a +[+]+a +[+]3a

0 1[2]5 5[3]10 0 0 +[+]7

0.072 <.0.001 <0.001 0.010 0.026 <0.001

Epiphyte stratum (0.1–2.5 m tall) Hypogymnia physodes (L.) Nyl. Usnea glabrata (Ach.) Vainio

0a 0a

0a 0a

0a 0a

+[+]+a +[+]+a

0a +[+]1a

1[1]9ab 1[1]9ab

+[+]5 1[1]7

<0.001 <0.001

Botanical characteristics Total species cover (%) Total tree species cover (%) Total Salix spp. cover (%) Total shrub cover (%)d Total herb cover (%) Total bryophyte cover (%)

123[12]a 32[14]a 12[11]c 2[3]a 15[8]ab 58[15]b

111[23]a 35[12]a 15[10]c 1[2]a 29[12]bc 28[15]ab

109[10]a 61[9]a +[1]ab 3[5]a 40[7]c 3[6]a

137[23]a 76[11]b 4[7]bc 25[19]b 29[13]c 2[3]a

120[18]a 75[14]ab 2[2]bc 14[11]ab 23[10]bc 4[5]a

127[26]a 45[11]a +[+]a 1[3]a 8[8]a 62[23]b

104[12] 41[8] 0 +[1] 6[2] 19[22]

0.004 <0.001 <0.001 <0.001 <0.001 <0.001

Low shrub stratum (<1 m tall) Achillea millefolium L. Arctostaphylos uva-ursi (L.) Spreng. Arnica lonchophylla Greene Bromus inermis ssp. pumpellianus (Scribn.) Wagnon Bupleurum americanum Coult. & Rose Calamagrostis purpurascens R.Br. Carex concinna R. Br. Chamerion angustifolium (L.) Holub Conioselinum cnidifolium (Turcz.) A.E. Porsild Galium boreale L. Geocaulon lividum (Richards.) Fern. Hedysarum alpinum L. Linnaea borealis L. Lupinus arcticus S. Wats. Mertensia paniculata (Ait.) G. Don Picea glauca (Moench) Voss Populus tremuloides Michx. Pulsatilla patens (L.) P. Mill. Rosa acicularis Lindl. Shepherdia canadensis (L.) Nutt. Solidago simplex Kunth Salix bebbiana Sarg. Salix glauca L. Salix planifolia Purch Vaccinium vitis-idaea L. Zigadenus elegans Pursh Ground stratum Bryum caespiticium Hedw. Ceratodon purpureus (Hedw.) Brid. Cladonia cornuta (L.) Hoffm. Cladonia ecmocyna Leighton Cladonia gracilis ssp. turbinata (Ach.) Ahti Cladonia mitis Sandst. Cladonia pyxidata (L.) Hoffm. Dicranum acutifolium (Lindb. & Arnell) C. Jens. ex. Weinm. Flavocetraria cucullata (Bellardi) Ka¨rnefelt & Thell Hylocomium splendens (Hedw.) Schimp. in B.S.G. Hypnum cupressiforme Hedw. Peltigera aphthosa (L.) Willd. Peltigera malacea (Ach.) Funck Peltigera ponojensis Gyelnik Peltigera rufescens (Weiss) Humb. Polytrichum juniperinum Hedw.

W.L. Strong / Forest Ecology and Management 258 (2009) 1110–1120

1114 Table 1 (Continued ) Variable

Total lichen cover (%) Richness per releve´ Dominance concentration (Dw) Age (years) Tree stems/ha (1000) Coarse woody debris (m3/ha) Number of releve´s

Vegetation typea

P

A

B1

B2

C

D

E

F

4[7]a 13[2]a 0.54[0.06]a 11[2]a 12.5[15.9]b 65(64)a 16

1[2]a 13[5]ab 0.51[0.09]a 14[4]ab 13.6[13.1]b 82(66)a 6

2[1]a 13[4]ab 0.65[0.04]ab 72[21]abc 2.2[0.7]ab 38(60)a 6

1[1]a 15[3]ab 0.61[0.08]ab 67[28]b 4.3[3.8]b 28(32)a 40

1[2]a 16[4]ab 0.60[0.06]ab 84[27]bc 2.6[1.4]ab 18(20)a 9

10[6]ab 18[3]b 0.68[0.05]b 125[25]c 1.8[1.0]a 28(33)a 19

37[8] 18[3] 0.59[0.07] 161[30] 1.1[0.6] 14(15) 4

<0.001 0.001 <0.001 <0.001 <0.001 0.050

Only species with >5% constancy among all releve´s included. Vegetation types compared using Kruskal–Wallis tests and within differences identified using Scheffe´ rank tests. Groups within a variable followed by the same Scheffe´ rank letters do not differ at the a 0.05 level. The F type was excluded from the Kruskal–Wallis tests because of small sample size. a Vegetation Type A: POTR/POTR/Chamerion angustifolium/Bryum caespiticium—Ceratodon purpureus type; B1: POTR/Salix spp./Calamagrositis purpurascens/Ceratodon purpureus—Bryum caespiticium; B2: POTR-PIGL//Calamagrostis purpurascens; C: POTR//Calamagrostis purpurascens—Arctostaphylos uva-ursi; D: PIGL—POTR//Calamagrostis purpurascens; E: PIGL//Hylocomium splendens; and F: PIGL//Flavocetraria cucullata; POTR—Populus tremuloides, PIGL—Picea glauca. b Constancy classes: + [<6%]; 1 [6–15%]; 2 [16–25%]; 3 [26–35%]; 4 [35–45%]; 5 [46–55%]; 6 [56–65%]; 7 [66–75%]; 8 [76–85%]; 9 [86–95%]; 10 [96–100%]. c ‘‘+’’ represents average and standard deviation values equal to or less than 0.55. d Excludes Salix species.

co-dominance between species. Consecutive slashes indicate the lack of one or more stratal dominants within the vegetation. Several growth-form variables (e.g., total herb, total tree cover) were compiled to evaluate broad changes in vegetation composition with time. These variables were created by summing individual species percent cover values by releve´. For total shrub cover, Salix species were excluded and separately tabulated due to their importance in early stand development. Richness, or number of occurring species, was determined by counting the number of unique taxa within each releve´, excluding differences associated with vertical structure. The percent occurrence of a species among releve´s is referred to as constancy (Daubenmire, 1968). Dominance concentration (Dw) among species by releve´ was calculated based on the following formula (Strong, 2002):   b i Dw ¼ maxi i  Q n where bi is the sequential cumulative totalling of ith species cover values ranked from largest to smallest; i is the ith species in the dataset, where i = 1 through n; n is the number of species in a sample; Q is the sum of ith species cover values; maxi is the largest calculated ith value. The product of this equation is on a 0–1 scale, where 0 implies all taxa have the same cover and 1 indicates maximum concentration among taxa (i.e., all cover occurs in one species). Kruskal–Wallis tests (Statsoft, 1995) were used to determine if composition differences occurred among recognized cluster analysis groupings, or vegetation types, and associated variables. This test was selected, rather than analysis of variance, because the cover values of most species and related variables did not conform to a normal distribution based on skewness (range of acceptance 0.9) and kurtosis (0.4 to +1.8, Wetherill, 1981, p. 9), and were difficult to normalized due a high proportion of zero values. Scheffe´ rank tests (Miller, 1966, p. 66—formula 110) at the a 0.050 level were used to identify group differences within significant (P < 0.050) Kruskal–Wallis tests. Pearson’s product– moment coefficient of correlation (r) was used to assess the intensity of association between stand age and selected variables. Regression analysis was used to display the relationships, including the estimation of POTR and PIGL site index when 50 years old (SI50). Linear regression, or the lowest order polynomial model for non-linear data was used, except when a >3% increase in explained variance occurred with the next higher order. Regression modelling and correlation analysis were based on Microsoft Office EXCEL edition 2003 software, but Scheffe´ rank tests were manually calculated.

3. Results A total of 100 sites were sampled from 23 June to 31 July 2008, inclusive. Sampled sites were distributed throughout the study corridor, except in major river valleys and where rough upland terrain occurred in the Carmacks areas (Fig. 1). Stands were 8–185 years old. No correlation (P > 0.05) occurred between stand location and age. At least five samples occurred in each of ten 20-year age-classes, except the two oldest classes (Fig. 2). A total of 113 taxa were recorded among all releve´s, with half being nonvascular species. All but four sites were associated with either glacial fluvial or aeolian surficial deposits (Klassen and Morison, 1987; Hughes, 1989; Jackson, 1997a,b). These deposits were commonly sandy to loam sand in texture (e.g., 30–50 cm thick) and underlain by gravelly materials with a sandy loam matrix. The four anomalous sites were associated with moraine deposits, which had a sandy to silty loam texture with few stones (Jackson, 1997a). Site in the southern half of the study corridor often had an overlay of silty-textured tephra 10–30 cm thick (Rostad et al., 1977). Site elevations ranged from 472 to 873 m ASL. Eluviated Eutric Brunisols (typically Ae, Bm, Bm or Btj, C horizon sequence, see Soil Classification Working Group (1998) for horizon attributes) and hemimor humus (Green et al., 1993; see Fons et al. (1998) for an alternative classification) occurred on most sites. Semi-mature and mature seral forest stands typically had combined humus layers 4–7 cm thick (range 1.5–12 cm), with submesotrophic to poor mesotrophic nutrient regimes. Stands <20 years old frequently had 1 cm of humus above the mineral soil surface.

Fig. 2. Age-class distribution of sampled vegetation plots from the central Yukon Territory.

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Half of all sites had slope gradients of 1% and 92% had 2% gradients with flat, inclined, or undulating surfaces. All sites were judged to be well drained with a submesic moisture regime due to soil texture and/or topographic position. 3.1. Vegetation types Seven groups were recognized within a generated cluster analysis dendrogram based on differences in stand structure and species abundance. Clustering was discontinued in the lower 30% of the dendrogram, with most individual releve´ amalgamations occurring in the lower 12%. The species composition within groupings was relatively homogeneous, except one group (B) contained three primary subdivisions (n = 2, 4, and 6 releve´s). Based on similarities in composition, the two smaller subdivisions were combined to create a separate vegetation type (B1) from the larger subdivision (B2). Three percent chaining occurred within the dendrogram. 3.1.1. POTR/POTR/Chamerion angustifolium/Bryum caespiticium— Ceratodon purpureus type This vegetation type represents the earliest stage of postfire vegetation development among the sampled stands. The tree stratum contained a scattered cover (12%) of POTR and lesser Salix spp. up to 5 m tall, with a similar amount of POTR and greater Salix cover in the tall shrub stratum (Table 1). The vascular component of the low shrub stratum contained a variety of low cover and low constancy species, only POTR and C. angustifolium had constancy values >70%. More than half of the ground stratum was consistently covered by B. caespiticium and C. purpureus (Table 1). Eleven of 16 stands contained PIGL seedlings. An average of 13 (SD 2) taxa occurred per releve´, with a moderate level of dominance concentration. Tree species reached a maximum density of 67,685 stems/ha in one releve´, but more commonly ranged from 2835 to 23,898 stems/ha. No obvious relationship occurred between the number of stems/ha and the type of prefire vegetation as determined by aerial photograph interpretation. Stands averaged 11.2 years old with 15 of 16 stands 10–14 years. 3.1.1.1. POTR/Salix spp./Calamagrositis purpurascens/C. purpureus— B. caespiticium type. Stands of Type B1 ranged from 10 to 20 years old. POTR canopy cover was similar to Type A (Table 1), except more trees reached a height of 5–6 m. Salix spp. <1 m tall had 8% cover, which was similar to Type A. Tree cover in the tall shrub stratum was less. Tree stem densities were similar to Type A based on overall averages. Type B1 differed from Type A by having greater C. purpurascens and C. angustifolium canopy cover, and less B. caespiticium cover (Table 1). Total herb cover (29%) tended to be greater than in Type A (15%). 3.1.1.2. POTR—PIGL//C. purpurascens type. Type B2 contained stands ranged from 15 to 76 years old (average 72 years). POTR represented two-thirds of the tree stratum (Table 1), with PIGL commonly occurring immediately beneath to above the main POTR canopy (10–11 m tall). The greater PIGL height often occurred despite being the same age or younger than the POTR. Tree densities averaged 2155 stems/ha. This vegetation lacked a welldeveloped tall shrub stratum, but had a low shrub stratum dominated by C. purpurascens (Table 1). Little lichen cover was present, and Bryum and Ceratodon covers were substantially reduced relative to Types A and B1. Species richness by releve´ was comparable to Types A and B1, although dominance concentration was elevated (Table 1). 3.1.2. POTR//C. purpurascens—Arctostaphylos uva-ursi type Type C was the most sampled vegetation within the study corridor. POTR trees up to 15 m tall, when 100 years old (SI50

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9.6 m, r = 0.898, P < 0.001, n = 151), dominated the upper canopy of this young to maturing seral forest type. Stands averaged 67 years old (range 13–115), with average densities of 4354 stems/ha. C. purpurascens and A. uva-ursi within the low shrub stratum were the dominant understory species, although a mixture of low constancy and low cover herbs and shrubs also occurred (Table 1). Shepherdia canadensis might also be a characteristic species of this vegetation type. Its frequency of occurrence was greater (92% constancy in 20-m  30-m plots) than detected by quadrat sampling, because of its clumped distribution and limited cover in most stands. Bryophytes and lichens in combination composed <3% cover. Within Type C occurred a gradation in species composition that was difficult to justify differentiating into two groups at the current level of analysis, because both portions had a similar floristic composition with few substantial understory differences. Although both portions were dominated by POTR with some PIGL in the overstory, one portion (designated C1) contained somewhat less PIGL (8% versus 14% cover). S. canadensis was the only understory species with a substantial difference in cover between C1 (26%) and C2 (2%). 3.1.3. PIGL—POTR//C. purpurascens type This vegetation had a co-dominance of PIGL and POTR. Tree cover averaged 72% or about two-thirds of the total species cover (Table 1). These stands averaged 84 years old, with 2625 tree stems/ha. Older stands had 14–16 m tall PIGL (SI50 11.4 m, r = 0.791, P < 0.001, n = 66). Understory plant cover in the low shrub stratum averaged 39%. Characteristic species included C. purpurascens, Linnaea borealis, and C. angustifolium. Nonvascular species had 5% cover with Hylocomium splendens composing the majority. An average of 16 taxa occurred among releve´s, with a mean dominance concentration value of 0.60. 3.1.4. PIGL///H. splendens type The overstory vegetation of Type E stands was dominated by PIGL trees, with an average cover of 39%. POTR trees occurred in more than half the sampled stands, but had very limited cover (Table 1). PIGL trees reached heights of 20–25 m, with maximum diameters of 30– 35 cm. Tree densities were low compared to younger vegetation types (Table 1). Gaps in the tree canopy created by the toppling of overly mature members were uncommon. This vegetation lacked a well-developed tall and low shrub stratum, but had a relative continuous ground stratum of H. splendens. Epiphytic lichens such as Hypogymnia physodes, Usnea glabrata, and Bryoria lanestris (Ach.) Brodo & D. Hawksw. commonly occurred on the dead lower branches of PIGL trees, but had limited cover (Table 1). Average maximum stand age was 177 years, with a typical range of 108–144 years. 3.1.5. PIGL///Flavocetraria cucullata type This vegetation type has an open-growing tree canopy composed of PIGL that often occurred in discontinuous patches. Larger trees reached 24–26 m tall with 35 cm diameters. Tree densities averaged 1119 stems/ha, which was the least among the seven vegetation types (Table 1). No tall shrub stratum occurred except a few PIGL saplings. C. purpurascens and PIGL seedlings consistently occurred within the low shrub stratum. The majority of understory vegetative cover occurred in the ground stratum, and included F. cucullata, Cladonia mitis (also referred to as Cladina mitis (Sandst.) Hustich, Brodo et al., 2001), as well as a variety of cupforming Cladonia species, Peltigera malacea, and various mosses. H. splendens occurred in this vegetation, but its cover in individual quadrats was inversely related to the abundance of F. cucullata. Nonvascular plants formed more than half of the total cover in this vegetation type (Table 1). Stands averaged 161 years old but ranged up to 185 years.

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Fig. 3. Scatter diagrams and stand age-based regression models for selected vegetation variables associated with a Populus tremuloides chronosequence from central Yukon Territory: (A) woody stem densities; (B) total tree cover; (C) Populus tremuloides canopy cover, vertical line indicates point of rapid canopy decline; (D) Picea glauca canopy cover; (E) proportion of Picea glauca in tree canopy; (F) total Salix canopy cover; (G) total shrub canopy cover, excluding Salix cover; (H) total herb canopy cover; and (I) total lichen cover. All regression models based on 100 samples; R/r, polynomial and simple regression coefficient, respectively; P, probability value; and SEE, standard error of estimate.

3.2. Age related development patterns A comparison of stand ages and stem densities indicated, after a rapid decline sometime prior to Year 14, that densities decrease gradually from 12,000 to <2000 stems/ha by Year 90, and then remained relatively constant (Fig. 3A). Total tree cover among the sampled stands increased steadily for 60–70 years after initiation to a peak of 75–80% then declined to what appears to be a stable level of about 35% after Year 170 (Fig. 3B). POTR followed a similar pattern of canopy cover increase, but reached a peak of 55%, 50 years after stand initiation, followed by a continual decline until Year 160 (Fig. 3C). In contrast, PIGL cover continuously increased in abundance overtime (Fig. 3D), equalling the abundance of POTR 95–100 years after stand initiation (Fig. 3E, i.e., 0.5 or 50% level on y-axis). Although the regression line in Fig. 3C indicates that POTR abundance consistently declined after a peak, a precipitous drop in cover appears to occur prior to Year 120. This rapid reduction in cover is at least partially supported by field observations. Eleven of 15 stands <120 years old contained either numerous standing dead trees or a forest floor littered with dead POTR trees that had fallen within a short period of time (e.g., <5 years). Salix cover followed the same general pattern of decline as tree densities (Fig. 3F). Both shrub and herb covers increased until stands were 50–60 years old (Fig. 3G and H). Lichen abundance steadily increased after stands reached 80 years old (Fig. 3I). Bryophyte cover was greatest during the first 20 years of postfire stand recovery, remained at a minimal level for 50 years, then increased to an average of >60% (Fig. 4A). After Year 100, bryophyte cover typically exceeded 40%. In the bryophyte

regression model, pyrophilic B. caespiticium and C. purpureus mosses were replaced by H. splendens and Dicranum acutifolium (cf. Table 1). Pleurozium schreberi (Brid.) Mitt., a normally common boreal forest moss species, only occurred in the two oldest vegetation types and had very limited cover. No significant difference occurred in total species cover in relationship to age (Fig. 4B), but richness (13–18 species) and dominance concentration increased (Fig. 4C and D). Vascular plant richness decline steadily from a peak about Years 50–60, whereas nonvascular species richness continuously increased with stand age (Fig. 4E). Equal floristic richness between these two growth-forms was reached about Year 130. Coarse woody debris declined from early highs of 100–223 m3/ha to values typically <50 m3/ha 20 years after stand initiation (Fig. 4F; Table 1). 3.3. Ordination Fig. 5 illustrates the relative compositional relationship among individual releve´s and among the seven recognized vegetation types. The youngest (A-regenerating POTR) and the oldest vegetation types (E and F-PIGL) occurred at opposing ends of the ordination. From left to right, the intermediate releve´s graded from POTR (Type B1) to POTR-dominated mixedwood (Type C) to PIGL-dominated mixedwood (Type D) stands. Types C1 and C2 with B2 occupied separate but adjacent locations within the central portion of the ordination. Releve´s of the C2 and B2 types appeared to be intermixed in the Axis 1–2 perspective (Fig. 4), but B2 occurred on the periphery of the C2 cluster of releve´s based on an Axis 1–3 perspective, not presented. Releve´s of the C2 and D types

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Fig. 4. Scatter diagrams and stand age-based regression models for selected vegetation and site variables associated with a Populus tremuloides chronosequence from central Yukon Territory: (A) total bryophyte cover; (B) total species cover; (C) number of species per releve´; (D) dominance concentration (Dw); (E) vascular (V, ~) and nonvascular (N, &) species richness; and (F) coarse woody debris. All regression models based on 100 samples; R/r, polynomial and simple regression coefficient, respectively; P, probability value; and SEE, standard error of estimate.

abutted, but did not substantially overlap along their common margin (Fig. 4). Axis 1 explained 70% of the variation in releve´ distributions within the ordination, with Axis 2 and Axis 3 each explaining 2%. Axis 1 was strongly and positively correlated with stand age (r = 0.893, P < 0.001, n = 100). 4. Discussion 4.1. Secondary succession The juxtaposition of different vegetation types and the strong correlation of ordination Axis 1 with stand age support the interpretation that Fig. 5 represents a postfire secondary successional chronosequence with two successional pathways: Type

Fig. 5. A detrended correspondence analysis of 100 vegetation releve´s from central Yukon Territory. Arrows indicate the pathway of development. Vegetation Type A: POTR/POTR/Chamerion angustifolium/Bryum caespiticium—Ceratodon purpureus type; B1: POTR/Salix spp./Calamagrositis purpurascens/Ceratodon purpureus— Bryum caespiticium; B2: POTR-PIGL//Calamagrostis purpurascens; C: POTR// Calamagrostis purpurascens—Arctostaphylos uva-ursi; D: PIGL—POTR// Calamagrostis purpurascens; E: PIGL—Hylocomium splendens; and F: PIGL// Flavocetraria cucullata; POTR—Populus tremuloides, PIGL—Picea glauca.

A ! B1 ! B2 ! D ! E; or A ! C ! D ! E. The A ! B1 pathway probably occurred due the earlier establishment of PIGL compared to the A ! C pathway. It is unclear whether Type F (PIGL///F. cucullata) vegetation fits within the chronosequence, because it has a substantially different botanical composition than Type E (PIGL///H. splendens) or other vegetation types. Type F could represent vegetation resulting from site conditions that differ slightly from those associated with Type E (Oswald and Brown, 1986, p. 39), or a later successional stage. The latter possibility would be most plausible if Type F vegetation were older than its member trees. Until additional data are available, Type F vegetation cannot be considered part of the described chronosequence. Succession of the early postfire vegetation from POTR to mixedwood then to PIGL forests within the chronosequence superficially follows the conventional development pattern associated with most western Canadian deciduous forests, but there were differences. The rate of succession to PIGL appeared to be quicker than on well to moderately well drained sites in the southern boreal, with PIGL cover exceeding the abundance of POTR about 90–95 years after stand initiation. Mixedwood stands (i.e., >25% relative conifer cover, Strong et al., 1990) begin to form about Year 60. In comparison, it is not uncommon for southern and mid boreal POTR stands of a similar age or older to contain few subdominant or co-dominant PIGL (e.g., Corns, 1983; Cumming et al., 2000; Strong and La Roi, 1983; Lieffers et al., 1986; Lee et al., 1995; Redburn and Strong, 2008), especially on well drained sites. This accelerated rate of succession probably occurs due to the early establishment of PIGL rather than just the differential in height growth rates (cf. Redburn and Strong, 2008). Early establish is probably a more critical factor because a similar height growth differential occurs between the species in both the northern and southern boreal (16–18%—SI50 11.4 m versus 9.6 m, this study; SI50—17.6 m versus 14.5 m, LF c ecosite phase, Beckingham et al., 1996, respectively). The rapid decline in POTR prior to Year 120 may have occurred because of age, or continual increase of PIGL cover. The latter can reduce the abundance of vascular understory plant and degrade the soil nutrient regime, impairing the health of POTR (Strong and La Roi, 1985).

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4.2. Stand dynamics Whether large numbers of POTR suckers were produced during the stand initiation stage is not known due the lack of data for the first 7 years of stand development. Oswald and Brown (1990) found 5-year-old POTR stands on well drained sites in the southern Yukon typically produced upper densities of 20,000–23,000 stems/ ha. These values are low compared to southern boreal clearcuts (44,000–225,000 stems/ha in Year 1, Peterson et al., 1989; 49,895 stems/ha in Year 5, Strong and Sidhu, 2005), although similar to densities reported for northern Alberta (18,700 stems/ ha, Year 3, Redburn and Strong, 2008). However, some evidence suggests that forest clearcutting and more severe fires (Keyser et al., 2005) tend to stimulate greater POTR suckering rates (Peterson and Peterson, 1992, p. 29). If it is assumed that the Johnstone et al. (2004, Fig. 1) postfire POTR establishment curve is characteristic of the central Yukon, then stem densities would have peaked at an average of 14,700 stems/ha by Year 7 based on regression analysis of the presented data. This indicates central Yukon POTR stands produce low sucker densities and peak production is delayed by 2–4 years compared to more southerly boreal stands. Competition from Salix spp., with a tall shrub and tree stature, could in part explain the lower POTR densities along with colder soils that reduce suckering rates (Maini and Horton, 1966). A relatively rapid decline in POTR stem densities after their peak suggests central Yukon POTR stands do go through a stem exclusion stage, which is consistent with the Oliver (1981) stand development model. This reduction in densities appears to occur after Year 7 and before Year 18, with subsequent reductions at a lower rate for 20–40 years. Because juvenile tree densities at their peak were not great (i.e., 1.47 POTR/m2), the modest ground vegetation that probably occurred (Oswald and Brown, 1990) was not substantially reduced in abundance. The persistence of early seral pyrophilic bryophytes with high cover for several years after peak stem densities supports this interpretation. Therefore, the vegetation does not appear to have undergone an understory reinitiation stage as predicted by the Oliver (1981) model. A similar finding was made by Strong (2004) and Redburn and Strong (2008) in west-central and northern Alberta, which suggests a consistent flaw in the Oliver model. The process of replacing POTR with PIGL (canopy transition stage, Chen and Popadiouk, 2002) was well established in the sampled vegetation when stands within the chronosequence reached 50–60 years old (See Types B2 and D) and lasted for an additional 50–70 years (Fig. 3E, Table 1). This replacement appeared more forced by PIGL within and directly below a healthy POTR tree canopy, rather than a passive filling of vacated niches as described by Chen and Popadiouk (2002, p. 140)—‘‘As [POTR] trees start to decline and die because of longevity or damage from nonstand-replacing disturbances, shade-tolerant coniferous trees [PIGL] from the understory and intermediate canopy now take over the main canopy’’. For this reason, it is suggested that the canopy transition stage should include the recognition of a ‘‘passive’’ and an ‘‘active’’ form to account for this inconsistency. The latter form would occur when shade-tolerate late-successional species intrude into the seral forest canopy as well as occupy a large proportion of the subcanopy. Active transition would be most common in stand with early and abundant late-successional tree species establishment (e.g., vegetation type B2), possibly including commercially planted site (e.g., Redburn and Strong, 2008). Excluding consideration of Type F vegetation, Type E stands were chronologically and successionally the oldest vegetation in the chronosequence. However, they lacked characteristics associated with old-growth forests (Oliver model) such as a diverse stratal and age-class structure, and the presence of snags and

deadfall (Hilbert and Wiensczyk, 2007). Only the oldest stand in this vegetation type (177 years old) had begun to develop oldgrowth forest characteristics. Therefore, the sampled Type E stands should be considered subclimax vegetation at the end of the canopy transition stage or beginning of the old-growth stage. The absence of Abies spp., a climax boreal tree that is often co-dominant with PIGL in older growth stands, may slow the development of climax stands due to presence of more vacant structural niches than would occur with two competing and ecological similar species. 4.3. Biodiversity considerations Among the sampled stands, the POTR//C. purpurascens—A. uvaursi vegetation type (C) represents the most mature POTR sere on well drained sites. This vegetation type has not been previously described along the east slopes of the Rocky Mountains elsewhere in Canada. It has some botanical and ecological similarities to the commonly described POTR/S. canadensis/Leymus innovatus type that occurs in southwest and west-central Alberta (ecosite LFb2.1, Archibald et al., 1996; UFc2.1, Beckingham et al., 1996). A comparable type (POTR/Arctostaphylos/Calamagrostis) was recognized by Oswald and Brown (1986) in southern Yukon and by Viereck et al. (1983, Stand 11) and Youngblood (1995, POTR/A. uvaursi stands) in central Alaska. These examples were all associated with well drained sites. The lack of examples from elsewhere implies this vegetation has a distribution that is probably limited to the northwest portion of the North American boreal forest. Young to mid-seral Yukon POTR stands differed in botanical composition from southern boreal vegetation in western Canada that occurred on well to moderately well drained sites. They lacked the arboreal diversity (see Section 1), with stands infrequently containing (i.e., P. contorta or P. balsamifera) tree species other than POTR and PIGL. Understory species that commonly occur in southern boreal stands (La Roi, 1992; Bork et al., 1997; Redburn and Strong, 2008) such as Amelanchier alnifolia (Nutt.) Nutt. ex M. Roemer, Maianthemum canadense Desf., Mitella nuda L., Rubus pubescens Raf., and Thalictrum venulosum Trel., and Vicia americana Muhl. ex Willd. were absent in the sampled stands, although they occur in the Yukon flora (Cody, 2000). In addition, C. purpurascens rather than Leymus innovatus (Beal) Pilger or Calamagrostis canadensis (Michx.) Beauv. was the dominant graminoid (cf. Archibald et al., 1996; Beckingham et al., 1996). Species richness by releve´ was less than the values reported for the southern boreal (e.g., 23–31 taxa, Archibald et al., 1984; 30–38 taxa, Downing and Karpuk, 1992), and somewhat fewer than in the mid boreal (24– 34 taxa, Hart and Chen, 2008; 16–22 taxa, Redburn and Strong, 2008). These differences support a trend of decreasing species richness and biodiversity with increasing latitude as well as represent a shift in the types of occurring plant communities (Strong and Redburn, 2009). Relative to previously reported stand development patterns, the sampled vegetation had a delayed peak in vascular understory species abundance (Years 50–60 as opposed to Year 20, Hart and Chen, 2006, Fig. 1a). Bryophyte and lichen abundances, excluding postfire Ceratodon and Bryum species, continuously increased in cover after about 75 years of postfire stand development instead of reaching an early plateau at a relatively low level (Hart and Chen, 2006). Furthermore, total species richness tended to continuously increase rather than remaining constant or declining with time after reaching an early peak (Hart and Chen, 2006, p. 384 and Fig. 1b). Coarse woody debris volumes in the 185-year chronosequence did not have, or at best had a weak, U-shaped temporal pattern of occurrence (Fig. 4F). This contrasts with patterns reported by Lee et al. (1995) in central Alberta and by various other researchers (Brassard and Chen, 2006). The latter difference could

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be due to either the chronosequence not being of sufficient length, or northern boreal forest stands follow different development patterns. These inconsistencies as well as reduced POTR suckering after burning, prominence of Salix spp. in early stands on well drained sites, and an accelerated canopy transition stage are likely responses to the cold climate associated with high-latitude environments. 5. Conclusions The composition and successional development of POTR stands in the northern boreal-cordilleran region of northwest Canada differ from comparable but more southerly boreal forest stands. Neither the Oliver (i.e., no understory reinitiation stage) nor Chen– Popadiouk (i.e., decline of seral vegetation not necessary for PIGLdominance establishment) stand dynamic models are fully satisfactory for characterizing the development of POTR vegetation in the central Yukon Territory, particularly the two latter stages in both models. Much synecological research will be required to document stand dynamics and associated biological development trends before common patterns can be established for northern boreal forests. It may be ultimately necessary to create region specific stand development models to adequately accommodate for the broad differences found in the boreal forest, as suggested by Chen et al. (2002). The inclusion of data from throughout the boreal forest, however, would likely improve the robustness of future composite models. Acknowledgements Funding for this project was provided by Yukon Environment. Thomas Jung and Karen Clyde (Fish and Wildlife Branch) provided helpful commentary on the study design and assisted with logistics, Val Loewen (Yukon Environment) provided access to the Territorial herbarium, and Miles Thorp (retired, Yukon Forest Management Branch) provided a fire history map for the studied area. References Annas, R.M., 1977. Boreal ecosystems of the Fort Nelson area of northeastern British Columbia. Ph.D. thesis. University of British Columbia, Vancouver, 409 pp. Archibald, J.H., Klappstein, G.D., Corns, I.G.W., 1996. Field Guide to Ecosites of Southwestern Alberta, Special Report 8. Canadian Forest Service, Edmonton, 525 pp. Archibald, J.H., Ferguson, N.B., Haag, R.W., Hay, W.K., O’Leary, D.J., 1984. An Integrated Resource Inventory of Deep Basin (NTS 83L), Report T/78, 3 vol. Alberta Energy and Natural Resources, Edmonton. Beckingham, J.D., Corns, I.G.W., Archibald, J.H., 1996. Field Guide to Ecosites of West-central Alberta, Special Report 9. Canadian Forest Service, Edmonton, 630 pp. Bork, E.W., Hudson, R.J., Bailey, A.W., 1997. Upland plant community classification in Elk Island national park, Alberta, Canada, using disturbance history and physical site factors. Plant Ecol. 130, 171–190. Brassard, B.W., Chen, H.Y.H., 2006. Stand structural dynamics of North American boreal forests. Crit. Rev. Plant Sci. 25, 115–137. Brodo, I.M., Sharnoff, S.D., Sharnoff, S., 2001. Lichens of North America. Yale University Press, New Haven, 795 pp. Chen, H.Y.H., Popadiouk, R.V., 2002. Dynamics of North American boreal mixedwoods. Environ. Rev. 10, 137–166. Chen, H.Y.H., Krestov, P.V., Klinka, K., 2002. Trembling aspen site index in relation to environmental measures of site quality at two spatial scales. Can. J. For. Res. 32, 112–119. Clark, D.F., Antos, J.A., Bradfield, G.E., 2003. Succession in sub-boreal forests in westcentral British Columbia. J. Veg. Sci. 14, 721–732. Cody, W.J., 2000. Flora of the Yukon Territory. NRC Research Press, Ottawa, 669 pp. Corns, I.G.W., 1983. Forest community types of west-central Alberta in relation to selected environmental factors. Can. J. Bot. 13, 995–1010. Cumming, S.G., Schmiegelow, F.K.A., Burton, P.J., 2000. Gap dynamics in boreal aspen stands: is the forest older than we think? Ecol. Appl. 10, 744–759. Daubenmire, R., 1968. Plant Communities. Harper & Row Publishers, New York, 300 pp.

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