Plant species diversity and composition along an experimental gradient of northern hardwood abundance in Picea mariana plantations

Forest Ecology and Management 198 (2004) 209–221

Plant species diversity and composition along an experimental gradient of northern hardwood abundance in Picea mariana plantations Robert Jobidon, Guillaume Cyr*, Nelson Thiffault Ministe`re des Ressources naturelles, de la Faune et des Parcs, Foreˆt Que´bec, Direction de la recherche forestie`re, 2700 Einstein, Sainte-Foy, Que., Canada G1P 3W8 Received 23 December 2003; received in revised form 14 April 2004; accepted 17 April 2004

Abstract Maintenance of biodiversity in managed forest ecosystem is an increasing concern, particularly in plantation forests. To test the two hypotheses that (i) past vegetation treatments modify species diversity and that (ii) a trade-off exists between plantation productivity and diversity, we measured plant species diversity in the upper, intermediate and lower strata of 16–17-year-old Picea mariana plantations along a gradient of hardwood abundance. The gradient was obtained with various intensities of nonchemical release and thinning treatments. P. mariana productivity linearly decreased as the proportion of hardwoods within the canopy increased. As the proportion of P. mariana increased, total species richness and plant diversity (Shannon index [H0 ]) first increased up to an intermediate level, and then decreased. Proportion of non-crop to crop tree species explained 66% of the variation in species diversity of the upper strata, 0% of the intermediate strata, and 20% of the lower strata. Intensity of past vegetation treatments slightly affected species composition, as evaluated by the Sørensen’s index of similarity. Maintaining an almost exclusive cover of P. mariana had marginal effects on species richness and understory diversity. But, a large increase in hardwoods proportion occurred at the expense of species richness and diversity of the understory stratum. Differences in canopy light transmission between covers could not explain this result. Variation in species richness and diversity along the gradient of hardwood abundance had a hump shape, a characteristic of the classic disturbancediversity hypothesis. But, the intensity of past vegetation control had a weak predictive capability for plant species richness and diversity. Thus, vegetation control to maximise crop tree productivity in P. mariana plantations is not likely to affect understory plant diversity or composition. It may however contribute to an increase in stand structural diversity. # 2004 Elsevier B.V. All rights reserved. Keywords: Canopy light transmission; Floristic diversity; Man-made disturbance; Stand structure; Vegetation control

1. Introduction Vegetation control is by far the most important and broadly applied silvicultural treatment in eastern *

Corresponding author. Tel.: þ1 418 643 7994; fax: þ1 418 643 2165. E-mail address: [email protected] (G. Cyr).

Canadian coniferous forest plantations. Vegetation control by means of chemical or motor-manual release treatments, or by means of pre-commercial thinning treatments, ensures survival and growth of young stand (Stewart et al., 1984), partially regulates hierarchy in tree dimension (Jobidon, 2000), and permits expression of site growth potential. These treatments offer high economic return by increasing potential

0378-1127/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2004.04.020

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crop tree yield. Important research efforts have been devoted over the past three decades to document the silvicultural benefits of these treatments in terms of growth and yield (Stewart et al., 1984; Walstad and Kuch, 1987; Wagner et al., 1999; Biring et al., 2003; Jobidon et al., 2003; Thiffault et al., 2003); their usefulness is not a subject of controversy. Effects of vegetation management practices on the plant community are an important environmental issue (Wagner, 1993; Wagner et al., 1998; Haeussler et al., 1999; Miller et al., 1999; Newmaster et al., 1999). Considering that understory layers contain most of the floristic diversity in boreal and sub-boreal forests (Thorpe, 1992), and are thus responsible for supporting much of the faunistic diversity, the consequences of any treatment that may alter the plant community is a major concern for foresters. Effect of vegetation management practices on plant species diversity has been identified as a research priority in Que´ bec, Canada (Ministe`re des Ressources naturelles du Que´ bec, 1996). In many regions, spruce (Picea spp.) forest plantations are managed for wood production. Detailed understanding of the effects of common silvicultural practices on plant species diversity in spruce plantations, such as release and pre-commercial thinning treatments, is essential to properly manage man-made forests for both biodiversity and wood production (Hansen et al., 1991; Burton et al., 1992). For example, wildlife diversity is closely linked to stand structure and plant diversity. Vegetation treatments that alter the habitat may therefore have significant effects on wildlife (Lautenschlager, 1993; Sullivan, 1994). Vegetation management in Picea mariana (Mill.) BSP plantations is primarily designed to reduce competition from shrubs (e.g., Rubus idaeus L. [red raspberry]) and shade-intolerant hardwoods (e.g., Betula papyrifera L. Marsh. [white birch] and Prunus pennsylvanica L. [pin cherry]) in order to maximise plantation productivity. Expected ecological effects of removing non-crop tree species by pre-commercially thinning spruce plantations are, first, a decrease in canopy tree species diversity, and second, an increase in resources availability that could alter competitive interactions among understory species. The Abies balsamea–Betula alleghaniensis bioclimatic domain in Que´ bec comprises an important part of the sawlog industry, and provides many benefits in

terms of site quality for wood production, wildlife habitat and recreation. In many instances, following clear-cut logging operations, sites are planted with spruce as part of the strategy to maintain forest productivity (Ministe`re des Ressources naturelles du Que´ bec, 1994). Without such management, sites can rapidly become occupied by numerous and aggressive competing species, and be unproductive for decades (Jobidon, 1995; Archambault et al., 1998). For some social groups, maintenance of diversity is an important environmental and economic issue. This, in some instances, seems to conflict with the productivity of plantations. Sustainability of all life forms in ecosystems is one step, among others, toward the integration of environmental perspectives and economic policy (Pulliam and O’Malley, 1996). Considering the limited carrying capacity of the environment, imprudent use may ultimately reduce the potential for generating material in the future, with consequences for economic activities (Arrow et al., 1995). Forest plantations that achieve yields corresponding to site potential are part of the economic growth of forest resource-dependent communities. Such economic growth should not be hampered by a lack of ecological information; the consequence too frequently being implementation of restrictive practices to attain a poorly defined ecological objective at the expense of an economic objective. There is critical need to perform research that will help define specific ecological objectives based on scientific evidence, and then enable decision makers to make justified commitments for specific economic and ecological trade-offs. Thus, foresters in Que´ bec and elsewhere (Simard and Hannam, 2000) face a dilemma when managing spruce plantations for both wood production and maintenance of a variety of services. Release and, more especially, pre-commercial thinning treatments are done to maintain a proportion of deciduous tree species within canopy of spruce plantations (Ministe`re des Ressources naturelles du Que´ bec, 1994). Notwithstanding the suitability of this approach, the proportion must be defined in terms of productivity and its contribution to diversity. Our study examines the effect of canopy cover condition (tree species composition) in black spruce (P. mariana) plantations on plant species richness, diversity, and composition. We first wanted to verify if canopy cover conditions, as created by past vegetation treatments, modify species diversity, and second,

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if a trade-off exists between spruce plantation productivity and diversity. For this purpose, we used a longterm experiment that had originally been designed to evaluate growth response of P. mariana to various intensities of mechanical vegetation control treatments. This former experiment created a gradient of northern hardwood abundance within the spruce canopy cover, ranging from almost pure non-crop cover to pure spruce cover conditions.

2. Materials and methods

211

positions within the spruce stands. Once pre-commercial thinning treatments completed, the following main treatment plots were available for the purpose of the actual study: (1) not released þ not thinned; (2) not released þ thinned; (3) released þ thinned; and (4) released þ not thinned. The various combinations of the release and the thinning treatments gave a unique gradient of stand structural conditions available within a given plantation, in which almost pure hardwood cover to almost pure P. mariana cover was present. Such conditions were essential for studying the relative influence of non-crop tree species present within the cover of P. mariana on plant diversity.

2.1. Study sites 2.2. Measurements We conducted the study in two experimental black spruce plantations of the same site quality index (16:2  1:5 m at age 50 based on ecological information and Be´ dard (2002)), and managed for competing vegetation research purposes by the Ministe`re des Ressources naturelles du Que´ bec. The two plantations are within the Abies balsamea–Betula alleghaniensis bioclimatic domain in the Te´ miscouata region of Que´ bec. The two plantations are on loamy and gravely loamy podzolic soils. The ‘‘Squatec’’ experimental site was planted with black spruce (bare-root 1-2 seedlings) in 1982, whereas the ‘‘Lac Anna’’ experimental site was planted with black spruce (bare-root 22 seedlings) in 1983. In 1984, the two sites were mainly occupied by R. idaeus and Epilobium angustifolium L. (fireweed), with a growing cover of shade intolerant northern hardwoods. These sites were primarily established to document the effect of various manual or motor-manual release treatments. The first release treatments were performed in 1984 at Squatec, and in 1985 at Lac Anna. A given release treatment was randomly assigned to 60 m  120 m sub-plots. A detailed treatment description is given in Jobidon and Charette (1997). In 1995, the plantations had various levels of competition mainly composed of shade intolerant hardwood species, depending on the relative success of each release treatment (Jobidon and Charette, 1997). In 1996 and 1997, a pre-commercial thinning treatment was carried out on half of the two original experimental designs. The thinned plots were randomly selected. Objective of the thinning treatment was to reduce the density of non-crop species occupying dominant and co-dominant social

In July and August 1999, we randomly established 100 vegetation sample plots distributed in the subplots of the two experimental plantations in order to cover a broad spectrum of non-crop tree species abundance within the canopy of the P. mariana plantations. Each sample plot was composed of three circular concentric sub-plots. First, we used a 100 m2 sub-plot for sampling trees in the upper stratum (canopy) (dbh [diameter at breast height: 130 cm above ground] 1.5 cm); a second sub-plot (25 m2) was used for vegetation sampling in the intermediate stratum (height >60 cm; dbh <1.5 cm); a third sub-plot of 4 m2 was used for vegetation sampling in the lower stratum (height 60 cm; nonvascular plants were not sampled). We measured tree diameter at dbh and recorded the number of each species within each circular sub-plot. During this same period (July and August 1999), we evaluated canopy condition by measurements of light transmission (photosynthetic photon flux density [PPFD] in mmol m2 s1) under clear sky conditions, between 11:00 and 14:00 h, local solar time, using a portable integrating radiometer (Sunfleck Ceptometer, model SF-80, Decagon Devices, WA). To meet sky condition requirement (clear sky, no clouds), we were only able to measure light transmission in a total of 71 sample plots. For a given plot, four equidistant PPFD measurements were taken 1 m above the ground along the perimeter of the 25 m2 circular sub-plot, and data averaged; the ceptometer was pointed toward the plot centre. We took an additional PPFD measurement in a clear area, usually an adjacent road, as a reference for calculating percent

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light transmission for a given plot. We took care to make this last measurement within a maximum of 10 min from the time the sample plot measurements were made. 2.3. Data analysis We examined plant diversity using two indices: species richness (the total number of species per sub-plot and the total number of species per plot), and species diversity (calculated as the Shannon index of heterogeneity [H0 ]) (Margurran, 1988): X H0 ¼  pi ln pi where pi is the proportion of individuals found in the ith species in each concentric plot or in the whole plot. This index considers both the number of species (species richness) and the evenness of their abundance, two components of diversity (Margurran, 1988). We also used data from the 100 m2 plots that were established to sample the upper stratum for calculation of total basal area (TBA) and black spruce basal area (SBA) for each plot. We used the ratio SBA/TBA to represent the proportion of non-crop tree species within the canopy of the P. mariana plantations. A value of 0 (or a proportion of 1:0) represents a pure non-crop cover, whereas a value of 1 (or a proportion of 0:1) represents a pure P. mariana cover. To examine the effect of non-crop tree species within the spruce canopy on plant diversity (species richness and H0 ) and spruce productivity (SBA), we fitted regressions to describe the relationship between the following:

We selected the best regression model based on polynomial regression analysis. We favoured a modelling approach which was more appropriate in order to meet our objectives than a descriptive approach such as principal component analysis (PCA). Not much stand variables were available to give interesting additional information by PCA. We examined plant species composition along the gradient of non-crop tree species within the canopy of the spruce plantations by partitioning the SBA/TBA ratio in three tree mixture classes, representing a high, medium, and low proportion of non-crop tree species within the spruce canopy. Classes were as follow— class 1: ratio < 0:33; class 2: 0:34 < ratio < 0:67; and class 3: ratio > 0:68. For a given stratum and class, we calculated frequency (f), and mean abundance (a) for each species. Concerning floristic similarity, we compared the three classes to each other by calculating the Sørensen’s index of similarity (IS):   2c IS ¼  100 AþB where c is the number of species in common between two classes having A and B number of species, respectively (Mueller-Dombois and Ellenberg, 1974). An IS value of 100 indicates classes with the same species, whereas an IS value of 0 indicates classes without species in common. Plant species turnover, that is, the number of species gained and lost between two classes, is given by 100  IS, which corresponds to floristic dissimilarity between two classes.

3. Results species richness versus SBA/TBA ratio; H0 versus SBA/TBA ratio; SBA versus NCBA, where NCBA represents the non-crop basal area. To examine the effect of canopy tree composition (SBA/TBA ratio) on canopy condition (percent light transmission) and the effect of canopy condition on plant diversity (species richness and H0 ), we fitted regressions to describe the relationship between the following: percent light transmission versus SBA/TBA ratio; species richness versus percent light transmission; H0 versus percent light transmission.

Plant species richness of the three combined strata was only moderately affected by the proportion of non-crop to crop tree species in the canopy ðr 2 ¼ 0:21Þ (Table 1 and Fig. 1A). Species richness peaks at a proportion in basal area of non-crop to crop tree species of about 1:1 (SBA/TBA ratio ¼ 0:5), and decreases with increasing proportions towards both 1:0 and 0:1. The main cause of decrease in species richness, noted when proportion moves from 1:1 toward a dominance of P. mariana (0:1), is mainly attributed to a decrease in species richness of upper stratum (Fig. 1B; r 2 ¼ 0:56, Table 1). We found no significant effect in species richness of the intermedi-

R. Jobidon et al. / Forest Ecology and Management 198 (2004) 209–221

213

Table 1 Regression models describing the relationship between species richness and ratio of spruce basal area to total basal area of canopy trees (see Fig. 1), Shannon diversity index (H0 ) and SBA/TBA (see Fig. 2), SBA and non-crop basal area (see Fig. 3), and between canopy percent light transmission (CPLT) and SBA/TBA, species richness, and H0 Model 2

SR (whole plot) ¼ 14.3422 þ 30.8725(SBA/TBA)  29.7711(SBA/TBA) SR (canopy) ¼ 7.3488 þ 0.9351(SBA/TBA)  6.0876(SBA/TBA)2 SR (intermediate stratum)  (SBA/TBA) SR (lower stratum) ¼ 8.0475 þ 23.5301(SBA/TBA)  20.4095(SBA/TBA)2 H0 (whole plot) ¼ 1.7546 þ 2.3222(SBA/TBA)  2.3818(SBA/TBA)2 H0 (canopy) ¼ 1.2382 þ 1.5157(SBA/TBA)  2.4170(SBA/TBA)2 H0 (intermediate stratum)  (SBA/TBA) H0 (lower stratum) ¼ 1.0562 þ 2.8769(SBA/TBA)  2.5377(SBA/TBA)2 SBA ¼ 1251.85  0.5647(NCBA) CPLT vs. (SBA/TBA) SR (whole plot) vs. CPLT H0 (whole plot) vs. CPLT

Canopy Species Richness

Whole Plot Species Richness

25 20 15 10 5 0 0

(A)

0.5 Spruce BA / total BA

p

RMSE

Corresponding figures

100 100 100 100 100 100 100 100 100 70 70 70

0.21 0.56 0.02 0.14 0.27 0.66 0.00 0.20 0.51 0.06 0.01 0.02

<0.001 <0.001 0.412 <0.001 <0.001 <0.001 0.960 <0.001 <0.001 0.040 0.351 0.212

4.83 1.68 2.01 4.22 0.35 0.30 0.41 0.42 342.18 13.77 1.80 0.40

Fig. 1A Fig. 1B Fig. 1C Fig. 1D Fig. 2A Fig. 2B Fig. 2C Fig. 2D Fig. 3 None None None

25 20 15 10 5 0

1

0

(B)

30 Lower Stratum Species Richness

Inter. Stratum Species Richness

r2

30

30

25 20 15 10 5 0

0.5

1

Spruce BA / total BA 30 25 20 15 10 5 0

0

(C)

n

0.5 Spruce BA / total BA

1

0

(D)

0.5 Spruce BA / total BA

1

Fig. 1. Relationship between species richness and the ratio of spruce basal area (spruce BA) to total basal area of canopy trees (total BA), in eastern Que´ bec P. mariana plantations 16 and 17 years of age: (A) whole plot; (B) canopy stratum; (C) intermediate stratum; (D) lower stratum. Regression models are presented in Table 1.

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R. Jobidon et al. / Forest Ecology and Management 198 (2004) 209–221

of species richness of the understory. Canopy tree species contributed a maximum of about 25% at maximum species richness, that is, at a 1:1 proportion, and contribution decreased at other proportions (Fig. 1). Total species richness was mainly explained by the species richness of the understory which is the stratum containing the highest number of species. Canopy condition, expressed in terms of light transmission, did not explain any of the variation in species richness of the understory, with r2 values less than 0.1 for lower and intermediate strata. The proportion of non-crop to crop tree species in the canopy explained only 27% of the variation found in total plant species diversity (Fig. 2A; Table 1). The curvilinear response obtained indicates that species diversity peaks at an almost equal proportion in basal area of non-crop to crop tree species in the canopy Canopy Shannon Diversity Index (H')

3 2.5 2 1.5 1 0.5 0 0

Inter. Stratum Shannon Diversity Index (H')

2.5 2 1.5 1 0.5 0

2.5 2 1.5 1 0.5 0 0.5

1

Spruce BA / total BA 0

0

0.5

1

Spruce BA / total BA

(B)

3

0

3

1

Spruce BA / total BA

(A)

(C)

0.5

Lower Stratum Shannon Diversity Index (H')

Whole Plot Shannon Diversity Index (H')

ate stratum (Fig. 1C). However, a relatively weak curvilinear relation (r 2 ¼ 0:14, Table 1) was detected for the lower stratum (Fig. 1D), but a slow decrease in species richness was associated with an increase in P. mariana dominance within the canopy. On the other hand, we could not attribute the decrease in species richness we observed when the proportion of non-crop tree species within the canopy increased to a decrease in species richness of the canopy cover (Fig. 1B), but rather to a decrease in species richness of the lower stratum. This is evidenced by the curvilinear relation we found (Fig. 1D). This indicates that maintaining an almost exclusive cover of the crop tree species within the canopy had a relatively small effect on species richness in the understory. At the opposite, increasing the proportion in basal area of non-crop tree species within the canopy cover beyond 1:1 was at the expense

3 2.5 2 1.5 1 0.5

(D)

0 0

0.5

1

Spruce BA / total BA

Fig. 2. Relationship between Shannon diversity index (H ) and the ratio of spruce basal area (spruce BA) to total basal area of canopy trees (total BA), in eastern Que´ bec P. mariana plantations 16 and 17 years of age: (A) whole plot; (B) canopy stratum; (C) intermediate stratum; (D) lower stratum. Regression models are presented in Table 1.

R. Jobidon et al. / Forest Ecology and Management 198 (2004) 209–221 2000

Spruce basal area (cm2)

(a proportion of 1:1), and decreases similarly at other proportions, that is, toward a dominance of the crop species or a dominance of non-crop tree species. Total plant species diversity was almost equal in pure P. mariana plots (0:1) and in pure non-crop tree plots (1:0), indicating that complete lack of vegetation management (no release treatment þ no pre-commercial thinning treatment) was comparable to a strategy aimed at favouring spruce growth. We observed that 66% of variation in species diversity of upper stratum was explained by the proportion of non-crop to crop tree species in canopy (Fig. 2B; Table 1). Also, none of the variation in species diversity of intermediate stratum was explained by canopy composition (Fig. 2C; Table 1). Only 20% of variation in species diversity of the lower stratum is explained by canopy composition (Fig. 2D; Table 1). We attribute the main cause of decrease in plant diversity at high proportions of P. mariana to a decrease in species diversity of the upper stratum only; plant diversity of understory was almost unaffected (Fig. 2). On the other hand, we could not attribute the decrease in plant diversity observed at high proportions of non-crop tree species within canopy to a decrease in plant diversity of the upper stratum (Fig. 2B), but rather to a decrease in plant diversity of the lower stratum (Fig. 2D). Canopy light transmission did not explain any variation in plant species diversity in the understory, with r2 values less than 0.1 for lower and intermediate strata. Productivity of P. mariana plantations, expressed in terms of basal area, decreased linearly as the basal area of non-crop tree species within the canopy increased (Fig. 3 and Table 1). Managing a P. mariana plantation to maximise its productivity had only a minor effect on total species richness and species diversity, as revealed by the relatively weak relationship between canopy composition and these indices. When considering only the canopy species richness and diversity, some loss was observed. We partitioned the proportion in basal area of noncrop tree species within the P. mariana canopy cover into three classes representing high (class 1), medium (class 2), and low (class 3) proportion of non-crop tree species. Sørensen’s index of similarity (IS) was high between classes, reaching 84.1% between classes 1 and 2, 89.4% between classes 2 and 3, and 82.7% between classes 1 and 3. This indicates that past

215

1500

1000

500

0 0

500

1000

1500

2000

2500

3000

Non-crop basal area (cm2)

Fig. 3. Relationship between spruce basal area and non-crop tree species basal area in eastern Que´ bec P. mariana plantations 16 and 17 years of age. Regression model is presented in Table 1.

vegetation treatments only slightly affected species composition of the P. mariana plantations. Also, this result indicates that species turnover from one basal area class to another was low. A total of 14 species belong to only one class, 13 of them from the lower stratum and 1 from the intermediate stratum (Nemopanthus mucronatus) (Table 2). All of these species appeared relatively rarely (frequency < 5%), except Solidago macrophylla, and their mean abundance was also relatively low, indicating that loss in similarity between classes was attributed to relatively scarce species (those most highly sensitive to competition or disturbance). Much unexpected, the highest number of species exclusive to one class (seven species) occurred in class 3, representing the highest intensity of past vegetation treatments (Table 2); three species were exclusive to each of the other two classes. Thus, increased intensity of vegetation treatments would not likely be a prelude to species exclusion, quite the contrary.

4. Discussion Plant species diversity, evaluated by two indices (species richness and Shannon index of heterogeneity [H0 ]), was slightly affected by mechanical release and pre-commercial thinning treatments carried out in P. mariana plantations. For any proportion of non-crop tree species within the canopy of the P. mariana plantations, plant diversity of the upper stratum repre-

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Table 2 Plant species frequency (f) (%) and mean abundance (a) occurring in the lower, intermediate, and upper stratum of group plots representing a high (class 1), medium (class 2), and low (class 3) proportion of northern hardwoods mixed with the canopy of P. mariana plantations 16 and 17 years of age Species

Abies balsamea Acer pensylvanicum Acer rubrum Acer saccharum Acer spicatum Actaea sp. Alnus rugosa Amelanchier sp. Anaphalis margaritacea Aralia nudicaulis Aralia racemosa Arctium sp. Aster acuminatus Aster macrophyllus Aster sp. Athyrium filix-femina Betula alleghaniensis Betula papyrifera Carex sp. Circaea alpina Cirsium sp. Claytonia caroliniana Clintonia borealis Coptis groenlandica Cornus canadensis Cornus stolonifera Corylus cornuta Diervilla lonicera Dryopteris cristata Dryopteris disjuncta Dryopteris noveboracensis Dryopteris phegopteris Dryopteris spinulosa Epilobium angustifolium Equisetum sp. Fragaria sp. Galeopsis tetrahit Gallium sp. Geum macrophyllum Gramine´ es Hieracium sp. Impatiens capensis Lactuca biennis Linnaea borealis Lonicera canadensis Lycopodium clavatum Lycopodium lucidulum Lycopodium obscurum

Class 1

Class 2

Class 3

Lower

Intermediate Upper

Lower

Intermediate Upper

Lower

Intermediate Upper

f

a

f

a

f

a

f

f

a

f

a

f

a

25 0 45 0 60 0 0 0 5 45 0 0 5 15 0 5 10 15 20 10 0 0 10 5 75 5 15 40 0 5 5 10 35 35 15 0 0 20 0 5 70 5 5 35 0 5 0 0

1 0 2 0 6 0 0 0 34 7 0 0 10 3 0 86 3 1 31 48 0 0 1 3 108 5 2 9 0 14 4 6 6 4 28 0 0 8 0 16 42 23 1 14 0 4 0 0

60 0 55 5 85 0 0 5 0 0 0 0 0 0 0 0 0 20 0 0 0 0 0 0 0 20 30 40 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

3 0 3 1 13 0 0 2 0 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 3 12 13 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

95 0 80 0 85 0 5 10 0 0 0 0 0 0 0 0 30 95 0 0 0 0 0 0 0 5 35 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

28 0 5 0 20 0 5 1 0 0 0 0 0 0 0 0 1 30 0 0 0 0 0 0 0 9 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

24 0 24 4 44 0 0 0 24 36 0 4 24 0 8 12 0 4 36 8 4 4 24 24 68 0 8 28 0 16 0 20 52 40 40 16 4 60 0 28 84 0 24 48 8 4 0 4

20 4 60 8 88 0 0 4 0 0 0 0 0 0 0 0 16 48 0 0 0 0 0 0 0 12 28 44 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 16 0 0 0

2 1 16 1 19 0 0 1 0 0 0 0 0 0 0 0 3 3 0 0 0 0 0 0 0 4 12 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 7 0 0 0

11 0 13 0 24 2 2 2 18 60 2 0 4 2 2 2 0 13 33 4 2 0 25 24 76 4 9 53 2 9 0 5 36 53 15 9 4 31 2 15 82 0 13 40 0 22 4 7

1 0 5 0 2 1 2 1 9 5 2 0 9 5 2 4 0 1 15 16 1 0 15 56 148 2 6 23 13 6 0 8 15 5 6 9 8 10 1 21 34 0 1 24 0 7 4 4

27 2 49 2 47 0 2 4 0 0 0 0 0 0 0 0 2 29 0 0 0 0 0 0 0 5 27 73 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 5 0 0 0

2 1 11 2 16 0 9 6 0 0 0 0 0 0 0 0 1 3 0 0 0 0 0 0 0 10 14 13 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 0 0 0

a 1 0 3 1 2 0 0 0 14 5 0 1 32 0 2 44 0 1 16 8 1 6 5 82 77 0 6 9 0 16 0 12 19 4 11 38 53 6 0 41 31 0 2 37 3 1 0 5

f

a 92 8 0 0 52 7 0 0 60 11 0 0 4 12 8 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 24 2 72 15 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 2 12 4 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

f

a 78 0 11 0 5 0 2 0 0 0 0 0 0 0 0 0 0 35 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

3 0 3 0 4 0 1 0 0 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

R. Jobidon et al. / Forest Ecology and Management 198 (2004) 209–221

217

Table 2 (Continued ) Species

Maianthemum canadense Mitella nuda Monotropa uniflora Nemopanthus mucronatus Osmunda claytoniana Oxalis montana P. glauca P. mariana Populus balsamifera Populus tremuloides Prenanthes sp. Prunella vulgaris Prunus pensylvanica Pteridium aquilinum Pyrola secunda Ribes glandulosum Ribes lacustre Rubus idaeus Rubus pubescens Salix sp. Sambucus sp. Solidago macrophylla Solidago sp. Sorbus sp. Streptopus roseus Taxus canadensis Thuja occidentalis Tiarella cordifolia Trientalis borealis Trillium undulatum Tussilago farfara Vaccinium sp. Veronica officinalis Viola sp.

Class 1

Class 2

Class 3

Lower

Intermediate Upper

Lower

Intermediate Upper

Lower

Intermediate Upper

f

a

f

f

a

f

a

f

60 5 5 0 0 25 0 0 0 0 5 0 10 0 0 5 10 80 55 5 0 15 15 0 5 0 0 0 75 0 10 0 5 20

65 338 7 0 0 11 0 0 0 0 3 0 6 0 0 1 4 9 77 2 0 1 6 0 1 0 0 0 13 0 14 0 5 108

0 0 0 0 0 0 5 45 5 15 0 0 10 0 0 0 0 0 0 30 25 0 0 0 0 0 5 0 0 0 0 0 0 0

76 20 2 0 2 15 0 4 2 9 13 4 11 24 15 33 11 60 58 7 9 0 24 9 4 0 4 0 75 2 9 2 4 36

60 118 1 0 2 47 0 1 1 1 8 12 3 13 11 2 7 17 74 11 1 0 4 2 2 0 1 0 8 1 5 1 9 10

0 0 0 4 0 0 0 29 7 24 0 0 58 0 0 4 0 0 0 40 42 0 0 15 0 0 2 0 0 0 0 0 0 0

0 0 0 3 0 0 0 1 3 3 0 0 7 0 0 2 0 0 0 10 3 0 0 7 0 0 1 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 100 20 7 3 35 2 0 0 0 0 22 2 0 0 0 0 0 0 0 0 0 0 0 0 29 5 2 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

a 0 0 0 0 0 0 1 2 1 3 0 0 2 0 0 0 0 0 0 4 2 0 0 0 0 0 1 0 0 0 0 0 0 0

f

a

f

a

f

a

f

a

0 0 0 0 0 0 5 95 15 40 0 0 75 0 0 0 0 0 0 35 0 0 0 40 0 0 5 0 0 0 0 0 0 0

0 0 0 0 0 0 16 10 4 4 0 0 8 0 0 0 0 0 0 10 0 0 0 4 0 0 1 0 0 0 0 0 0 0

64 28 0 0 0 32 0 0 0 8 12 4 4 8 12 24 16 80 72 4 12 0 24 0 20 4 0 4 48 0 24 0 4 44

27 115 0 0 0 32 0 0 0 3 12 7 13 5 19 11 4 16 99 3 1 0 6 0 1 2 0 35 8 0 12 0 5 34

0 0 0 0 0 0 0 32 12 20 0 0 20 0 0 8 4 0 0 32 24 0 0 16 0 0 12 0 0 0 0 0 0 0

0 0 0 0 0 0 0 2 16 4 0 0 10 0 0 5 2 0 0 16 3 0 0 10 0 0 1 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 100 17 20 7 64 5 0 0 0 0 32 10 0 0 0 0 0 0 0 0 0 0 0 0 68 14 0 0 0 0 0 0 12 3 0 0 0 0 8 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0

a

Note—class 1: spruce BA/total BA ¼ 0:0–0.33; class 2: spruce BA/total BA ¼ 0:34–0.67; class 3: spruce BA/total BA ¼ 0:68–1.0.

sented only a small fraction of total plant diversity. We know that in North America tree species diversity is considered to be very low, and most floristic diversity is among understory species (Thorpe, 1992). It was mostly plant species diversity of the upper stratum that explained lost in species diversity observed when the crop species becomes dominant. Intermediate and lower strata did not or only slightly responded, respectively, likely because species richness and diversity were not a function of canopy cover condition (light transmission) and were weakly linked to cover composition (proportion of tree species). In addition, the

vegetation treatments scarcely affected floristic composition. Thus, the variation in plant diversity observed along the gradient occurred with only a minor change in composition. The pattern of variation in plant species diversity, observed along the gradient of non-crop tree species, has a characteristic hump shape (unimodal curve), similar to the pattern generally described in relation to a productivity gradient (Rosenzweig and Abramsky, 1992). In our study, the variation in plant species diversity (both richness and H0 ) is likely the expression of a disturbance-diversity pattern. The general model

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of diversity proposed by Petraitis et al. (1989) suggests that disturbance is a key to maintain diversity by preventing competitive exclusion. The general model predicts that highest diversity should occur at intermediate disturbance levels, which is called the ‘‘intermediate-disturbance hypothesis’’ (Grime, 1977; Connell, 1978; Huston, 1979; Roberts and Gilliam, 1995). Highest level of diversity in a community subject to disturbance is maintained at intermediate levels of disturbance (van der Maarel, 1993), which creates conditions favouring competitive species and those tolerating disturbance. At high or low frequency of disturbance, species richness decreases because species intolerant to disturbance become locally extinct or because dominant species eliminate weak competitors. Results we report here agree with this hypothesis. But, predictive capability of the specific models presented in Table 1 is relatively limited, because of the weak effect on plant diversity and floristic composition of disturbance induced by the treatments. In a study to evaluate understory plant species cover and richness in thinned or fertilized Pseudotsuga menziezii (Douglas-fir) plantations, Thomas et al. (1999) found that species richness increases monotonically with the level of thinning. But, they note that their study did not include highly intense thinning treatments, for which a decline in species richness may have occurred. It is well documented that vegetation treatments applied in young conifer stands improve survival and growth of the crop species, and along with thinning treatments contribute to ensure the conifer yield expectations (e.g., Newton et al., 1992; Brissette et al., 1999). In our study, black spruce basal area was lowest in control (untreated) plots (not released þ not thinned) and maximised in the intensively treated plots ðreleased þ thinnedÞ. Other treatment combinations (released þ not thinned; not released þ thinned) resulted in a mixed dominance of conifers and hardwoods. Increasing the proportion in basal area of non-crop tree species within the canopy of spruce plantations showed a positive curvilinear relation for both species richness and H0 , up to a proportion of about 1:1. A similar relationship was reported by Schabenberger and Zedaker (1999) for Pinus taeda (loblolly pine) in Virginia Piedmont plantations. Additionally, our study showed that a further increase in non-crop basal area

beyond the 1:1 proportion led to a decrease in species richness and H0 . Conversely, black spruce basal area productivity linearly decreased as basal area of noncrop tree species increased. An admixture of companion species in spruce plantation, obtained by a lack of vegetation control treatment, increased the total productivity at age 20 years by maximising the occupation of the space by the tree component. At the same time, it allowed for a shift in productivity from spruce to the companion species. For the first few years following plantation establishment, Jobidon (2000) depicted the relationship between spruce growth and the abundance of non-crop tree species as a negative hyperbolic form of density dependence. In view of the potential for a large loss in spruce growth when northern hardwoods are present, Jobidon (2000) recommended to manage diversity in spruce plantations only at the time of the pre-commercial thinning treatment. Results from our study show that vegetation management of spruce plantations for maximising spruce productivity is not likely to affect plant diversity of the understory, despite the trade-off at age 20 years between managing vegetation for spruce performance and for overstory diversity. Therefore, the reason for maintaining non-crop tree species within canopy of planted spruce should be considered using two factors, the first related to productivity, that is the specific competitive effect of each species on spruce growth, and the second to diversity, that is the specific role intended to be played by each companion tree species. During the first few years following the thinning treatment, a decrease in plant species diversity of the upper stratum was noted, associated with canopy dominance by P. mariana. It should be evaluated whether such a decrease has ecological consequences. It is important to consider that past vegetation control treatments did not involve any chemical herbicides. Hartley (2002) stated that the less perfectly the competing vegetation is eliminated from the stand, the better for biodiversity. Mechanical tending is rightly less drastic than chemical: vegetative reproduction of most northern hardwood tree species by means of stump sprouting or root suckering occurs after cutting (Bell, 1991; Jobidon, 1995, 1997). In a natural forest ecosystem, diversity increases in stands with an increasing number of tree species, each in several size and age classes, corresponding to the structural diver-

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sity of a stand (Buongiorno et al., 1994; La¨ hde et al., 1999). In spruce plantations, the age of dominant species does not vary, and variations in size classes are kept to a minimum. But, a pre-commercial thinning treatment could contribute to increase age and size classes of non-crop tree species component of plant diversity by means of vegetative reproduction. The treatment would help increase structural diversity, which is of primary importance to protect biodiversity (Hansen et al., 1991). For example, decreases observed in species richness of the upper stratum when increasing the proportion in basal area of P. mariana will likely be compensated for over time by the ingrowth of non-crop tree species occupying the intermediate stratum at the time of sampling. Within the intermediate stratum of the highest P. mariana productivity class (class 3 in Table 2), we found deciduous tree species, mainly Acer rubrum, Betula papyrifera, Populus balsamifera, P. tremuloı`des, and Prunus pensylvanica at frequencies of 49, 29, 7, 24, and 58%, respectively. Thus, pre-commercial thinning treatments in spruce plantations could be viewed as a disturbance that improves the structural diversity of the stand, as well as its growth. Importance of silvicultural treatments for increasing diversity in coniferous plantations was noted by Lust et al. (1998) and Hartley (2002). Lust et al. (1998) reported an increase in the structural diversity of maturing Pinus sylvestris (Scots pine) plantations, attributed to the ingrowth of several hardwood species. Considering that diversity is related to disturbance, managed spruce plantations subjected to frequent disturbances (such as thinning treatments) are likely to maintain higher levels of diversity compared to undisturbed plantations (for example, Thomas et al., 1999). A strategy aiming to maximise spruce plantation productivity by means of release and pre-commercial thinning treatments will contribute to maintain or even increase stand structural diversity, which helps protect biodiversity, without affecting plant species diversity of the lower stratum. However, standards still need to be defined (sensu Spellerberg and Sawyer, 1996) for species diversity to be reached within the canopy cover of spruce plantations, to satisfy specific criteria. Long-term productivity of forest ecosystems is closely linked to nutrient cycles, and species composition might affect the stability of this relationship

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(Larsen, 1995). Maintenance of hardwood tree species was recommended to prevent a decrease in Picea glauca (white spruce) plantation productivity, as a result of soil acidification and decreased N mineralization (Brand et al., 1986; Hendrickson, 1990). However, their abundance within a spruce plantation has not yet been established, nor linked to site fertility, to achieve that role. Still, Longpre´ et al. (1994) found no differences among mixed (aspen [Populus tremuloides] or white birch with jack pine [Pinus banksiana]) and pure natural jack pine stands on a clay soil for net N mineralization. Also, beneficial effects of broadleaves on conifer nutrition appear to be less common than ecosystem theory and conventional wisdom make us believe (Rothe et al., 2003). More research effort is needed to predict specific competitive interactions between northern hardwood species and conifer crop tree species. Similarly, the contribution of the former to ecosystem stability, which could help alleviate establishing an arbitrary level of hardwood abundance within a conifer plantation, also needs further investigation. Otherwise, spruce plantation productivity could be compromised, considering its sensitivity to the presence of hardwoods (Fig. 3; Jobidon, 2000).

Acknowledgements We gratefully acknowledge Jacques Carignan and Re´ jean Poliquin for their involvement in field work, and Daniel D. Kneeshaw for helpful comments on an earlier version of the manuscript.

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