Effects of simulated post-harvest light availability and soil compaction ...

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Effects of simulated post-harvest light availability and soil compaction on deciduous forest herbs Christine J. Small and Brian C. McCarthy

Abstract: To better understand the response of eastern deciduous forest herbs to microenvironmental changes associated with logging, the effects of experimental light and soil compaction treatments were examined in six herbaceous plant species characteristic of varying successional stages. We found severe growth reductions and increased mortality of Osmorhiza claytonii (Michx.) C.B. Clarke, a shade-tolerant forest perennial, when grown in full sun and greater soil compaction. Deeply shaded conditions, similar to those beneath regenerating forests, resulted in reduced growth of early successional species such as Galium aparine L., and Eupatorium rugosum Houtt. Growth of other species such as Geum canadense Jacq., and Elymus hystrix L. appeared to increase in the patchy, intermediate light treatment mimicking mature eastern deciduous forests. Soil compaction caused severe reductions in height and biomass of Eupatorium rugosum and O. claytonii, early- and late-successional species, respectively. While harvested stands experience relatively uniform light environments, canopy gaps and sunflecks in mature eastern deciduous forests create heterogeneous light environments often correlated with recruitment, growth, and diversity of understory herbs. Therefore, management approaches that minimize alteration of forest environments and mimic natural disturbance patterns may be important to the maintenance and regeneration of forest herbs. Résumé : De façon à mieux comprendre la réponse des plantes herbacées des forêts feuillues de l’Est aux changements micro-environnementaux associés à la coupe, les effets de traitements expérimentaux sur la disponibilité en lumière et la compaction du sol ont été examinés sur six plantes herbacées typiques de plusieurs stades de succession. Nous avons observé de fortes réductions de croissance et une augmentation de la mortalité de Osmorhiza calaytonii (Michx.) C.B. Clarke, une espèce pérenne tolérante à l’ombre, lorsque celle-ci était exposée à la pleine lumière et croissait dans un sol plus compact. Des conditions fortement ombragées, similaires à celles présentes dans les forêts en régénération, ont réduit la croissance d’espèces de début de succession comme Galium aparine L. et Eupatorium rugosum Houtt. La croissance d’autres espèces comme Geum canadense Jacq. et Elymus hystrix L. a semblé augmenter dans le traitement avec une luminosité hétérogène de niveau intermédiaire semblable à celle des forêts feuillues matures de l’Est. Des réductions importantes de hauteur et de biomasse de Eupatoruim rugosum et O. claytonii, qui sont respectivement des espèces de début et de fin de succession, ont été observées suite à la compaction du sol. Alors que les peuplements coupés sont soumis à un environnement lumineux relativement uniforme, les trouées du couvert et les trouées de lumière des forêts feuillues matures de l’Est créent un environnement lumineux hétérogène qui est souvent corrélé avec le recrutement, la croissance et la diversité des espèces herbacées du sous-bois. Des approches sylvicoles qui minimisent l’altération de l’environnement forestier et qui imitent les patrons naturels de perturbation peuvent donc jouer un rôle important dans le maintien et la régénération des plantes herbacées en forêt. [Traduit par la Rédaction]

Small and McCarthy

Introduction Eastern deciduous forests are characterized by small-scale disturbances associated with the formation of canopy gaps and soil pits and mounds (Collins et al. 1985; White and Pickett 1985). These natural disturbances produce variable light, moisture, and nutrient environments often related to high herb layer diversity (Bratton 1976; Beatty 1984). Anthropogenic disturbances such as clear-cut harvesting are Received 7 September 2001. Accepted 11 June 2002. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 18 September 2002. C.J. Small1,2 and B.C. McCarthy. Department of Environmental and Plant Biology, Ohio University, Athens, OH 45701, U.S.A. 1 2

Corresponding author (e-mail: [email protected]). Present address: Department of Botany, P.O. Box 5508, Connecticut College, 270 Mohegan Avenue, New London, CT 06320, U.S.A.

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of much greater extent and intensity than most natural forest disturbances and typically lead to reductions in the heterogeneity of forested systems. The high microhabitat specificity and sensitivity of many forest herbs to changes in environmental and edaphic conditions may limit growth and regeneration following such anthropogenic disturbances (Moore and Vankat 1986; Meilleur et al. 1992; Meier et al. 1995). Consequently, the disturbance response of understory herbs has become of increasing concern (Meier et al. 1995; Roberts and Gilliam 1995b). Clear-cut harvesting is a stand-level disturbance, impacting hydrologic, biogeochemical, and other ecosystem-level properties in forest ecosystems. Harvesting causes direct nutrient losses through biomass removal and increases nutrient leaching and soil erosion by reducing transpiration. Removal of the forest canopy increases the duration and intensity of light reaching the soil surface, elevating soil temperatures and rates of organic matter decomposition (Bormann and Likens 1979). Compaction of forest soils during mechanical harvesting increases soil bulk density and reduces porosity

DOI: 10.1139/X02-099

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and infiltration capacity (Greacen and Sands 1980). Of all vegetation strata, the understory appears most sensitive to disturbance and microenvironmental change (McCarthy and Facelli 1990; Meier et al. 1995). Seed production, germination, and growth rates of many forest herbs are altered by increased light availability (Reader and Bricker 1992; Maschinski et al. 1997). Soil compaction further affects recruitment, growth, and survival of understory species (Greacen and Sands 1980; Cussans et al. 1996). Thus, changes in microsite availability and the forest environment may be important in determining species responses to disturbance and pathways of post-harvest forest development (MacLean and Wein 1977; Mou et al. 1993; Roberts and Gilliam 1995a). Light is often the most limiting factor in forest understory communities (Canham et al. 1990), controlling germination and growth of many forest herbs and woody seedlings (Collins et al. 1985). Following clear-cutting, light availability in the understory changes dramatically. Immediately after harvesting, understory plants experience increased duration and intensity of light. With the development of a dense, early successional canopy, light reaching the forest floor becomes severely limiting (Oliver and Larson 1990). Canopy gaps, providing patches of increased sunlight and habitat variability to understory herbs, typically do not occur until the development of a mature forest canopy (Oliver and Larson 1990; Valverde and Silvertown 1997). These changes in light availability during forest recovery can significantly influence seedling recruitment, establishment, and growth and reproduction through altered photosynthetic rates, chlorophyll and nutrient concentrations, and water relations (Collins et al. 1985; Reader and Bricker 1992; Maschinski et al. 1997). Thus, varying light environments may play an important role in the growth or survival of understory species and influence the composition and rate of vegetation recovery following disturbance. Soil compaction is also a principal form of damage associated with logging (Williamson and Neilsen 2000). The use of heavy machinery (e.g., skidders) during timber removal can greatly modify soil structure, hydrology, and chemical properties (Greacen and Sands 1980; Vora 1988; Ballard 2000). The loose, friable, porous soils typically associated with forest vegetation are particularly sensitive to soil compaction (Froehlich et al. 1986a, 1986b). Such compaction increases bulk density and decreases macroporosity and soil aggregation, negatively impacting water infiltration, retention, drainage, and soil aeration and increasing surface runoff and erosion potential (Greacen and Sands 1980; Woodward 1996; Ballard 2000). These structural changes in turn restrict root penetration and growth and limit mycorrhizal association, reducing overall plant growth, survival, and site productivity (Froehlich et al. 1986a; Woodward 1996; Nadian et al. 1997). As moisture availability and rooting depth are frequently correlated with the abundance and diversity of forest herbs and woody seedlings (Huebner et al. 1995; McCarthy et al. 2001), increases in soil compaction may greatly influence forest regeneration and the growth and proliferation of many mature forest herbs (Mou et al. 1993; Roberts and Gilliam 1995a). Such changes in resource availability and the physical environment have been recognized as important determinants

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of community response to disturbance (White and Pickett 1985; Roberts and Gilliam 1995a). Plants respond to environmental changes by altering growth or biomass partitioning to maximize the capture of limiting resources. Shifting growth patterns allow plants to compensate for resource limitations by increasing allocation to organs or functions most closely related to acquisition of the limiting resource (Reynolds and Thornley 1982; Mooney and Winner 1991; Rastetter and Shaver 1992). Thus, plant life-history characteristics and resource-allocation patterns are often strongly related to species disturbance response (Abrahamson 1979; Sousa 1980; Halpern 1989; Jonsson and Esseen 1998). While a number of studies have focused on herb layer recovery following disturbance in eastern forests (e.g., Duffy and Meier 1992; Gilliam et al. 1995; De Grandpré et al. 2000), few have directly examined the effects of post-harvest site conditions on growth and resource allocation of understory herbaceous species. In particular, while growth reductions due to soil compaction have been reported for many forest tree species (e.g., Froehlich et al. 1986a; Woodward 1996; Kozlowski 1999), relatively few studies have considered effects on herbaceous species (Maschinski et al. 1997; Nadian et al. 1997). Thus, to better understand regeneration patterns and the effects of management disturbances on understory herbs, this study examined germination and growth responses of six herbaceous plant species to varying levels of light and soil compaction associated with pre- and postharvest forest environments.

Materials and methods Determination of pre- and post-harvest conditions Measurement of pre- and post-harvest light and soil compaction conditions were performed at the Waterloo Wildlife Experimental Station (WWES; 39°21′08′′N, 82°16′57′′W) in Athens County, Ohio, U.S.A. The WWES lies in the Low Hills Belt section (Braun 1950) of the Unglaciated Allegheny Plateau physiographic province in southeastern Ohio (Brockman 1998). The climate of Athens County is temperate continental, with mean annual precipitation of 102.5 cm, relatively evenly distributed throughout the year (NCDC– NOAA 1999). Soils of upper slopes and ridges are primarily Ultic or Aquic Hapludalfs, consisting of moderately deep to deep, moderately well- to well-drained silt loam surface soils, formed in residuum or from local colluvium of interbedded shale, siltstone, sandstone, or limestone. Soils of lower slopes are generally Typic Dystrochrepts, composed of moderately deep to deep, well-drained loam or silt loam surface soils formed in sandstone residuum (Lucht et al. 1982). The vegetation of WWES lies within the mixed mesophytic forest association (Braun 1950), a component of the unglaciated eastern deciduous forest region. Upper slopes and ridgetops of mature forests are dominated by closed-canopy mixed-oak (e.g., Quercus prinus L., Quercus velutina Lam., Quercus alba L.) or oak–hickory (Quercus– Carya spp.) forests, while lower and north-facing slopes are dominated by mixed mesophytic forests (e.g., Fagus grandifolia Ehrh., Acer saccharum Marsh., and Tilia americana L.; Braun 1950). Pre- and post-harvest soil compaction levels were determined from soil penetrometer readings (n = 400; MPa; Lang © 2002 NRC Canada



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penetrometer) taken in permanent sample plots in 12 upland mature second-growth forests (>125 years; understory reinitiation stage; Oliver and Larson 1990) and revegetating clearcuts (-7 years; late stand initiation – early stem exclusion stage; Oliver and Larson 1990) at WWES (Small and McCarthy 2002). Soil compaction was also measured within skidder tracks, just outside skidder tracks (approximately 20 cm), and on landing pads (where logs are stacked prior to pick-up or hauling) at a recently clear-cut site. Pre- and postharvest and skidder track compaction measurements were as follows: mature second-growth forest, 1.44 ± 0.03 MPa (mean ± SE); 7-year regenerating clearcut, 2.11 ± 0.04 MPa; skidder tracks, 3.48 ± 0.002 MPa). Light-intensity measurements in mature forests (heterogeneous, sunflecked understory), regenerating clearcuts (deeply shaded, closed canopy), and active clearcuts (full sunlight) were taken at the same sites as above, 0.5 m above the ground, using a quantum photometer model No. LI-189 (LI-COR, Inc., Lincoln, Nebr.; n = 50). Measurements were taken at mid-day under minimal cloud cover in August 1999. Values for light intensity were as follows: mature secondgrowth forest, 662.99 ± 66.43 µmol·m–2·s–1; regenerating clearcut, 179.53 ± 38.46 µmol·m–2·s–1; and active clearcut, 1999.01 ± 27.54 µmol·m–2·s–1. Study species Growth responses of six understory herbaceous plant species common to recently clear-cut or mature secondgrowth mixed mesophytic forest stands of the WWES (southeastern Ohio) were examined relative to light and soil compaction conditions associated with forest harvesting. Species represented a range of life histories and successional stages associated with forest regeneration following clear-cut harvesting. Freshly matured seeds were collected during the 1999 growing season from clear-cut and forest stands at WWES. Seeds of each species were collected within single populations. Study species were Eupatorium rugosum Houtt. (Asteraceae), a relatively shade-intolerant perennial of open fields and rich woods; Galium aparine L. (Rubiaceae), a shade-tolerant, often weedy, winter annual of moist, shady woodlands, thickets, fields, and waste places; Elymus hystrix L. (Poaceae), a relatively shade-intolerant perennial of well-drained, rich deciduous forests and thickets; Aster divaricatus L. (Asteraceae), a shade-tolerant perennial of dry open woodlands and clearings; Geum canadense Jacq. (Rosaceae), a shade-tolerant forest perennial, typical of mesic forests but also found in dry or moist open thickets; and Osmorhiza claytonii (Michx.) C.B. Clarke (Apiaceae), a shade-tolerant perennial forest herb of moist, rich woods (Gleason and Cronquist 1991). Most early successional study species (e.g., Eupatorium rugosum, Galium aparine, A. divaricatus) were present in mature stands (as well as clearcuts) but at much lower abundances. Of the later successional species, Elymus hystrix was found along mature forest edges and in rich clearcuts while Geum canadense and O. claytonii were common only in mature forest understories (Small and McCarthy 2002). Experimental light and soil compaction treatments Experimental light and soil compaction treatments were performed in an open field at the Ohio University Experi-

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mental Gardens in Athens, Ohio. On September 18, 1999, ten seeds of each species were planted in 11.43 cm diameter by 11.43 cm deep round plastic pots containing one part Sunshine MixTM (Sun Gro Horticulture Canada Ltd., Bellevue, Wash.) and three parts upper mineral field soil (taken from the site of seed collection). Seeds were placed on the soil surface and covered with -0.5 cm soil mix. Prior to planting, the soil in each pot was compacted to one of three levels, based on field measurements: uncompacted (1.03 MPa), moderately compacted (2.07 MPa), or heavily compacted (3.45 MPa), determined using a Lang soil penetrometer. Twelve replicates of each compaction treatment per species were used. Two pots at each of the three soil compaction levels (six pots per species) were randomly arranged in six rectangular shade houses (1.82 L × 2.43 W × 1.07 m H; 4.78 m3 total volume; 36 pots per house). Two shade houses received a low light treatment created using two layers of overlapping wood lattice (5 cm square openings in each layer; light intensity 124.68 ± 25.82 µmol·m–2·s–1) to mimic the deeply shaded, closed canopy of a regenerating clearcut in the late stand initiation – early stem exclusion stage of stand development. Two shade houses received an intermediate light treatment (one layer of lattice, 5 cm openings; 761.91 ± 119.37 µmol·m–2·s–1) to mimic the heterogeneous, sunflecked understory light environment of a mature forest (understory reinitiation stage). The final two shade houses received full ambient sunlight (house frame only; 2020.00 ± 6.35 µmol·m–2·s–1) to represent a recently clear-cut site. Light measurements were taken at 5-cm intervals 0.5 m above the ground (n = 41) on a cloud-free day in September 1999 using a quantum photometer model No. LI–189 (LICOR, Inc., Lincoln, Nebr.). The three shade house treatments differed significantly in the quantity of light reaching the ground (ANOVA, P < 0.0001; Bonferroni multiple comparisons, P < 0.01). Low, intermediate, and full sun light levels in the shade houses did not differ significantly from corresponding field measures of light availability in regenerating clearcuts, mature forests, and active clearcuts (one-way ANOVA, ns). Pots were watered as necessary to maintain adequate moisture. After germination, plants were thinned to one plant per pot. All other emerging plant species were removed. Within each shade house, pots were rearranged randomly each month to minimize the influence of withinhouse position. Percent germination, shoot height (or length, e.g., Galium aparine), and percent survival were determined monthly for each plant during the growing season (March to September 2000). On September 20, 2000, all plants were harvested. Washed roots and shoots were oven-dried to a constant mass at 80°C for 72 h to determine root, shoot, and total biomass for each plant. Data analysis Mean plant height over time was examined relative to light (3 levels, 2 replicates/level; fixed main treatment effect) and soil compaction (3 levels, 12 replicates/level; fixed subtreatment effect) using a two-way repeated measures analysis of variance (ANOVA) for each species. Shade house was treated as a nested replicate within each light treatment, and the full set of interaction terms were included in each © 2002 NRC Canada



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ANOVA model. Before analysis, height measurements were log10 transformed to meet normality and variance assumptions. At the conclusion of the experiment, mean final plant height, dry mass shoot and root biomass, root to shoot ratio, and total biomass were determined for each species in each treatment combination. Data were analyzed using protected split-plot general linear model ANOVA tests (Steel and Torrie 1980; Dowdy and Wearden 1991) to evaluate the effects of light availability (3 levels, 2 replicates/level; fixed main treatment effect), soil compaction (3 levels, 12 replicates/level; fixed subtreatment effect), and the interaction of light and compaction on plant growth and allocation variables (shoot height and shoot and root biomass) for each study species. Because total biomass and root to shoot ratio values were direct derivations of other morphological measures, these variables were not analyzed in ANOVA tests. Shade house was treated as a nested replicate within each light treatment, and the full set of interaction terms were used in each model. Before analysis, dependent variables were log10 transformed to meet ANOVA normality and variance assumptions. Untransformed means and standard errors are reported throughout the paper. A Bonferroni correction (α/k) was used to adjust for the likelihood of type I (experimentwise) error (Scheiner 1993). Treatment means for significant ANOVAs were compared within species relative to light, soil compaction, and interactions of these factors using a posteriori Bonferroni multiple comparison tests corrected for pairwise experimental error (α/k(k – 1)). All analyses were conducted using NCSS 2000 software (Hintze 2000).

Results Of the six study species, A. divaricatus had particularly low germination and high mortality rates (1 plant germinating in low light, 2 plants in intermediate light, and 10 plants in high light treatments). As a result, this species was excluded from analyses. In general, A. divaricatus germination, plant height, and survival appeared greatest in full sun and intermediate or low soil compaction treatments (not shown or tested). For most other study species, significant effects of light and soil compaction on plant height growth over the growing season were found. Changes in plant height were significantly influenced by light availability in all species except Geum canadense (all others P < 0.001; Fig. 1). Most of these species (e.g., Elymus hystrix, Galium aparine, Eupatorium rugosum) had lower growth rates under shaded conditions (low and (or) intermediate light treatment). Only Osmorhiza claytonii showed enhanced growth under shaded conditions (Fig. 1). Height growth of all species except Elymus hystrix responded to soil compaction treatments (P < 0.001; Fig. 1). Soil compaction generally had a negative effect, with reduced height growth in O. claytonii, Galium aparine, and Eupatorium rugosum plants grown in severely compacted soils. Significant interaction of light and compaction effects on height growth were found in O. claytonii (P < 0.001), Geum canadense (P < 0.001), and Eupatorium rugosum (P = 0.003; Fig. 1), however, making interpretation of main treatment effects alone (as described above) some-


what problematic. These combined treatment effects were particularly evident in plants grown under low-light, highcompaction conditions and resulted in reduced height growth in each of these species. Split-plot ANOVA results indicated that plant growth was significantly influenced by light availability in the early successional species Eupatorium rugosum and Galium aparine (Table 1). Biomass reductions in Eupatorium rugosum resulted primarily from reduced allocation to roots in the low light treatment, with significantly lower root biomass under shaded conditions (Table 2, Fig. 2). Root to shoot ratios (R/S) for Eupatorium rugosum also appeared to decline with reduced light levels (R/S = 1.85 ± 0.30 for high light; 2.05 ± 0.16 for intermediate light; 1.37 ± 0.23 for low light). Both shoot height and root biomass of Galium aparine declined dramatically in response to low light levels relative to intermediate or full sunlight (Table 2). As in Eupatorium rugosum, root/shoot ratios suggest reduced root allocation under lower light conditions (R/S = 0.779 ± 0.088 for high light; 0.579 ± 0.091 for intermediate light; 0.521 ± 0.099 for low light; Fig. 2). The late-successional herb Osmorhiza claytonii, alternatively, showed significant growth reductions (reduced height) when grown in full sun (Tables 1 and 2, Fig. 3). Soil compaction also influenced the growth of Eupatorium rugosum and O. claytonii, as evidenced by splitplot ANOVA results (Table 1). Compaction negatively influenced shoot parameters in Eupatorium rugosum (Table 1), with significantly lower plant height and shoot biomass in highly compacted relative to uncompacted soil (Table 2, Fig. 2). Allocation to above- and below-ground structures of Eupatorium rugosum was similar in highly compacted and uncompacted soils (R/S = 1.55 ± 0.20 for high compaction; 1.68 ± 0.15 for no compaction). In O. claytonii, both aboveand below-ground plant growth were negatively influenced by compaction (Table 2; intermediate and low light levels of Fig. 3). In this species, root biomass allocation appeared to increase in response to compacted soils (R/S = 3.05 ± 0.75 high compaction; 1.68 ± 0.35 no compaction; Fig. 3). A significant interaction of light and soil compaction was also found in O. claytonii, with increased shoot biomass under the combined effects of moderate or low light and soil compaction levels (Table 1, Fig. 3).

Discussion Influence of light availability Experimental light and soil compaction treatments used in this study were chosen to represent disturbances associated with clearcut harvesting. High light intensities following canopy removal increase soil temperatures and reduce water availability. Many shade-tolerant forest herbs subjected to high light and temperature are limited by low maximum photosynthetic rates and high metabolic costs. Thus, higher light levels may lead to photoinhibition, reducing relative growth rates (Lovelock et al. 1994). Effects of high light may be further intensified by water stress (Björkman and Powles 1984; Araus and Hogan 1994). Meier et al. (1995) suggest that such high light conditions cause direct mortality of some forest herbs. In our study, we found significant growth reductions (shoot height) and increased mortality of © 2002 NRC Canada



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Fig. 1. Mean plant height (±SE) of study species grown under three soil compaction and three light intensity treatments during the 2000 growing season (March–September). Statistical probability (P) values for light (L), compaction (C), time (T), and all interaction effects derived from the repeated measures ANOVA are shown in right margin.

O. claytonii, a shade-tolerant mesic forest herb, when grown in full sun treatments. Low light levels on the forest floor following canopy closure (stem exclusion stage) can also limit light-demanding, early successional species and forest herbs dependent on canopy gaps for growth and reproduction (Oliver and Larson 1990; Chazdon and Pearcy 1991). In our study, root and shoot growth of early successional, shade-intolerant species (e.g., Eupatorium rugosum and Galium aparine) significantly declined under deeply shaded conditions. Growth of other species appeared to increase in intermediate light conditions, those mimicking mature eastern deciduous forests. Shoot height of O. claytonii, a mature forest understory herb, increased dramatically in intermediate and low light treatments relative to full sun. Height increases in

Geum canadense, Galium aparine, and Elymus hystrix across the growing season were often greater under patchy, intermediate light levels than deeply shaded conditions. While harvested stands experience fairly uniform, high-light environments, canopy gaps and sunflecks in mature eastern deciduous forests create heterogeneous light environments ranging from patches of deep shade to full sunlight (Canham et al. 1994). Such gaps have been associated with enhanced recruitment and growth of understory herbs and woody seedlings (Beatty 1984; Meier et al. 1995; Robison and McCarthy 1999). In our study, the high spatial variability of the intermediate and low light environments (see Materials and methods for standard errors or variability of shade house light values) may contribute to the enhanced growth of several of our study species. It should be noted, however, that © 2002 NRC Canada



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Can. J. For. Res. Vol. 32, 2002 Table 1. F ratios for split-plot general linear model analysis of variance tests for plants grown under three light and three soil compaction treatments. Source

Light

Compaction

Eupatorium rugosum Shoot height 15.52 (0.026) Shoot biomass 6.60 (0.079) Root biomass 486.87 (< 0.001) Galium aparine Shoot height 54.22 (0.004) Shoot biomass 19.94 (0.019) Root biomass 58.24 (0.004) Elymus hystrix Shoot height 3.34 (0.173) Shoot biomass 0.31 (0.752) Root biomass 0.45 (0.676) Geum canadense Shoot height 1.16 (0.422) Shoot biomass 2.73 (0.211) Root biomass 1.90 (0.292) Osmorhiza claytonii Shoot height 11.68 (0.018) Shoot biomass 4.85 (0.115) Root biomass 1.92 (0.290)

Light × compaction

7.05 (0.027) 9.62 (0.013) 3.87 (0.083)

0.82 (0.557) 0.73 (0.601) 0.88 (0.526)

2.86 (0.134) 1.87 (0.233) 3.49 (0.099)

0.89 (0.523) 0.72 (0.608) 0.57 (0.694)

0.27 (0.770) 0.94 (0.443) 0.51 (0.625)

1.03 (0.463) 0.64 (0.653) 0.08 (0.985)

0.01 (0.994) 0.59 (0.583) 0.06 (0.946)

4.24 (0.057) 4.82 (0.044) 2.78 (0.127)

7.85 (0.021) 28.52 (< 0.001) 15.86 (0.004)

2.45 (0.156) 8.23 (0.013) 4.56 (0.049)

Note: Values in parentheses are probability (P) values associated with each F test. Effects were tested using 2 and 3 degrees of freedom (numerator and denominator df, respectively) for the main treatment effect (light), 3 and 18 df for the shade house effect (nested replicate within each main treatment), 2 and 6 df for the subtreatment effect (soil compaction), and 4 and 6 df for the light × compaction interaction.

Table 2. Means and standard errors (in parentheses) of main treatment (light) and subtreatment (compaction) effects. Light High Eupatorium rugosum Shoot height 13.31 (1.80)ab Shoot biomass 0.306 (0.034)a Root biomass 0.508 (0.048)a Galium aparine Shoot height 9.00 (1.71)a Shoot biomass 0.114 (0.040)a Root biomass 0.105 (0.042)a Elymus hystrix Shoot height 28.74 (2.08)a Shoot biomass 1.18 (0.08)a Root biomass 1.73 (0.17)a Geum canadense Shoot height 4.65 (0.88)a Shoot biomass 0.389 (0.094)a Root biomass 0.322 (0.080)a Osmorhiza claytonii Shoot height 0.942 (0.512)a Shoot biomass 0.004 (0.002)a Root biomass 0.020 (0.011)a

Compaction Intermediate

Low

None

Moderate

High

14.91 (1.70)a 0.303 (0.047)a 0.597 (0.082)a

9.11 (1.58)b 0.145 (0.041)a 0.223 (0.071)b

18.13 (1.75)a 0.355 (0.043)a 0.552 (0.058)a

10.08 (1.07)ab 0.228 (0.043)ab 0.476 (0.098)a

9.12 (1.24)b 0.171 (0.034)b 0.301 (0.071)a

9.03 (1.98)a 0.059 (0.023)ab 0.045 (0.019)a

1.68 (0.97)b 0.006 (0.004)b 0.003 (0.002)b

7.88 (2.06)a 0.085 (0.035)a 0.065 (0.027)a

8.73 (1.99)a 0.080 (0.034)a 0.079 (0.040)a

3.11 (1.07)a 0.015 (0.006)a 0.009 (0.004)a

34.33 (2.22)a 1.00 (0.12)a 3.89 (1.42)a

24.93 (1.19)a 1.02 (0.15)a 1.04 (0.17)a

29.92 (1.86)a 1.12 (0.103)a 3.01 (1.25)a

29.28 (2.46)a 1.127 (0.14)a 2.09 (0.86)a

28.80 (2.27)a 0.94 (0.11)a 1.57 (0.31)a

7.31 (0.83)a 0.372 (0.080)a 0.264 (0.069)a

4.18 (1.41)a 0.189 (0.082)a 0.097 (0.045)a

5.55 (1.33)a 0.376 (0.094)a 0.271 (0.073)a

5.41 (1.09)a 0.268 (0.062)a 0.194 (0.052)a

5.18 (1.00)a 0.305 (0.105)a 0.217 (0.087)a

10.51 (1.44)b 0.131 (0.036)a 0.192 (0.061)a

7.35 (2.08)b 0.085 (0.042)a 0.077 (0.036)a

10.09 (2.04)a 0.137 (0.046)a 0.175 (0.060)a

6.47 (1.62)ab 0.069 (0.030)a 0.085 (0.042)ab

2.14 (1.15)b 0.013 (0.009)b 0.028 (0.014)b

Note: Values with different letters are significantly different among treatments (split-plot ANOVA; Bonferroni-corrected (α/k(k – 1)) multiple comparisons for comparisons within each species). Degrees of freedom for each term are provided in Table 1.

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Fig. 2. Effect of soil compaction and light availability on mean (±SE) growth and allocation variables measured for the earlier successional species Eupatorium rugosum, Galium aparine, and Elymus hystrix.

only light quantity was manipulated in this study. Effects of shading associated with light quality (reduced red/far red ratios) could further influence these species (Mitchell and Woodward 1988). Influence of soil compaction Soil compaction negatively influenced plant growth in several of our study species. Shoot biomass and height were significantly lower in Eupatorium rugosum, a midsuccessional species, when grown in severely compacted soils. All measured variables for O. claytonii, including root biomass, decreased in response to soil compaction. Snider

and Miller (1985) found similar reductions in belowground biomass of understory plant species on skid trails. Anaerobic root environments and the greater sensitivity of forest herbs to soil disturbance may contribute to these growth declines (McIntyre et al. 1995). Water stress associated with soil compaction may be as limiting to plant growth as reduced root penetration and growth. Negative effects on plants have been associated with reduced water infiltration, oxygen availability, diffusion of nutrients, and decreased root penetration (Greacen and Sands 1980; Rab 1996). In our study, soil compaction coupled with low light conditions appeared to cause growth de© 2002 NRC Canada



1760 Fig. 3. Effect of soil compaction and light availability on mean (±SE) growth and allocation variables measured for the later successional species Geum canadense and Osmorhiza claytonii.


et al. 1995; Roberts and Gilliam 1995b; De Grandpré et al. 2000). Like these studies, our results suggest that management activities that minimize alteration of the forest environment and mimic natural forest disturbance patterns by maintaining high microsite variability reduce impacts on forest understory communities.

Acknowledgements We thank the Sigma Xi National Research Society and the Department of Environmental and Plant Biology at Ohio University for financial support, D. White and C. Longbrake for assistance with field and laboratory work, and P. Roback for assistance with statistical analyses.

References

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