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Aspen plant community response to organic matter removal and soil compaction Sybille Haeussler and Richard Kabzems
Abstract: Organic matter removal and reduced soil aeration porosity during logging are important factors influencing the sustained productivity of managed forest ecosystems. We studied the 4-year effect of these factors on diversity and composition of a trembling aspen (Populus tremuloides Michx.) plant community in northeastern British Columbia, Canada, in a completely randomized experiment with three levels of organic matter removal (tree stems; stems and slash; stems, slash, and forest floor) and three levels of soil compaction (none; intermediate (2-cm impression); heavy (5-cm impression)). Tree stem removal caused the greatest change in species diversity (30% of variance; ANOVA p ≤ 0.01), increasing the dominance of aspen and Calamagrostis canadensis (Michx.) Beauv. over other species. Slash removal had little effect. Forest floor removal caused the greatest compositional change (37% of variance; MANOVA p = 0.001), favoring ruderal over bud-banking species. Presence or absence of forest floor better explained these changes than any soil physical or chemical parameter. Although dominance of aspen over Calamagrostis was positively correlated with soil aeration porosity (R2 = 0.50, n = 27, p < 0.001), there were few differences between intermediate and heavy compaction. In this ecosystem, disturbances that reduce forest floor thickness without compacting soils will likely optimize plant species diversity and enhance aspen regrowth. Résumé : L’enlèvement de la matière organique et la réduction de la macroporosité du sol associés à l’exploitation forestière sont des facteurs importants qui influencent le rendement soutenu des écosystèmes forestiers sous aménagement. Nous avons étudié leurs effets après quatre ans sur la diversité et la composition d’une communauté végétale de peuplier faux-tremble (Populus tremuloides Michx.) dans le nord-est de la Colombie-Britannique, au Canada. Le dispositif expérimental était complètement aléatoire et comprenait trois niveaux d’enlèvement de la matière organique (seulement la tige des arbres, la tige et les déchets de coupe, la tige, les déchets de coupe et la couverture morte) et trois niveaux de compaction du sol (aucune, intermédiaire (empreinte de 2 cm) et forte (empreinte de 5 cm). L’enlèvement de la tige des arbres a causé le changement le plus important dans la diversité des espèces (variance de 30 %; valeur de p d’ANOVA ≤ 0,01), entraînant la dominance du peuplier et de Calamagrostis canadensis (Michx.) Beauv. sur les autres espèces. L’enlèvement des déchets de coupe a eu peu d’effet. L’enlèvement de la couverture morte a entraîné le changement de composition le plus important (variance de 37 %; valeur de p de MANOVA = 0,001), ce qui a favorisé les espèces rudérales plutôt que celles qui ont des bourgeons latents. La présence ou l’absence de couverture morte expliquait ces changements mieux que n’importe quel paramètre physique ou chimique du sol. Bien que la dominance du peuplier sur le Calamagrostis soit positivement corrélée avec la macroporosité du sol (R2 = 0,50, n = 27, p < 0,001), il y avait peu de différences entre la compaction intermédiaire et forte. Dans cet écosystème, les perturbations qui réduisent l’épaisseur de la couverture morte sans compacter le sol devraient optimiser la diversité des espèces végétales et favoriser la régénération de peuplier. [Traduit par la Rédaction]
Haeussler and Kabzems
Introduction Research in forest ecology must find reliable indicators of the sustainability of forest practices and of the health or integrity of forest ecosystems (Adamowicz and Burton 2003). Vegetation and soils are among the most useful integrators of forest ecosystem conditions, but both are dynamic components of ecosystems that change substantially over the Received 18 February 2005. Accepted 12 June 2005. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 24 September 2005. S. Haeussler. Skeena Forestry Consultants, 2041 Monckton Road, Smithers, BC V0J 2N4, Canada. R. Kabzems. British Columbia Ministry of Forests, 9000 17th Street, Dawson Creek, BC V1G 4A4, Canada. 1
Corresponding author (e-mail:
[email protected]).
Can. J. For. Res. 35: 2030–2044 (2005)
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course of forest succession and in response to forest disturbances. It is difficult, therefore, to select easily measurable properties (e.g., rates of tree growth or quantities of soil organic debris) that can serve as unambiguous indicators of ecosystem condition and of the success or failure of forest management practices. Furthermore, soils and vegetation interact in complex, nonlinear ways that are inadequately understood. The North American Long-Term Soil Productivity (LTSP) cooperative research program was established in 1989 to determine how harvest practices affect the sustained productivity of forest ecosystems and to develop soil-based productivity indicators of sustainable forest management (Powers et al. 1990). The LTSP program includes at least 62 core installations across the United States and Canada and 40 affiliated sites in and outside of North America, making it the world’s largest network of studies on the relationship between soil
doi: 10.1139/X05-133
© 2005 NRC Canada
Haeussler and Kabzems
disturbance and ecosystem productivity (Powers 2002). Organic matter losses and decreased aeration porosity resulting from soil compaction are considered two primary factors contributing to observed declines in forest productivity (Powers et al. 1990). Thus, the core LTSP experimental design consists of a two-factor experiment with three levels of organic matter removal — (1) tree stem removal, (2) tree stem and woody debris (slash) removal, and (3) tree stem, slash, and forest floor removal — and three levels of soil compaction — (1) none, (2) intermediate, and (3) heavy. Soil properties and growth rates of planted crop trees are closely monitored on all LTSP sites. With early results from LTSP studies now appearing, two important findings have emerged. First, the early response of forest ecosystems to the standard set of treatments seems highly dependent on local soil, vegetation, and weather conditions. For example, tree growth and plant water stress on contrasting sandy- and clay-textured sites in California showed radically different responses to compaction (Gomez et al. 2002). Secondly, indirect effects of the experimental treatments on noncrop vegetation often mask the response of crop tree indicators (Powers 2002). On several LTSP sites, initial tree growth on severely disturbed soils exceeded that on undisturbed soils because of reduced competition from noncrop vegetation (Powers and Fiddler 1997). These findings underline the importance of a holistic understanding of ecosystem response to the treatments and to disturbance phenomena in general. One implication is that tree growth response to soil physical or chemical properties cannot be properly understood without reference to the larger plant community. This study describes the 4-year response of plant communities at an LTSP installation on a mesic trembling aspen (Populus tremuloides Michx.) dominated ecosystem with moderately fine textured glacial till soils in southern boreal forests of northeastern British Columbia, Canada. Trembling aspen is a cornerstone of North America’s boreal forest industry. Its abundance, prolific regeneration, high productivity, and rates of nutrient cycling have generated great interest in management of aspen for sustained crop productivity. Current research suggests regular removal of organic matter through logging has less negative impact on aspen ecosystems than on more nutrient-limited boreal forest types (Paré and Munson 2000; Reich et al. 2001). However, soil compaction from logging equipment is recognized as an important threat to sustained aspen productivity (Zasada and Tappeiner 1969; Shepperd 1993; Smidt and Blinn 2002; Frey et al. 2003). Reduced aspen sucker density and growth rates after soil compaction can result from direct physical damage to aspen root systems, reduced soil aeration, and increased grass competition (Navratil 1996). Our objective was to assess relationships between changes in organic matter levels and soil aeration porosity and changes in the diversity and composition of the plant community. Aspen communities are known for their rapid recovery and high resistance to changes in species composition after wildfire and logging (De Grandpré and Bergeron 1997; Reich et al. 2001). From prior work in similar ecosystems (Haeussler et al. 1999, 2002; Haeussler and Bergeron 2004), we predicted that stem and slash removal would cause only minor changes in vascular plant composition and diversity. Forest floor removal was expected to cause substantial short-term
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change because bud banks and seed banks responsible for rapid recovery of the plant community are concentrated in this layer (Schimmel and Granström 1996; Lee 2004). Strong correlations between changes in plant communities and changes in soil physical or chemical properties were not expected because short-term community response is driven primarily by light availability and the residual capacity for vegetative regeneration (Haeussler et al. 2002; Haeussler 2004). For soil compaction, on the other hand, we hypothesized a direct correlation between changes in soil physical properties (bulk density, aeration porosity) and the competitive balance between aspen and Calamagrostis canadensis (Michx.) Beauv., a highly competitive rhizomatous grass that is ubiquitous in the western boreal region (Navratil 1996; Landhäusser and Lieffers 1998). Trembling aspen does not grow well in wet, poorly aerated soils (Maini and Horton 1964), whereas Calamagrostis is optimally adapted to such conditions (Lieffers et al. 1993). More generally, we hypothesized that increased soil compaction would shift the overall composition of the plant community towards species with higher moisture tolerance. Significant synergies or interactions between organic matter removal (which affects competition for light and the capacity for vegetative and seed regeneration) and compaction (which affects belowground tolerances) may also occur. We postulated that the combined effect of complete forest floor removal plus additional soil compaction could overwhelm the capacity of the aspen plant community to recover within a 4-year period.
Materials and methods Study area The study is located north of the Kiskatinaw River (55°58′N, 120°28′W; elevation 720 m), 25 km northwest of Dawson Creek in the Boreal White and Black Spruce biogeoclimatic zone (BWBSmw1 variant, Delong et al. 1990) of northeastern British Columbia, Canada. Slopes average 4% with a slight south aspect. Soils are Orthic Luvic Gleysols (Soil Classification Working Group 1998) formed on glaciofluvial veneers over glacial till. They have 20–30 cm of eluviated silt loam over mottled clay loam and a coarse fragment content below 5%. The soil moisture regime was assessed as mesic, but drainage is only moderate owing to clay-enriched B horizons. Prelogging forest floor depth averaged 7 cm. The 54-ha even-aged stand of trembling aspen was approximately 100 years old in 1994. Aspen accounted for more than 97% of basal area (32.8 m2·ha–1). White spruce (Picea glauca (Moench) Voss), balsam poplar (Populus balsamifera L.) and lodgepole pine (Pinus contorta Dougl. ex Loud.) were minor components. The moderately developed shrub layer (
