Understory conifer seedling response to a gradient of root and ...

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Abstract: The possible benefit of common mycorrhizal network linkages to seedling growth was tested in a low light partial-cut forest understory. Naturally ...


Understory conifer seedling response to a gradient of root and ectomycorrhizal fungal contact J.M. Kranabetter

Abstract: The possible benefit of common mycorrhizal network linkages to seedling growth was tested in a low light partial-cut forest understory. Naturally regenerated western hemlock (Tsuga heterophylla Raf.) and hybrid spruce (Picea glauca × Picea sitchensis [Moench] Voss) seedlings were transplanted directly into soil or within bags of different pore sizes to restrict the amounts of root and ectomycorrhizal contact. The 5-year study included “full contact” (no bag), “moderate contact” (250-µm openings), and “low contact” (4-µm openings) treatments. Height increment was lowest for full contact seedlings over most of the experiment, and highest for low contact seedlings by years 4 and 5. Full contact seedlings also had slightly lower foliar N content than moderate and low contact seedlings. There were no significant interactions in growth detected between tree species and treatments, despite the higher potential for common mycorrhizal network linkages between a western hemlock understory and canopy. Fifty-eight ectomycorrhizal fungal morphotypes were identified on the seedlings, including many with smooth mantles or with only sparse emanating hyphae, which likely reduced the potential for common mycorrhizal network linkages. These results would support the more traditional concepts of competition for scarce resources by isolated seedlings as the primary interaction for the understory of these mature forests. Key words: common mycorrhizal networks, facilitation, shade tolerance, competition. Résumé : L’auteur a vérifié l’existence d’un bénéfice potentiel provenant de réseaux de liens mycorhiziens communs sur la croissance des plantules, dans les conditions de faible luminosité du sous étage d’une forêt partiellement coupée. Il y a transplanté des plantules de la pruche de l’ouest (Tsuga heterophylla Raf.) et d’épinettes hybrides (Picea glauca × Picea sitchensis [Moench] Voss), soit directement dans le sol, ou soit dans des sachets comportant diverses ouvertures de mailles, de manière à restreindre les quantités de contacts racinaires ou ectomycorhiziens. L’étude, d’une durée de 5 ans, inclut les traitements « contact complet » (sans sachet), « contact modéré » (ouvertures de 250 µm) et « faible contact » (ouvertures de 4 µm). Les accroissements en hauteur sont plus faibles chez les plantules du traitement complet, comparativement à la majorité des autres traitements, et les plus élevés avec le traitement de faible contact, vers les années 4 et 5. Les plantules en plein contact ont également des teneurs en azote légèrement plus basses, que celles des traitements avec contacts modérés et faibles. Il n’y a pas d’interactions significatives de croissance décelables entre les espèces d’arbres et les traitements, en dépit du fort potentiel d’établissement de liens mycorhiziens communs, entre les pruches de l’ouest de la canopée et celles du sous étage. L’auteur a identifié cinquante-huit morphotypes de champignons ectomycorhiziens sur les plantules, incluant plusieurs avec des manchons lisses, ou portant seulement quelques hyphes dispersés, ce qui réduit possiblement le potentiel pour l ’établissement de liens mycorhiziens communs. Ces résultats tendent à supporter les concepts traditionnels de la compétition pour des ressources limitées par les plantules isolées, comme interaction primaire au niveau du sous étage de ces forêts matures. Mots clés : réseaux mycorhiziens communs, facilitation, tolérance à l’ombre, compétition. [Traduit par la Rédaction]



Introduction The expectation that competition for resources best characterizes belowground interactions has been tempered in recent years by the evidence for common mycorrhizal network (CMN) linkages that can ameliorate resource scarcity between plants (Newman 1988; Read 1997; Perry 1998). The Received 5 January 2005. Published on the NRC Research Press Web site at http://canjbot.nrc.ca on 29 June 2005. J.M. Kranabetter. B.C. Ministry of Forests, BAG 6000, Smithers, BC V0J 2N0, Canada (e-mail: [email protected]). Can. J. Bot. 83: 638–646 (2005)

sharing of resources via ectomycorrhiza or arbuscular mycorrhiza is not limited to soil effects, such as water, nitrogen (N), or phosphorus (P), but also includes the key limitation of photosynthate carbon (C) (Read et al. 1985; Simard et al. 2002; He et al. 2003). The experimental evidence for mycorrhizal linkages in field settings has become stronger (Simard et al. 1997a; Lerat et al. 2002), but questions remain as to extent of C transfer from donor plant to fungus to receiver plant, and the degree to which this transfer might offset competitive interactions (Robinson and Fitter 1999; Wu et al. 2001; Kytöviita et al. 2003; Pfeffer et al. 2004). Trenching experiments testing the effects of root competition have generally produced positive growth responses for a

doi: 10.1139/B05-035

© 2005 NRC Canada


variety of understory woodland plants, especially on soils deficient in nutrient supply or water (Coomes and Grubb 2000). Some modification of these competitive interactions by CMN is plausible, however, for ectomycorrhizal (ECM) tree species in a forest understory. Sharing of photosynthate C in mature forests is possibly facilitated by strong gradients in light availability from canopy trees in full light to juvenile trees in shaded understories (Simard et al. 2002). Many ECM fungal species have rhizomorphs or abundant emanating hyphae that allow translocation of resources through the soil (Cairney 1992; Agerer 2001). ECM fungal species with these features are often host-generalists (colonize numerous tree species) and relatively abundant, allowing for numerous potential linkages between understory and canopy trees (Jonsson et al. 1999; Kennedy et al. 2003). Light availability has generally been a good proxy for predicting juvenile tree growth in northern latitude forests (e.g., Wright et al. 1998; Coates and Burton 1999; Claveau et al. 2002), but the degree to which CMN linkages influence shade tolerance and subsequent patterns in understory development is unclear. For example, recruitment into undisturbed interior cedar–hemlock stands is generally limited to western hemlock (Lepage et al. 2000; Coates 2002), perhaps in part because of resource sharing between germinants and the conspecific overstory. Further studies are therefore needed to demonstrate whether species-dependent benefits of belowground contact can be detected in forest ecosystems, via CMN linkages, and the relative significance of this facilitation compared with the limitations of light availability. This field experiment was designed to manipulate both root and ECM hyphal contact, creating a gradient of belowground interactions for seedlings in a low light forest environment. Naturally regenerated western hemlock (Tsuga heterophylla Raf.) and hybrid spruce (Picea glauca × Picea sitchensis [Moench] Voss) seedlings from partial-cut forests were transplanted directly into soil or bags of different pore sizes. Seedling response was monitored for 5 years, and differences in growth, nutrition, and ECM fungal communities of the transplanted seedlings are reported here. My hypothesis was that seedlings with unrestricted ECM hyphal contact but limited rooting contact would have the highest growth rates because of the reduction in root competition and maintenance of CMN resource transfer. A second hypothesis was that western hemlock seedlings would have a relatively larger growth response to CMN manipulations than hybrid spruce because of the greater potential for resource sharing with a western hemlock overstory.

Materials and methods Site description The 4000-ha Date Creek research forest is located in the coast–interior transitional forests of the Interior Cedar– Hemlock zone, moist-cold subzone (Banner et al. 1993) near Hazelton, British Columbia, Canada (55°22′N, 127°50′W, elevation 450 m). The average climatic characteristics of Hazelton are 535 mm annual precipitation (238 mm during the growing season), 4.4 °C mean annual temperature, 1267 growing degree-days >5 °C, and 176 frost-free days (Banner et al. 1993). Mature forests (originating from wildfire in


1855) are dominated by western hemlock (65% of the basal area) but mixed with western redcedar (Thuja plicata (Dougl. ex D. Don) (18%), hybrid spruce (8%), subalpine fir (Abies lasiocarpa [Hook.] Nutt.) (2%), paper birch (Betula papyrifera Marsh.) (5%), and lodgepole pine (Pinus contorta var. latifolia Dougl. ex Loud.) (2%). Mesic ecosystems (western hemlock – step moss; Banner et al. 1993) are characterized by a continuous cover of mostly coniferous trees, sparse shrub and herb layers, and a continuous and well-developed ground cover of feathermosses. Soils are Eluviated and (or) Orthic Dystric Brunisols and Orthic Humo-Ferric Podzols developed in morainal blankets, with soil textures ranging from sandy loam to clay loam (Soil Classification Working Group 1998). Forest floors are Hemimors (Green et al. 1993), 4–14 cm thick. Experimental design The experiment had three treatments: (1) a “full contact” treatment, where the seedlings were transplanted directly into the soil, with no restrictions on root and ECM hyphal contact between seedlings and canopy trees; (2) a “moderate contact” treatment, where seedlings were transplanted within fibreglass mesh bags with 250-µm openings. The mesh bags minimized root contact but allowed ECM hyphal growth into and out of the bags; and (3) a “low contact” treatment where seedlings were transplanted into polyester bags with very limited pore openings. This material (a polyester dress lining) was made of tightly woven threads, with widely spaced pores (4 µm in diameter), that restricted both root and ECM hyphal contact between the seedlings and the mature trees. The bags were constructed as cylinders, 10 cm wide and 20 cm long, open at the top and bottom, and double-sealed at the seam with a heat press. The experiment took place in the “light removal” silviculture system (30% volume removed in either single stems or small groups of stems) in the mature forest (approximately 150 years old) blocks at Date Creek (Coates et al. 1997). I chose microsites in the forest where the removal of single stems had maintained light conditions only slightly greater than undisturbed forests. Holes were excavated through the forest floor and mineral soil down to 20 cm, and roots from mature trees were clipped at the edge and removed when necessary. Mineral soils and forest floors were excavated from the surrounding forest and mixed in separate buckets to create a homogeneous soil for transplanting. A bag was fitted into the hole, and 10 cm of mineral soil was packed into the bag. Naturally regenerated western hemlock and hybrid spruce, approximately 12 cm tall, were excavated from the surrounding partial-cut matrix, next to mature trees, for transplanting. The roots of the seedlings were gently washed in water to remove adhering soil, and the roots were splayed out into the partially filled hole for planting. A small amount of mineral soil and forest floor was added around the root systems, and then forest floor material was packed in until the hole was level with the ground surface. A depth of 7 cm was maintained for forest floors, similar to the undisturbed forest. Approximately 0.5 cm of the mesh and polyester bags were left above the soil surface. Mineral soil and forest floor was also packed around the outside of the bags where needed to maintain contact with the surrounding © 2005 NRC Canada


soil. For the “full contact” treatment, a hole was dug to the same dimensions, and the same transplant procedure was followed as the other treatments. Transplanted seedlings of full, moderate, and low contact were spaced 1 m apart, in a triangular arrangement, for each species. This replicate was established three times for each species within a 5-m-radius circular plot in the partial-cut mature forest (3 treatments × 2 species × 3 replicates = 18 seedlings per plot). These plots were repeated five times within each of three “light-removal” blocks at the Date Creek research forest. The experiment had a total of 270 seedlings, or 45 seedlings per species per treatment. The transplanting took place in the spring, 20–28 May 1999, just before the commencement of leader growth. Seedling and light measurements Seedling height, height increment, and root collar diameter were measured at the end of the growing season, in midSeptember, from 1999 (year 1) to 2003 (year 5). Seedling height was also measured at the time of transplanting. In mid-September 2003 (year 5), the current year’s foliage was sampled from lateral branches on the seedlings. These needles were bulked, by species, from the three treatment replicates per plot (n = 90). Foliar samples were oven-dried (70 °C for 24 h) and ground with a Wiley mill. Foliar specific mass was determined as mass (g) of 50 needles. Foliar N was analyzed by dry combustion with the Leco CHN-600 analyzer (Leco Corp., St. Joseph, Michigan). Macro- and micro-nutrients were analysed by ICP-AES following microwave digestion (Kalra and Maynard 1991; Carter 1993). Available light for growth over the growing season (midApril to mid-September) was assessed using hemispherical canopy photos. The digital photo was taken in the centre of each 5-m-radius plot, at a height of 1 m. The growing season light availability, expressed as a percentage of full sun, was computed from each photograph using the Gap Light Analyzer (GLA) 2.0 software, following Frazer et al. (1999). Ectomycorrhizal fungi assessment A subset of seedlings was excavated in June 2004 for ECM fungi assessment. One replicate of 3 seedlings (full, moderate, and low contact) per species from each experimental plot was randomly chosen for a total of 90 seedlings (15 seedlings per treatment per species). The seedlings were excavated, and each was placed into a plastic bag, along with some forest floor material to keep the root tips fresh. The seedlings were refrigerated until the ectomycorrhizal assessment. The root system of each seedling was washed gently in warm water to remove most of the soil and organic debris. Once all surface debris was removed, the clean roots were cut into approximately 2.5-cm-long sections and placed in a Petri dish filled with water. Sections were randomly selected, and the number of root tips colonized by each ECM morphotype was determined. Successive root sections were examined until 200 root tips had been counted from each of the seedlings. Each root tip was examined macroscopically (10×–40× magnification) for features such as colour, shape, size, and texture of the root tip, as well as emanating elements, if present. The root tips were examined at 400× and 1000×

Can. J. Bot. Vol. 83, 2005

magnification for characteristics of the mantle layers and emanating elements such as mantle type, ornamentation, cell contents, clamp frequency, and lengths and widths of hyphal cells. Slides were prepared using either mantle squashes or mantle peels, depending on the thickness of the mantle layers. When necessary, the root tips were stained with either 10% (m/v) KOH, Meltzer’s reagent (Largent et al. 1977) or 0.1% (m/v) aqueous toluidine blue O to emphasize the mantle features. Root tips were designated “undetermined” if mantle features were not apparent, either because the root tip had not been colonized or the mantle was too young and undeveloped to describe. I named the morphotype if it matched published descriptions (Agerer 1988; Ingleby et al. 1990; Goodman et al. 1996). In addition, I have matched some morphotypes to fungal species at Date Creek through repeated sampling of sporocarp–root linkages (Kranabetter and Friesen 2002). Statistics Differences in seedling responses were tested by ANOVA using a randomized block design with a split-plot for species (Table 1) (SAS Institute Inc. 1988). Repeated measures analysis was not used because of the inability to test tree species interactions, and to allow for closer examination of responses by year. Differences between means were tested in pairwise t test comparisons using “tdiff” and an error term of block × treatment (SAS Institute Inc. 1988). Residuals were tested for normal distributions using the Shapiro–Wilk test and visually examined for unusual patterns and outliers. The test of foliar nutrients and ectomycorrhizal richness was without subsamples, so the ANOVA was modified by replacing rep(block × plot × treatment) with plot × treatment(block) and removing plot × treatment × species(block). ECM community similarity based on morphotype abundance (% root colonized) was determined by percentage of similarity (Pielou 1984). The modified Shannon–Wiener diversity index was used to compare the relative distribution of morphotypes on each seedling (Krebs 1989). The distribution of individual morphotypes was not statistically tested because of their infrequent occurrence and small number of seedlings sampled per treatment.

Results Seedling response Available light in the partial-cut understory averaged 16.5% (range 11%–19%) of full sunlight. A total of 18 seedlings died during the course of the experiment (7% of the total), and the mortality was evenly spread between species (10 for western hemlock and 8 for hybrid spruce) and treatments (8 for full contact, 4 for moderate contact, and 6 for low contact). Seedling growth was quite poor in the forest understory, with an average total height and diameter of approximately 30 cm and 4 mm, respectively, after 5 years (Fig. 1). The full contact seedlings had significantly reduced height and diameter compared with the moderate and low contact seedlings (Table 2). Western hemlock seedlings were taller than hybrid spruce (36 cm (SE 0.9) versus 27 cm (0.6), respectively), but diameter differences were not significant by years 4 and 5 (Table 2). Treatment growth responses were consistent be© 2005 NRC Canada


641 Table 1. ANOVA model for tests of seedling response. Source


Block Plot(block) Treatment Block × treatment Rep(block × plot × treatment) Species Block × species Plot × species(block) Treatment × species Block × treatment × species Plot × treatment × species(block)

2 12 2 4 90 1 2 12 2 4 24

Error term

Block × treatment

Block × species

Block × treatment × species

Fig. 1. Seedling total height (a) and root collar diameter (b) over the 5-year experiment (western hemlock and hybrid spruce combined; error bars are SE). Year 0 is seedling height at the time of transplant. Treatments marked by an asterisk are significantly different from the other treatments in that year of the experiment (p < 0.05).




Height (cm)



25 20 15 10 5

Full contact Moderate

Root collar diameter (mm)



4 *

3 2

Full contact Moderate


Low contact

Low contact


0 year year year year year year 0 1 2 3 4 5

tween species (no significant species × treatment interaction). Height increment was lowest for full contact seedlings in years 2 and 3 of the experiment, with no significant differences detected between low and moderate contact seedlings (p = 0.166 and 0.612 for years 2 and 3, respectively) (Table 2; Fig. 2). By years 4 and 5, the trends shifted to better growth for low contact seedlings (p values between low and moderate contact seedlings were 0.076 and 0.062 for years 4 and 5, respectively, and between low and full contact seedlings, they were 0.014 and 0.025, respectively). There was also a significant species effect, with more growth by western hemlock under low light levels than hybrid spruce (23 cm (SE 0.8) versus 16 cm (0.5) over 5 years, respectively), but no significant species × treatment interaction (Table 2). Diameter increment per year was so low that it was difficult to measure precisely, and subsequently no statistical analysis was undertaken. The western hemlock and hybrid spruce seedlings showed severe deficiencies in foliar N concentrations across all treatments (Table 3) (adequate N concentrations of 13.5 and 14.5 g·kg–1 for western hemlock and hybrid spruce, respectively; Carter 1992). There were no strong differences in foliar nutrient concentrations between treatments, but N, when

year 1

year 2

year 3

year 4

year 5

converted to mass, was significantly different between treatments (p = 0.059). Low and moderate contact seedlings had an 8% increase in foliar N content over full contact seedlings. P nutrition was more sufficient than N (adequate P concentrations of 2.5 and 1.6 g·kg–1 for western hemlock and hybrid spruce, respectively; Carter 1992), but an interaction in foliar P across treatments was detected between species (Table 3). Neither species had a significant treatment effect for P concentrations when tested separately (p = 0.209 and p = 0.208 for western hemlock and hybrid spruce, respectively). In addition, when expressed as foliar P content, there were no longer significant interaction (p = 0.925) nor treatment effects (p = 0.242). Ectomycorrhizal communities Five years after transplanting, I identified 58 morphotypes on 90 seedlings, with 28 of these morphotypes found on both hybrid white spruce and western hemlock (Table 4). The ECM communities of full contact western hemlock and hybrid white spruce had a similarity of 57%, based on morphotype abundance. Shared and abundant morphotypes included Mycelium radicis atrovirens (MRA), Cenococcum geophilum, and Piloderma fallax, while unique or hostpreferred morphotypes included Lactarius pseudomucidus © 2005 NRC Canada


Can. J. Bot. Vol. 83, 2005 Table 2. ANOVA results (p > F) for seedling growth response over the 5-year experiment. Year 1

Year 2

Year 3

Year 4

Year 5

Height Treatment Species Treatment × species

0.621 0.012 0.797

0.386 0.021 0.268

0.250 0.037 0.140

0.115 0.040 0.120

0.030 0.040 0.346

Diameter Treatment Species Treatment × species

0.066 0.010 0.816

0.382 0.018 0.311

0.057 0.020 0.777

0.054 0.112 0.911

0.040 0.214 0.398

Height increment Treatment Species Treatment × species

0.368 0.310 0.164

0.063 0.092 0.300

0.035 0.109 0.169

0.034 0.044 0.931

0.054 0.002 0.373

Fig. 2. Seedling height increment (year 1 – year 5) for seedlings (western hemlock and hybrid spruce combined) across full, moderate and low contact treatments (error bars are SE). Treatments marked by different letters are significantly different in that year of the experiment (p < 0.10).

Height increment (cm)

6 5

a a








Full contact Moderate


Low contact

0 year 1

year 2

year 3

year 4

year 5

and Russula occidentalis. Many of the more abundant morphotypes were either contact or short-distance exploration types, while long-distance exploration types were quite rare (classification following Agerer 2001). The ECM communities were influenced by the treatments, with slightly higher levels of average morphotype richness (p = 0.007) and evenness (Shannon–Wiener index) (p = 0.062) for full contact seedlings compared with moderate or low contact seedlings (Table 4). Most of the more abundant morphotypes were found in each treatment, resulting in an average community similarity between full, moderate, and low contact treatments of 65% for western hemlock and 73% for hybrid spruce.

Discussion In this experiment I attempted to control the amount of root and ECM fungal contact between seedlings and overstory trees with bags of differing pore sizes. An examination of the seedlings and bags at the conclusion of the experiment demonstrated a clear gradient in the relative amounts of rooting and ECM hyphal overlap. Full contact seedlings were completely enmeshed in roots from the ma-

ture trees, and were difficult to extricate for morphotype examination. ECM hyphae and rhizomorphs had clearly passed through the coarser mesh of the moderate contact treatment, along with an occasional root (roots penetrated approx. 1% of the bag surface area). Low contact bags were undisturbed and easily removed from the soil, with no root penetration and no obvious ECM hyphae or rhizomorphs growing through the polyester fabric. There was little evidence of roots entering or exiting the bottom of the bags after 5 years because of the shallow rooting depths in these soils. The only methodological difficulty noted was that roots were able to grow over the top of the bags as moss accumulated. This was found on a small number of seedlings, and probably only occurred in the final year or two of the experiment, but should be controlled for in any future studies of this nature. The transplanted seedlings in the understory grew very slowly and were approaching complete stagnation, demonstrating the strong growth limitations of light and possibly N availability (Chapin et al. 1987; Canham et al. 1996; Messier et al. 1999). Seedling performance improved with diminished belowground contact, which suggested there was little net transfer of photosynthate C or N to offset growthlimiting factors. Seedlings in the moderate contact treatments grew relatively well initially, but apparently over time enough ECM hyphal biomass from canopy trees entered the bags to increase belowground competition and cause significant reductions in height increment. In addition, both tree species responded consistently to the treatments, despite the more limited potential for CMN linkages between hybrid spruce seedlings and a western hemlock overstory. These results would support the more traditional concepts of competition for scarce resources by isolated seedlings as the primary interaction for the understory of these mature forests (Tilman 1988; Casper and Jackson 1997; Coomes and Grubb 2000). Much of the experimental evidence for CMN linkages is based on seedlings grown in controlled environments (microcosms) or recently harvested forests (Simard et al. 1997a; McKendrick et al. 2000; Wu et al. 2001; Querejeta et al. 2003). The ectomycorrhizal communities under these conditions tend to be less complex than mature forests, often with only a few ECM species dominating root systems. The possibility of hyphal contact and sharing of resources between root systems in the mature forests of the present experiment was reduced by the high species richness and relatively even © 2005 NRC Canada

36 (3) 35 (2) 36 (2) 32 (4) 41 (8) 32 (4) 1.85 (0.2) 1.85 (0.2) 1.89 (0.2) Note: ANOVA results (p > F) are given for treatment, species, and the interaction. Values are means with SE in parentheses.

11 (0.9) 11 (0.9) 11 (0.8) 0.87 (0.05) 0.84 (0.07) 0.85 (0.03) 6.8 (0.4) 7.2 (0.6) 7.8 (0.4) 3.0 (0.3) 2.6 (0.1) 2.7 (0.1) 0.59 (0.03) 0.66 (0.04) 0.66 (0.03) 1.7 (0.08) 1.8 (0.10) 1.9 (0.07) 0.040 (0.002) 0.041 (0.002) 0.037 (0.001) Hybrid spruce Full contact Moderate contact Low contact

8.3 (0.4) 8.7 (0.4) 8.9 (0.3)

7.8 (0.7) 8.3 (0.6) 8.0 (0.7) 31 (3) 36 (4) 32 (2) 1.92 (0.1) 2.06 (0.1) 1.96 (0.1) 16 (0.9) 19 (1.5) 17 (1.2) 1.4 (0.06) 1.4 (0.06) 1.4 (0.03) 7.8 (0.4) 8.2 (0.3) 8.0 (0.3) 2.6 (0.1) 2.5 (0.1) 2.5 (0.1) 0.72 (0.03) 0.73 (0.04) 0.70 (0.02) 2.2 (0.06) 2.2 (0.06) 2.0 (0.07) 0.049 (0.002) 0.054 (0.002) 0.054 (0.002) Western hemlock Full contact Moderate contact Low contact

9.9 (0.3) 9.5 (0.4) 9.6 (0.3)

0.464 0.364 0.464 0.452 0.349 0.535 0.086 0.012 0.292 0.411 0.003 0.943 0.190 0.155 0.457 0.129 0.005 0.151 0.075 0.071 0.463 0.544 0.022 0.001 0.814 0.001 0.560 0.128 0.014 0.293 Treatment (p > F) Species Treatment × species

Fe (mg·kg–1) Cu (mg·kg–1) B (mg·kg–1) Mg (g·kg–1) K (g·kg–1) Ca (g·kg–1) S (g·kg–1) P (g·kg–1) N (g·kg–1) 50 needle mass (g)

Table 3. Foliar mass and nutrient concentrations at year 5 across full, moderate, and low contact treatment for western hemlock and hybrid spruce.

0.460 0.005 0.701

643 Zn (mg·kg–1)


distribution of this ECM community. In addition, morphotypes with smooth mantles (contact exploration type) were quite prevalent, and, in the case of western hemlock, colonized over 40% of the root tips. A number of other morphotypes had only sparse emanating hyphae, with no rhizomorphs, so overall the potential for widespread mycorrhizal networks would be relatively moderate to low in these mature forests (Kranabetter et al. 1999). The functional significance of CMN linkages might therefore change with ECM fungal succession as stands mature, or by the nature of ECM communities across specific host species (Wilkinson 1999; McKendrick et al. 2000; Dickie et al. 2002). Limiting the amount of root and ECM hyphal contact will not only reduce the likelihood of CMNs, but will also have some effect on the ECM fungal communities colonizing the seedlings (Fleming 1983; Simard et al. 1997b). Naturally regenerated seedlings from the mature forest, rather than nursery or nonmycorrhizal planting stock, were used in this experiment to minimize the possible confounding influences of ECM dispersal on seedling response. The use of naturally regenerated seedlings limited the scope of the study to resource transfer, and excluded the possible benefits of CMN linkages on seedling establishment (Horton et al. 1999; Newbery et al. 2000; Onguene and Kuyper 2002; Dickie et al. 2002; Booth 2004). I expect the differences in morphotype richness and distribution were small enough to assume the improvements in growth were primarily a result of diminished root and hyphal contact. This better growth response was likely caused by reduced competition for moisture and small improvements in N status, although this interpretation was complicated by the differing patterns in foliar mass response between species. P interactions were likely from physiological differences between the species in foliar attributes under shade and not an influence of CMNs. Other potential factors related to the experimental design should be considered in the interpretation of these results. Although western hemlock predominated in the canopy, other tree species, including arbuscular mycorrhizal western redcedar (Carpenter and Trappe 1970), were present and would have reduced the likelihood of CMN linkages with the understory. In addition, transplanting seedlings would have caused a disruption in any existing hyphal linkages between root systems, and the initial ECM fungal community could be quite different than that found in the planted microsites. The experiment was run for 5 years to enhance the probability of building hyphal linkages as feeder roots were replaced, but low contact seedlings were instead growing slightly better over time. Functional CMN linkages might only develop on a small percentage of seedlings, rather than across the whole population, which would be less detectable but still ecologically significant (Watkins et al. 1996). In this experiment, however, I did not find skewed distributions in height increment or differences in maximum height between treatments that would support the possibility of enhanced growth for a minority of full contact seedlings. Any manipulation of belowground interactions is subject to possible secondary effects of the partitions (McPhee and Aarssen 2001), such as an improvement in growth by reducing exposure to pathogenic soil fauna or fungi. Soils for the transplant were taken directly from the forest, presumably with a full community of fauna, and the bags were open at © 2005 NRC Canada


Can. J. Bot. Vol. 83, 2005 Table 4. Ectomycorrhizal community attributes at year 5 post-transplant across tree species and root contact treatments. Western hemlock Typea Average richness Total richness Shannon–Wiener index % roots colonized MRAb Cenococcum geophilumc Lactarius pseudomucidusd Laccaria laccatad Russula occidentalisd Amphinema byssoides-likec Rozites caperatad Piloderma fallaxc Russula decoloransd ITE-2b Cortinarius muscigenusd Cortinarius cinnamomeusd Cortinarius vibratilis-liked Hygrophorus piceaed “Green globs”d Lactarius rufusd Hebeloma crustuliniformed Hygrophorus bakerensisd Leccinum aurantiacumd Unknown “stringy”


Hybrid spruce

Full contact

Moderate contact

Low contact

Full contact

Moderate contact

Low contact

6.5 (0.7) 33 4.39 (0.5)

5.3 (0.6) 31 3.62 (0.4)

5.5 (0.6) 28 4.19 (0.6)

7.4 (0.5) 32 5.39 (0.4)

6.0 (0.4) 29 4.17 (0.3)

6.0 (0.5) 28 4.25 (0.5)

12.6 10.7 8.2 2.0 7.5 4.6 6.1 5.7 5.0 5.3 4.8 4.0 5.5 2.6 1.0 0 1.9 1.1 0.5 0

11.7 12.6 11.9 1.3 22.2 5.2 0.3 4.6 3.5 4.3 1.8 0.3 0 0.9 0.9 0 0.1 0 2.4 0.7

5.3 9.5 15.1 5.9 19.5 6.6 2.1 5.3 3.6 2.6 0.6 4.5 0 2.6 1.7 3.4 1.0 3.1 0 0.1

12.6 10.3 0 13.4 1.4 11.9 3.0 7.5 8.3 7.6 0.1 3.4 0 1.7 2.6 0.4 1.4 0 0 1.8

9.4 10.8 0 8.6 0.6 11.3 0 6.6 16.1 6.8 0 1.6 0 0.3 2.2 2.3 3.6 0 1.6 4.4

11.7 (2.9) 9.2 (3.4) 0 13.4 (5.6) 0.4 (0.4) 19.9 (8.4) 2.9 (2.9) 4.4 (1.1) 3.9 (3.9) 6.3 (2.9) 1.9 (1.4) 1.5 (0.9) 0 2.9 (1.8) 3.6 (1.8) 2.5 (1.8) 1.3 (0.9) 0 1.2 (1.2) 3.5 (2.8)

(2.1) (3.0) (3.1) (1.2) (6.6) (3.2) (5.9) (2.7) (5.0) (2.7) (3.8) (3.0) (4.2) (1.8) (0.7) (1.3) (0.8) (0.4)

(4.1) (4.7) (4.5) (0.7) (9.8) (2.8) (0.3) (1.9) (3.5) (2.6) (1.8) (0.2) (0.7) (0.6) (0.1) (2.0) (0.5)

(1.6) (2.6) (4.9) (2.9) (9.3) (4.7) (2.1) (2.1) (3.6) (1.4) (0.6) (2.0) (1.4) (1.0) (2.4) (0.7) (2.3) (0.1)

(2.3) (1.4) (3.4) (1.4) (5.2) (3.0) (2.8) (4.3) (4.1) (0.1) (1.6) (1.0) (1.3) (0.4) (0.9)


(1.7) (2.8) (4.0) (0.6) (4.9) (2.8) (7.6) (4.2) (0.9) (0.3) (1.6) (1.1) (3.6) (1.0) (3.0)

Note: Morphotype abundance (% roots colonized across 15 seedlings = 3000 root tips per treatment) is listed for the more dominant ectomycorrhizal fungi. Values are means with SE in parentheses. a Exploration type from Agerer (2001); C, contact exploration; S, short-distance exploration; M, medium-distance exploration; L, longdistance exploration. b Based on descriptions from Ingleby et al. (1990). MRA, Mycelium radicis atrovirens; ITE-2, Institute of Terrestrial Ecology. c Based on descriptions from Goodman et al. 1996. d From sporocarp–root linkage descriptions of morphotypes at Date Creek.

the top and bottom to allow some migration. Western hemlock is less susceptible than hybrid spruce to a Tomentosus root disease common to this area (Lavender et al. 1990), but mortality (which was quite low) between species and among treatments was similar. The root systems and seedlings looked healthy among treatments, so I have no reason to suspect that pathogens were playing a role in the results. Nevertheless a mix of experimental approaches with partitions would be useful to build some consensus on CMN effects. To date at least the general consistency in response (40 out of 47 root-trenching experiments resulted in a positive plant response; Coomes and Grubb 2000) would suggest that CMNs in many forests do not provide enough net benefits to understory plants to ameliorate the effects of shade or belowground competition.

Acknowledgements Marcel Lavigne, Anne-Marie Roberts, and Karen Geertsema assisted me in transplanting seedlings for this experiment. Marcel Lavigne also assisted in the seedling measurements and foliar sampling and preparation for analysis. Clive Dawson and Dave Dunn from the B.C. Ministry of Forests Analytical laboratory undertook the foliar nutrient

analysis. Peter Ott of the B.C. Ministry of Forests was consulted on the statistical analysis. Graeme Hope provided useful comments on an earlier draft of the manuscript. Funds for the research project were provided by the B.C. Ministry of Forests.

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