Effects of wood decomposer fungi on tree seedling establishment on coarse woody debris

Forest Ecology and Management 266 (2012) 232–238

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Effects of wood decomposer fungi on tree seedling establishment on coarse woody debris Yu Fukasawa ⇑ Laboratory of Forest Ecology, Graduate School of Agricultural Science, Tohoku University, Naruko-onsen, Osaki, Miyagi 989-6711, Japan

a r t i c l e

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Article history: Received 27 September 2011 Received in revised form 14 November 2011 Accepted 17 November 2011 Available online 15 December 2011 Keywords: Decay type Rotted log Safe sites Seed size Tree recruitment Wood decay fungi

a b s t r a c t Although decomposer fungi have been recognized to play important roles in forest carbon and nutrient cycling as well as soil humus formation, their effects on forest dynamics, such as tree regeneration, are far less well understood than the effects of symbiotic and pathogenic fungi. In this study, I focused on tree regeneration on fallen woody debris, ‘‘nurse logs’’. I examined the effects of wood decomposer fungi on species composition and population densities of tree seedlings (height <50 cm) established on these logs. In an abandoned tract of coppice forest in Honshu, Japan, a thick litter layer had accumulated on the forest floor and seedlings of the small-seeded pioneer tree species Clethra barbinervis (Ericales) were found to be preferentially established on rotting fallen logs of the Japanese red pine Pinus densiflora. C. barbinervis seedling establishment was considerably reduced on soil probably because there were impediments to colonization on the ground, such as the thick litter layer, which was less well developed on logs. In contrast, larger-seeded species such as Aphananthe aspera, Carpinus spp., Quercus serrata, and Rhus trichocarpa preferentially established on soil. Characteristics of wood decomposition by fungi varied among logs, and this variability significantly influenced C. barbinervis seedling density. Seedling density was significantly higher on brown-rotted logs than that on logs belonging to other decay types. Wood pH was lower in brown-rotted logs than that in logs belonging to other decay types and was negatively correlated with seedling density. Thus, pine coarse woody debris and the functional diversity of wood inhabiting fungi influence the establishment of diverse tree seedlings in this abandoned Japanese coppice forest. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Soil fungi (e.g. symbionts, pathogens, and decomposers) have strong direct and indirect effects on aboveground vegetation dynamics (Bardgett and Wardle, 2010; Van der Heijden et al., 2008). Symbiotic mycorrhizal fungi are crucial agents for plant growth and survival. These fungi improve water and nutrient uptake and boost defenses of host plants against root pathogens (Smith and Read, 2008). In contrast, pathogenic fungi are responsible for mortality and reduced fecundities of individual plants and drive host population dynamics (Gilbert, 2002). In some cases, pathogenic fungi have important roles in the maintenance of local species diversity in natural plant communities (Packer and Clay, 2000). These effects of symbiotic and pathogenic fungi are examples of direct interactions between fungi and plants. The effects on aboveground vegetation dynamics, including forest regeneration, are well researched for both symbiotic (Dickie et al., 2002; McGuire, 2009; Nara, 2006; Nara and Hogetsu, 2004; Perry et al., 1989) and pathogenic interactions (Packer and Clay, 2003; Seiwa ⇑ Tel.: +81 229 84 7397; fax: +81 229 84 6490. E-mail address: [email protected] 0378-1127/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2011.11.027

et al., 2008; Yamazaki et al., 2009). Decomposer fungi may also influence aboveground plant regeneration through indirect interactions because they play an important role in the breakdown and transformation of organic matter to soil humus. Furthermore, decomposer fungi create safe sites (e.g. decayed logs) for tree seedling establishment (Bardgett and Wardle, 2010; Harmon et al., 1986), but little is known about the mechanisms of decomposer fungal activity that influence plant regeneration, especially of trees. Coarse woody debris (CWD) is a major decay substrate and energy source for forest decomposer fungi (Rayner and Boddy, 1988; Swift et al., 1979). Wood decomposition by fungi is classified into different decay types such as white-rot, brown-rot, and soft-rot, which indicate lignocellulose decomposition capabilities of the different fungal species (Eaton and Hale, 1993). Because decomposition of lignocellulose is a key factor controlling soil humus formation on the forest floor (Stevenson, 1982), decay type has significant effects on species composition of organisms in soil detritus-based food webs such as bacteria (Folman et al., 2008; Jergensen et al., 1989), ectomycorrhizal fungi (Tedersoo et al., 2008), termites (Cornelius et al., 2002), and beetle larvae (Araya, 1993; Wardlaw et al., 2009) that inhabit dead wood. Wood

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chemical properties including pH and organic content may control communities of organisms in woody debris (Jergensen et al., 1989). Since soil food web properties have feedback effects on aboveground plant growth and community composition (Bardgett and Wardle, 2010), I postulated that wood decay type would also influence plant seedling establishment on fallen logs via altering physicochemical and biotic conditions of logs. Heterogeneity of forest understory microsites is critically important for seedling colonization by various plant species and for the determination of subsequent forest structure (Nakashizuka, 2001). Among microsites on forest floors, ‘‘nurse log’’ CWD provides important safe sites for seedling establishment in various forest ecosystems such as boreal and subalpine (Doi et al., 2008; Harmon and Franklin, 1989; Iijima and Shibuya, 2010; Narukawa and Yamamoto, 2003; Sugita and Nagaike, 2005; Yano and Shibuya, 2010), temperate (Bellingham and Richardson, 2006; Christie and Armesto, 2003; Heinemann and Kitzberger, 2006; Papaik and Canham, 2006), and tropical woodlands (Sanchez et al., 2009; Santiago, 2000; Van der Meer et al., 1998). Properties of logs, such as diameter (Takahashi, 1994), moss cover (Iijima and Shibuya, 2010; Nakamura, 1992), and decay class (Doi et al., 2008; Mori et al., 2004; Takahashi et al., 2000) influence seedling establishment on logs. However, effects of functional differences among wood decomposer fungi on seedling establishment have yet to be reported. Japanese red pine (Pinus densiflora Sieb. et Zucc.) is a dominant canopy tree species in temperate secondary forests of Japan. The pine wilt disease has killed many adult pine trees over recent decades (Takemoto and Futai, 2008), resulting in accumulation of substantial quantities of pine CWD on forest floors (Kato and Hayashi, 2006). Evaluations of the roles of pine CWD in forest regeneration and maintenance of biodiversity in subsequent forest recovery process are particularly relevant at present. In the present study, I particularly focused on (1) seedling species composition and densities on pine logs, (2) the effects of diverse log properties, including wood decay type, on seedling densities of the most dominant species on logs, (3) relationships between log properties and wood decay types, and (4) mycorrhizal infection of seedlings on CWD and soil. Based on the above, I discussed about the effects of wood decay types on seedling densities via log properties and mycorrhizal infection.

2. Materials and methods 2.1. Study area and background The study site was located in Higashiyamato Park (35°450 N, 139°260 E; 114–122 m a.s.l.), 40 km west of Tokyo, Japan. The mean annual temperature at the nearest meteorological station in Tokorozawa (35°460 N, 139°250 E; 119 m a.s.l.) from 1979 to 2000 was 14.1 °C. The mean monthly temperature ranged from 3.6 °C in January to 25.5 °C in August. The mean annual precipitation was 1443.9 mm, and there was rarely snow in winter (Japan Meteorological Agency, 2011). The park area of 184 ha was surrounded by residential district. The forest was an abandoned coppice of Quercus serrata and P. densiflora that had been thinned on a 20–25 year cycle until the late 1970s. The park was established in 1979 and the most recent clear cut of this area (excluding P. densiflora) took place from 1982 to 1986. There was widespread mortality in the dominant P. densiflora canopy as a result of the pine wilt disease, which began in the 1980s and is still continuing. These tree deaths provided abundant quantities of woody substrate on the forest floor (Kato and Hayashi, 2006). Presently, the canopy is dominated by P. densiflora and Q. serrata (Table 1), and the understory is dominated by the herb Pertya scandens, the

Table 1 Community composition in basal area (BA) and stem number for all woody species P1 cm diameter in study plots. Species

BA (m2 ha1)

Number ha1

Pinus densiflora Quercus serrata Carpinus tschonoskii Prunus jamasakura Clethra barbinervis Carpinus laxiflora Prunus buergeriana Vaccinium oldhamii Styrax japonica Aphananthe aspera Ilex macropoda Prunus grayana Mallotus japonicus Swida controversa Lyonia ovalifolia var. elliptica Magnolia praecocissima Fraxinus sieboldiana Pourthiaea villosa var. laevis Rhus trichocarpa

9.36 ± 3.52 9.24 ± 2.57 1.57 ± 0.84 1.29 ± 0.80 0.99 ± 0.37 0.92 ± 0.46 0.78 ± 0.46 0.71 ± 0.75 0.64 ± 0.23 0.60 ± 0.41 0.50 ± 0.25 0.45 ± 0.30 0.34 ± 0.24 0.32 ± 0.34 0.25 ± 0.17 0.15 ± 0.16 0.07 ± 0.07 0.03 ± 0.03 0.02 ± 0.01

80 ± 31 270 ± 67 40 ± 17 60 ± 23 390 ± 176 120 ± 62 50 ± 32 40 ± 42 130 ± 42 30 ± 16 170 ± 85 40 ± 23 30 ± 22 10 ± 11 210 ± 168 10 ± 11 10 ± 11 30 ± 32 50 ± 32

Mean ± S.E. (n = 10 plot).

grass Lophatherum gracile, the dwarf bamboo Pleioblastus chino, and lianas such as Parthenocissus tricuspidata and Trachelospermum asiaticum. 2.2. Field measurements Ten 10  10 m plots were randomly selected within an approximately 1 ha tract on a gentle north slope at the study site. The number and basal area of adult trees (diameter at breast height >1 cm) was recorded at each site. Substrate patches on the forest floor were categorized as either P. densiflora CWD (fallen logs and rotten stumps >10 cm diameter) or soil. Diameters of logs and stumps ranged from 10 to 47 cm and 25 to 60 cm, respectively. In the study area, few CWD were derived from broad leaved trees. Logs were classified into five subcategories according to decay classes by employing the criterion of Heilmann-Clausen (2001) after minor modifications as follows: (I) wood hard, penetrable with a knife to only a few mm, bark and twigs (diameter <1 cm) intact; (II) wood rather hard, penetrable with a knife to less than 1 cm, bark and twigs begin to shed away, branches (diameter 1–4 cm) intact; (III) wood distinctly softened, penetrable with a knife to approximately 1–4 cm, bark and branches partially lost, original log circumference intact; (IV) wood considerably decayed, penetrable with a knife to approximately 5–10 cm, bark lost in most places, original log circumference begins to disintegrate; (V) wood disintegrating either to a very soft crumbly texture or is flaky and fragile, penetrable with a knife to more than 10 cm, original log circumference barely recognizable or not discernable. To eliminate within-stem variation in decay stages (Pyle and Brown, 1999), a stem section was selected for each CWD (ca. 2 m along the stem) where decay stage is uniform. The projected areas of all substrates in each plot were estimated as the ground area directly covered by each substrate. Physicochemical properties (environmental variables) for each log in decay class IV were measured from June to July 2009. Presence or absence of certain decay type in a log was recorded as binary data for sapwood and heartwood separately, regardless of wood mass occupied by certain decay type. In this study, decay types were classified according to the macroscopic criterion of Araya (1993). Brown-rot is reddish-brown and easily breakable into cubical fragments; white-rot is whitish and bleached (yellowish- or grayish-white) and breakable into fibrous fragments; and soft-rot is dull-gray to brown with a mud-like surface. Soil contact

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(%), bark cover (%), and moss cover (%) were visually estimated to within 10%. Heights were measured from the ground to the top of the logs. Moisture content (%) was measured with a wood moisture meter (Turku, Kett Electronic Laboratory, Tokyo) and pH was measured with a hand-held pH meter (DM–13, Sato Shouji Inc., Yokohama). In September 2009, the number of tree seedlings (height <50 cm without reference to age; Streng et al., 1989) growing on each substrate within each of the 10 plots was counted by species. I counted all seedlings on CWD. To count seedlings growing on the ground, I randomly deployed three 1  1 m subplots within each plot and counted seedlings in the subplots.

occupied by Q. serrata and P. densiflora, followed by Carpinus tschonoskii and Prunus jamasakura. All other species constituted relatively small proportions of the total basal area. C. barbinervis had the largest number of live stems, suggesting that it is a dominant subcanopy species in the study area.

2.3. Mycorrhizal colonization

3.3. Seedling densities on logs and soil

As biotic conditions of the logs for seedling establishment, mycorrhizal colonization was measured for seedlings growing on logs and the effect of wood decay types on mycorrhizal colonization was examined. In September 2010, I sampled seedlings of Clethra barbinervis (the dominant tree species on logs) growing on logs and on the soil in the study plots (28 seedlings in total). Seedlings in each category include 1-5 year seedlings. After sampling, the aboveground shoot length was measured. Roots were separated from shoots and used to measure mycorrhizal colonization. C. barbinervis forms arbuscular mycorrhizae (Kubota et al., 2001). Roots were rinsed with tap water and cleared by heating in 10% KOH at 100 °C more than 1 h. Cleared roots were rinsed with distilled water and bleached in 0.1% H2O2 for 15 min. Bleached roots were then rinsed with tap water and fixed in 2% HCl. The fixed roots were stained with trypan blue and stored in lactoglycerol (lactic acid 875 mL, glycerin 63 mL, made up to 1000 mL with distilled water). Colonization was assessed (following McGonigle et al., 1990) under 200 magnification to obtain percentage of root length colonized by fungal structures. Because the hyphal coil is the main component of the arbuscular mycorrhizae colonizing C. barbinervis (Kubota et al., 2001), hyphal coils were recorded as indicators of mycorrhizal colonization.

A total of 38 woody species were recorded as seedlings (height <50 cm) growing on logs or soil in the 10 plots surveyed (Table 3). Total seedling density was highest on soil. Among logs, the majority of the seedlings were found on decay class IV logs, while no seedlings were found on decay class I logs. Five species, Aphananthe aspera, Carpinus spp., C. barbinervis, Q. serrata, and Rhus trichocarpa, occurred at densities of more than 1 seedling per m2 on logs or soil, and were considered to be frequent species. Among these five species, only C. barbinervis was dominant on logs (particularly on decay stage IV), and seedlings of this species were rarely encountered on soil. On the other hand, seedlings of the other four species were preferentially established on soil.

2.4. Statistical analysis Seedling densities across all species and within frequent species on soil and different decay classes of logs were compared by Kruskal–Wallis tests. Multiple regression was used to evaluate the relationship between environmental variables and seedling densities of the most frequent species on logs. Forward selection was used to determine the variables for inclusion in the multiple regression models. Student’s t-test with Bonferroni adjustment of the significance level was employed to test for effects of log decay types on seedling density and pH of logs. Linear regression analysis was employed to detect the relationships between shoot length and AM colonization rate on seedlings growing on logs or on soil separately. Analysis of covariance (ANCOVA) was performed to detect the effects of wood decay type on mycorrhizal colonization rate. Data of a certain decay type were compared with the combined data of the other decay types. Shoot length of seedlings was set as the covariate. Percent data were arcsine transformed before proceeding with analyses. All statistical analysis was performed with JMP version 5.0 software (SAS Institute Inc., 2004). 3. Results 3.1. Species composition of adult trees A total of 19 adult woody species (dbh >1 cm) were found in the 10 plots surveyed (Table 1). Most of the community basal area was

3.2. Relative areas of CWD on the forest floor Relative areas of P. densiflora logs and stumps and forest floor soil are presented in Table 2. Logs covered 5.88% of the total area. Decay class IV logs occupied the highest relative area among decay classes. Stumps occupied only a small portion of the area.

3.4. Effects of environmental condition on seedling density Three environmental variables studied showed significant correlation with C. barbinervis seedling density (Table 4). Basal area of conspecific adults and log height were positively correlated; log pH was negatively correlated with seedling density. Multiple linear regression models explained up to 33% of variation in seedling density. The moisture content, soil contact, moss cover, and bark cover of a log had no significant correlation with seedling density. Log decay type influenced C. barbinervis seedling density (Fig. 1). Among white-rot, brown-rot, and soft-rot of sapwood and heartwood logs, C. barbinervis seedling density was significantly higher on brown-rot heartwood logs than on logs belonging to other decay types. White-rot and soft-rot had no significant effects on seedling density. In addition, brown-rot sapwood significantly influenced log pH, but other decay types did not (Fig. 2). 3.5. Arbuscular mycorrhizal (AM) colonization Fig. 3 demonstrates the relationships between AM colonization and shoot length in Clethra seedlings. AM colonization significantly increased as shoot length of seedlings on logs increased, but this

Table 2 Relative area of each substrate in the study plot. Substrate

Relative area (%)

Total CWD DC I DC II DC III DC IV DC V Stump Soil

5.880 ± 1.336 0.751 ± 0.382 1.599 ± 0.859 0.786 ± 0.297 2.312 ± 0.987 0.432 ± 0.169 0.324 ± 0.078 93.797 ± 1.294

Mean ± S.E. (n = 10 plot); DC, decay class.

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Y. Fukasawa / Forest Ecology and Management 266 (2012) 232–238 Table 3 Seedling densities of 38 woody species on CWD of Pinus densiflora in five decay classes (DC) and soil. Seedling density (No./m2)

Species

pa

DC I (n = 27)

DC II (n = 62)

DC III (n = 32)

DC IV (n = 106)

DC V (n = 22)

Soil (n = 30)

Frequent species Aphananthe aspera Carpinus spp. Clethra barbinervis Quercus serrata Rhus trichocarpa

– – – – –

– – 1.41 ± 1.15 – –

0.23 ± 0.16 – 7.34 ± 4.66 – –

0.57 ± 0.30 0.21 ± 0.11 7.18 ± 1.69 0.08 ± 0.06 –

0.74 ± 0.55 – 0.21 ± 0.22 0.14 ± 0.15 –

1.60 ± 0.74 5.97 ± 2.36 0.27 ± 0.12 4.90 ± 1.81 1.37 ± 0.54

<0.0001 <0.0001 0.0003 <0.0001 <0.0001

Infrequent species Ardisia crenata Ardisia japonica Aucuba japonica Callicarpa japonica Celtis sinensis var. japonica Clerodendrum trichotomum Euonymus alatus f. stiatus Eurya japonica Fatsia japonica Fraxinus sieboldiana Hedera rhombea Ilex crenata Ilex macropoda Ligustrum lucidum Magnolia obovata Magnolia praecocissima Mallotus japonicus Osmantus heterophyllus Parthenocissus tricuspidata Pinus densiflora Pourthiaea villosa var. laevis Prunus spp. Quercus glauca Quercus myrsinaefolia Rhododendron obtusum var. kaempferi Sambucus racemosa ssp. sieboldiana Styrax japonica Swida controversa Symplocos coreana Vaccinium oldhamii Viburnum dilatatum Viburnum phlebotrichum Zanthoxylum piperitum Total

– – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

– – – – – – – – – – – – – – – – – – 0.02 ± 0.02 – – – – – – – – – – – – – – 1.45 ± 1.15

0.10 ± 0.10 – – – – – – – 0.22 ± 0.16 – – – – – – – – – – 0.23 ± 0.16 – – – – 0.09 ± 0.09 – – – – 0.05 ± 0.05 – – – 8.25 ± 4.80

– – 0.04 ± 0.04 – 0.10 ± 0.07 – – 0.32 ± 0.20 – 0.02 ± 0.02 0.05 ± 0.05 0.03 ± 0.03 0.02 ± 0.02 – – – 0.05 ± 0.05 – 0.16 ± 0.08 0.04 ± 0.04 – – – – – – – 0.18 ± 0.11 – – – – – 9.07 ± 1.78

0.21 ± 0.21 – – – – – – 0.14 ± 0.15 – – – – 0.14 ± 0.15 – – – – – – – – 0.43 ± 0.31 – – – – – – – – – – – 2.02 ± 0.89

0.47 ± 0.34 0.17 ± 0.14 0.10 ± 0.10 0.03 ± 0.03 0.30 ± 0.15 0.07 ± 0.07 0.07 ± 0.05 0.87 ± 0.23 0.20 ± 0.15 0.43 ± 0.19 – 0.40 ± 0.14 0.93 ± 0.36 0.07 ± 0.05 0.03 ± 0.03 0.07 ± 0.07 0.23 ± 0.13 0.03 ± 0.03 – – 0.10 ± 0.07 0.10 ± 0.07 0.03 ± 0.03 0.20 ± 0.09 0.53 ± 0.23 0.03 ± 0.03 0.23 ± 0.09 – 0.07 ± 0.07 – 0.37 ± 0.19 0.70 ± 0.25 0.03 ± 0.03 20.97 ± 3.35

n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. <0.0001

–, 0.00 ± 0.00. n.a., not applicable. a Kruskal–Wallis test.

Table 4 Stepwise multiple regression models for seedling density of Clethra barbinervis individuals growing on logs of Pinus densiflora in decay class IV. Variable

Coefficient

Sum of square

P

R2

BA of conspecific adults +Height of log +pH of log

8.32 0.68 6.56

6430.973 2507.625 1073.772

<0.001 0.0037 0.0495

0.2148 0.2985 0.3344

was not the case for seedlings growing on soil. Decay type did not significantly affect AM colonization of Clethra seedlings growing on logs (ANCOVA). 4. Discussion 4.1. Differences in seedling establishment among species C. barbinervis seedling density was the highest on logs in advanced stages of decay; densities of the other four frequent tree species were highest on soil. These differences in seedling densities between substrates may be partly attributable to differences in seed size among species. Small-seeded species preferentially

establish on logs and stumps, while large-seeded species establish on undisturbed forest floor (Lusk, 1995). Thick litter layers on soil tend to reduce seedling establishment of small-seeded species because plants are unable to penetrate through or germinate beneath the litter layer on the ground (Seiwa and Kikuzawa, 1996); litter layer impediments to colonization are reduced on logs (Kanno and Seiwa, 2004; Mori et al., 2004). C. barbinervis has small wind-dispersed seeds (0.056–0.1 mg), and its seedlings occur mainly on disturbed soil without litter layer cover (Kobayashi and Kamitani, 2000; Nakashizuka, 1989). Colonization of another small-seeded Clethra species , Clethra occidentalis, occurs disproportionately on elevated microsites in Jamaican montane rain forests (Newton and Healey, 1989). In contrast, species with seedlings occurring frequently on soil, including A. aspera (seed weight 19 mg, Tsuji et al., 2011), Carpinus spp. (3 mg, Hori and Tsuge, 1993), Q. serrata (1460 mg, Xiao et al., 2004), and R. trichocarpa (20 mg, Osada, 2005), have larger seeds that are more suited for germination on litter-covered forest floors. Seed size differences are likely determinants of differential utilization of forest floor heterogeneity among tree species, and these differences probably promote the coexistence of plant species (Lusk, 1995).

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Fig. 1. Effects of decay type of Pinus densiflora logs of decay class IV on seedling density of Clethra barbinervis. Figures in the upper row (a–c) show results of sapwood, and that in the lower row (d–g) show results of heartwood. ‘‘Others’’ means all other kind of decay types combined. Differences were tested with the student t-test: (n.s.) p > 0.1, (+) p < 0.1, () p < 0.001, with Bonferroni adjustment of the significant level.

Fig. 2. Effects of decay type of Pinus densiflora logs of decay class IV on pH of the logs. Figures in the upper row (a–c) show results of sapwood, and that in the lower row (d–g) show results of heartwood. ‘‘Others’’ means all other kind of decay types combined. Differences were tested with the student t-test: (n.s.) p > 0.1, () p < 0.05, with Bonferroni adjustment of the significant level.

Several decades after heavy damage caused by the pine wilt disease, the abandoned pine stands in Japan were widely dominated by deciduous Quercus such as Q. serrata (Fujihara et al., 2002) and Quercus crispula (Kato and Hayashi, 2006, 2007). At present, evergreen Quercus such as Quercus glauca are replacing the deciduous species, especially in southern Japan (Fujihara et al., 2002). Although C. barbinervis is a common canopy or shrub tree species in these modern stands (Fujihara et al., 2002; Kato and Hayashi, 2006), its regeneration is inhibited and species diversity of seedlings is reduced (Higo et al., 1995). Different from well-managed traditional coppice stands where the litter layer was collected for use as soil conditioner or fuel (Fujihara et al., 2002), abandoned

coppices such as the tract examined in this study accumulate deep litter layers. Under these circumstances, decaying logs may provide important microsites for small-seeded species. 4.2. Factors affecting establishment of Clethra seedlings on logs Although C. barbinervis seedling density on logs was most strongly explained by the presence of conspecific adults, properties of logs such as height and pH also significantly influenced seedling density. The height of a log used as an establishment site by individual seedlings may be a factor influencing their survival; high logs will provide elevated refuges from shading by understory

Y. Fukasawa / Forest Ecology and Management 266 (2012) 232–238

Fig. 3. Correlation between AM infection (% root length colonized by hyphal coil) and shoot length of Clethra seedlings growing on logs (a) and on soil (b).

vegetation (Duchesneau and Morin, 1999; Harmon and Franklin, 1989; Takahashi, 1994). Seed germination of the light-demanding pioneer species Weinmannia tinctoria (Cunoniaceae) increases with height along standing tree fern trunks in rain forests of the tropical Mascarene Archipelago of East Africa (Derroire et al., 2007). Soil pH is also a determinant of tree regeneration. Many species of Ericales, including Clethra, have adapted to acidic soil through metabolic process evolution in their symbiotic mycorrhizal associations. Phenolic acids released by plant decay residue reduce and often stop natural regeneration of tree species in other taxa (such as conifers) through inhibition of primary root growth in seedlings whose mycorrhizal symbioses are less well adapted to acidic conditions (Mallik, 2003). This study demonstrated that brown-rot wood significantly stimulates establishment of Clethra seedlings on logs. Fruiting bodies of the brown-rot fungi Cystidiophorus castaneus and Neolentinus lepideus are frequently observed on sapwood and heartwood of fallen pine stems in the study area (Fukasawa, unpublished data). These species may have central roles in pine CWD brownrot in this abandoned coppice. To my knowledge, this is the first report that demonstrates the effects of wood decomposer fungi on seedling establishment on nurse logs. Although some wood decomposer fungi directly transport nutrients to plants, including orchids, via mycorrhizal symbioses (Ogura-Tsujita and Yukawa, 2008; Yagame et al., 2008), belowground decomposer communities more generally influence aboveground plant community organization indirectly through their roles in breakdown and transformation of organic matter as well as liberation of plant growth-limiting nutrients (Bardgett and Wardle, 2010; Van der Heijden et al., 2008). However, the difference in nutrient content between brown- and white-rotted woods is very small (Ostrofsky et al., 1997), and differences in wood pH among wood decay types may better explain the effects on seedling density, as demonstrated in this study. Brown-rots produce organic acids (that affect plant regeneration) in larger quantities than other decay types; these acids have roles in non-enzymatic degradation of wood cellulose by brown-rot fungi (Espejo and Agosin, 1991). A further possibility is that decay types influence seedling density through effects on the composition of the microbial community, including symbiotic and pathogenic fungi (O’hanlon-Manners and Kotanen, 2004; Tedersoo et al., 2008). This would represent an indirect pathway from wood decomposer fungi to plants via soil biotic interactions (Bardgett and Wardle, 2010). Although no difference in AM colonization was observed between seedlings growing on brown- and white-rotted pine CWD, the positive correlation between seedling size and AM colonization suggests the importance of mycorrhizal colonization for seedling growth on CWD compared to that on soil. CWD is a nutritionally poor substrate compared to mineral soil, and mycorrhizal colonization may allow

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plants to extract sufficient nutrients from the rotting wood (Baier et al., 2006; Harmon et al., 1986). Mycorrhizal colonization is critical for seedling growth, especially under stressful conditions (Smith and Read, 2008), and individuals that are unable to form mycorrhizae have higher mortality risk (Onguene and Kuyper, 2002). For improved understanding of wood decay mechanisms that determine seedling establishment success, it will be necessary to describe seed germination rates and seedling demography on CWD belonging to different decay types, while at the same time examining the development of associated microbial communities, especially mycorrhizal symbionts. In conclusion, this study clearly demonstrated that wood decay type influences seedling establishment on CWD in temperate forest. Importance of CWD and its decay types as microsites for seedling establishment would be more widely understood by revealing the relationships between wood decay types and seedling establishment in more diverse forest types. Acknowledgments I thank staffs of Higashiyamato Park for kindly providing study area. I am also grateful to Dr. K. Takahashi for a wood moisture meter, and Dr. M. Saito, Dr. R. Tajima and Ms. T. Suzuki for technical advices on mycorrhizal measurement. Thanks are extended to Dr. K. Seiwa and Dr. C. Kobayashi for critical reading of draft manuscript, and Dr. G. Rambold and two anonymous reviewers for valuable comments. This study was supported by Grants related to the Fujiwara Natural History Foundation to Y.F. References Araya, K., 1993. Relationship between the decay types of dead wood and occurrence of Lucanid beetles (coleoptera: lucanidae). Appl. Entomol. Zool. 28, 27–33. Baier, R., Ettl, R., Hahn, C., Göttlein, A., 2006. Early development and nutrition of Norway spruce (Picea abies (L.) Karst.) seedlings on different seedbeds in the Bavarian limestone Alps – a bioassay. Ann. For. Sci. 63, 339–348. Bardgett, R.D., Wardle, D.A., 2010. Aboveground-belowground linkages: biotic interactions, ecosystem processes, and global change. Oxford University Press, Oxford. Bellingham, P.J., Richardson, S.J., 2006. Tree seedling growth and survival over 6 years across different microsites in a temperate rain forest. Can. J. For. Res. 36, 910–918. Cornelius, M.L., Daigle, D.J., Connick, W.J., Parker, A., Wunch, K., 2002. Responses of Coptotermes formosanus and Reticulitermes flavipes (Isoptera: Rhinotermitidae) to three types of wood rot fungi cultured on different substrates. J. Econ. Entomol. 95, 121–128. Christie, D.A., Armesto, J.J., 2003. Regeneration microsites and tree species coexistence in temperate rain forest of Chiloé Island, Chile. J. Ecol. 91, 776–784. Derroire, G., Schmitt, L., Rivière, J.N., Sarrailh, J.M., Tassin, J., 2007. The essential role of tree-fern trunks in the regeneration of Weinmannia tinctoria in rain forest on Réunion, Mascarene Archiperago. J. Trop. Ecol. 23, 487–492. Dickie, I.A., Koide, R.T., Steiner, K.C., 2002. Influences of established trees on mycorrhizas, nutrition, and growth of Quercus rubra seedlings. Ecol. Monogr. 72, 505–521. Doi, Y., Mori, A.S., Takeda, H., 2008. Conifer establishment and root architectural responses to forest floor heterogeneity in an old-growth subalpine forest in central Japan. For. Ecol. Manage. 255, 1472–1478. Duchesneau, R., Morin, H., 1999. Early seedling demography in balsam fir seedling banks. Can. J. For. Res. 29, 1502–1509. Eaton, R.A., Hale, M.D.C., 1993. Wood: Decay, Pests and Protection. Chapman & Hall. Espejo, E., Agosin, E., 1991. Production and degradation of oxalic acid by brown rot fungi. Appl. Environ. Microbiol. 57, 1980–1986. Folman, L.B., Klein Gunnewiek, P.J.A., Boddy, L., de Boer, W., 2008. Impact of whiterot fungi on numbers and community composition of bacteria colonizing beech wood from forest soil. FEMS Microbiol. Ecol. 63, 181–191. Fujihara, M., Hada, Y., Toyohara, G., 2002. Changes in the stand structure of a pine forest after rapid growth of Quercus serrata Thumb. For. Ecol. Manage. 170, 55– 65. Gilbert, G.S., 2002. Evolutionary ecology of plant disease in natural ecosystems. Ann. Rev. Phytopathol. 40, 13–43. Harmon, M.E., Franklin, J.F., 1989. Tree seedling on logs in Picea-Tsuga forests of Oregon and Washington. Ecology 70, 48–59. Harmon, M.E., Franklin, J.F., Swanson, F.J., Sollins, P., Gregory, S.V., Lattin, J.D., Anderson, N.H., Cline, S.P., Aumen, N.G., Sedell, J.R., Lienkaemper, G.W., Cromack, K., Cummins, K.W., 1986. Ecology of coarse woody debris in temperate ecosystems. Adv. Ecol. Res. 15, 133–302.

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