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1363 Twenty years of community dynamics in a mixed conifer – broad-leaved forest under a selection system in northern Japan Toshiya Yoshida, Mahoko Noguchi, Yukio Akibayashi, Masato Noda, Masahiko Kadomatsu, and Kaichiro Sasa Abstract: Single-tree selection has been employed widely in northern Japanese mixed forests, but management-induced changes in forests are not well understood. This study examined demographic parameters of major tree species during a 20-year study of a 68 ha stand in which single-tree selection has been conducted since 1971. Results showed that growth and survival of conifers (mostly Abies sachalinensis (Fr. Schm.) Masters) was the most strongly positively affected by the treatment. Nevertheless, recruitment of conifers was not sufficiently improved, suggesting their decreased dominance over the longer term. Instead, shade-intolerant broad-leaved species (mainly Betula ermanii Cham.) will gradually increase because of their higher recruitment rates after the treatment. Shade-tolerant broad-leaved species (mainly Acer mono Maxim. and Tilia japonica (Miq.) Simonkai) appeared to experience the most distinct negative effects, especially on survival. These trends differed markedly from those reported in previous papers concerning partial harvesting systems, which predicted an increase in dominance of shade-tolerant species. The results shown here should be generalized carefully because we have investigated only one stand without repetition of the control area. Nevertheless, trends described in this large-scale, long-term study could provide a basis for simulating stand dynamics. We discussed possible reasons for the observed patterns and provided implications for sustainable management in the region. Résumé : Le jardinage par pied d’arbre a été largement utilisé dans les forêts mixtes du nord du Japon, mais les changements induits par l’aménagement des forêts ne sont pas encore bien compris. Cette étude se penche sur les paramètres démographiques des principales espèces arborescentes sur une période de 20 ans dans un territoire d’étude de 68 ha à l’intérieur duquel le jardinage par pied d’arbre est appliqué depuis 1971. Les résultats montrent que la croissance et la survie des conifères (surtout Abies sachalinensis (Fr. Schm.) Masters) ont le plus bénéficié du traitement. Néanmoins, le recrutement des conifères n’a pas été suffisamment amélioré, ce qui suggère une diminution de leur dominance à long terme. En contrepartie, la présence de feuillus intolérants à l’ombre (surtout Betula ermanii Cham.) augmentera graduellement à cause de leur taux de recrutement plus élevé après le traitement. Les feuillus tolérants à l’ombre (surtout Acer mono Maxim. et Tilia japonica (Miq.) Simonkai) semblent connaître les effets négatifs les plus marqués, particulièrement dans le cas du taux de survie. Ces tendances vont clairement à l’encontre de celles qui ont été publiées précédemment au sujet des systèmes de coupe partielle et qui prévoyaient plutôt une augmentation de la présence d’espèces tolérantes à l’ombre. Les résultats devraient être généralisés avec précaution parce que les auteurs n’ont étudié qu’un seul peuplement sans répétition de la zone témoin. Néanmoins, les tendances décrites dans cette étude à long terme sur un grand territoire pourraient servir de base pour simuler la dynamique des peuplements. Ils discutent des raisons qui pourraient expliquer les résultats qu’ils ont observés et faitent état des implications pour l’aménagement durable dans la région. [Traduit par la Rédaction] Yoshida et al. 1375 Received 14 August 2005. Accepted 23 January 2006. Published on the NRC Research Press Web site at http://cjfr.nrc.ca on 11 May 2006. T. Yoshida.1 Uryu Experimental Forest, Field Science Center for Northern Biosphere, Hokkaido University, Moshiri, Horokanai 074-0741, Japan. M. Noguchi. Faculty of Environmental Earth Science, Hokkaido University, Tokuda 250, Nayoro 096-0071, Japan. Y. Akibayashi and K. Sasa. Field Science Center for Northern Biosphere, Hokkaido University, Sapporo 060-0809, Japan. M. Noda. Wakayama Experimental Forest, Field Science Center for Northern Biosphere, Hokkaido University, Hirai, Kozakawa 649-4563, Japan. M. Kadomatsu. Nakagawa Experimental Forest, Field Science Center for Northern Biosphere, Hokkaido University, Otoineppu 098-2501, Japan. 1 Corresponding author (e-mail: [email protected]). Can. J. For. Res. 36: 1363–1375 (2006) doi:10.1139/X06-041 © 2006 NRC Canada 1364 Introduction Recently, the use of partial harvesting systems is raising concerns about managing forests both for timber production and ecosystem functions (Oliver and Larson 1996; Kohm and Franklin 1997; von Gadow et al. 2002) because it often engenders stands with complex structure and composition (O’Hara 1998; Deal 2001; Deal and Tappeiner 2002; Harvey et al. 2002). In a northern Japanese mixed conifer – broadleaved forest (including Abies, Picea, Acer, Tilia, Quercus, Betula, etc.), a typical forest type of that region (Tatewaki 1958), a selection system (single-tree selection) has been widely employed since its exploitation began in the early 20th century as a superior option to manage natural forests. Most forests have experienced at least one instance of harvesting in the past. However, despite basic silvicultural principles (e.g., Smith et al. 1997), actual harvesting practices were often excessive and showed preferential extraction of large trees (Nagaike et al. 1999), thereby degrading regional forests. Most previous studies of harvesting practices in northern Japan have emphasized sustainable yields (Ohgane et al. 1988; Nigi et al. 1998); few studies have examined broader objectives, including maintenance of biodiversity. Contributing factors for ecosystem integrity (Hunter 1999), such as changes in stand structure and tree species composition, have not been well understood. Sustainable forestry attempts to mimic natural disturbances through management practices (Spence 2001; Franklin et al. 2002; Mitchell et al. 2002). The natural disturbance regime of northern Japanese mixed forests is a combination of rare large-scale blowdown (>10 000 m2; note that fire is unimportant in this region) and frequent small-scale tree death (<1000 m2: Ishikawa and Ito 1989; Kubota 1995; Nakashizuka and Iida 1995). Although little information is available regarding the large-scale disturbances in this region, several previous studies have predicted that the establishment of some major component tree species, including Betula ermanii Cham., Quercus crispula Bl., and Abies sachalinensis (Fr. Schm.) Masters, is closely related to largescale disturbances (Suzuki et al. 1987; Sano 1988; Ishikawa and Ito 1989; Osawa 1992; Ishizuka et al. 1998). On the other hand, as shown in many other regions (Hibbs 1982; Runkle 1985; Tanaka and Nakashizuka 1998), small-scale disturbances have been shown to have a fundamental influence on the structure of these natural forests. Natural tree mortalities in terms of basal area observed in this region were frequently less than 1% per year (Hiura and Fujiwara 1999; Kubota 2000); some major tree species (Abies sachalinensis, Acer mono Maxim., and Sorbus commixta Hedl.) regenerate in the resultant canopy gaps (Ishizuka 1984; Kubota et al. 1994; Hiura et al. 1996; Kubota 2000; Takahashi et al. 2003). The objective of this study was to present implications for sustainable harvesting systems, particularly through evaluating the effects of harvesting on tree species composition. Single-tree selection superficially resembles a small-scale disturbance regime (creation of canopy gaps), but it engenders some fundamental differences, such as removal of stems and the consequent reduction of fallen trees (Noguchi and Yoshida 2004). In a mixed forest, we can therefore predict that single-tree selection generally enhances the estab- Can. J. For. Res. Vol. 36, 2006 lishment and growth of particular species that adapt ecologically to small canopy gaps (cf. Runkle 1982; Canham and Marks 1985; Runkle and Yetter 1987; Abe et al. 1995; Messier et al. 1999). In fact, some North American studies have reported that dominance of shade-tolerant species gradually increases after single-tree selection (Leak and Fillip 1977; Jenkins and Parker 1999; Webster and Lorimer 2002; Shuler 2004). Nevertheless, such a trend in compositional change may differ among forest types. Constituent species probably adapt to inherent environmental conditions and interspecific relationships, both of which can be strongly affected by harvesting treatment. The northern Japanese mixed forest is characterized by a relatively large gap area in the canopy layer, which is often more than 20% (Kubota 1995, 2000; Takahashi et al. 2003). This characteristic is conspicuously apparent in heavy-snowfall regions; it is inferred to be related to slow recruitment rate caused by dense undercover of dwarf bamboo species (Sasa kurilensis (Rupr.) Makino & Shibata or Sasa senanensis (Franch. & Savat.) Rehd.) (Takahashi 1997; Umeki and Kikuzawa 1999). Small trees may not respond directly to single-tree selection because dwarf bamboos increasingly dominate the canopy gaps (Noguchi and Yoshida 2005; cf. Messier et al. 1998). We addressed the following questions in this study: (1) Does single-tree selection change tree species composition? (2) If change occurs, does it correspond to the expectations derived from characteristics (e.g., shade-tolerance) of constituent species? We examined 20 years of community dynamics of a mixed conifer – broad-leaved forest in northern Japan under a selection system by which single-tree selection at 10-year intervals had been conducted since 1971. We compared demographic parameters of major component tree species (and species groups) between treated and untreated areas based on a repeated census of ca. 27 000 individuals in a large (68 ha) plot. We summarized species recovery for that period and discussed long-term effects of single-tree selection on stand structure and species composition in the region. Methods This study was conducted in the Nakagawa Experimental Forest of Hokkaido University (44°48′N, 142°14′E, 150 m elevation; Fig. 1). The forest is located in a cool-temperate region with heavy snowfall in winter. Typical vegetation in the Nakagawa forest is mixed conifer – broad-leaved forest, as described by Tatewaki and Igarashi (1971). This type of forest is widely distributed in northeastern Asia as representative natural vegetation (Tatewaki 1958) and is responsible for regional biodiversity. The mean annual temperature is 5.4 °C and the mean precipitation is 1449 mm. Nearly half of the precipitation falls in the nongrowing season (November–April). Snow cover usually extends from early November to early May, with a maximum depth of 200 cm. The soils are classified as Inceptisol (acidic brown forest soil), and the predominant bedrock is Cretaceous sedimentary rock (Tatewaki and Igarashi 1971). Since 1967, a long-term study has been conducted at a 110 ha study site (Fig. 1, Table 1). Few large-scale disturbances or outbreaks were recorded during the study period, enabling us to clearly evaluate the effects of selection har© 2006 NRC Canada Yoshida et al. 1365 Fig. 1. Location and topography of the study area. Compartments 5–10 (62.1 ha) and the control (5.9 ha) were studied. Table 1. Description of the study forest. Basal area (m2/ha) Compartment* Area (ha) Initial First period Second period Census year† 5 6 7 8 9 10 Control 11.1 5.6 8.2 6.7 12.9 17.6 5.9 27.8 34.0 22.6 27.9 25.3 31.4 26.6 6.8 4.4 3.2 2.9 5.3 6.8 0.0 3.2 5.7 4.2 3.8 3.4 4,2 0.0 1970, 1971, 1972, 1974, 1976, 1977, 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1990 1990 1991 1992 1993 1994 1995 *Compartments 1–4 were not analysed in this study, because their trees were censused without individual identification. † The census years were in the year before harvesting. vesting. The primary objective of the original study was to develop a relevant management system to extract sustainable yield (Ohgane et al. 1988; Nigi et al. 1998). The “check method” presented by Knuchel (1953), which was intended to maintain timber volume and quality in the stand, was applied (see later paragraph for detailed practices). The original stand had a multiaged structure that was similar to that of other mixed forests in the region (Suzuki et al. 1987; Takahashi et al. 2003). Before plot establishment, the stand had been partially harvested at very low intensity for domestic fuel materials. The sum of the basal area at the first census (i.e., preharvest basal area) was 29.9 m2·ha–1, which was typical of the region. There were 27 tall-tree species in all (4 evergreen conifers and 23 deciduous broadleaved species), of which Abies sachalinensis shared nearly half of the total basal area, with lesser areas occupied, in descending order, by Acer mono, Tilia japonica (Miq.) Simonkai, Quercus crispula, Betula ermanii, and others (see Table 2 for a complete list). The forest was divided into 11 compartments (areas ranging from 5.6 to 17.6 ha) including a control compartment (5.9 ha). The dominant species showed no strong preferences in their spatial distributions (T. Yoshida, unpublished data), and tree species compositions before the treatments were similar among the compartments. A selection system was employed in which single-tree selections were conducted at 10-year intervals (one compartment per year). This study analyzed 20 years of data (i.e., two rounds of harvesting) from six compartments and the control (68 ha in all), within which 27 300 trees were censused individually and identified with numbered tags. Compartments 1–4 were not included in this study, because their censuses were conducted without individual identifications. The area, the sum of the basal area, and the harvested level of the compart© 2006 NRC Canada 1366 Can. J. For. Res. Vol. 36, 2006 Table 2. Preharvest composition of trees with DBH ≥12.5 cm in the study forest. Basal area Species Abbreviation Group* m2/ha Abies sachalinensis Acer mono Tiljajaponica Quercus crispula Betula ermanii Α1nus hirsuta Magnolia obovata Ulmus davidiana Ulmus laciniata Kalopanax pictus Sorbus alnifolia Sorbus commixta Prunus sargentii Phellodendron amurense Prunus ssiori Betula maximowicxiana Picea jezoensis, Taxus cuspidata Fraxinus mandshurica Salix hultenii Swida controversa Picea glehnii Betula platyphylla Acanthopanax sciadophylloides Juglans mandshurica Total Conifer Shade-tolerant broad-leaved Shade-intolerant broad-leaved AS AM TJ QC BE AH MO UD UL KΡ SA SC PS PA PR BM PJ TC FM SH SW PG BP AC JM CF BT BT BI BI BΙ BI BT BT BI BT BT BT BI BT BI CF CF BT BI BI CF BI BT BT 14.68 3.10 3.06 3.06 2.17 0.64 0.56 0.49 0.44 0.28 0.23 0.23 0.21 0.18 0.12 0.10 0.08 0.07 0.05 0.04 0.04 0.03 0.02 0.01 0.01 29.9 14.9 8.0 7.1 % 49.1 10.4 10.2 10.2 7.2 2.1 1.9 1.6 1.5 0.9 0.8 0.8 0.7 0.6 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.1 0.1 0.04 0.02 100.0 49.7 26.6 23.7 Density (stems/ha) Max. DBH (cm) 80 70 75 150 95 75 45 80 65 100 50 45 45 50 50 75 55 60 65 40 45 55 50 40 25 150 80 80 150 212.9 78.9 50.4 33.1 27.1 12.3 16.2 6.8 6.0 3.5 6.1 7.3 6.4 4.8 3.2 1.0 1.0 1.4 0.9 1.6 1.2 0.3 0.5 0.4 0.3 484 216 167 101 *CF, conifer; BI, shade-intolerant broad-leaved species; BT, shade-tolerant broad-leaved species. The classification was based on Koike (1988) and Masaki (2002). ments are shown in Table 1. The mean harvested level in terms of basal area was 19% (range 10%–28%) for the first treatment and 16% (range 12%–18%) for the second treatment. These intensities were determined carefully in consideration of the census data, to maintain a harvested volume equivalent to the growth (i.e., volume increment) of the compartments. In these harvests, trees with a lower timber value (i.e., with injury, crack, etc.) were selected mainly to improve the future economic quality of the stand. Nevertheless, the harvested trees were selected from a broad range of size classes and showed no bias for any particular tree species; the total amounts of harvested conifers and harvested broad-leaved species were roughly equal. Harvests were conducted in the winter. Trees were felled and their major branches cut immediately using a chainsaw. Then the logs were extracted using a tractor (a sled was used only in a few early years). Snow, normally 100–200 cm deep, covered the forest floor. Consequently, soil disturbances that typically accompany treatments were minor. Although supplemental plantings were conducted for some large nonwooded openings (e.g., canopy gaps larger than 400 m2), we did not consider them in this study because planted trees did not reach the lower limit size set for the census (see later paragraph). More detailed descriptions of the practices are available in Ohgane et al. (1988). In the 68 ha area, we identified 27 300 trees with a diameter at breast height (DBH) ≥12.5 cm. The DBH classes (5 cm intervals) of all living trees were determined by measurement in the year before treatment. Checks of survival or death, repeated measures of the DBH class, and records of newly recruited trees were carried out in the second and third censuses. The basal area of individuals was calculated using the median of the DBH class. We calculated the following demographic parameters. [1] Tree DBH class promotion rate ⎡(N + N p) ⎤ = ⎢ ⎥ ⎦ ⎣ N 0.1 −1 01 . [2] ⎡(N − N d ) ⎤ Tree mortality = 1 − ⎢ ⎥ ⎦ ⎣ N [3] ⎡(N + N r ) ⎤ Tree recruitment rate = ⎢ ⎥ ⎦ ⎣ N 0.1 −1 where N represents the initial tree number (except for harvested trees); Np is the number of trees that are promoted to the next DBH class; Nd is the number of naturally dead © 2006 NRC Canada Yoshida et al. 1367 Fig. 2. The DBH class promotion rates in the treated area (first and second periods) and the control. 0.05 0.05 DBH class promotion rates (year-1) All 0.04 0.04 0.03 0.03 0.02 0.02 First period Second period Control 0.01 0.01 Conifer 0 0 0.05 0.05 Shade-tolerant broad-leaved 0.04 0.04 0.03 0.03 0.02 0.02 0.01 0.01 0 0 12.5-22.4 22.5-32.4 32.5-42.4 42.5-52.4 Shade-intolerant broad-leaved 12.5-22.4 22.5-32.4 32.5-42.4 42.5-52.4 DBH class (cm) trees; and Nr is the number of trees recruited during the 10-year period. Compartments with exceptional intervals (8 or 9 years) in the first period were calculated separately; then the weighted averages by area were used as a representative value. We calculated these indices for four DBH classes (12.5–22.4, 22.5–32.4, 32.5–42.4, and over 42.4 cm (in the case of the DBH promotion rate, 42.5–52.4 cm)) by species. In addition, species were grouped into conifer, shade-tolerant broad-leaved species, and shade-intolerant broad-leaved species to summarize the results. The conifer group consists of four shade-tolerant species, represented mostly by Abies sachalinensis. The broad-leaved species were classified with reference to previous studies of physiological traits (Koike 1988) or regeneration mode in an oldgrowth stand (i.e., species with abundant smaller trees are assumed to be shade tolerant: Masaki 2002) (Table 2). Conifers shared nearly half of the total basal area: shade-tolerant (dominated by Acer mono and Tilia japonica) and shadeintolerant broad-leaved species (dominated by Quercus crispula and Betula ermanii) respectively shared a quarter. Results Growth The DBH class promotion rates of the treated area were lower than those in the control (about 0.8 times) in the first period, but became higher (about 1.5 times) in the second period in all the DBH classes (Fig. 2). Improvement in the second period was particularly apparent for conifers, but not so distinct for broad-leaved species. A markedly high promotion rate was observed in the largest DBH class (42.5– 52.4 cm) of shade-intolerant broad-leaved species in the control, but it was not followed by that in the treated area, even in the second period. In the first period, nearly half the species had lower DBH promotion rates in the treated area than in the control (Fig. 3). In the smallest DBH class (12.5– 22.4 cm), shade-tolerant broad-leaved species tended to show higher promotion rates, whereas shade-intolerant broad-leaved species tended to show lower rates. Nevertheless, in the larger DBH classes (≥32.5 cm), a shade-intolerant broad-leaved species, Quercus crispula, which generally showed the highest promotion rate among species in the control, showed a considerably lower rate. In the second period, although the apparent decrease in the treated area was observed only for Quercus crispula in the largest DBH class (42.5–52.4 cm), most species showed an improved promotion rate. A shade-tolerant conifer, Abies sachalinensis, showed the highest promotion rates among species in the treated area in all the DBH classes in the second period. Natural mortality In the second period, natural mortality for the larger DBH classes (≥32.5 cm) in the treated area was considerably lower than in the control when considering all species (Fig. 4). This trend was largely reflected by conifers, and the respective mortalities of broad-leaved (both shade-tolerant and shade-intolerant) species in the treated area were rather higher than of those in the control. Among shade-tolerant broad-leaved species, the increase in mortality in the smallest DBH class (12.5–22.4 cm) was significantly large. The species’ trends (Fig. 5) demonstrated that the range of natural mortality among species was generally smaller in the treated area than in the control: species with higher mortality in the control tended to show a lower mortality rate, and vice © 2006 NRC Canada 1368 Can. J. For. Res. Vol. 36, 2006 Fig. 3. DBH class promotion rates for major tree species in the treated area (first and second periods) plotted against those in the control. Species abbreviations are defined in Table 2. 0.1 0.1 (a) First period 12.5 ⱕ DBH < 22.5 AS QC AH UD KP SA QC AS TJ BE SB SB SC MO SA 0.0 TJ AM PR MO PR (b) Second period 12.5 ⱕ DBH < 22.5 PA 0.0 0.1 0.1 DBH class promotion rates (year–1) in the treated area SC UD AM PA KP AH BE AS (c) First period 22.5 ⱕ DBH < 32.5 UD QC AS QC AH TJ AH AM UD MO TJ (d) Second period 22.5 ⱕ DBH < 32.5 MO AM 0.0 0.0 0.1 0.1 (e) First period 32..5 ⱕ DBH < 42.5 AS QC AS UD TJ QC UD AM TJ (f) Second period 32.5 ⱕ DBH < 42.5 AM 0.0 0.0 0.1 0.1 (g) First period 42..5 ⱕ DBH < 52.5 AS QC AS TJ QC TJ 0.0 0.0 0.1 0.0 (h) Second period 42.5 ⱕ DBH < 52.5 0.0 0.1 –1 DBH class promotion rates (year ) in control conifer shade-tolerant broad-leaved versa. Trees in the larger DBH classes (≥ 32.5 cm) exhibited generally lower mortality in the second period. In contrast, for trees in the smaller DBH classes (<32.5 cm), the change between the periods was small; it showed no general trend among species. Recruitment For the three species groups, recruitment rates in the shade-intolerant broad-leaved treated area in the first period were similar to that in the control and double that in the second period (Fig. 6). The recruitment rate of shade-intolerant broad-leaved species was highest: in the second period, recruitment was roughly two and three times higher than that of shade-tolerant broadleaved species and conifers, respectively. Although shadetolerant broad-leaved species had the lowest recruitment rates among the groups in the control, they showed the © 2006 NRC Canada Yoshida et al. 1369 Fig. 4. Tree mortality in the treated area (first and second periods) and the control. 0.010 0.010 First period All Conifer Second period Tree mortality (year-1) Control 0.005 0.005 0.000 0.000 0.010 0.010 Shade-tolerant broad-leaved Shade-intolerant broad-leaved 0.005 0.005 0.000 0.000 12.5-22.4 22.5-32.4 32.5-42.4 42.5- 12.5-22.4 22.5-32.4 32.5-42.4 42.5- DBH class (cm) greatest improvement. In contrast, the improvement was smallest for conifers. Five species (four shade-intolerant broad-leaved and one conifer) had lower recruitment rates in the treated area (Fig. 7). However, in the second period, almost all the subjected species were over the equivalence line (i.e., the treated area showed higher rates than the control), with considerable improvement of shade-intolerant broad-leaved species (such as Betula ermanii and Salix sp.). In contrast to the control, Abies sachalinensis showed the lowest rate among all species in the treated area. Shade-tolerant broad-leaved species did not show a clear trend among species. Change in basal area Table 3 summarizes changes in basal area of species (and species groups) during the 20-year study period. In total, 87% of the harvested basal area was recovered in that period; conifers showed considerably higher recovery (112%). Three minor conifers (Picea jezoensis (Sieb. & Zucc.) Carr.), Picea glehnii (Fr. Schm.) Masters, and Taxus cuspidate Sieb. et Zucc.) showed a similar trend to that of Abies sachalinensis. These high recoveries were mainly attributable to high growth, and less so to recruitment. In contrast, the recovery of shade-tolerant broad-leaved species remained at only 48%, reflecting both low growth and high mortality. In particular, species with high dead basal area (Acer mono, Sorbus alnifolia (Sieb. & Zucc.) C. Koch, and Prunus sargentii Rehd.) exhibited the lowest recovery. Shade-intolerant broad-leaved species showed intermediate recovery (85%), with high a contribution of recruitment. The species with lowest recoveries (Magnolia obovata Thunb., Alnus hirsuta Turcz., and Betula maximo- wicziana Regel) in this group also showed high dead basal area. Discussion Demographic parameters of tree species and species groups differ widely between the treated area and the control; the response to single-tree selection changed considerably between the first and second harvests. Such a temporal change in the response may be attributable to a delay in the response of trees (e.g., Oliver and Larson 1996; Youngblood and Ferguson 2003) and a cumulative effect of repetitive harvests. General expectations of selection harvesting are increased growth, recruitment, and survival of trees, all positive effects, to compensate the loss of harvested trees. Actually, opposite and negative effects were dominant in the first period. Many previous studies have suggested that partial harvesting systems can maintain structural complexities of the stand (Deal 2001; Deal and Tappeiner 2002). We can expect that the multiaged structure of this stand was generally maintained, because harvested trees were selected from a broad range of size classes. Nevertheless, our results showed clearly that selection harvesting changes tree species composition, even if basal area levels are nearly maintained. Although longer term trends do not necessarily follow the observed characteristics of demography, we could applied those results for predicting stand dynamics. In the short term (i.e., several decades), Abies sachalinensis will become more dominant relative to broad-leaved species because its growth and survival were the most strongly positively affected by selection harvesting. Such a positive response to small-scale canopy disturbances has already been reported in natural for© 2006 NRC Canada 1370 Can. J. For. Res. Vol. 36, 2006 Fig. 5. Tree mortality for major tree species in the treated area (first and second periods) plotted against those in the control. Species abbreviations are defined in Table 2. 0.02 0.02 (a) First period 12.5 ⱕ DBH < 22.5 (b) Second period 12.5 ⱕ DBH < 22.5 PS SH AH AM AM UD BE TJ TJ SA UD MO PA PA SC AS SA SC BE QC KP 0.00 PS AH AS SH QC KP MO 0.00 0.02 0.02 (d) Second period 22.5 ⱕ DBH < 32.5 (c) First period 22.5 ⱕ DBH < 32.5 AH Mortality (year-1) in the treated area AH AM MO TJ UD QC AS QC AM TJ 0.00 0.00 0.02 0.01 UD MO AS 0.00 0.00 0.02 0.01 (f) Second period 32.5 ⱕ DBH < 42.5 (e) First period 32.5 ⱕ DBH < 42.5 AM AM UD UD AS TJ QC AS QC 0.00 TJ 0.00 0.01 0.01 (g) First period 42.5 ⱕ DBH (h) Second period 42.5 ⱕ DBH TJ AS QC TJ QC AS 0.00 0.00 0.01 0.00 0.00 0.01 Mortality (year-1) in the control conifer shade-tolerant broad-leaved ests (Hiura et al. 1996; cf. Veblen 1986), probably because of the physiological ability of Abies spp. to rapidly respond to increased light availability (Noguchi et al. 2003; cf. Kneeshaw et al. 1998; Youngblood and Ferguson 2003). Nevertheless, we found that recruitment of this species was not sufficiently improved after the treatment: it was the lowest among major tree species. This trend should engender a shade-intolerant broad-leaved lack of small-sized trees and decreased dominance of this species over the long term (i.e., more than 100 years). Past studies have shown two different regeneration modes of Abies sachalinensis: continuous age distribution, suggesting dependence of its regeneration on small-scale disturbances (Ishizuka 1984; Takahashi et al. 2003), and discontinuous distribution, suggesting importance of large© 2006 NRC Canada Yoshida et al. 1371 Fig. 6. Tree recruitment rate in the treated area (first and second periods) and the control. Tree recruitment rate (year–1) 0.03 First period Second period Control 0.02 0.01 0 All Shade-tolerant broad-leaved Conifer Shade-intolerant broad-leaved Fig. 7. Tree recruitment rate for major tree species in the treated area (first and second periods) plotted against those in the control. Species abbreviations are defined in Table 2. Recruitment rate (year–1) in the treated area 0.06 0.06 (b) Second period (a) First period KP PA SH PA BE KP SC SH UD UD SC BE MO SA PR PS QC TJ AM MO AH SA PR AM 0.00 AS 0.00 0.06 PS TJ AH QC AS 0.00 0.00 0.06 Recruitment rate (year –1) in the control conifer shade-tolerant broad-leaved scale disturbances (Suzuki et al. 1987; Ishikawa and Ito 1989). The present results are, at least, not consistent with the former. We presume that the limited recruitment can be explained as follows: (1) contrary to growth improvement (see previous paragraph), photosynthesis of Abies sachalinensis was found to decrease in response to severe increased light availability, especially in the dry summer (Noguchi et al. 2003; cf. Kneeshaw et al. 2002; Bourgeois et al. 2004); (2) because seedlings and saplings of this species are frequently distributed around the stems of overstory trees (Suzuki et al. 1987; Takahashi 1997), they may be affected greatly by physical damage accompanying the treatments (Harvey and Bergeron 1989). In general, conifers are predicted to be more vulnerable to such physical damage than are broad-leaved species because of the general inability of conifers to sprout (Greene et al. 1999). Furthermore, because conifers base their establishment largely on fallen logs as a preferable substrate (Hiura et al. 1996), the possible decrease in fallen logs resulting from repetitive selection harvestings (Noguchi and Yoshida 2004) may accelerate that decreasing trend over a longer term. This tendency should be more apparent in minor conifer species in this forest (but shade-intolerant broad-leaved common in the region), Picea jezoensis and Picea glehnii, which show a particular dependency upon fallen logs (Kubota et al. 1994; Takahashi 1997). Despite increased light availability after the selection harvesting, growth improvement of shade-intolerant broadleaved species was less than that of Abies sachalinensis. Even in the second period, large-sized trees in the treated area had considerably lower growth than those in the control. These facts seem to greatly reflect the trend observed for Quercus crispula, but its small-sized trees showed an improved response (also see Yoshida and Kamitani 1998). A great difference might exist in response to canopy opening between size classes. Quercus crispula had discontinuous age distribution in natural mixed stands (Sano 1988; Takahashi et al. 2003), suggesting the lower contribution of small-scale disturbances. Our results are consistent with this hypothesis. Regarding other shade-intolerant broad-leaved species, we found improved growth in small-sized Betula ermanii and Alnus firma and decreased mortality in midsized Magnolia obovata and Kalopanax pictus (Thunb.) Nakai. These are expected results in consideration of responses to high light availability (Ishikawa and Ito 1989). © 2006 NRC Canada 1372 Can. J. For. Res. Vol. 36, 2006 Table 3. Changes in basal area during the 20-year study period. Basal area (m2/ha) Species Abbreviation Group* Abies sachalinensis Acer mono Tilia japonica Quercus crispula Betula ermanii Alnus hirsuta Magnolia obovata Ulmus davidiana Ulmus laciniata Kalopanax pictus Sorbus alnifolia Sorbus commixta Prunus sargentii Phellodendron amurense Prunus ssiori Betula maximowicziana Picea jezoensis Taxus cuspidata Fraxinus mandshurica Salix hultenii Swida controversa Picea glehnii Betula platyphylla Acanthopanax sciadophylloides Juglans mandshurica Total Conifer Shade-tolerant broad-leaved Shade-intolerant broad-leaved AS AM TJ QC BE AH MO UD UL KP SA SC PS PA PR BM PJ TC FM SH SW PG BP AC JM CF BT BT BI BI BI BI BT BT BI BT BT BT BI BT BI CF CF BT BI BI CF BI BT BT Harvested (H) Grown (G) Recruited (R) Naturally dead (D) 4.454 1.299 0.842 0.689 0.803 0.245 0.248 0.204 0.017 0.049 0.094 0.117 0.097 0.043 0.024 0.035 0.004 0.013 0.004 0.010 0.014 0.001 0.002 0.005 0.002 9.315 4.471 2.705 2.139 5.189 0.521 0.546 0.738 0.365 0.157 0.083 0.143 0.103 0.102 0.037 0.044 0.026 0.062 0.026 0.014 0.039 0.010 0.024 0.021 0.010 0.012 0.017 0.003 0.005 8.296 5.250 1.477 1.569 0.286 0.136 0.114 0.101 0.137 0.056 0.046 0.036 0.009 0.069 0.012 0.032 0.013 0.093 0.009 0.013 0.003 0.003 0.016 0.022 0.024 0.001 0.010 0.002 0.002 1.245 0.292 0.381 0.571 0.526 0.277 0.127 0.118 0.052 0.071 0.034 0.006 0.085 0.006 0.012 0.009 0.016 0.010 0.012 0.012 0.003 0.002 0.005 0.005 0.001 0.000 0.004 0.002 0.001 1.396 0.532 0.552 0.313 Recovered (G+R+D)/H 1.11 0.29 0.63 1.05 0.56 0.58 0.38 0.85 1.59 3.34 0.39 0.57 0.24 3.32 1.00 0.44 9.79 0.87 9.37 3.70 2.36 11.92 10.43 0.40 3.69 0.87 1.12 0.48 0.85 *CF, conifer; BI, shade-intolerant broad-leaved species; BT, shade-tolerant broad-leaved species. The classification was based on Koike (1988) and Masaki (2002). These species also showed higher recruitment rates after the treatment, which may contribute considerably to their recovery. They may gain a relative advantage because their potential high growth rate occasionally enables them to escape faster from the understory to a lighter environment. We can expect these species to gradually increase in density after several decades. In contrast, shade-tolerant broad-leaved species appeared to incur the most distinct negative effects from the treatment. Recovery of the basal area (percentage of the increment to that lost) in the 20-year period was only 48%; in particular, recovery rates of Acer mono, Sorbus alnifolia, and Prunus sargentii were less than 40%. These low recoveries were mainly attributable to high mortality, presumably resulting from increased risks of wind or frost damage through canopy exposure (Peltola et al. 1999). In addition, we found no distinct positive effects of the treatment on growth of these species. Despite some exceptions (e.g., small-sized Ulmus davidiana Planch. var. japonica (Rehd.) Nakai and Sorbus alnifolia), growth rates of dominant species (Acer mono and Tilia japonica) were unchanged or decreased in the treated area. This trend is consistent with a previous finding that these species showed low plasticity of growth against high light availability (Umeki 2001). In contrast, recruitment of this species group increased in the second period, showing the greatest improvement, compared with the control, among the three species groups. However, the absolute amount of recruitment was still half that of shade-intolerant species. Our study demonstrated complicated responses of the mixed-species community under the selection system. The observed trends of change in species composition differ markedly from those reported in North America (Leak and Fillip 1977; Jenkins and Parker 1999; Webster and Lorimer 2002; Shuler 2004). We suspect that this difference is attributable to large canopy gap areas in the overstory and dominance of dwarf bamboo species in this forest’s understory. Selection harvesting, and its consequent creation of canopy gaps, increases dwarf bamboos (Noguchi and Yoshida 2005), which thereby suppresses tree regeneration (Nagaike 1999; Umeki and Kikuzawa 1999). Those two distinct characteristics are closely correlated. We presume that the results are informative, even for management of stands with rich understory vegetation (e.g., Harvey and Bergeron 1989; Messier et al. 1998; Beckage et al. 2000; Bourgeois et al. 2004), where © 2006 NRC Canada Yoshida et al. such an indirect negative effect of harvesting on light availability seems to occur. The present results should be generalized carefully because we have investigated only one 68 ha stand without repetition of the control area. Nevertheless, trends described in this large-scale, long-term study present implications for improving current harvesting practices. Attention should be devoted to the possible change in species composition toward shade-intolerant broad-leaved species, which might alter the ecosystem integrity of these mixed forests. Management plans must mitigate the negative effects of selection harvesting on growth, survival, and recruitment of particular species. For conifer species, supplemental planting or site preparation for natural regeneration (Wurtz and Zasada 2001; Yoshida et al. 2005) should be included to enhance their recruitment. At the same time, while considering the distinct negative effects, harvesting level of shadetolerant broad-leaved species should be limited to preserve their relative dominance. We need more detailed mechanisms by which observed patterns and processes are produced. In particular, we note that the amount of recruitment in this study might depend largely on preharvesting accumulation of a seedling bank because most of the recruited trees seemed to be older than 20 years (H. Miya, unpublished data). Longer term studies or those examining dynamics of seedlings and saplings (M. Noguchi and T. Yoshida, in preparation) are needed. In addition, we should consider site variations in the stand. Further studies that include responses of individual trees with spatial consideration (M. Noguchi and T. Yoshida, in preparation) are expected to yield additional important information. Together with the results of the current study, these extension studies can form an appropriate basis for adaptive management in this region. Acknowledgements We would like to thank anonymous reviewers for their helpful comments and suggestions. We gratefully acknowledge S. Taniguchi, E. 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