「森林総合研究所研究報告」(Bulletin of FFPRI), Vol.6, No.4 (No.405), 239 - 244 , Dec. 2007

短 報（Note）
Flowering culm dynamics in sporadic floweringof Sasa cernua MAKINOKITAMURA Keiko1)* and KAWAHARA Takayuki1)
Abstract

Dwarf bamboo species are monocarpic, flower simultaneously, and die at intervals of several decades. The typeof flowering ranges from sporadic to gregarious. We investigated sporadic flowering in Sasa cernua, one of the major
dwarf bamboo species in central Hokkaido, by mapping all culms and studying their flowering status. An important
finding of this study is that a single culm flowers twice over two consecutive years. In the major flowering year,
some culms were significantly larger than the rest and showed evidence of preceding flowering. This indicated thatat maturity size is one requirement for the flowering potential of S. cernua. All individual culms that have flowered
once died after the major flowering year. Secondary flowering was observed in July of major flowering year in small
study plot where seed fertilization did not take place. We also observed that new shoots with inflorescence alone were
emerged in the following year to the major flowering. The unit size of the flowering patch of S. cernua was assumed
to be 250 to 1000m2 in the study area.

Key words : dwarf bamboo, monocarpic, Sasa, simultaneous flowering, sporadic flowering
Introduction(2004) reported that the synchronous flowering of P. pubescens

Dwarf bamboos are distributed widely in temperate andis genetically regulated. However, we do not yet know the fac-

subarctic forests in Japan, often dominating the forest floor andtors regulating simultaneous flowering process of dwarf bam-preventing seedling establishment and successful regenerationboo species, such as the age of flowering, or what regulates theof other plants. They are monocarpic and take several decades

flowering range.

to flower. The simultaneous flowering and subsequent death of

Regionally synchronized large-scale gregarious floweringdwarf bamboos has a crucial impact on forest dynamics (Yama-and death of dwarf bamboos is prominent and captures the at-

moto et al., 1995; Nakashizuka, 1988). These impacts include

tention of scientists (e. g. Makita, 1992). In contrast, small-altered light and soil conditions of the forest floor where seed-scale sporadic flowering rarely merits detailed study. However,lings of various plant species are expected to establish (Kudoh

sporadic flowering can be considered the minimum unit size

& Ujiie, 1990; Yamazaki & Nakagoshi, 2005; Abe et al., 2005)of a flowering patch, and possibly a good indicator for genet

and provide a large number of viable seeds for insects (Kudoh

size if flowering were genetically controlled for dwarf bamboo

et al., 1994; Nishiwaki & Makita, 1998) and mammals (Abe etspecies. Yet, individual identification using genetic markers isal., 2001) as a nutrition. Therefore, knowledge of regenerationnecessary to conclude whether a flowering unit is a genet or not.

process is critical to understand the long-term forest dynamics

It is also important to obtain detailed information on the spatialin terms of the flowering behavior of dwarf bamboos on the for-

distribution of flowering culms and flowering dynamics.

est floor.
The magnitude of simultaneous flowering and subsequent

In the broadleaf forest in Sapporo, central Hokkaido, we
observed sporadic flowering of Sasa cernua, one of the major

death for monocarpic dwarf bamboo varies from the large-scale

dwarf bamboo species. Sasa cernua is known to be monocarpic

gregarious flowering (Makita, 1992; Makita et al., 1988) to the(Kudoh & Ujiie, 1990; Janzen, 1976), although the flowering

small-scale sporadic flowering (Kudoh et al., 1994; Yamazaki

phenomenon is not yet fully understood. We took advantage& Nakagoshi, 2005). The flowering interval of the long-livedof this opportunity to investigate the distribution of flowering

monocarpic bamboo (Phyllostachys pubescens) has been deter-

culms in sporadic flowering patches of S. cernua in order tomined to be 67 years by observing the whole life cycle twice

understand small-scale flowering dynamics of dwarf bamboo(Watanabe et al., 1982; Nagao & Ishikawa, 1998). Isagi et al.

species.

原稿受付：平成 18 年 1 月 18 日 Received Jan. 18, 2006
原稿受理：平成 19 年 10 月 25 日 Accepted Oct. 25, 2007
* Hokkaido Research Center, Forestry and Forest Products Research Institute (FFPRI), Hitsujigaoka-7, Toyohira, Sapporo, Hokkaido 062-8516,
Japan1) Hokkaido Research Center, Forestry and Forest Products Research Institute (FFPRI)KITAMURA K and KAWAHARA T

240

Materials and Methods
Study species
The genus Sasa is known to have 35 species distributed inestry and Forest Products Research Institute, 1983). These two
dwarf bamboo species are mixed and densely cover the forest
floor of the study site. A somewhat gradual shift of abundanceJapan, Sakhalin, the Kuriles, and Korea (Suzuki, 1978). Sasa

is observed; however, according to altitude, the proportion of S.

section Macrochlamys NAKAI is distributed mainly in northerncernua increases in over approximately 200m of elevation.

Japan.

We established three study plots for sporadic flowering of S.

The study species, S. cernua, has been confused with S. ku-

cernua in the experimental forest (Plots 1, 2, and 3). Plot 1 wasrilensis in central Hokkaido. The species at this study site has

a flowering patch, contiguous to S. senanensis vegetation withsometimes been identified as S. kurilensis (e. g. Morita, 1975).

a few culms of S. senanensis established within the boundaries.

We took this opportunity to identify this species using spikelet

In Plot 2, S. cernua was sparsely distributed and co-occured

characters. The study species was identified as S. cernua MAKINO

with S. senanensis; however, the majority of S. senanensis had

because it has minute hairs on culm-sheaths and leaf-sheaths,

flowered and subsequently died. The forest floor is completely

and 13 veins on the lower glume (Mikio Kobayashi pers. com.)covered with dense S. cernua in Plot 3. Plot 1 flowered over anwhereas, the vegetative organ of S. kurilensis are glabrescent (Su-area of approximately 625 m2 in May 2004 (Fig. 1a). Plots 2

zuki, 1978). S. cernua is of putative hybrid origin (Usui, 1961)

and 3 flowered in May 2006 over areas of approximately 250

between section Macrochlamys and section Sasa.

m2 and 1600 m2, respectively (Fig. 2a and b). For Plots 1 and 2,
all culms were in the flowering patch, while for Plot 3 only a 10

Study sitem x 50 m patch in the middle of the flowering area was selected

We observed a couple of sporadic flowering patches of S.

for study due to the large size of the flowering patch. The loca-

cernua from 2003 to 2006 in the experimental forest of Hokkai-tions of all living culms within the plots, and the status of each

do Research Center, Forestry and Forest Products Research In-

culm—flowering or vegetative—were recorded. The size of

stitute, Sapporo, Hokkaido (42°58’ 32’’ N, 141°23’ 46’’ E, 130each culm was determined to be the longest length of the living−260 m above sea level). Sasa cernua flowers in mid-May andshoot regardless of the main shoot or substituted shoots. The

produces seeds in July in this forest. This is a secondary forest

major flowering year was the year in which the highest number

with various broadleaf tree species, such as Betula platyphylla,of flowering culm within each study site was observed; 2004 forB. maximowicziana, Acer pitum (A. mono), A. japonicum, Quer-

Plot 1, and 2006 for Plots 2 and 3. We also surveyed precedingcus crispula, Magnolia hypoleuca (M. obovata), and Kalopanax

flowering culms by identifying the remains of dead inflores-

pictus. The maximum snow depth is approximately 75cm. Twocences in the main flowering year. We also observed secondary

dwarf bamboo species, S. cernua, and S. senanensis [(FRANCH.

flowering when new flowering shoots or inflorescences were

& SAVAT.) REHD.] co-occur at this site (Hokkaido Branch, For-observed after May of the main flowering year. In addition, we
checked the survival of flowering culms in Plot 1 in June 2005

Fig. 1. Culm positions in Plot1 in 2004 (a) and 2005 (b). Scales are given in meters. Red circle, flowering culm; green,vegetative culm; grey, dead culm; yellow, new culm
森林総合研究所研究報告 第 6 巻 4 号 , 2007

Flowering culm dynamics in sporadic flowering of Sasa cernua MAKINO

241

Table 1. Three study plots for sporadic flowering of S. cernua

Major flowering year
Average culm length (cm)
Flowering patch area (m2)Surveyed area (m2)
Total number of culms
Culm density (culms/m2)
Average culm length (cm)Number of culms
Proportion (%)
Density (culms/m2)

Plot1
2004
114.8
625.0
625.0
1715
2.74
flowering vegetative
132.2
104.0
657
1058
38.3
61.7
1.05
1.69

and Plots 2 and 3 in June 2007.
Significant t-test was carried out for differences in culm size
between status (flowering or vegetative).
Results

Plot2
2006
124.8
250.0
250.0
415
1.66
flowering vegetative
128.2
119.3
259
156
62.4
37.6
1.04
0.62

Plot3
2006
125.4
1600.0
500.0
2529
5.05
flowering vegetative
126.6
123.7
1529
1000
60.5
39.5
3.06
2.0

Table 2. The number of preceding flowering culms for eachstudy plot.

Status for the major
flowering year
Vegetative
Flowering

Plot1

Plot2

Plot3

47
247

1
5

97
52

Flowering culms were found in the proportion of 38.3%,
62.4%, and 60.5% in Plots 1, 2, and 3, respectively (Table 1).
The average length of flowering culms in the major flowering

were observed in June 2007.

year was significantly larger than vegetative culms in Plots 1

Preceding flowering culms were observed in all the study

and 2 (p<0.01), but not significant in Plot 3. Seeds were pro-plots, and appeared on both flowering and vegetative culms in

duced in July in Plots 1 and 3, but not in Plot 2.

the major flowering year (Table 2). The size of preceding flow-

We did not observe secondary flowering in Plot 1. Secondary flowering was observed in Plot 2 for two culms, which

ering culms was significantly larger than both flowering andvegetative culms for all study plots (p<0.01) (Fig. 3).

flowered in May and bore three new shoots with inflorescences

The census of Plot 1 for 2005 revealed that all culms thatalone in mid-July 2006. However, fertilization of seeds did not

had flowered once were dead, and there was no flowering after

take place in Plot 2. In Plot 3, there was no secondary flowering

the major flowering year (Fig. 1b).

on existing culms, but 47 new shoots with inflorescences alone

Fig. 2. Culm positions in Plot2 (a) and Plot3 (b) in 2006. Scales are given in meters.
Red circle, flowering culm; green, vegetative culm
Bulletin of FFPRI, Vol.6, No.4, 2007

KITAMURA K and KAWAHARA T

242

Discussion

tial to produce inflorescences; however, a quantitative study of

A partial flowering, which precedes the gregarious flower-

seed production may be necessary to confirm this relationship.

ing, has been reported for a couple of dwarf bamboo species

Within the flowering area, approximately 40 to 60% culms(Nishiwaki & Makita, 1998; Kobayashi, 2000; Makita et al.,were vegetative culms and did not flowered. This phenom-

1988, 2004), but whether a single culm flowered twice in con-enon has also been reported for mass-flowering of S. kurilensis

secutive years, or different culms flower once in different years(Makita, 1992; Makita et al., 2004).

had not been determined. The two-year consecutive flowering

We have also observed other sporadic flowering sites of S.

on the same culm for S. cernua is a major finding of this study.

cernua adjacent to these study sites from 2003 to 2006. The

Moreover, significantly larger culms tend to flower prior to thesize of these flowering patches are 294 m2, 868 m2, and 958 m2,

major flowering year. Yamazaki & Nakagoshi (2005) reported awhich are similar to the size of Plot 1 (625 m2) and Plot 2 (250similar result for S. kurilensis, where the living culm height wasm2). We could also assume expansion of underground rhizome

lower than dead culm height, indicating taller mature culms hadconnections of flowering culms from the culm position map offlowered and died. These results suggest that size at maturation

Plot 1 (Fig.1a) and Plot 2 (Fig. 2a) because Plot 3, the largestof the culm is one of the pre-requisites for the flowering of S.

flowering area (1600m2), was twice as large as the second larg-

cernua.

est flowering patch observed in the surrounding area. However,Preceding flowering in dwarf bamboo species was reported

the location of flowering culms in Plot 3 shows a low density ofalong with the secondary flowering in small areas after a gre-flowering culms at approximately 35 m in the x-axis (Fig. 2b).

garious flowering year (Nishiwaki & Makita, 1998; Kobayashi,

If the unit of sporadic flowering of S. cernua is between 250

2000; Makita et al., 1988, 2004). However, we observed no sec-

m2 and 1000m2, Plot 3 might be assumed to be two flowering

ondary flowering after the major flowering year in Plot 1. Re-

patches joined at the middle of the plot.

gardless of the flowering year and frequency, all culms that had

Synchronous flowering units have been shown to be a ge-

flowered in Plot 1 were dead and regenerated culms (Kobayashi

netic trait for P. pubescens, another monocarpic bamboo species& Nomura, 2001) were not observed until 2006. On the other(Isagi et al., 2004). Whether the sporadic flowering unit forhand, secondary flowering was observed in July of the major

S. cernua has a genetic basis can be elucidated using genetic

flowering year in Plot 2 and in June of the subsequent year inmarkers, and this will form the basis of our next study.

Plot 3.
The flowering season of S. cernua in Sapporo is primarily

Acknowledgements

mid-May. In this study, we observed a secondary flowering

We thank Mikio Kobayashi of Utsunomiya University forin mid-July in a small flowering patch (Plot 2). The other two

taxonomic identification of the study species. This study wasstudy sites produced seeds after the flowering in mid-May, but

supported by a Grant-in-Aid for Scientific Research (18580153)

Plot 2 did not produce any seeds from the first flowering. This

from the Japan Society for the Promotion of Science.

may indicate a relationship between fertilization and the poten-

Fig. 3. Average culm length of different flowering status for each study plot. Standard
errors are given in bars. Small case letters indicate significant difference by t-test(p<0.01), calculated independently for each study plot. V-V, vegetative for two
years; V-F, flowering culm without preceding inflorescence; F-V, vegetative culm
with preceding inflorescence; F-F, 2-year sequential flowering culm
No statistical test was carried out for status F-V in Plot2, where only a single
culm was observed.
森林総合研究所研究報告 第 6 巻 4 号 , 2007

Flowering culm dynamics in sporadic flowering of Sasa cernua MAKINO

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244

オクヤマザサ部分開花集団における開花稈の動態
北村 系子 1）*・河原 孝行 1）

要旨
一回結実性植物として知られているササは、数十年に一度一斉開花し枯死する。ササにおける開花の範
囲は、部分開花から大面積にいたるまでさまざまである。本研究では北海道の林床に広く分布するオクヤ
マザサ（Sasa cernua）の部分開花集団を対象に稈単位の詳細な位置図と開花の有無を調査した。調査を行った３プロットすべてにおいて開花の前年に部分的な開花があり、開花年には前年開花で枯死しなかった稈が再び開花する現象が見られた。つまり稈単位で開花性の調査を行うことによって、同一稈が連続して２年間咲くという事実が初めて明らかになった。また、前年開花を行う稈のサイズは行わない稈にくらべて有意に大きかった。これらの結果から、オクヤマザサの開花には稈サイズがなんらかの影響を及ぼしていることが示唆された。開花後の追跡調査では、過去に一度でも開花した稈はすべて枯死しており、再生稈は認められなかった。結実しなかった小面積の開花パッチでは開花同年の７月に再開花が認められた。
また、開花翌年に花穂のみからなる新たな稈の発生が認められた。本調査地におけるオクヤマザサ部分開花の範囲は、周辺の開花集団も含めて約 250 〜 1000m2 と推定された。
キーワード：一回結実性植物、ササ属、開花同調、部分開花、林床性ササ＊森林総合研究所北海道支所
1) 森 林 総 合 研 究 所 北 海 道 支 所〒 062-8516 札 幌 市 豊 平 区 羊 ケ 丘 ７ 番 地

森林総合研究所研究報告 第 6 巻 4 号 , 2007