Litterfall in forests dominated by an alien woody species, Casuarina equisetifolia, on Chichijima Island
Kenji HATA
1*, Hidetoshi KATO
2and Naoki KACHI
11
Department of Biological Sciences, Graduate School of Science and Engineering, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo, 192-0397, JAPAN
2
Makino Herbarium, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo, 192-0397, JAPAN
*
Author for correspondence (e-mail: [email protected])
ABSTRACT
To quantify litterfall in forests invaded by the alien woody species Casuarina equisetifolia in the Bonin Islands, the amount of annual litterfall and its seasonal changes at forested sites dominated by C. equisetifolia (n = 30 sites) were compared with those at sites dominated by native species (n = 30) on Chichijima Island. Litterfall collected in traps was sampled every two months between August 2007 and August 2008, oven-dried, and weighed. The total amount of annual litterfall at the C. equisetifolia sites was significantly larger than that at the sites dominated by native species. Approximately 70% of the total litterfall at the C. equisetifolia sites was dead leaves of that species, indicating that the greater accumulation of litter in forests dominated by C.
equisetifolia is due to large amounts of dead C. equisetifolia leaves.
Keywords: exotic plants, invasive trees, litter accumulation, Ogasawara (Bonin) Islands
INTRODUCTION
Several studies have reported negative correlations between the amount of litter of alien plants and the density of native plants (Chen et al. 2004; Hartman and McCarthy 2007; Nilsson et al.
2008; Williams and Wardle 2008). These findings suggest that accumulated litter of alien plants
can suppress the establishment of native plants in a variety of ways: by acting as mechanical
barriers (Rotundo and Aguiar 2005; Navarro-Cano 2008), via allelopathy (Bosy and Reader
1995; Yirdaw and Leinonen 2002; Raniello et al. 2007), and by modifying environmental factors
such as soil temperature and moisture (Becerra et al. 2004). Thus, clarifying the processes of litter accumulation is important for effective eradication or control of alien plants and for the conservation of native plant communities.
The alien woody species Casuarina equisetifolia Forst. (Casuarinaceae) has been introduced to tropical and subtropical islands such as those in the Caribbean and the Mascarene, Galapagos, and Hawaii Islands; large amounts of litter accumulates under the dense canopy of this species (Weber 2003). In the Ogasawara (Bonin) Islands, the amount of accumulated litter in forests dominated by C. equisetifolia is much larger than that in adjacent forests dominated by native species (Hata et al. 2009). Greater litter accumulation can inhibit seed germination and seedling growth of native woody species (Hata et al., submitted), which would result in a decrease in species richness of native saplings in forests dominated by C. equisetifolia (Hata et al. 2009). By suppressing the establishment of native woody species under its canopy, C. equisetifolia could change successional pathways of the native forests (e.g., Yoshida and Oka 2000). Therefore, this alien tree should be eradicated or controlled in order to conserve the native forests of the Ogasawara Islands. For effective control of C. equisetifolia, it is essential to quantify the process of litter accumulation.
The larger amount of accumulated litter in forests dominated by C. equisetifolia could be explained by greater litterfall from this species, a lower decomposition rate of its litter, or a combination of the two factors (Berg and McClaugherty 2003). Preliminary studies showed that there were no significant differences in the litter decomposition rates between forested sites dominated by C. equisetifolia and those dominated by native species on the Ogasawara Islands (Hata, unpublished data). Therefore, we predicted that the amount of litterfall at forested sites invaded by C. equisetifolia would be larger than at sites that had not been invaded by this species. To test this prediction, we compared the amount of litterfall in forested sites dominated by C. equisetifolia with that in patches of native forest on Chichijima Island.
METHODS Study sites
This study took place in forested sites at Suzaki (27°03–04'N, 142°11'E, 40–80 m above sea level) on Chichijima Island. The mean annual temperature on the island is about 23 °C, and the mean annual precipitation (1970–1999) was 1234.9 mm (Oka et al. 2000). Some sites in the forest are dominated by C. equisetifolia and others by several native species, notably Schima mertensiana (Hata et al. 2009).
We selected 30 sites dominated by C. equisetifolia and 30 sites dominated by native species.
There was a distance of at least 10 m between sites (for the distribution of the 60 sites, see Fig.
1 in Hata et al. 2009). Among the 60 sites, woody plants taller than 1.3 m (hereafter, “trees”) of
C. equisetifolia, Psidium cattleianum, Machilus kobu, Pandanus boninensis, Pouteria obovata, Rhaphiolepis wrightiana, and S. mertensiana were observed at more than 30 sites. We defined a site as dominated by C. equisetifolia if the basal area of this species accounted for more than 40%
of the total basal area of all trees at the site. At sites dominated by native species, S. mertensiana had the greatest basal area, accounting for more than 30% of the total basal area of all trees at the site. The numbers of trees of C. equisetifolia, R. wrightiana, and S. mertensiana were highest at the sites dominated by C. equisetifolia, whereas at sites dominated by native species, R.
wrightiana and S. mertensiana showed the highest values.
Litter collection and analysis
A litter trap (0.8-m diameter, 0.8-m depth) was placed randomly at each site in August 2007. All litter that fell in each trap was sampled every two months until August 2008. The litter was sorted into the following six categories: leaves of C. equisetifolia, leaves of other species, reproductive parts (seeds, seedpods, and flowers) of C. equisetifolia, reproductive parts of other species, branches of both C. equisetifolia and other species, and undistinguished parts. After sorting, the litter was dried at 70 °C for 72 h and then weighed.
We used t-tests to compare the total amount of annual litterfall between sites dominated by C.
equisetifolia and those dominated by native species. All statistical analyses were carried out with the software R 2.9.1 (R Development Core Team 2009).
RESULTS
There were obvious differences in the amount of litterfall and its composition between the sites dominated by C. equisetifolia and those dominated by native species (Table 1). The total amount of annual litterfall at the C. equisetifolia sites was significantly larger than that at the native
Table 1 Amounts of annual litterfall (t ha –1 year –1 ) at sites dominated by Casuarina equisetifolia and those dominated by native species. Mean values and standard errors are shown.
C. equisetifolia sites Native sites (n = 30) (n = 30)
Total 11.13 ± 0.48 7.59 ± 0.25 ***
Leaves C. equisetifolia 8.15 ± 0.35 0.51 ± 0.08
Other species 0.44 ± 0.07 4.71 ± 0.17 Reproductive parts C. equisetifolia 1.18 ± 0.19 0.09 ± 0.02 Other species 0.04 ± 0.04 1.05 ± 0.14
Branches 0.84 ± 0.08 0.66 ± 0.08
Undistinguished parts 0.48 ± 0.04 0.56 ± 0.05
Figure 1. Seasonal changes in (a) total litterfall, (b) leaves, (c) reproductive parts (seeds, seedpods, and flowers), and (d) branches. (a, d) Closed triangles and open squares indicate dry weight of litterfall at sites dominated by Casuarina equisetifolia and that at sites dominated by native species, respectively. (b, c) Closed and open triangles indicate dry weight of litterfall of C. equisetifolia and that of other species at C. equisetifolia sites, respectively; closed and open squares indicate dry weight of litterfall of C. equisetifolia and that of other species at native sites, respectively. Mean values and standard errors are shown.
sites (t-test, p < 0.001). Among the six categories of litter, dead leaves of C. equisetifolia was the largest category. At sites dominated by C. equisetifolia, that species’ dead leaves accounted for more than 70% of total litterfall. At sites dominated by native species, dead leaves of other species showed the highest value and accounted for approximately 62% of the total litterfall.
Most of the dead leaves of other species at both types of sites were those of native woody species such as S. mertensiana, except for leaves from the alien tree Pinus luchuensis.
0 1 2
3 (a) Total
O ct. D ec.Feb. Apr. Jun. Aug.
2007 Aug. 2008
0 1 2
O ct. D ec.Feb. Apr. Jun. Aug.
(b) Leaf
2007 Aug. 2008
0 0.2 0.4
O ct. D ec.Feb. Apr. Jun. Aug.
(c) R eproductive
2007 Aug. 2008
0 0.2 0.4
O ct. D ec.Feb. Apr. Jun. Aug.
(d) Branch
2007 Aug. 2008
Dry weight (t ha -1 )
Seasonal changes in the amount of litterfall were observed at both sites (Fig. 1). Total litterfall at the C. equisetifolia sites was highest between August and October 2007 and between April and August 2008 (Fig. 1a). At the native sites, the highest value of total litterfall was observed between April and June 2008. The seasonal changes in the amount of dead leaves at both types of sites were similar to changes in total litterfall. The amount of dead leaves of C. equisetifolia at the C. equisetifolia sites was highest between August and October 2007, and that of other species at the native sites was highest between April and June 2008 (Fig. 1b). Litterfall of reproductive parts was highest between June and August 2008 at the C. equisetifolia sites and between October 2007 and February 2008 at the native sites (Fig. 1c). Litterfall of branches at both types of sites was highest between August and October 2007 (Fig. 1d).
DISCUSSION
Our results indicate that the greater accumulation of litter in C. equisetifolia forests than in native forests (Hata et al. 2009) is due to larger amounts of litterfall at the former sites, especially dead leaves of C. equisetifolia. This greater accumulation of C. equisetifolia leaf litter would likely inhibit native woody species’ seed germination and initial growth of seedlings (Hata et al., submitted). Therefore, the removal of C. equisetifolia litter in forests dominated by that species could be an effective method to promote the re-establishment of native trees in the forests. For the effective removal of C. equisetifolia litter, it is necessary to control the amount of litterfall as well.
At Suzaki, there were no differences in the total basal area of trees between the sites dominated by C. equisetifolia and those dominated by native species (Hata et al. 2009).
Therefore, the amount of litterfall per unit biomass at the C. equisetifolia forested sites could be larger than that in the native forests, which suggested that turnover of matter, such as nitrogen, at the C. equisetifolia sites could be faster than that at the sites dominated by natives.
Furthermore, the greater ability of C. equisetifolia to fix atmospheric nitrogen (Weber 2003)
would likely increase the nitrogen included in this species’ dead leaves. Indeed, the nitrogen
content in dead leaves of C. equisetifolia was much higher than that in dead leaves of native
woody species (Hata, unpublished data). With the invasion of C. equisetifolia, increases in the
amount of nitrogen and its turnover rate could affect the functions of invaded ecosystems (e.g.,
Vitousek and Walker 1989; Ley and D’Antonio 1998). To test the effects of C. equisetifolia on
ecosystem functions, it is necessary to quantify this species’ primary production, decomposition
rates of litterfall, and the release of nitrogen from the litter as it decomposes. Our present findings
regarding the greater litterfall of C. equisetifolia as compared to that of the native species of
Chichijima Island is a first step in this important research.
ACKNOWLEDGMENTS
The Environmental Agency and the Department of National Forests in the Ogasawara Islands allowed us to conduct this study on Chichijima. This study was financially supported in part by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science and the Fund for Global Environmental Research of the Ministry of the Environment of Japan (grant no.
F-51). We thank the Department of National Forests in the Ogasawara Islands and the Ogasawara Subtropical Agriculture Center for providing locations for the field experiment. We are grateful to Ms. Motomi Suda for help in producing the litter traps.
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ABSTRACT IN JAPANESE
父島において外来木本種モクマオウが優占する 林分におけるリターフォール
畑 憲治1・加藤英寿2・可知直毅1
1首都大学東京理工学研究科生命科学専攻
2首都大学東京牧野標本館
小笠原諸島において、外来木本種であるモクマオウが侵入した林分におけるリター フォールを定量的な情報に基づいて把握するために、父島の洲崎の二次林の中で、モ クマオウが優占する林分（モクマオウ林、30 調査区）とモクマオウが優占せずに在来 種が優占する林分（在来林、30 調査区）において年間のリターフォール量とその季 節的な変化を比較した。2007 年 8 月に各調査区においてリタートラップを設置して、
2008 年 8 月まで約 2 カ月おきにトラップ内のリター回収し、各器官ごとに乾燥重量を 測定した。モクマオウ林における年間総リターフォール量は、在来林におけるそれより も有意に大きかった。モクマオウ林における総リターフォールの約 70%は、モクマオ ウの落葉であった。これらの結果は、モクマオウ林における厚いリターの堆積は、大量 のモクマオウの落葉によることを示唆する。
キーワード：外来種、侵略的外来木本種、リターの堆積、小笠原諸島
