Vegetation recovery patterns in early volcanic succession

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<ul><li><p>J. Plant Res. 108 : 241-248, 1995 Journal of Plant Research ~) by The Botanical Society of Japan 1995 </p><p>Invited Article </p><p>Vegetation Recovery Patterns in Early Volcanic Succession </p><p>Shiro Tsuyuzaki* </p><p>Graduate School of Science and Technology, Niigata University, Niigata, 950-21 Japan </p><p>Permanently plots were monitored from 1983 to the present on Mount Usu after the eruptions of 1977-78 which destroyed the pre-eruption vegetation by 1-3 m thick accumulations of ash and pumice in order to clarify the processes and mechanisms of succession. Until now, 163 species were recorded in the summit area. Most of these species were derived from vegetative reproduction throughout the volcanic deposits. Vegeta- tive reproduction plays a major role on increases in cover. Although long-distance seed-dispersal species could immigrate to the crater basin, their cover increase was slow. Seedbank species only established in gullies where the original topsoil was exposed by erosion. Most annuais were supplied by the seedbank in the original topsoil and woody species originated via immigration, suggesting that the source greatly determines the species composition of establishing vegetation. Annual seedlings showed low survival, while overwintering perennial seedlings steadily established. Ground sur- face movements strongly restricted increases in plant cover and the distance from source vegetation was the principal determinant of plant density. Due to differ- ences in disturbance intensity, successional rates were higher in the stable substrates outside gullies and lower on the exposed original topsoil in some gullies. </p><p>Key words : Immigration m Mount Usu m Permanent p lot - - Seedbank-- Seedling establishment - - Species composition ~ Vegetative reproduction ~ Volcanic suc- cession </p><p>Since the first major ecological theory "ecological succession" was proposed (Clements 1916), a large num- ber of studies have been conducted (see, Glenn-Lewin et al. 1992, Miles and Walton 1993). There are some trends in succession (Whittaker 1975, Tilman 1988, Glenn-Lewin eta/. 1992) : 1) it is a time dependent process with chang- ing vegetation characteristics such as density, cover, and species richness, diversity, and composition, 2) it results from a modification of the stress and disturbance regimes, and 3) it changes ecosystems which are unstable to </p><p>* Recipient of the Botanical Society Award of Young Scien- tists, 1994 </p><p>stable, concerning cover, biomass, and/or diversity infor- mation content. </p><p>Plant succession is categorized into primary and secon- dary succession (Clements 1916, Tsuyuzaki 1993a). Pri- mary succession, which occurs following complete destruction of biosystems where the ground surface is covered by rocks and/or inorganic soil substrates (Vitousek and Walker 1989, del Moral and Bliss 1993), is considerably different from secondary succession which may be initiated by burns or abandoned fields (Walker et al. 1982, McCune 1988, Tsuyuzaki et al. 1994). The most important process of primary succession is the accumula- tion of nutrients including nitrogen in the soil (Whittaker 1975, Tilman 1982). Primary succession was previously considered to be principally initiated by blue-green algae, mosses, lichens, etc., while secondary suceession begins with more or less mature soils containing a sizable bank of seeds and vegetative propagules (Crawley 1986, Fenner 1992). Therefore, primary suceession proceeds very slowly in its early stages, due to a sterile ground surface and the lack of a seedbank and vegetative propagules (Tilman 1982, Miles and Walton 1993). Therefore, to clarify successional trends, we must determine plant origins (Tsuyuzaki 1987). </p><p>Sites that support primary succession have been pro- vided following glacial retreat (Bormann and Sidle 1990, Matthews 1992), massive landslides (White 1979, Naka- shizuka et al. 1993), and volcanic eruptions (Matson 1990, Tagawa et al. 1985, Whittaker et al. 1986, del Moral and Bliss 1993, Tsuyuzaki and del Moral 1995). To confirm changes over time in successional seres community structure must be assessed for the long-term using per- manent plots, however, these have been rare (Tsuyuzaki 1993). I have annually monitored revegetation on a recently-erupted volcano, Mount Usu, for more than 10 years by measuring vegetation cover and density and also by examining the fates of individual plants belonging to various life form types. The present study was conduct- ed in the crater basin of volcano Usu where many plant species of various life forms were growing. These plants were growing in spite of high seedling mortality induced mainly by ground surface instability. </p></li><li><p>242 S. Tsuyuzaki </p><p>Mount Usu </p><p>Mount Usu, located on the northernmost Japancse Island, Hokkaido (42~ 140~ is composed of two peaks, O-Usu (727 m) and Ko-Usu (552 m) which are enclosed by a caldera rim and crater basin, with an area of ca. 2 km. The pre-eruption summit area had been covered mostly with forests of Populus maximowiczfi and Betula platyphylla var. japonica and partly with seeded pasture of Dactylis glomerata, Trifolium repens and T. pratense (Tsuyuzaki 1987). In 1977-78 when I was a high school student, the eruptions completely destroyed the vegetation with thick accumulations of tephra. Soon after the eruptions, the northwest slope of O-Usu and inner wall of the caldera rim were covered by a thick layer of volcanic rocks and the crater basin was ovcrlain by volcanic ash and pumice with some deeply eroded gullies in which the original topsoil was exposed. The inner wall of the caldera rim was inhabited by a community of Petasites japonicus var. giganteus and Polygonum sacha- linense, both of which recovered vegetatively soon after the erosion of volcanic deposits, whereas in the crater basin the ash and pumice layer is still thick and revegeta- tion is very slow. </p><p>Based on geographical physiognomy, the crater basin was divided into three habitat types. Deep, stable tephra </p><p>is the dominant landform outside gullies (i.e., hereafter, T) (Fig. 1). In other locations, erosion was moderated and formed gullies (G) with altered surface conditions but erosion had not reached the original topsoil. Extreme erosion has removed large quantities of ash and pumice, exposing the original surface (E) creating a third type of habitat (Tsuyuzaki 1989a, 1991a). Distance from the cal- dera rim, which was surrogate for the distance from major plant resources, ranged from 50.0 to 400.0 m (Tsuyuzaki and del Moral 1994). Ground surface movements de- creased from -I-41 and --162 cm in 1983 to -I-15 and --2 cm in 1992 in the gullies (positive values mean the accu- mulation of volcanic deposits and negative ones mean the erosion) (Tsuyuzaki 1989a). Ground surface movements were intense in the gullies, but were less than __.10 cm outside the gullies in 1984. On ski slopes in Hokkaido ground surface movements were mostly derived from snow-melt (Tsuyuzaki 1990). Similar processes might occur on Mount Usu (Tsuyuzaki unpublished data). </p><p>Organic matter status expressed as loss on ignition ranged from 0.2 to 1.1% in the volcanic deposits and from 2.5 to 13.2% in the original topsoil in 1984 (Tsuyuzaki 1989a), indicating that organic matter content was much greater in E. The loss on ignition of volcanic deposits was less than 5% even in 1994 (Tsuyuzaki unpublished data). These values were only 1/3 of loss on ignition </p><p>Species richness and diversity Habitat preference Life form </p><p>Contribution and/or Plant origin recovering pace </p><p>Medium and high,~-..--- </p><p>Rich and high </p><p>Poor and low ~" </p><p>Plant sourcN~ </p><p>Immigratior~ </p><p>G </p><p>Microhabitat rills cracks </p><p>~-- Slow </p><p>~ egetation ~ Rapid and most reproduction ~ contributive </p><p>sion) - ~(R p ) </p><p>X Seedbank ,,,,,,,,,,,,4~Rapid but decreased \ </p><p>Ground surface Original topsoil instability </p><p>(=Nutrients) </p><p>Fig. 1. Revegetation dynamics in the crater basin of the volcano Usu after 1977-78 eruptions. T : outside gully dominated by thick tephra. E: original-topsoil-exposed gully. G: inside gully without original topsoil. Relationships are connected by solid lines (strong relationships are shown by thick solid lines). Woody species were mostly derived from immigration, and annuals were from the seedbank. Annuals, most of which established in the gullies, and well-rooted perennials, that could reach their roots to the original topsoil even on the thick volcanic deposits, utilized nutrients in the original topsoil. Due principally to ground surface movements, most woody species did not establish inside gullies and annuals disappeared after 1989. Ground surface instability restricted plant growth, while perennials could spread their roots by means of long rhizomes and/or stolonierous shoots in spite of the instability. Therefore, perennial species contributed the most to revegetation. Species diversity is higher in habitats T and E than G, due to seedbank and immigrant species. </p></li><li><p>Vegetation Recovery Patterns in Early Volcanic Succession 243 </p><p>values from low-productivity abandoned pasture in north- ern Japan (Tsuyuzaki et al. 1994). </p><p>Origin of recovering plants </p><p>On the summit areas of Mount Usu, 163 vascular plant taxa were observed during the years of 1983 to 1994 (in detail, see Appendix). Four major plant origins were recognized (Tsuyuzaki 1987): vegetative reproduction, seedbank, immigration and artificial introduction as fol- lows (Fig. 1). </p><p>Vegetatively-reproducing species Most perennials originated vegetatively from buried </p><p>plants throughout the volcanic landscape, e.g., Angelica ursina, Aralia cordata, Petasites japonicus var. giganteus and Polygonum sachalinense. All the species in this group are perennial herbaceous and woody species. </p><p>Clonal expansion effectively succeeds in colonization in highly disturbed sites (Fahrig et al. 1994). On Mount St. Helens, vegetative recovery was conspicuous in areas where volcanic deposits were less than 15 cm deep (Antos and Zobel 1985ab). On Mount Usu, P. sachalinense and P. japonicus var. giganteus often recovered from under- ground organs buried by the volcanic deposits more than 50cm deep (Tsuyuzaki 1989a). Therefore, species characteristics such as life form and root system deter- mine the success rates of vegetative recovery. For example, most needle-leaved species could not repro- duce vegetatively after the eruption of Mount St. Helens, while well-rooted perennial herbs Epilobium angustifolium and Anapharis margaritacea rapidly resprouted vigorously from beneath the tephra (Halpern et al. 1990). </p><p>Seedbank species Using a K2CO3 floatation technique which can extract </p><p>nearly 100% of the viable seeds from soil samples (Tsu- yuzaki 1993b, 1994a), in 1987 viable seeds of 17 species were extracted from the seedbank of original topsoil buried beneath 65-130 cm of volcanic deposits, indicating that seed viability is at least 10 years (Tsuyuzaki 1989b). The most common seedbank species was Rumex obtusifolius. Seed volume of most species tested was less than 2.0 mm 3 and smaller seeds had a greater rate of survival than larger seeds (Tsuyuzaki 1991b). (The vol- ume was calculated by assuming simple ovoid shapes.) Small seeds survive for longer periods in the soil, and their dormancy is frequently associated with a requirement for light (Cook 1980). However, seed survival rates of a few species including R. obtusifolius in the seedbank of Mount Usu were positively correlated with the thickness of volcanic deposits, suggesting that stable soil tempera- ture with little diurnal fluctuations allowed for long-term survival (Thill et al. 1985, Tsuyuzaki 1991b). In addition, the species composition of the seedbank might be related to the pre-eruption vegetation, although the relation was weak (Tsuyuzaki 1989b). Based on these analyses, I concluded that at least 14 species were derived from the </p><p>seedbank (Tsuyuzaki 1994b). While the distribution of seedbank species was </p><p>restricted to the deeply eroded gullies of the crater basin where the original topsoil became exposed, these species greatly contributed to increased species richness there (Tsuyuzaki 1989c). The seedbank included both annual and perennial herbaceous species. All the annuals except for Senecio vulgaris were derived from the seed- bank, indicating that the seedbank is an important deter- minant of initial stages of revegetation (Tsuyuzaki 1989c, 1994b). Buried seeds contribute greatly to secondary succession starting with abandoned pastures, post-fire forests, etc. (Archibold 1981, Hill and Stevens 1981). Due to the seedbank containing a large number of the seeds of annuals, secondary succession often starts with the dominance of annual plants (Leck et al. 1989). Revegetation processes on Mount Usu suggest that typi- cal secondary succession could be recognized if seed- rich soil re-appears immediately after the disturbance. </p><p>Immigration Most immigrating species produce long-distance wind- </p><p>dispersed seeds. Most. woody species such as Salix hultenii var. angustifolium, Populus maximowiczii, Betula plaryphylla var. japonica, and Larix kaempferi were immi- grants into the crater basin. Herbaceous species, e.g., Senecio vulgaris, Anaphalis margaritacea var. angus- tifolior, Aster ageratoides var. ovatus, and Epilobium montanum also recovered via immigrant seeds (Tsuyuzaki 1987). Animal-dispersed species such as Prunus sar- gentii and Fragaria vesca were infrequently observed and they did not persist in the crater basin. </p><p>Floristic sources surrounding the crater basin strongly determined the rate of vegetation recovery (Tsuyuzaki 1991a, 1994b). For example, due to the paucity of annuals in the intact vegetation, annuals were rare in the crater basin. Similar trends were also observed after the erup- tion of Mount St. Helens (del Moral 1988). Vegetation recovery was faster in areas where plant sources were closely available (Wood and del Moral 1987, del Moral and Wood 1988). Therefore, wind-dispersal is often the most important determinant of vegetation recovery (Dale 1985, Nakashizuka et al. 1993). On newly-emerged islands such as Anak Krakatau in Indonesia and Surtsey in Iceland, all seeds must immigrate from outside of the island. In these cases, water-dispersal as well as wind -dispersal contribute to make up the new species compo- sition (Fridriksson 1992). </p><p>Artificially-introduced species A few grass and legumes were introduced for artificial </p><p>erosion control. The major species were Festuca rubra and F. elatior (Tsuyuzaki 1987). These species were aerial sprayed shortly after the eruptions, and to a much lesser extent invaded from outside of the caldera rim. However, their total cover was very small, i.e., less than 3% even in 1989 (Tsuyuzaki 1989a), indicating that the effects of artificial introduction to prevent ground surface </p></li><li><p>244 S. Tsuyuzaki </p><p>movements are minimal. </p><p>Determinants of vegetation dynamics </p><p>Based on eight environmental factors, loss on ignition of volcanic deposits collected from the ground surface, tephra deposition a...</p></li></ul>