Vegetatio 122: 151-156, 1996. 151 @ 1996 Kluwer Academic Publishers. Printed in Belgium.

Species diversities analyzed by density and cover in an early volcanic succession

Shi ro T s u y u z a k i Graduate School of Science and Technology, Niigata University, Niigata 950-21 Japan

Accepted 5 September 1995

Key words: Cover, Density, Diversity, Succession, Volcano

Abstract

To evaluate alpha diversities, various variables such as density, cover, volume, and weight have been used. How- ever, density is often a distinct variable from the remaining three. To clarify differences in diversity measured by those two kinds of variables, the data collected in fourteen 2 x 5 m permanently-marked plots on Mount Usu, Japan, which erupted during 1977 and 1978 in growing seasons from 1983 to 1989 was analyzed, using Shannon's species diversity (H') that is represented as a result of combination of species richness and evenness (J'). H' and J' were evaluated by density (density H' and jr) and cover (cover H t and jr). Cover H r and J' were significantly lower than density H ~ and f , indicating that cover H ~ has different characteristics from density H/. Those differences are due to differences in evenness, because species richness is the same. The rank orders of species density are different from those of cover. The predominance of a few perennial herbs greatly decreases cover evenness, while seedling establishment success influences density evenness. Therefore, I propose that, during the early stages of succession on harsh environments such as volcanoes, density diversity represents seedling establishment success rate while cover diversity expresses vegetative reproduction success rate.

Nomenclature Ohwi (1975)

Introduction

Since the concept of species diversity was proposed for the expression of community structures, there has been considerable research on plant community diver- sity (Cody & Diamond 1975; Newman 1982; Peet 1974; Pielou 1975; Shannon & Weaver 1949; Tilman 1988; Whittaker 1977). In plant ecology, species diver- sifies have been discussed based on various parame- ters, e.g., density, area or cover, volume, and weight (Gough et al. 1994; del Moral & Wood 1988; Rein- ers et al. 1971; Tsuyuzaki 1989a; Zahl 1977). Reiners et al. (1971) selected basal areas and densities for trees and tall shrubs, and cover for low shrub-herbs and for bryoid-thalloids to evaluate species diversities. In general, cover, volume and weight are strongly corre- lated with each other, and subsequently those variables tend to show similar patterns of diversity fluctuation (Numata 1956; Zahl 1977).

In harsh environments such as volcanoes that cre- ate fresh volcanic substrates, vegetative reproduction significantly contributes to revegetation, and seedling establishment is strongly restricted (Canham & Marks 1985; del Moral & Bliss 1993; Tsuyuzaki 1991). Even though seed bank species were dominant in the gul- lies where the former topsoil was exposed on Mount Usu, Japan, soon after the 1977-78 eruptions, their contribution to revegetation annually decreased except for nitrogen-fixing seed bank species such as Trifolium repens and Lotus corniculatus var.japonicus (Tsuyuza- ki 1994). Therefore, density fluctuation would not be expected to increase cover (Tsuyuzaki 1994), suggest- ing that the characteristics of diversity evaluated by density (hereafter, i.e., density diversity) are different from those by cover (cover diversity). In spite of dif- ferences between diversity based on cover and density such differences have rarely been discussed in the lit-

152

erature. Therefore, I try to find out the causal effects of those two variables on diversity measurement.

The Shannon-Wiener function is a common index used to evaluate species diversity, because this function is comparatively independent of sample size and does not have an upper limit of the value (Hurlbert 1971; Pielou 1975). This function, which is a kind of het- erogeneity index, is based on a combination of species richness and evenness (Peet 1974). Therefore, I expect that this function strongly reveals the different char- acteristics between two variables, density and cover. Here, I use the Shannon-Wiener function to compare among species richness, density and cover diversities, and density and cover evenness and to examine the differences of those variables in the early stages of volcanic succession.

Study area

Mount Usu (42 ° 32' N, 140 ° 50' E) on the island of Hokkaido, northern Japan, consists of two peaks, O- Usu (727 m) and Ko-Usu which are enclosed by a caldera rim and crater basin. The 1977-78 eruptions destroyed the vegetation on the summit area which was dominated by Populus maximowiczii and Betu- la platyphylla var. japonica, by a 1-3 m thick accu- mulation of volcanic deposits. Due to ground surface movements soon after the eruptions, the crater basin produced three habitat types, i.e., gully inside where former topsoil was exposed, gully inside where vol- canic deposits still accumulated, and gully outside (Tsuyuzaki 1989b). Revegetation on the summit area started mostly with vegetative regrowth of large peren- nial herbs such as Polygonum sachalinense and Peta- sites japonicus var. giganteus (Tsuyuzaki 1987). The main aspect of recovery is the increase in total cover from zero at the eruptions to more than 50% at the present (Tsuyuzaki 1989b, 1994).

Methods

Density and percentage cover of each species were monitored in 14 permanently-marked 2 x 5 m plots in the crater basin during 1983 and 1989. Six plots were set up gully outside, four were set up gully inside where the former topsoil was exposed, and four were set up gully inside where thick volcanic deposits still accumulated. Based on excavations near the plots, I confirmed that closely aggregated clumps were mostly

one individual (Tsuyuzaki 1987). So, closely aggregat- ed clumps were counted as one individual. Percentage cover was estimated at 1% intervals when cover was more than 1%. If less than 1%, cover was recorded at 0.01% intervals (Tsuyuzaki 1991).

To compare between the variables of density and cover, Shannon-Wiener species diversity /4' (Shan- non & Weaver 1949) and evenness J ' (Pielou 1966) were evaluated for each plot. The equations are as fol- lows:

H ' = ~ In(pi), and S' = H'/ln(S), (1) i

wherepi is the proportion of a species i to the total den- sity or cover, and S is number of species, i.e., species richness.

Correlation coefficients (r) were obtained for data from Kendall's rank order for each parameter on each year, to test relationships between density and cover (Zar 1984). Differences between the density and cover diversities and between the two evenness values are tested by pairwise t-test in each year.

Results

Species richness, diversity, and evenness

The species richness ranged from 3 to 28, and diver- sity and evenness also showed wide ranges (Table 1). Those five parameters, i.e., species richness, density and cover diversity, and the two evenness remained relatively unchanged. Density diversity and evenness, respectively, ranged from 0.59 to 2.68 and from 0.30 to 0.92. Cover diversity and evenness, respectively, ranged from 0.17 to 2.78 and from 0.09 to 0.96. Mean density diversity and evenness were slightly higher than mean cover diversity and evenness every year; in particular there were significant differences in 1983 and 1985. The lowest values of density and cover evenness in a plot were, respectively, 0.17 and 0.09 in 1985.

Relationships between density and cover diversities

During the period from 1983 to 1985, density and cov- er diversities were highly significantly related to each other (r= + 0.760 to + 0.958, significant at p<0.01). Thereafter, those relationships became not significant

153

Table I. Yearly values of species richness, species diversity and evenness evaluated by density and cover. Maximum and minimum values are shown under each row of mean values. The differences between density and cover diversities and between density and cover evenness are tested by pairwise t-test

Year 1983 1984 1985 1986 1987 1988 1989

Species richness 124-8 9 ± 4 13+6 12±5 124-4 13±3 124-3 28-3 19-3 23-4 22-6 18-6 17-7 19-7

Density diversity 1.58 4- 0.56 1.56 4- 0.48 1.654-4- 0.49 1.67 4- 0.55 1.63 4- 0.53 1.73 4- 0.52 1.65 4- 0.61 2.54-0.90 2.42-0.82 2.58-0.77 2.68-0.75 2.47-0.59 2.56-0.78 2.49-0.75

Cover diversity 1.41-0.52 1.51 4- 0.59 1.44 -4- 0.67 1.26 4- 0.48 1.41 4- 0.41 1.54 4- 0.30 1.55 4- 0.26 2.03-0.71 2.78-0.76 2.52-0.17 2.22-0.54 2.11-0.78 2.03-1.03 2.11-1.16

Significance * NS NS * NS NS NS

Density evenness 0.71 4- 0.09 0.76 4- 0.06 0.67 4- 0.13 0.69 4- 0.15 0.66 4- 0.15 0.67 4- 0.16 0.66 4- 0.21 0.85-0.58 0.86-0.64 0.85-0.42 0.90-0.38 0.87-0.30 0.90-0.40 0.92-0.36

Coverevenness 0.634-0.10 0.74±0.16 0.574-0.22 0.544-0.17 0.584-0.14 0.614-0.10 0.644-0.11 0.79-0.46 0.96-0.38 0.84-0.09 0.79-0.24 0.88-0.34 0.82-0.42 0.92-0.49

Significance • * NS NS * NS NS NS

**: significant difference atp<0.01. *: p<0.05. NS: non-significant, n=14.

Table 2. Number of plots (n=14) showing significance of Kendall's rank correlation coefficients between density and cover in each year

Year 1983 1 9 8 4 1985 1 9 8 6 1 9 8 7 1 9 8 8 1989

Significance level /9<0.01 6 5 3 2 4 0 2 /9<0.05 4 1 4 3 4 5 2 Non-significant 4 8 7 9 6 9 10

( r = +0.464 to + 0.658). Because species r ichness is the same for both density and cover diversity, diversity dif-

ferences were der ived f rom their evenness. In fact, the

values of densi ty evenness differed from those of cover evenness (Table 1). Densi ty diversity and cover even-

ness relat ions and cover diversity and density evenness relat ions showed no signif icant relationships.

Species r ichness was highly significantly correlat- ed with densi ty diversity, a l though those correlations tended to decrease annua l ly from + 0.927 to + 0.689.

Species r ichness was not correlated with cover diver- sity in 1985, 1987 and 1988; i.e., species r ichness inf luenced densi ty diversi ty more than cover diversi- ty. Densi ty diversi ty decreased the correlation coeffi-

cients (r) with species r ichness from +0 .884 in 1983 to +0 .341 in 1989 (significant at p < 0 . 0 1 from 1983 to 1985, and non-s igni f icant thereafter), bu t became more significantly related with density evenness from + 0.464 in 1983 to + 0.952 in 1989 (non-s ignif icant in 1983, and significant at p < 0 . 0 1 after 1984). Cover

diversity was significantly related with species rich-

ness f rom 1983 to 1985 but was less related after 1986.

The relat ionship be tween cover diversity and evenness gradually decreased from + 0.925 to + 0.760, a l though

those values were signif icant at p < 0 . 0 1 . Those results

indicated that species richness, i.e., n u m b e r of species, was more important de terminant for density diversi ty

than for cover diversity, in particular; in the earlier stages of succession.

Relationships between density and cover

Two large herbaceous perennia l species Polygonum sachalinense and Petasites japonicus vat. giganteus were dominan t in terms of both densi ty and cover. The cumula t ive dominance of these first 2 leading species ranged from 35.6% to 69.2% on density and f rom 43.1% to 69.6% on cover in each year. Due most- ly to the p redominance of those two species, peren- nials showed high relat ive dominance in both cover

154

and density over the entire study period 1983 to 1989 (Fig. 1). The predominance of the two species greatly contributed to cover more than density. Therefore, low cover evenness (see Table 1) was mostly due to the predominance of the two leading species, and vice ver- sa. However, cover order was not significantly related to density order in more than 1/3 of the plots each year (Table 2), indicating that cover increase was not necessarily dependent on density increase. Rank cor- relations showed that the number of plots that showed non-significant relationships between density and cov- er gradually increased, showing that over time cover growth became independent of density increase. For example, P. sachalinense increased the relative cover dominance from 13.2% in 1983 to 36.0% in 1989, but fluctuated the relative density dominance ranging from 5.1% to 26.7%. P. japonicus var. giganteus annually decreased the relative cover dominance from 47.0% to 14.0%, while the relative density dominance was 27.8% in 1983 and 56.3% in 1989. Relative density dominance irregularly fluctuated in most species. For annual species, the relative dominance was ca 30% in 1983 but was less than 10% after that (Fig. 1). Annuals decreased in cover with a drastic decrease in densi- ty.

Woody and shrub species annually decreased in rel- ative density but increased in relative cover (Fig. 1). Populus maximowiczii, one of the dominant species, yearly decreased in density relative dominance from 4.0% to 0.9% but fluctuated in cover relative domi- nance during 1.5% to 4.0%. The fate of other woody species was similar with that of P. maximowiczii. In 1989, woody species such as P. maximowiczii, Sal- ix hultenii var. angustior and Betula pIatyphylla var. japonica had a density of 1.1-2.2/plot but a cover of 2.6-3.2%. Salix integra, a shrub species, showed a density of less than l/plot but a cover of 1.3%. These results showed that the cover increase of those species was mostly dependent on vegetative reproduction.

Discussion

Based on the same species richness in each year, both the density and cover diversities were evaluated. Therefore, any differences between the two diversities are derived from the differences of density and cov- er evenness. Most annual species originated from the seed bank in the old topsoil and approximately 70% of individuals were derived from the seed bank inside the gullies where the former topsoil was exposed in 1983

~, 100

80 16o 40

20

0

Densi ty

1983 1984 1985 1986 1987 1988 1989

Year

100

8 80

40

20

0

Cover

1983 1984 1985 1986 1987 1988 1989

Year

Fig. 1. Percentage relative dominance on density (upper column) and cover (lower column) of four life history types. Closed, open, dotted and hatched column show annuals, perennials, shrubs, and trees, respectively.

(Tsuyuzaki 1987). However, those annuals gradually decreased in both density and cover and almost van- ished after 1990 (Tsuyuzaki 1994). Of all life forms, annual density and cover fluctuated the most in parallel, due to the absence of vegetative reproduction. Owing to the high contribution of the seed-bank derived annu- als to the density and cover fluctuations from 1983 to 1985, cover diversity should be highly correlated with density diversity. Therefore, when the revegeta- tion is mostly conducted by seed immigration, e.g., soon after catastrophic disturbances such as eruption, cover diversity could be analogous to density diversi- ty.

While rare species are an important component to determine evenness (Bulla 1994; Medina & Huber 1992), rare species frequently turnovered through time on Mount Usu (Tsuyuzaki 1991). Those species show low relative density and cover and seem not to influ- ence the differences between density and diversity.

The cover increase of perennial species is not nec- essarily dependent on density. Vegetative reproduction has a prominent role in community development, since

seedling establishment is highly restricted on harsh environments such as tundra and volcanoes (Tsuyuza- ki 1991; Van der Valk 1992; Wood & del Moral 1987). Once seedlings can overwinter, the survival rates are very high due to well-developed root expan- sion (Tsuyuzaki 1989b). Vegetation development on the crater basin of Mount Usu was mostly accom- plished by large perennial plants such as Polygonum sachalinense and Petasites japonicus var. giganteus (Tsuyuzaki 1987, 1989b). Those two species are the first two main dominants in both density and cover, while the orders of density and cover were reversed. Woody species developed relatively high cover in spite of low density, due to the ground surface movements (Tsuyuzaki 1989b; Tsuyuzaki & del Moral 1994). As well as large perennial plants, the cover increase of most woody species was through vegetative reproduc- tion. As time has passed, those perennial species grad- ually increases in cover without seedling supply and thus cover diversity became inconsistent with density diversity.

Density diversity is significantly related to species richness, while cover diversity is not or weakly relat- ed to the species richness after 1986. The relation- ships between diversity and evenness evaluated by density increased annually, while those evaluated by cover gradually decreased. Those results show that cover evenness decreases due mostly to the predomi- nance of a few species (Tsuyuzaki 1991). Well-rooted species that are predominant often control the com- munity structure (Bornkamm 1981). Therefore, cover diversity was lower than density diversity when cover evenness was significantly lower than density even- ness, due to the predominance of a few species.

On the basis of density and cover analyses, dif- ferent environmental factors influenced density and cover on Mount Usu; viz. distance from colonizing source affected plant density while erosion affected cover (Tsuyuzaki & del Moral 1994). Diversity eval- uated by each variable is also considered to be influ- enced by those distinct factors. Therefore, I propose that density diversity expresses seedling establishment success diversity and cover diversity is related to veg- etative reproduction in disturbed habitats such as vol- canoes. In this sense, cover diversity is a better tool to express successional fates on habitats where environ- mental conditions are rather unstable, because density fluctuates irregularly (Tsuyuzaki 1989b). The cover index has one more advantage when individuals are not able to be counted. Generally, cover, volume and biomass are strongly correlated to each other (Numata

155

1956; Zahl 1977). The other variables such as vol- ume and biomass may express different characteristics of vegetation structure but those diversity differences seem to less serious.

Acknowledgments

I thank Prof S Yoshida for his invaluable suggestions and Dr H Okada and all members of UVO for support- ing me in the field work. This work is partly supported by the Ministry of Education, Science and Culture of Japan.

References

Bomkamm, R. 1981. Rates of change in vegetation during secondary succession. Vegetatio 47: 213-220.

Bulla, L. 1994. An index of evenness and its associated diversity measure. Oikos 70: 167-171.

Canham, C. D. & Marks, E L. 1985. The response of woody plants to disturbance: Patterns of establishment and growth. In: Pickett, S. T. A. & White, E S. (eds) The ecology of natural disturbance and patch dynamics. Academic Press, San Diego, USA.

Cody, M. L. & Diamond, J. M. 1975. Ecology and evolution of communities. Belknap Press, Cambridge, UK.

del Moral, R. & Wood, D. M. 1988. Dynamics of herbaceous vege- tation recovery on Mount St. Helens, Washington, USA, after a volcanic eruption. Vegetatio 74:11-27.

del Moral, R. & Bliss, L. C. 1993. Mechanisms of primary succes- sion: Insights resulting from the eruption of Mount St. Helens. Adv. Ecol. Res. 24: 1-66.

Gough, L., Brace, J. B. & Taylor, K. L. 1994. The relationship between species richness and community biomass: the impor- tance of environmental variables. Oikos 70: 271-279.

Hurlbert, S. H. 1971. The nonconcept of species diversity: a critique and alternative parameters. Ecology 52: 577-586.

Medina, E. & Huber, O. 1992. The role of biodiversity in the func- tion of savanna ecosystems, pp. 139-158. In: Solbrig, O. T., van Emdem, H. M. & van Oort, E G. W. J. (eds.) Biodiversity and global change, International Union of Biological Sciences, Paris, France.

Newman, E. I. 1982. The plant community as a working mechanism. Blackwell, Oxford, UK.

Numata, M. 1956. The developmental process of weed communities - - Experimental studies on early stage of a secondary succession, II - - . Jap. J. Ecol. 6: 62-66, 89-93.

Ohwi, J. 1975. Flora of Japan (rev. ed.). Shibundo, Tokyo, Japan. Peet, R. K. 1974. The measurement of species diversity. Ann. Rev.

Ecol. Syst. 5: 285-307. Pielou, E. C. 1966. The measurement of diversity in different types

of biological collections. J. Theor. Biol. 13:131-144. Pielou, E. C. 1975. Ecological diversity. Wiley-Interscience,

New York, USA. Reiners, W. A., Worley, I. A. & Lawrence, D. B. 1971. Plant diversity

in a chronosequence at Glacier Bay, Alaska. Ecology 52: 55-69. Shannon, C. E. & Weaver, W. 1949. The mathematical theory of

communications. Univ. Illinois Press, Urbana, USA.

156

Tilman, D. 1988. Plant strategies and the dynamics and structure of plant communities. Princeton University Press, Princeton, New Jersey, USA.

Tsuyuzaki, S. 1987. Origin of plants recovering on the volcano Usu, northern Japan, since the eruptions of 1977 and 1978. Vegetatio 73: 53-58.

Tsuyuzaki, S. 1989a. Buried seed populations on the volcano Mt. Usn, northern Japan, ten years after the 1977-78 eruptions. Ecol. Res. 4: 167-173.

Tsuyuzaki, S. 1989b. Analysis of vegetation dynamics on the vol- cano Usu, northern Japan, deforested by 1977-78 eruptions. Am. J. Bot. 76: 1468-1477.

Tsuyuzaki, S. 1991. Species turnover and diversity during early stages of vegetation recovery on the volcano Usu, northern Japan. J. Veg. Sci. 2: 301-306.

Tsuyuzaki, S. 1994. Fate of plants from buried seeds on volcano Usu, Japan, after the 1977-78 eruptions. Am. J. Bot. 81: 395-399.

Tsuyuzaki, S. & del Moral, R. 1994. Canonical correspondence analysis of early volcanic succession on Mount Usu, Japan. Ecol. Res. 9: 143-150.

Van der Valk, A. G. 1992. Establishment, colonization and persis- tence, pp. 60-102. In: Glenn-Lewin, D. C., Peet, R. K. & Veblen, T. T. (eds.) Plant succession. Theory and prediction. Chapman & Hall, London, UK.

Whittaker, R. H. 1977. Evolution of species diversity in land com- munities. Evol. Biol. 10: 1-67.

Wood, D. M. & del Moral, R. 1987. Mechanisms of early primary succession in subalpine habitats on Mount St. Helens. Ecology 68: 780-790.

Wood, D. M. & del Moral, R. 1988. Colonizing plants on the Pumice Plains, Mount St. Helens, Washington. Am. J. Bot. 75: 1228- - 1237.

Zahl, S. 1977. Jackknifing an index of diversity. Ecology 58: 907- 913.

Zar, J. M. 1984. Biostatistical analysis (2nd. ed). Prentice-Hall, Englewood Cliffs, New Jersey, UK.