Rates of change in vegetation during secondary succession*

R. Bornkamm Institute of Ecology, TU Berlin, Rothenburgstr. 12, D 1000 Berlin 41, B.R.D.

Keywords: Community Coefficient, Percentage Similarity, Rates, Succession

Abstract

For a number of experiments on secondary succession on different soils (lasting 9-23 yr) calculations were made of: a) rates of floristic change as measured with the community coefficient of Sorensen (CC), b) rates of cover-based change by the percentage similarity coefficient of Dahl & HadaE (PS), c) rates of change of stages (dominant growth forms). In the calculation of CC and PS the first year of observation and the preceding year have been used as reference years for a given year. Rates o f > 65% have been ranked as very rapid, rates of 35-65% as rapid, rates of 5-35% as slow, whereas rates of <5% (similarity >95%) indicate temporal stability. Rapid changes of CC are to a large extent confined to the first year only, rapid changes of PS may occur in the following years too. After 5 yr, in many cases CC and PS are as high as ca. 90%, but the values do not exceed 95%. The examples show that rates of change are a useful tool in the description of the succession process.

Introduction

Succession is a time depending process. Many vegetation parameters change with time. Since the beginning of succession research it is known that successions may be 'slow' or 'fast' (see Ltidi, 1930). Only few papers try to evaluate the speed of succession. This may be due to the fact that it is difficult to generalize the different rates of change of different vegetation properties.

Major (1974a) listed kinds of changes in vegetation and their durations (see also Major, 1970, 1974 b, c, d, e). Three groups can be distinguished: 1. Successional (unidirectional) changes of the

succession process as a whole 2. Change of seral stages

* Nomenclature follows F. Ehrendorfer: Liste der Gef~ss- pflanzen Mitteleuropas, 2. ed., Stuttgart 1973.

Vegetatio47,213-220(1981). 0042 3106/81/0471-0213/$1.60. © Dr W. Junk Publishers, The Hague. Printed in The Netherlands.

3. Phasic or cyclic changes within a stable community or a succession. For all types of changes we can describe

chronofunctions and calculate rates of change. A great number of parameters can be used in order to detect rates: plant cover, species number (modal values, see Yodzis, 1978), biomass and productivity (Major, 1974 a, c), and chemical constituents as well (Major, 1974 d, e).

The durations of sequences from the beginning of a succession to the terminal stage (the hypothetical climax stage) can be very different (Major, 1974 b). If we use a chronofunction v = fit) for the description of vegetation, a stable situation at the end of a succession is indicated by dv/dt~O. It is supposed that the terminal stage is

dv reached when - - ~ 5% (Major, 1974a, see also

dt Olsen, 1958; Tagawa, 1964). As a consequence of time limitation these problems cannot be solved through the study of permanent plots and therefore

214

will not be discussed further here. The sequence of stages of a sere (Knapp, 1974b) is

defined by floristic change and by change of the dominant life forms, i.e. mainly by qualitative categories. Rates of change at the community level can be determined by measuring the time, needed for transitions of given stages into next ones. An analysis at the population level makes clear that the transition process consists of a multiplicity of small-scale changes ('Kleinsukzessionen', Born- kamm, 1961a). These changes may be uni- directional or fluctuating (cyclic or irregular, see van der Maarel, 1980); they may be quantitative only or at the same time qualitative (replacement, Horn, 1976).

In phasic or cyclic changes within a stable community or a succession (Knapp, 1974a) rates of change may refer to the processes within a cycle, or to the frequency of cycles. Cyclic changes give rise to special problems, which will not be discussed here.

Succession processes in C. European vegetation concern - with very few exceptions - secondary succession, i.e. sequences following a disturbance or the termination of a state of disturbance. Here stages are often characterized by the dominance of resp. annuals, biennials, perennials, shrubs, pioneer trees, and trees of later stages. In the present paper for a number of experiments rates of change for both stages and small scale processes within the stages will be discussed.

Examples

Means o f comparison

A few examples of succession experiments are discussed. These experiments are of different design; they last 10 20 yr. Quantitative and qualitative changes of vegetation are described with the help of two indices: The presence/absence based community coefficient according to Sorensen,

2a 1948, CC -- where a is the number of

2 a + b + c , species occurring in both the given and the reference year, b the number occurring only in the given year, and e the number occurring only in the reference year. The reference year may be the first year of the succession, or the preceding year (year-to-year values).

The percentage similarity according to Dahl &

HadaE, 1941, PS : 2Zmin (Xi, Yi) X 100, is based X(Xi + Yi)

on the cover of species. Here x i andyiare the cover % values of given species in years x and y; and minx i or min Yiis the lower cover value of species occurring in both years.

Experiments

The first experiments were located in the Institute's experimental garden at Berlin-Dahlem, where three different soil types were brought in from elsewhere: sand, loam and clayey loam. It comprised 15 plots of 1 m 2 on each soil type. The vegetation was left alone from 1968-1976; it was recorded yearly. The mean values of the replicates were used for calculation (for details see Born- kamm & Hennig, in press). Since the succession started with a ruderal weed vegetation it will be referred to as the ruderal succession experiment.

The second experiment comprised only ten 1 m 2 plots on horticultural soil in the experimental field of the Botanical Institute of the University of Cologne, recorded (and harvested) year by year without replicates. During the experiment (1967- 1976) a vegetation of high forbs developed (for details see Bornkamm, 1981). It will be called the horticultural succession experiment.

The third experiment was carried out in a dry meadow near G~ttingen set up in 1953 in two parts: Grassland experiment I started with destruction of the vegetation on a 4 m 2 plot; grassland experiment II was set up by transplanting a very small plot (0.7 X 0.8 m 2) from a moist meadow into a dry meadow (for details see Bornkamm, 1961b, 1974, 1975).

Stages

The succession stages detected in the different years are plotted in Figure 1. It shows, that in the ruderal experiment on loam and clayey loam an annual vegetation (characterized by Senecio vulgaris and Conyza canadensis) merged in the second year into a perennial stage dominated by Solidago canadensis. On sand Solidago was not as successful; for this reason the annual stage lasted longer and comprised higher contributions of

215

NNNNNNN I NN NNN

BERLIN

~ ~ ~ - - ~ / J S S Y ~ ~ 140L N

(SAND) ~'-'! ANNUALS • e____~J

(LOAM) o BIENNIALS o

(CLAYY LOAM) ~ PERENNIAL GRASSES

...... PERENNIAL -- FORBS

(LOAM) ~ SHRUBS oeooe

N \ \ ~ . \ \ \ ' % ~ , " ~ \ \ \ ~ . N \ \ \ \ ~ \ N

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 YEARS OF SUCCESSION

Fig. 1. Dominant growth forms characterizing the stages of different succession experiments. Berlin = ruderal experiment; KOln = horticultural experiment; G6ttingen = grassland experiment.

b iennia l species (esp. Oenothera biennis). The plots then sudden ly changed into a shrub stage d o m i n a t e d by Sarothamnus scoparius.

In the hor t i cu l tu ra l expe r imen t the perennia ls s ta r ted at the same t ime as the annuals , and were a l r e a d y d o m i n a n t in the second year (esp. Urtica dioica). 100- %

In the g rass land expe r imen t I a smal l n u m b e r of annua ls af ter the r emova l of vege ta t ion es tabl ished only in the first year, whereas perenn ia l grasses 80 were d o m i n a n t near ly f rom the beginning. Gras s l and expe r imen t II showed a rep lacement of grasses of mois t meadows (e.g. Arrhenatherum 60 elatius, Lolium perenne, Dactylis glomerata) by grasses of d ry meadows (esp. Bromus erectus). D u r i n g this process herbs were for a n u m b e r of years successful in compe t ing with the grasses. Even 40 af te r more than 20 yr there is no sign of a shrub layer. This is p r o b a b l y due to rabbi t grazing.

Indices o f similarity

In the rude ra l e x p e r i m e n t on sand the c o m m u n i t y coeff ic ient is < 60% in the second year as c o m p a r e d with the first one, decreas ing to 23% in 1975 (CC, Fig. 2a, full points) . The yea r - to -yea r values are > 75% a l r e a d y af ter two years, i nd ica t ing a ra ther slow rate of change (Fig. 2a, circles). The percentage s imi lar i ty values (PS) decrease much faster than the CC values: The s imi la r i ty be tween year 3 and 1 is a l r eady < 10%

(Fig. 2b, full points)! The s imi lar i ty of one year to the preceding year ' s value increases s lower with the PS values (Fig. 2b, circles) than with the CC values. But finally, af ter 9 yr, a very high value of more

20

(I 0

100 °/o

8O

6O

&0

2O

0 67 69 71 73 75 67 69 71 73 75

Fig. 2. Indices of similarity in the ruderal experiment on sand. a. Community Coefficient; b. Percentage Similarity. Full points: Reference to the first year; circles: Reference to the preceding year (year-to-year values). Secondary reference lines indicate the relation to the other years. In order to obtain a clearer distinction reference to years 2,4 and 6 is given by dotted lines, reference to years 3,5 and 7 by broken lines.

100- %

80

60

40

20

0 67

C1 69 71 73 75

216

100 I O0 ]~ °]° 1 100. %

%

80 .... ~ !........ S 80 80

,, -,, ,<~:)iii'" . . . . . ,y... ,,,, " + '~ 60 ............... ',;...~.< 6o 6°t . . . . . . . . . . . . - ..... ,, .~ -'~

' . . . : ~ • ". . :" q

40 40 " \. 40

2C 20 20

0 . . . . . 0 0 68 70 72 74 "16 67 69 71 73 75 67 69 71 73 75

Fig. 3. Indices of similarity in the ruderal experiment on loam. a. Community Coefficient; b. Percentage Similarity. Further explanation see Figure 2.

Fig. 4. Community Coefficients in the horticultural experiment. a. Community coefficient; b. percentage similarity. Further explanation see Figure 2.

than 90% is reached, indicating an extremely slow rate of change. This is caused by the stable dominance of Sarothamnus scoparius. The se- condary reference lines show that in a given year the plots are more similar to the plots of one year before than to plots of two years before, more similar to the plots of two years before than to the plots of three years before, and so on. This means that there is no influence of the different climatological condit ions in the different years, e.g., dry and wetness.

On loam the CC values related to the first year (Fig. 3a) are comparab le to those of sand. The values related to the preceding year show more variation. In some cases the secondary reference lines cross: In those years the plots were more similar to plots of earlier years that were moist (1970, 1972) than to more recent years that were dry (1971, 1973). Thus the effect of the cl imatological condit ions is more apparent on loam than on sand. With the PS-values the similarity to the first year is already < 10% by the third year.

On clayey loam we recognize the same trends, but here exists a very high year- to-year stability caused by the early and steady dominance of Solidago canadensis (data not presented here).

In the hort icul tural experiment the decrease of CC-values related to the first year (Fig. 4) is slower than in the ruderal experiments. This is due to the fact that the dominat ing perennial forb, Urtica dioica, was present f rom the beginning. The CC- values related to the preceding year show a rather stable si tuation after 4-6 yr, caused by the dominance of Urtica. The decrease after 7-9 yr is caused by a methodological problem: In plots 1-7 Urtica was dominant , in plots 8-10 Symphytum officinale was more dominan t than Urtica. Since one plot was harvested every year, starting with plot 1, only the remaining plots could be used for vegetat ion analysis. For this reason the cover of Symphytum became more and more apparent in the calculations (for details see Bornkamm, 1981) The decline of the PS-values is also not as rapid as in the ruderal experiment; still > 20% after 9 years.

In the grassland experiments the picture is different. In experiment I the vegetation was coml~letely removed in one half of the' plot, in the other half the two most dominan t grasses, Bromus erectus and Brachypodium pinnatum, were left. The CC-values (Fig. 5a) show two phases. In phase I (up to the early sixties) the decrease of values related to the first year is faster than in phase II (1964-1973). This is not unusual. The CC-values

related to the preceding year are 78,5 ___ 1,4% in phase I (average for 1954-1963) and 85,5 + 2,6% in phase II (average for 1964-1973), the difference being significant with P < 0,05. The first phase is characterized by the spread of the most common dry meadow plants and the occurrence of some annual weeds. The second phase is characterized by the occurrence of seedlings of woody plants (which do not succeed); a more complicated pattern combined with a tendency towards a more homogeneous distribution of the species. These tendencies result in a lower degree of floristic change. The secondary reference lines of both phases are well separated. The similarity to the dry years of 1957 and 1959 is mostly lower than to the more distant, but moister years of 1956 and 1958. The PS-values (Fig. 5b) are rather high because the most frequent species, B r o m u s erectus, was present from the beginning. The values related to the first year oscillated between 50 and 70% in phase I, and between 45 and 60% in phase II. The values related to the preceding year oscillate between 70 and 90%. Here, too, it can be seen that the similarity to dry years like 1957 and 1959 is mostly lower than to moist years like 1956 and 1958.

in the grassland experiment II greater changes take place (see Fig. 7a, b). In the first phase (1953-1958) most of the transplanted species of moist meadows are still present (little floristic change, high CC-values). But there is already a change in cover by species already present that grow in moist a n d dry meadows (decrease of PS- values related to the first year). In the second phase (for details see Bornkamm, 1974) these species are most frequent, but species of dry meadows invade. This results in a decrease of CC-values related to the first year. The third phase (since 1962) is characterized by a stable dominance of B r o m u s

erectus, leading to high year-to-year CC-values (80-90%). In the PS-values the secondary reference lines show two groups with a gap at about 1962/63 and a close parallelity since 1966. Since here the vegetation was changing from a moister to a dryer type, apparently the dry years were 'pushing' the succession. CC- and PS-similarity to the earlier but dry year 1957 is greater than to the more recent but moist year 1958, and with the CC-values it is greater for 1959 than for 1960. In most cases the plots in the dry years (e.g. 1959, 1963, 1969) were less similar to the plots in the preceding years than in the moist years.

217

1oo /o

8 0

60

2O

0 53

£1 5 8 6 3 . 6 8 73

"". . . ...... ,.,.. '".... ,,..""~"... / ""

• .~: -" • "" ""~,'.i!, 80 V ~ .." . . . . ,,' " ~,..i~. ' i ̂ , t " - ~.~.~ ~

:- ' &.,' ", ',~";v=~' ~".'%:-

40

2C

b o

5 : ' ' ' 5'8 . . . . 63 . . . . 6 8 . . . . '7'3

Fig. 5. Indices of similarity in the grassland experiment I. a. Community Coefficient; b. Percentage Similarity. Further explanation see Figure 2.

C o n c l u s i o n s

The rates of change discussed in this paper are measured as floristic change (CC) and as change of species cover (PS). In the examples used two types of dependence have been found. One type.(ruderal and horticultural experiment) is outlined in Figure 7, where the mean values for the three chosen parts of the ruderal experiment are plotted. It shows a

218

100 1

8 0 "'-. :, / ....... ".~ . . . . . . . :__,,~

0'1 . . . . . . . . . i . . . . . . . . . . 53 58 63 68 73

80 ...... . . . . . ~ ......

...... -~ ,~-:i~ ,", "~i~"~: ..-..-:,.. ~,~.. ~.., '~,..,-~

60 " ~ ' _. ., ~ \ : ,, . , , ' , I ~, -'..,.

oI b` . . . . . . . . . ....... . . . . . . . . . . . " - " """ .....

53 58 63 68 73

Fig. 6. Indices of similarity in the grassland experiment II. a. Community Coefficient; b. Percentage Similarity. Further explanation see Figure 2.

r ap id f loris t ic change in the first years, and the CC-va lue wi th in 10 years decreases to ca. 30%. Dur ing this t ime the f lorist ic compos i t i on becomes more and more s table and the s imi la r i ty between two subsequent years is a b o u t 80%. This change is par t ly due to the progress of succession, (the s imi lar i ty to the first year is still declining). I t is also due to the var ia t ion in dry and wet years. The rate of cover based change is even more p ronounced . Af te r

100 ,

°/o

70-

60- \ /

x

. CC, , I - .

\

o

' ~ P S

0

r e f e r e n c e y e a r

Fig. 7. Indices of similarity, mean values for the three ruderai experiments. CC (broken lines) = Community Coefficient; PS (solid lines) = Percentage Similarity; Circles: reference to the first year; X: Reference to the preceding year•

5 yr the PS-va lue re la ted to the first year i s < 10%. Here ye a r - t o -ye a r changes of 20% occur, while the s imi lar i ty to the first year decreases very litt le fur ther . This type co r re sponds to the dominance con t ro l l ed communi t ies , accord ing to Yodzis, 1978.

In the g rass land expe r imen t I the domina t i ng perennia ls con t r ibu t ed to the vege ta t ion compos i - t ion and cover f rom the beginning (Fig. 5). Here the rates of change are not as high. The yea r - to -yea r va r i a t ion is more p rominen t than the succession process itself. This type co r re sponds with the ' founder con t ro l led ' communi ty , accord ing to Yodzis, 1978.

Grass l and expe r imen t II holds an in te rmedia te pos i t ion (Fig. 6). In the first phases un id i rec t iona l f lorist ic and cover changes are detectable . In the last phase (since 1968, see Fig. 6b) the s i tua t ion resembles exper imen t I very much.

F requen t ly only quant i ta t ive changes of vegeta- t ion have been called f luc tua t ions in con t ras t to the qual i ta t ive and quant i ta t ive changes between the succession stages, which are called succession in a

219

strict sense (Knapp, 1974a; Rabotnov, 1955, 1974; Barkman, 1958; Braun-Blanquet, 1964; Miles, 1979; van den Bergh, in press). F rom an analytical point of view it is important to distinguish yearly fluctuations from the trend of the succession process as a whole. But in fact, fluctuations are integrated parts of this process. The fluctuation of one year, even if it is governed by the weather (e.g. dry years) influences the development of the next year. (Van der Maarel, this symposium) Fluctua- tions may sometime change species cover to zero if annual species do not germinate in dry years, but emerge in the next moist year (Runge, 1963). In this case we have qualitative changes. The succession is the total of all fluctuations and not something additional.

It has been shown that parameters of similarity are a means of evaluating rates of change. These rates are different, if different parameters are used. If we call rates of change of 5-35% per year slow, 35-65% rapid and >65% very rapid we can rank the

events of the experiments discussed (Fig. 8). Rapid changes occur in the first years of the ruderal and horticultural experiment, and likewise in the grassland experiment II, whereas grassland ex- periment I shows slow rates. Schmidt (in press) determined rates of change in succession experi- ments in old fields with different treatments. He found rapid changes of the PS-values in the first year under all treatments, of the CC-values in the first year with shading only. In all other cases rates of change were slow.

If we compare the change from stage to stage, the grassland experiment develops very rapid, because the annual stage hardly exists. In the ruderal experiment the change to the perennial stage is slow, but the conversion into shrubs is rapid. On the contrary the investigated grassland is nearly stable as a perennial grassland and does not show any signs of woody vegetation.

In conclusion we may say that it is possible to speak of succession rates if it is clearly indicated which parameter of change is chosen.

s t a b l e

for CC ÷ PS: > 95°/o

slow

for CC + PS : 65- 90 %

rapid

for CC + PS: 35 - 65 %

dramatic

for CC + PS: < 35%

CC

Ha S3-Sn Ru S S2-Sn

L S2-Sn C Sz-Sn

Gr I 51-Sn If S1-Sn

Ha $1 +52 Ru S $1

L $1 C S~

PP

Ha $ 2 - Sn Ru S $ 5 - Sn

L $ 5 - Sn C S 3 - S n

Gr ! $1 - Sn ]1 $ 6 - Sn

Ha $1 Ru S $1 - $3

L $ 2 - $4 C $1 - $2

Gr TLI $ 1 - $5

perenniats

Ru S

Ho Ru L

Gr I Gr II

Ru L $I

stages shrubs

Gr I Gr II

H a

Ru L Ru C

Ru S

Fig. 8. Class i f i ca t ion of success ion rates . R u = r u d e r a l e x p e r i m e n t ; H a = h o r t i c u l t u r a l e x p e r i m e n t ; Gr . I, II = g r a s s l a n d e x p e r i m e n t I, II; S = sand ; 1 = l o a m ; C = c l a y e y l o a m ;

S~ . . . . . S n ---- yea rs o f success ion .

220

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Accepted 28.7.1981.