Vegetatio 123: 91-100, 1996. 91 (~) 1996 Kluwer Academic Publishers. Printed in Belgium.

Reproductive effort and phenology of seed production of savanna grasses with different growth form and life history

E.M. V e e n e n d a a l 1,2,*, W.H .O . E rns t 1 & G.S. M o d i s e 2 J Department of Ecology and Ecotoxicology, Faculty of Biology, Vrije Universiteit, De Boelelaan 1087, 1081 HV Amsterdam, The Netherlands; 2National Institute for Development Research and Documentation, University of Botswana, Private Bag 0022, Gaborone, Botswana; (*present address: Department of Plant and Soil Science, The University, Aberdeen AB9 2UD, Scotland

Accepted I0 October 1995

Key words: Annual grasses, Grassland disturbance, Perennial grasses, South Eastern Botswana, Seed size

Abstract

The phenology of seed production in natural savanna grasslands was studied in the grass species Aristida congesta, Cymbopogon plurinodis, Cynodon dactylon, Digitaria eriantha ssp. pentzii, Eragrostis rigidior, Eragrostis superba, Panicum coloratura, Schmidtia pappophoroides, Tragus berteronianus and Urochloa panicoides. Maximum seed production varied according to life history strategy and growth form from 0.03 mg seed g-~ shoot dry weight in the perennial D. eriantha ssp. pentzii which produces long stolons and 14.8 mg seed g-lshoot in E rigidior, which produces short geniculate stolons, to 169.1 mg g-1 in the annual T. berteronianus. Seed production was in most species divided over several peaks during the season. Peaks of seed production were observed 3 to 7 months after the onset of the growth season depending on the start of the rains and the life history strategy and growth form of the species. Seed production varied from maxima of 180 seeds m -2 in D. eriantha ssp. pentzii to 47000 seeds m -2 in annual stands of T. berteronianus. Except for annual grasslands with U. panicoides, seedling emergence data reported are smaller by at least a factor of 10 than the observed seed production. Among other factors, a low quality of produced seeds, predation by birds and insects and previous grazing by livestock may have contributed to this difference.

Introduction

The persistence of species and the maintenance of bio- diversity in semi-arid savanna grasslands depends on the annual recruitment by seedlings in annual grass- es and on the survival, and vegetative and generative reproduction in perennial grasses. Grazing by cattle has a marked effect on species establishment and survival and hence on the composition of savanna grasslands, especially during periods of drought, and generally induces a change in the proportion of perennials to annuals (Breman & Ciss6 1977, Noy-Meir et al. 1989, Skarpe 1986, Van Vegten 1981). Annual grasses may be successful in disturbed habitats because of their large reproductive effort and adaptation to unshaded microsites (Bazzaz 1979, Grime 1979, Jackson & Roy 1986) but perennials which reproduce by forming long

stolons or rhizomes may also be able to maintain them- selves under grazing pressure (Tietema et al. 1991). Perennial grass species that depend only on seeds for their regeneration may however be vulnerable to graz- ing, which prevents seed maturation (O'Connor 1991). It can be hypothesized that savanna grasses, which show a wide variation in tiller formation ranging from tufted species to long stoloniferous types, will also vary widely in seed production.

Different phenological types of flowering and seed setting have been proposed for savanna grasses. These range from precocious flowering and early seed set- ting, or delayed flowering and seed setting, to species which show an opportunistic behaviour in phenology in relation to environmental factors such as rainfall and fire (Sarmiento & Monasterio 1983). Relating flower production to seed production must be done cautiously.

92

Dispersal structures often form a considerable propor- tion of the mass of the dispersal unit and are quite often empty, particularly in perennial grasses (Ernst et al. 1992). Observations on flower production only may not be representative as an estimate for actual seed production during the year, particularly if seeds have been aborted.

Savanna grasslands in South Eastern Botswana experience low (long term mean 450-550 mm per annum) and highly variable rainfall (Vossen 1988) and have for several decades been subjected to heavy graz- ing (Arntzen & Veenendaal 1986). Species commonly observed in these grasslands will have reproductive strategies that enable them to survive these conditions. Both annuals and perennials with different growth forms are common (Field 1976). However observa- tions on the timing and quantity of seed production in relation to growth form, which could explain their per- sistence, are thus far completely lacking. This paper therefore focuses on: (A) a description of the seed pro- duction phenology of individual savanna grass species in relation to rainfall and (B) the relation between seed production and life history strategy and plant growth form.

Materials and methods

Ten grass species (Table 1) common to South Eastern Botswana were selected for the study. On the basis of growth form and life history they were categorized into the following groups: (1) tufted annuals or short-lived perennials with a generative reproductive strategy, (2) tufted perennials with erect tillers, (3) stoloniferous perennials with short geniculate or decumbent tillers and (4) perennials with long prostrate stolons (Field 1976 and pers. obs.). Observations were made whether species were mainly found in areas where runoff water collected and whether species were associated with tree canopy cover.

The study site was located in a savanna grassland in an Acacia tortilis - Dichrostachys cinerea savanna (Timberlake 1980) at Diklim communal grazing area, 24 km South East of Gaborone, Botswana. An area of 0.4 ha with a 0-1% slope on a sandy loam soil with a profile depth of at least 1.5 m was fenced at the beginning of the rainfall season to prevent interference with the experiment by grazing cattle. Rainfall was recorded daily at the site with a manual raingauge.

Seed production was monitored throughout the rainfall season of 1988/1989 and 1989/1990. In each

season five plants were randomly selected per species and ripe seeds and dispersal units were collected every other day from each plant. Samples were stored in paper bags and lumped for every first or second half of each month to gain sufficiently large samples for further analysis. "Seed" in this study is used as a syn- onym for caryopsis. At the end of the rainfall season in April/May, or earlier in the case of annuals, the shoots were also harvested. Dry weight of the sam- ples was determined after drying at 80 °C to constant weight.

Well-mixed subsamples of the ripe dispersal units (2-4 subsamples per plant from different times of col- lection, ranging from 0.1-0.5 g) were separated into flower parts and seeds by gently rubbing the samples between two sheets of paper and separating the seeds under a microscope. The total number of fully devel- oped seeds in each subsample was counted. Average seed weight was determined for the samples on an electronic balance (1 mg range). Differences in the number of seeds realised in early and late collections per mass of collected flowers were statistically anal- ysed with Wilcoxon's signed rank test for paired obser- vations after combining data of both seasons. Differ- ences between species in seed mass and the proportion of flowers and seeds produced per shoot mass were analysed with Tukey's HSD procedure. Data on the proportion of flowers and seeds per shoot mass were first logtransformed to reduce heterogeneity of vari- ances. Statistical analyses follow standard procedures outlined in Sokal & Rohlf (1981).

Variation of individual seed weight was analysed by weighing 100 early or late produced seeds of a prolif- ically seed-producing specimen of Aristida congesta, Cynodon dactylon, Eragrostis rigidior, E. superba and Tragus berteronianus collected during the 1988/1989 season. Individual seeds were weighed on a METT- LER ME 30 balance (1 #g range). Similarity of mass distribution in early and late produced seeds was tested with Student's t-test after testing for normality.

The range of seed production for different grassland types per unit area was calculated by measuring shoot biomass at the end of the season in populations of each species at the site. At least 5 randomly allocated 0.25 m 2 plots in the study area were sampled. Samples were dried at 80 °C to constant weight. The maximum and minimum aboveground biomass was used to calculate an estimate for the range of seed production per unit area.

93

Table 1. Life history, growth type, grouping on this basis (see methods) and preference for soil moisture and shade of the investigated savanna grasses.

Species name* Life history Growth** Group Preference** group type number soil moisture shade

Tragus berteronianus Schultes Urochloa panicoides Beanv. Aristida congesta Roemer & Schultes Cymbopogon plurinodis Stapf ex Burtt Davy Eragrostis rigidior Pilger Eragrostis superba Peyr Schmidtia pappophoroides Steudel Cynodon dactylon (L.) Pers. Digitaria eriantha subsp. Pentzii Steud. Panicum coloratura L.

annual annual annual/~erennial perennial perennial perennial perennial perennial perennial perennial

tufted 1 dry open tufted 1 moist shaded tufted 1 dry open tufted 2 dry open geniculate short stolons 3 dry open geniculate short stolons 3 intermediate open geniculate short stolons 3 dry/moist open long stolons/rhizome 4 moist open tufts with long stolons 4 moist shaded/open long stolons 4 moist shaded/open

* Nomenclature follows Gibbs Russell etal. (1985); ** Source Field (1976) and observations at the study site.

Results

Timing of seed production in relation to rainfall

The rainfall pattern (Fig. 1) differed in the two years that seed production was monitored. During 1988/1989 rainfall started in September and total rain for the sea- son was 796 mm, which is at least 250 mm above average. In 1989/1990 the first rain fell in November and the season 's total was 543 mm, which is near the long term average (450-550 mm, Vossen 1988).

During 1988/1989, A. congesta, and T. berteroni- anus (group 1; annuals/short l ived perennials) and E. rigidior, E. superba and S. pappophoroides (group 3; perennials with short geniculate stolons) produced the first ripe dispersal units in early December, 3-4 weeks after flowering (Fig. 1). The tufted perennial Cymbo- pogon plurinodis (group 2) and the long stoloniferous Cynodon dactylon and D. eriantha ssp. pentzii (group 4) produced seeds mostly towards the end of the rainfall season from February onwards. Most species showed more than one period of seed production associated with flowering of primary and secondary shoots.

In 1989/90 fruiting was monitored in ten species (Fig. 1). In T. berteronianus and A. congesta (group 1 species) and E. rigidior, E. superba and S. pap- pophoroides (group 3 species) seed set was delayed compared to the previous year, although the differ- ence of t iming in seed set was at around 4 weeks, less pronounced than the difference of 8 weeks in the onset of rainfall. Two species of group 4, the stoloniferous perennial Cynodon dactylon and the tuft-

ed Cymbopogonplurinodis produced seeds earlier than in 1988/89. The annual U. panicoides and the stolonif- erous producing perennial P. coloratum which were only monitored during 1989/90 produced ripe disper- sal units from late February onwards. As in the previ- ous year, most species showed more than one period of seed production associated with flowering of primary and secondary shoots.

The number of seeds realised per gram flowers was investigated in the species which produced sufficient numbers of seeds on each individual for this analy- sis (Table 2). The number of seeds per gram flow- ers remained constant in the species of A. congesta, T. berteronianus and U. panicoides (group 1). The geniculate perennial species, E. rigidior, E. superba S. pappophoroides (group 3) produced more seeds per gram flowers in the first period of seed setting com- pared to later periods. This was not the case in the tufted perennial C. plurinodis and in 1988/1989 even the opposite

There was a considerable spread in seed mass in the samples collected from individual plants (Fig. 2). In the case of E. rigidior and E. superba, the largest seeds were up to I0 times heavier than the smallest seeds. Except for the E. rigidior specimen, there was a significant (at least p<0 .05 ) increase in the numbers of seeds in the smaller size classes in later, as compared to earlier produced seeds. Mean seed mass is thus not necessarily constant during the season.

94

E

. I

r-,

4Y

E ¢..

o q , ,

O

21)t/q

"°1 1001

5101 -lql [-] _ I I I I I I I I I

1 1 2 1 2 1 2 1 2 1 2 1 2 1 2 100 75 -] Eragrostis rigidior

25

0 I I I [ I I I I I

1 2 1 2 1 2 1 2 1 2 1 1 2 1 2

1~ ~ Eragrostis superba

0 I I I I I I I I I 1

[ ] 88/89

I 89/90

100 75 -] Aristida congesta

25

0 , , , , , , ¢ 1 I I I I I I I I I I

1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2

100-] Tragus berteronianus 75t ~ !

Ill 25

, , j , 0 i i i , , , ' , , 4 1 - , I I i I ~ - i i

loo-]

50

25

0 i | I I I I

1 2 1 2 1 1 2 1 2 1 2 1 2 1 2

Sep Oct Nov Dec Jan Feb Mar Apr

1 1 2 1 2 1 2 1 2 1 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1°°t 75 Schmidt~ papp~horoides 75 -] UrochIoa panicoi~s

50 50

l | l l l l l l l i

1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 100 q

0 , , ~ 7 , ,7 i

1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2 ihi i ' n ' ' ' ' ' ' ' ' " 2 1 2 1 2 1 2 1 2 1 2

Digi~ria erianNa 100 q Panicum colora~m

,1 25

, I i i o l l , i , , , , i ~ U 1 2 1 2 1 2 1 2 1 2 1 2 1 2 1 2

Sep Oct Nov Dec Jail Feb Mar Apr

Calendar month (1= first half; 2 = second half)

Fig. 1. Distribution of rainfall and seed production of ten grass species during the rainy seasons 1988/1989 (I-]) and 1989/1990 ( I ) . Data (average values per plant) are expressed as percentage of total seasonal production of numbers of seeds; n = 5; error bar = 1 standard error).

95

Table 2. Number of seeds per gram flowers in samples collected in early or later peaks of seed production (n = 5).

Species Group Year of Number of seeds per gram collection flowers (-4- S.E.)

first peak second peak

T. berteronianus 1 1988/1989 1445±154 16574-122 1989/1990 15884-65 18334-171 n.sJ

U. panicoides 1 1989/1990 824-23 994-26 A. congesta 1 1988/1989 969+160 9104-257

1989/1990 4864-111 4614-173 n.s. C. plurinodis 2 1988/1989 54-14 954-14

1989/1990 224-9 23-t-8 * E. rigidior 3 1988/1989 11834-302 3764-139

1989/1990 34464-883 6464-155 * E. superba 3 1988/1989 3474-50 1414-44

1989/1990 4354-66 1974-62 *** S. pappophoroides 3 1988/1989 1614-20 814-11

1989/1990 1894-27 1004-11 ***

I indicates signifiant differences between first peak of seed setting and later samples according to Wilcoxon's signed rank test. * p<0.05, *** p<0.005, n.s. = not significant. Data for both years were combined. U. panicoides was not tested due to low sample size.

Reproductive effort

Species with different life history and growth form var- ied widely in individual mass and flower and seed pro- duction (Table 3). Variation in shoot mass was signif- icant (p>0.05) between years in Cymbopogon plurin- odis, with the largest plants harvested in 1989/1990. Flower and seed production was between the two years not significant with the exception of E. rigidior, which produced a larger quantity of seeds in 1989/1990 com- pared to the previous year.

Although there was often overlap between species, the annuals/short lived perennials (group 1) T. bert- eronianus, U. panicoides and A. congesta had the smallest tuft size and realised the largest proportion of their shoot biomass as flowers and seeds ranging from 44.7% (flowers and seeds) or 16.91% (seeds) in T. berteronianus in 1989/1990 to 21% (flowers and seeds) or 1.93% (seeds) in A. congesta in 1989/1990 (Table 3).

The tufted perennial C. plurinodis (group 2) and the perennials with geniculate short stolons (Group 3; E. rigidior, E. superba and S. pappophoroides) generally realised a lower proport ion of their shoot as flowers and seeds, varying between 12.2% (flowers and seeds) or 1.85% (seeds) in E. superba, (1988/1989)and 2.7%

(flowers and seeds) or 0.14% (seeds) in E. rigidior (1988/1989). This latter species produced the smallest seeds and as a result the largest quantities (1.48% in 1989/1990, which results in more than 47000 seeds per individual plant). This species produced in 1989/1990 significantly more seeds than in 1988/1989.

The long stoloniferous species (group 4; C. dacty- lon, D. eriantha ssp. pentzii and P. coloratura) pro- duced the fewest seeds per shoot mass compared to the other species ranging from between 0.03% in P. col- oratum (1989/1990) and 0.003% in D. eriantha ssp. pentzii (1989/1990).

The heaviest seeds (mean weight 1.45 mg) were produced by the annual U. panicoides fol lowed by the tufted perenial Cymbopogon plurinodis. In general, seed mass was not related to life history or growth form.

Seed production on an area basis

The seed production per area is both determined by standing biomass, reproductive effort and seed mass (Table 4). Seed production varied from as high as 4000--47000 in stands of the annual T. berteronianus to as low as 10-180 seeds m -2 on sites with D. eriantha ssp pentzii as the dominating species. Seed produc-

96

Table 3. Average dry mass of shoot, flowers and seeds and flowers and seeds as a proportion of total shoot produced per plant (n = 5) in 10 savanna grasses during two consecutive seasons.

Species Group Year of Mass of shoot, flowers + seeds seeds mean seed mass

collection flowers & seeds (g) (% of total mass) (% of total mass) (mg)

Tragus 1 1988/1989 7.6 ° -4-1.1 38.7 ~ -t- 1.8 12.47 a 4-1.18 0.19 ̀ /4-0.003

berteronianus 1989/1990 8.0°-4-1.0 44.7'~ q-2.8 16.91a+1.83 0.19aq-0.013

Urochloa 1 -

pan±co±des 1989/1990 9.4ab-4-1.4 30.1ab'4-1.8 3.89ab-+-1.31 h45a'4-0.05 Aristida 1 1988/1989 37.2be4-7.1 27.0ab4-1.9 5.75°b-4-0.41 0.18de 4-0.01

congesta 1989/1990 47.9bc+4.7 21.0a~c-t-5.0 1.93b±0.95 0.16'/~ ±0.02 Cymbopogon 2 1988/1989 122.9e±14.5 9.1a~±1.6 0.55bc±0.16 0.73t'±0.05 plurinodis 1989/1990 459.2'/±112.2 5.4a~Y-t- 1.6 0.10~±0.04 0.62b±0.07

Eragrostis 3 1988/1989 218.8a±41.7 2.7f±0.2 0.14c±0.025 0.09~ ±0.01

rigid±or 1989/1990 258.1'/±18.8 6.4d~f 4-0.4 1.48b-4-0.47 0.0Be ±0.002 Eragrostis 3 1988/1989 180.2cd'4-64.3 12.3bcd±l.2 1.19b±0.23 0.42c ±0.03

superba 1989/1990 335.3'/4-68.3 12.2bc`/±1.6 1.85b±0.37 0.45~ 4-0.05

Schmidtia 3 1988/1989 165.7c'/-4-66.4 7.9~`/~ 4-1.1 0.29bc ±0.05 0.47c±0.04 pappophoroides 1989/1990 277.3'/4-58.5 9.5~`/~ 4-1.9 0.40be ±0.07 0.38c±0.02

Cynodon 4 1988/1989 253.0'/4-30.9 3.0f~ 4-0.4 0.008`/4-0.002 0.18*4--

dactylon 1989/1990 358.8`/4-113.1 2.8-f 4-0.4 0.011`/±0.006 0 .14*±-

Digitaria eriantha. 4 1988/1989 331.6~`/+222.3 0.3g 4-0.1 0.004`/4-0.002 0 .26*±- ssp. pentzii 1989/1990 280.8`/±46.6 0.8g 4-0.1 0.003'/4-0.001 0 .27*±-

Pan±cure 4 -

coloratum 1989/1990 405.8`/4-70.3 2.7Y4-0.7 0.03~`/±0.001 0.36~±0.02

* based on one sample and not analysed; values in each column sharing superscripts do not vary significantly (p<0.05) according to Tukey's HSD method: analysis on logtransformed data except for seed mass.

tion was lowest in the grassland dominated by the long stolon-forming (group 4) species, but was also com- paratively low in the annual stands of U. pan±co±des, which produces heavy seeds. Seed production was, with the exception of U. pan±co±des, at least 10 to 100 times higher than emergence of seedlings from the seed bank in 1989/1990 in the same grasslands, as reported by Veenendaal et al. (1996).

Discussion

Life history, growth form and seed production

In recent years attempts have been made to understand selection processes in plants considering larger num- bers of plant traits at the same time, and to formulate adaptive strategies with regard to reproduction and fit- ness (e.g. Grime 1979, Ernst 1981, Venable & Brown 1988). Savanna grass species differ not only in life his- tory but, in the case of perennials, also in vegetative or generative reproductive strategy. Seed size and num-

ber, plant size and growth form may to some extent vary irrespective of life history.

In our study we distinguished a priori 4 differ- ent types in terms of life history and growth form all represented by species which successfully maintain themselves in heavily grazed ecosystems. The annu- als and short-lived perennials produced large quanti- ties of seeds in terms of biomass and number, char- acteristic for semi-arid annual grasslands (De Ridder et al. 1981) and typical of successional species (Harp- er 1967, Jackson & Roy 1986). The second group, tufted perennials without vegetative stolons, was rep- resented by Cymbopogon plurinodis, which produced a much lower quantity of seeds. This species is consid- ered a climax species (Leistner 1976) and this strategy of seed production is sensitive to grazing (O'Connor 1991).C. plurinodis however has chemical defences (vernacular name: "turpentine grass"; Field 1976), that may help it escape heavy grazing. E. rigid±or and S. pappophoroides, species with short geniculate stolons, realise a mostly intermediate seed production and are particularly dominant in our study area. The 4th group

Table 4. Estimated seed production and observed seedling emergence during 1989/1990 in some savanna grassland types

Grassland Group Sprout Production* Emergence** main species biomass (g m -2) (seed m -2) (seedling m -z)

T. berteronianus 1 5-60 4000-47000 24-2920 U. panicoides 1 4-60 200-2400 4-984 A. congesta 1 50-150 11000-33000 12-488 Cym. plurinodis 2 40-500 180-2200 12-140 E. rigidior 3 150~50 13500-40000 0-92 E. superba 3 20-350 800-12600 0-116 S. pappophoroides 3 150-450 1200-3600 no data Cyn. dactylon 4 100-600 50-300 0 D. eriantha 4 50-1200 10-180 0 P. coloratum 4 25~600 80-500 no data

* based on average seed production per g shoot; **source Veenendaal et aL 1996.

97

consists of perennials with long stolons, which produce very few seeds and thus rely on a vegetative reproduc- tive strategy. Seed production as measured is thus in line with the proposed grouping.

Life history, growth form and seed size

Several species in our study show trade-offs in repro- ductive traits in relation to habitat choice. A larger seed size can be an advantage under semi-arid con- ditions (Baker 1972) and in shaded habitats (Venable & Brown 1988, Veenendaal et al. 1993) and corre- lates positively with seedling survival (Veenendaal et al. 1996). The annual U. panicoides growing in shad- ed habitats shows a larger seed size than the annual T. b.erteronianus, a species of generally open, heavily grazed habitats (Ernst & Tolsma, 1988). The perenni- al E. rigidior produces small seeds and particularly in the second year produced more seeds than the peren- nial E. superba. Smaller seed size favours dispersal (Ernst etal. 1992), but is disadvantageous in grassland with few available patches, unless the vegetation is often disturbed (Gross & Werner 1982, Gross 1984). E. rigidior is by far the most dominant grassland species and is a colonizer of abandoned arable lands (Miller & Veenendaal 1991). Once established in a dense sward, it expands through its short stolons. In contrast, E. superba produces only very short geniculate stolons at our study site plant and its re-establishment may rely more on fitness of the seedling within the grassland vegetation, which again would favour larger seed size (Gross & Werner 1982, Gross 1984).

Phenology o f seed production

The phenological classification proposed by Sarmien- to & Monasterio (1983) can to some extent be applied to our savanna grasses. In our study area the onset of the rainfall season is very variable and first rains can occur any time between the beginning of Septem- ber and November as demonstrated in this study. New growth was not observed before the first rainfall at the study site, and elsewhere only if plants had access to water. A precocious strategy would have to rely on water resources and can only be found in some peren- nial geophytes and trees (pers. obs.). In most perennial savanna grasses in this study, ripe seeds were pro- duced 3 to 4 months after the first rains. This could be classified as delayed flowering (Sarmiento & Monas- terio 1983), even though a considerable variation still existed between species. The annuals in our study ger- minate and establish at the onset of the rainfall season (Veenendaal et al. 1996), but their period of fruiting is either extended throughout the season or toward the end and they also can be best classified as having a delayed strategy.

The period of seed production coincides for peren- nials with the peak in nutrient concentrations in the plant shoot (Tolsma et al. 1987, Ernst & Tolsma 1988). Peak growth rates have been reported for various species between the beginning of December and Febru- ary (Randall & Cresswell 1983, Pieterse & Grunow 1985, Danckwerts 1987a, 1987b). While the annu- als continue to produce the same number of seeds per gram of dispersal units throughout their reproductive

98

50 A. congesta • December 0.310 mg

[] March0.253 mg***

10 -

o 0.05 0. I 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6

60- 50- C. dactylon I • December0.120 mg

4 ° ill [] March 0.101mg*** 30-

II 20- 10- 0 , ~ , j i'm ,

0.03 0.06 0.09 0.'12 0.'15 0.'18 0.21 0.24 0.27 0.3 6O 50 E. rigidior • December 0.052 mg

e.-

.~ 40- [ ] March 0.052 mg n.s. e~ 30-

- 1-11qlq 20-

,nl il,q 00100:0.03 0.;4 0. 5 o.b6 0. 7 0.;8 0. 9 0.'12 0.',3

60- 50- E. superba • December 0.367 mg

40" [ ] March 0.309 mg*

20- ~ - ~

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 60- • December 0.199mg 50- T. berteronianus 40- [ ] January0.165 mg***

30-10. _ _ [ ' 7 ~ ~ ~ _ ] 20-

0 I I I I I

0.03 0.06 0.12 0.15 0.18 0.27 013 0.21 0.24 0.09

Seed mass class (rag)

Fig,. 2. Distribution of seed mass in early (December, I-1) and late season samples (March, • or Jan. for T. berteronianus) of selected species (samples in each case taken from the same plant. Seed mass class value is the upper limit of each class; mean 4- S.E.; * p>0.05, • ** p>0.001),

life cycle, the grass species with short stolons appear to reduce the seed production after January, coincid- ing with a decrease in growth rate and a recycling of nutrients.The data on seed mass distribution suggest that mean seed mass may also be reduced towards the end of the season, possibly negatively affecting via- bility and increasing dormancy of late produced seeds (Tolsma 1989).

The vegetative reproduction strategy appears to coincide with mostly late but minimal seed produc- tion. Pieterse and Grunow (1985) report a growth peak for D. eriantha ssp. pentzii and S. pappophoroides lat- er (mid to end of January) than for E. rigidior. A shift towards vegetative production is possibly associated with prolonged growth. Species with such a strategy are in our study area often growing in moister areas where run-off water collects. No data appear to be available on nutrient cycling in these plants.

Seed production per unit area and seedling emergence

Maxima of seed production in our study site varied between 47000 seeds m -2 in annual stands of T. bert- eronianus and 180 in D. eriantha ssp pentzii. Seed production estimates for tropical grasslands dominat- ed by annuals are often of a similar order of magnitude at 103-104 seeds m -2 (Hobbs & Mooney 1986,Mclvor & Gardener 1991, O'Connor & Pickett 1992, Penning de Vries & Djit~ye 1982). The number of seedlings which actually emerge is however mostly lower by at least a factor 10. Few studies actually report soil seed counts, but data collected for our study area during the dry season for T. berteronianus (0-20.000 seeds m -e) and U. panicoides (0-3500 seeds m -2) stands (Ernst et al. 1992) suggest that at times a large proportion of seeds of some species actually reaches the soil seed bank. The discrepancy between production and emer- gence indicates the existence of large seed sinks.

First of all, the seed quality among plants of the same grass species affects germination, with a high drop-out of seeds with a low mass (Ernst & Tolsma 1988, Ernst et al. 1991). Secondly some species (e.g. T. berteronianus) show a prolonged high dormancy (Ernst & Tolsma 1988, Ernst et al. 1992, Veenendaal & Ernst 1991) as is common for many grass species in semi-arid ecosystems (Elberse & Breman 1989, Mott 1978). However the time between seed shedding and the start of the new rainy season is for most species suf- ficient for the loss of dormancy in at least a significant proportion of seeds (Veenendaal & Ernst 1991).

Predispersal infestation by insects and fungi may diminish seed quality of some species (Lock & Mil- burn 1970) and has been observed in Urochloa species in our study area (Ernst & Veenendaal unpublished results). Postdispersal predation of seeds by harvester ants (Marsh 1987, Capon & O'Connor 1990), birds (Fry 1983, Ernst et aL 1992) and rodents is another source of large losses in savanna ecosystems. In our study area harvester ants consume large amounts of seeds of T. berteronianus and to a much lesser degree those of other grasses (Ernst et al. 1992). The post- dispersal effect of fungal attack on seeds present in the soil seed bank remains undocumented in savanna grasslands.

Finally in the present experiment grazing by cattle and wild ungulates was excluded, which would nor- mally have diminished the seed rain. Grazing intensity will affect particularly the reproductive output of palat- able tufted grasses, as demonstrated for several species of two grasslands in South Africa (O'Connor & Pickett 1992). In Botswana heavy grazing by cattle has strong- ly diminished Panicum maximum and Themeda trian-

dra so that grazing pressure may cause local extinction (O'Connor 1991) and thus change the species diversity in savanna grassland.

Acknowledgements

S. Mpoloka, K. Phaatswane and M. Rampha are thanked for their assistance with the analysis of the samples. R.T. Lecha and two anonymous referees are thanked for comments on earlier drafts. This research was sponsored by The Free University Amsterdam, the University of Botswana and SAREC.

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