Journal of Blackwell Science, Ltd Ecology 2002 Effects of plant size on reproductive output and offspring 90, 958–966 performance in the facultative biennial Digitalis purpurea

NINA SLETVOLD Department of Biology, Division of Botany and Plant Physiology, University of Oslo, PO Box 1045 Blindern, N-0316 Oslo, Norway

Summary 1 Monocarpy in facultative biennials can be favoured by selection when there is a more than proportional increase in fitness with size. The possible contributions of larger reproductive output and higher offspring quality to such an increase were investigated in the facultative biennial plant Digitalis purpurea. Straw mass (rosette leaves and flower- ing stem excluding fruits and seeds) was determined for 100 individuals of varying size and correlated with the number of flowers and seeds per flower to examine whether sex- allocation is size dependent. 2 To determine whether maternal size influences offspring growth, and whether this effect is more pronounced in the presence of competition, seeds from 32 maternal plants were grown alone, and with one, two or four competitors. 3 Total seed number increased proportionally with size. Seed quality, measured as seed size, germination percentage and speed also increased with maternal size. The combined fitness measures of total seed mass (seed number × seed size), number of seedlings (seed number × germination percentage) and total female fitness (seed number × germination percentage × mean offspring dry mass after 14 weeks) all increased disproportionally with size. 4 Sex-allocation was size dependent, with large plants emphasizing the female function. 5 Offspring dry mass after both 8 and 14 weeks of growth was strongly influenced by competition, whereas the effect of maternal size was significant only at the first harvest date. Competition did not accentuate the effects of maternal size. 6 The results support that monocarpy exists in D. purpurea because of increased offspring quality rather than a disproportionate advantage in seed output of large individuals. Key-words: delayed reproduction, maternal size-effects, monocarpy, offspring quality, seed size variation Journal of Ecology (2002) 90, 958–966

monocarpic species that can complete their life cycle Introduction within 2 years, but where a delay of reproduction until Individual plant size is usually one of the best predic- the third or later year is commonly observed (Werner tors of current and future reproductive output, and the 1975; van der Meijden & van der Waals-Kooi 1979; relative advantages or disadvantages of reproducing as Gross 1981; de Jong et al. 1986; Klinkhamer et al. a large individual have been broadly discussed (Stearns 1987a, b, 1991; de Jong & Klinkhamer 1988; Campbell 1992). The question of whether there exists an optimal 1997). A monocarpic perennial life-history could be size for reproduction has been of particular interest in of selective advantage when there is a more than pro- the study of facultative biennials (Kachi & Hirose portional fitness gain with size (Schaffer 1974; Schaffer 1985; de Jong et al. 1989; Kachi 1990). These are & Gadgil 1975). This gain could be caused by a more than proportional increase in reproductive output with plant size, or by a higher quality of the offspring pro- Correspondence: Nina Sletvold, Department of Biology, Division of Botany and Plant Physiology, University of Oslo, duced by larger individuals (Stearns 1992). © 2002 British PO Box 1045 Blindern, N-0316 Oslo, Norway (fax 47 22 85 46 Initiation of reproduction in facultative biennial spe- Ecological Society 64; e-mail [email protected]). cies depends on size rather than on age (Lacey 1986),

959 and the individual plant has to reach a certain thresh- reproductive output is proportional to size in most Plant size and old size before any reproduction can be initiated Cynoglossum officinale populations in most years reproduction (Wesselingh et al. 1993). Plants with identical thresh- (Klinkhamer & de Jong 1987; Klinkhamer et al. 1990). old sizes could differ in timing of reproduction, due to A more than proportional increase was found in differences in local habitat quality. Whenever growth is Verbascum thapsus (original data in Reinartz 1984; restricted, due to competition and/or low habitat qual- reanalyses based on population means by Klinkhamer ity, it will take longer to reach the threshold size. The et al. 1990), but a decrease in Carlina vulgaris proximate cause of population variation in timing of (Klinkhamer et al. 1992) shows that, in biennials too, reproduction is, however, often plant-to-plant vari- convincing support is not available. ation in the size requirement for vernalization and/or The evidence of a positive correlation between photoinduction (Wesselingh & de Jong 1995; maternal size and offspring quality is predominantly Wesselingh & Klinkhamer 1996; van Dijk et al. 1997), zoological (Stearns 1992). Many studies on plants and evolution of an optimal size for reproduction will have, however, demonstrated considerable maternal occur through selection on these traits. Delayed repro- influence on seed size and quality. This maternal influ- duction can be caused by selection for an increased ence on seed provisioning/offspring phenotype is medi- threshold size for reproduction, and this will be advant- ated through other mechanisms than nuclear gene ageous when the demographic fitness loss due to transmission, and can be separated into genetic and longer pre-reproductive growth is more than compen- environmental effects. Genetic maternal effects can be sated by the fitness gain achieved by reproducing as a caused by strictly maternal inheritance of mitochon- large individual (Harper 1967; Schaffer & Gadgil 1975; dria, chloroplasts and/or plastids (Roach & Wulff Hart 1977; Silvertown 1983). 1987; Platenkamp & Shaw 1993). Environmental A more than proportional increase in reproductive maternal effects may be caused by several characteri- output with size could arise through a disproportion- stics such as temperature, light, soil nutrient level and ate access to resources by large plants. Alternatively, water availability (Gutterman 1992), as well as levels of large plants may attract more pollinators, and this pollination (Quesada et al. 1993, 1996) or herbivory could result in higher levels of both pollen deposition (Crawley & Nachapong 1985; Sills & Nienhuis 1995). and removal, increasing both female and male repro- Environmental maternal effects on offspring fitness ductive output. Large individuals of monocarpic spe- often seem to be transient (Miao et al. 1991; Wulff & cies tend to emphasize the female function (de Jong & Bazzaz 1992; Schmid & Dolt 1994; Andalo et al. 1999), Klinkhamer 1989), indicating that increased size is but may still have great impact if they influence life more important for seed production than for pollen cycle stages that are crucial for fitness, e.g, probability removal. The higher quality of offspring produced by and speed of germination (Alexander & Wulff 1985; large plants could arise through the same processes as Platenkamp & Shaw 1993). Competition has also been the higher reproductive output. A more effective shown to increase the influence of environmental resource acquisition by a large mother plant could maternal effects (Stratton 1989). The evidence of a pos- result in better provided seeds, particularly so for itive correlation between seed size and germination and monocarpic species that invest all available resources in subsequent seedling growth is substantial (Gross 1984; a single reproductive event. Alternatively, if large Stanton 1984; Wulff 1986a,b; Aarssen & Burton plants receive more pollinator visits per flower, this 1990; Schmid & Dolt 1994; Weiner et al. 1997), but the could lead to higher outcrossing rates and improved relationship between maternal size and seed size and seed quality. The relationship between outcrossing rate quality has received little attention. Klinkhamer & de and plant size is, however, problematic, as larger plants Jong (1987) found no significant relation between may also receive more geitonogamous pollinations maternal size and seed mass or germination percent- resulting in higher selfing rates (Iwasa et al. 1995) and age in the facultative biennial species Cynoglossum fitness loss due to inbreeding depression (Darwin officinale. 1876). The reasons for the evolution of the facultative bien- In contrast to most zoological studies, reproductive nial life-history strategy (Klinkhamer et al. 1997) are output explanations have generally been emphasized in still largely unresolved, and there are few data on the studies of plants. Optimal allocation models have relationship between size, reproductive output and shown that, in the absence of a more than proportional offspring quality in biennial species. In this study, I increase in reproductive output with plant size, the examine the relationship between plant size and optimal life-history is either annual or polycarpic per- reproductive output in the facultative biennial ennial, but never monocarpic perennial (Pugliese 1988; species Digitalis purpurea L., to address the following Iwasa & Cohen 1989). The empirical evidence of any questions: such disproportionate increase in reproductive output • Is there a more than proportional increase in repro- with size is, however, mixed (Samson & Werk 1986). ductive output with size in this species? © 2002 British Ecological Society, Decreasing, constant and increasing relationships have • Is offspring quality (measured as seed size, germina- Journal of Ecology, been found in both annuals and perennials, but data for tion percentage, germination speed and offspring size) 90, 958–966 facultative biennials are relatively rare. However, positively related to the size of the maternal plant?

960 • Does this result in a disproportionate increase in ing period. After seed ripening in August 1998, the N. Sletvold three estimates of female fitness (seed number × seed height of the flowering stem was recorded together with size; seed number × germination percentage; and seed the total number of capsules produced (which was number × germination percentage × offspring size) equal to the number of flowers). All above-ground with maternal plant size? parts of the 100 experimental plants were brought to • Does competition accentuate maternal size-effects the laboratory. One of the 10 lowermost capsules on on offspring performance? each plant was collected for later seed germination tests in a growth chamber. This sampling was restricted to one of the lowermost fruits to minimize the influence of Materials and methods seasonal variation in fruit/seed size. All remaining parts of the plants were dried at 80 °C for 72 h, and   then weighed. The term ‘straw mass’ was used for the Digitalis purpurea L. is a facultative biennial herb, total mass of rosette leaves and flowering stem exclud- which is widespread in coastal areas of south-western ing fruits and seeds. The total mass of the fruits with Norway. Large populations are found in areas where seeds constituted the reproductive mass. The seeds the vegetation has been disturbed, e.g. in roadcuts, were dissected from the capsules and weighed separ- woodland clearings and pasture-land. The species has ately. A random sample of 100 seeds from each plant a persistent seed bank, from which abundant germina- was bulk-weighed and divided by 100 to estimate mean tion and plant establishment take place after soil dis- individual seed mass. This was used to calculate the turbance (van Baalen 1982). Individuals form basal total number of seeds produced per plant. One plant rosettes that usually produce a flowering stem during failed to develop any seeds and was excluded from the the second summer, but also have the ability to delay analysis. flowering for several seasons (van Baalen & Prins 1983). The species is usually monocarpic, but a small     percentage of the plants may survive flowering (Nina Sletvold, unpublished data). The pink to purple The capsules collected for germination tests were air- protandrous flowers are produced on terminal dried at 25 °C for 48 h, and stored at 4 °C until Febru- racemes, whose flowers open sequentially from the bot- ary 1999. One hundred seeds from each individual were tom upwards. The male phase of each flower lasts on then weighed and sown in a standard soil mixture in average 3 to 4 days, and the stigma opens on the 5th or 1.5-L pots in a greenhouse. The temperature was 20 °C/ 6th day, giving a day in between where the flower is 15 °C, respectively, during 18 h day/6 h night. Addi- functionally sterile (Percival & Morgan 1965). At any tional light was supplied during the day. Germination given time, a flowering stem thus consists of a number was scored when the radicle elongated, and germinated of female-phase flowers at the bottom, a few neuter seeds were removed from the pot. Recordings were per- flowers above them, and then the male-phase flowers formed every second day for 3 weeks, except in two pots followed by buds. There is a seasonal decline in flower that were removed after 1 week when a mould infection size (Percival & Morgan 1965) and fruit/seed size (N. interfered with germination. Sletvold, unpublished data), with the largest, early ini- To determine whether maternal size-effects on off- tiated flowers/fruits at the bottom of the inflorescence. spring performance decline over time, and whether Plants are completely self-compatible and primarily competition accentuates the maternal effects, a com- visited by bumblebees (J.M. Grindeland, unpublished petition experiment was set up. Thirty-six maternal data). Maximum seed dry mass is attained 36 days plants covering the total range of size variation (straw after flowering (Hay & Probert 1995), then remains mass 0.60–39.4 g; seed size 40–115 µg) were selected as constant until capsule dehiscence. The average number focal individuals. From each of these plants, seeds were of seeds per capsule is 1029 (Salisbury 1942), with aver- randomly chosen and sown at four different competi- age individual seed mass 0.090 mg. tion intensities (one focal seed in each pot): (1) alone, (2) with one competitor, (3) with two competitors, and (4) with four competitors. This yielded a total of 144     different maternal plant × competition treatment com- This study was conducted in Ulvik (Hordaland binations, each of which was replicated 12 times. One County) in south-western Norway. In a pasture-land hundred seeds from each non-focal maternal plant in population of D. purpurea, five size categories of repro- categories IV and V were pooled and constituted the ductive plants were identified based on their rosette subset from which competitors were randomly drawn. diameter (I, 0–19 cm; II, 20–29 cm; III, 30–39 cm; IV, In January 2000, the experiment was set up in a growth 40–49 cm; V, > 50 cm). The first 20 randomly selected chamber with 18 h daylight, at temperatures 20 °C plants in each size category were included in the study, (day) and 15 °C (night). All seeds were sown in © 2002 British × × Ecological Society, yielding a total of 100 individuals. Rosette diameter 7 7 7 cm pots filled with standard soil mixture, and Journal of Ecology, ranged between 12 cm and 67 cm. All plants were the pots were arranged in 12 blocks with one of the 12 90, 958–966 tagged in June 1998, at the very beginning of the flower- replicates of each treatment combination randomly

961 assigned to each block. Blocks were rotated once a used to compare the relative importance of the different Plant size and week. No seeds germinated from four of the maternal factors. reproduction plants, and these plants were excluded from analyses. The 32 remaining maternal plants yielded from 3 to 12 Results surviving replicates. A total of 88 treatment combina- tions (22 maternal plants) had eight or more surviving   replicates after 6 weeks of the experiment. To evalu- ate possible changes in maternal size-effects over the Rosette diameter, the measurement used for estimating course of the experiment half of the replicates (from size in the field, was strongly correlated with total dry

four to six) from these plants were harvested 8 weeks mass (straw plus reproductive), rs = 0.94, P < 0.0001, n into the experiment. The remaining replicates, together = 100. Straw dry mass ranged from 0.43 g to 42.5 g, with the 10 maternal plants which at the 6-week stage with a mean of 9.6 g ± 10.1 (mean ± SE), and reproduc- had less than eight replicates, were harvested at the end tive dry mass ranged from 0.067 g to 24.1 g, with a of the experiment, after 14 weeks of growth. All above- mean of 5.4 g ± 5.4 (mean ± SE, n = 100). ground parts were dried at 80 °C for 72 h and then weighed.  ,           The total number of flowers produced by an individual All statistical analyses were conducted with SPSS 9.0 ranged from 3 to 164, with a mean of 55 ± 43 (mean ± (SPSS 1996). Correlations between variables were SE, n = 99). Most of the variation in number of flowers quantified as the Pearson product moment correlation produced was accounted for by straw mass (Fig. 1a).

coefficient (rp) when assumptions on bivariate normal The regression coefficient of the log-log relationship distributions were met, in all other cases the Spearman was significantly less than one (b = 0.75, P < 0.0001),

rank order correlation coefficient (rs) was used. which means that small plants produced more flowers The effects of plant size (X) on reproductive traits per unit of straw mass than large plants. (Y) were analysed using the model Y = aXb (model 2 in The mean number of seeds per flower varied sub- Klinkhamer et al. 1992). This model becomes linear stantially between plants, ranging from 58 to 969 seeds after log transformation: log Y = log(a) + b log(X). per flower, with a mean of 497 ± 184 (mean ± SE). Linear regression was used to determine the regression There was a significant positive correlation between coefficient b. If there was a linear relation with b equal mean number of seeds per flower and straw mass,

to one after the transformation, a linear relation rs = 0.76, P < 0.0001, n = 99. through the origin was present in the original data The phenotypic femaleness (Gp) of individuals (Klinkhamer & de Jong 1987), and the relative invest- varied from 0.10 to 0.64. There was a significant posi-

ment in Y was proportional to plant size. If b < 1, tive correlation between straw mass and Gp, rs = 0.76, investment in Y decreases with increasing plant size, P < 0.0001, n = 99. whereas b > 1 implies the reverse pattern. Deviations from unity of the regression coefficients were tested       with the t-test. Standardized phenotypic femaleness of individuals The total number of seeds per plant ranged from 291 to

was calculated as Gp = (di/li)/[E + (di/li)], where E more than 140 000. Mean seed output per plant was ± ± measures the ratio of maternal (di = number of seeds) to 32 957 32 213 (mean SE, n = 99). Most of the vari-

paternal investment (li = number of flowers) in the ation in total number of seeds per plant was accounted Σ Σ whole population (E = di/ li) (Lloyd & Bawa 1984). for by straw mass (Fig. 1b). The regression coefficient Gp varies between 0 and 1 and relates individual spe- for the log-log relation did not deviate significantly cialization in one sex to the average ratio in the whole from one (b = 1.03, P = 0.4), and seed number thus population. increased proportionally with straw mass. Maternal size-effects on offspring performance in Mean mass of individual seeds per plant varied relation to competition intensity were analysed using more than threefold, from 0.036 mg to 0.111 mg, with an analysis of variance model. Size of the offspring at a mean of 0.082 mg ± 0.015 (mean ± SE). There was harvest at the two different dates was analysed with a significant positive correlation between straw mass

, with competition treatment as a fixed factor, and mean individual seed mass, rs = 0.55, P < 0.0001, block as a random factor and maternal straw mass as a n = 99. covariate. The interaction term between treatment and Total seed mass per plant ranged from 0.031 g to maternal mass was included to see whether competi- 12.8 g, mean total seed mass was 2.91 g ± 2.98 (mean ± tion accentuated the maternal size-effects (Model: SE, n = 99). Variation in straw mass also accounted for © 2002 British Ecological Society, Plant dry mass = Treatment + Block + Maternal mass most of the variation in total seed mass in a log-log 2 Journal of Ecology, + Treatment × Maternal mass). ‘Effect sizes’ (η = SS regression (Fig. 1c). The regression coefficient was 90, 958–966 factor/[SS factor + SS residual]; Cohen 1977) were significantly larger than one (b = 1.12, P = 0.0007), 962 N. Sletvold

Fig. 1 The relation between straw dry mass and several components of reproductive success, all on a log-log scale. (a) Number of flowers, linear regression: y = 0.75x + 1.05, r2 = 0.95, P < 0.0001, n = 99. (b) Total number of seeds, linear regression: y = 1.03x + 3.51, r2 = 0.90, P < 0.0001, n = 99. (c) Total mass of seeds, linear regression: y = 1.12x − 0.65, r2 = 0.91, P < 0.0001, n = 99. (d) Seedling number (calculated as seed number × germination percentage), linear regression: y = 1.30x + 5.14, r2 = 0.85, P < 0.0001, n = 97. (e) Seedling number × size (calculated as seed number × germination percentage × mean offspring dry mass after 14 weeks), linear regression: y = 1.25x + 5.33, r2 = 0.84, P < 0.0001, n = 32.

demonstrating that total seed mass increased more seedlings was accounted for by straw mass (Fig. 1d). than proportionally with straw mass. The regression coefficient for the log-log relation was signi- ficantly larger than one (b = 1.30, P < 0.0001, n = 97).    Germination was generally highly synchronous. The first seeds germinated on the 4th day of the experiment, Dry mass of offspring after both 8 and 14 weeks of and germination was registered in 75 of the pots on this growth was strongly influenced by competition date, with 50% germination reached, on average, in treatment (Fig. 2, Table 1). Maternal straw mass 6 days. By the end of the experiment a total of 85 pots had a significant effect only at the first harvest date had reached more than 90% germination. The number (Table 1). The effect size of the competition treat- of germinated seeds after 1 week was positively correlated ment doubled from 8 weeks to 14 weeks, whilst the

with both straw mass (rs = 0.52, P < 0.0001) and mean effect size of block identity was halved (Table 1).

individual seed mass (rp = 0.47, P < 0.0001, n = 97). There was no significant interaction between com- The total germination percentage was also very high. petition treatment and maternal straw mass at either of It ranged from 16% to 100%, with a mean of 94.4%, the two harvest dates (Table 1), demonstrating that n = 97. There was a positive correlation between competition did not accentuate the maternal influence total germination percentage and both straw mass on offspring size.

(rs = 0.37, P < 0.0001, n = 97) and mean individual seed The total female fitness of maternal plants measured mass (r = 0.37, P < 0.0001, n = 97). as number of seeds × germination percentage × mean © 2002 British s Ecological Society, The number of seedlings produced, estimated as number offspring dry mass after 14 weeks varied more than Journal of Ecology, of seeds × germination percentage, ranged from 22 116 100-fold (from 140 537 to 18 527 943). Most of the 90, 958–966 to 13 618 800. Most of the variation in number of variation in total fitness was accounted for by straw Table963 1  of the effects of treatment, maternal straw mass, block and treatment ×Plant maternal size strawand mass on offspring dry mass after 8 and 14 weeks of growth. η2 is the effectreproduction size

Source of variation SS d.f. FP η2

8 weeks Treatment 2.19 3 13.55 < 0.001 0.077 Maternal mass 0.23 1 8.03 0.042 0.008 Block 2.16 5 4.17 < 0.001 0.076 Treatment × maternal mass 0.008 3 0.50 0.685 0.003 Residual 26.35 489 14 weeks Treatment 279.35 3 44.00 < 0.001 0.159 Maternal mass 1.01 1 0.48 0.491 0.001 Fig. 2 Plant dry mass in relation to competition intensity at Block 61.55 11 2.64 0.003 0.040 the two harvest dates; individuals grown alone and with one, Treatment × maternal mass 5.28 3 0.83 0.477 0.004 two or four neighbours. Residual 1477.19 698 reproductive success (seed production). The pollen export model they developed did, however, show that mass (Fig. 1e). The regression coefficient of the log-log large plants would suffer more through geitonogamous relation was significantly larger than one (b = 1.25, pollinations, and supported the view that male fitness is P < 0.0001, n = 32). a decelerating function of reproductive investment (Devlin et al. 1992). In D. purpurea, geitonogamy is uncommon due to sequential, protandrous flowering Discussion combined with upwards pollinator movements (Best & This study demonstrates that large individual size in Bierzychudek 1982), and the fact that most individuals facultative biennial plants may be associated with a in natural populations only have one flowering stem more than proportionate increase in fitness due to a (N. Sletvold, personal observation). In a field study in higher quality of the offspring produced. Estimates of this population during the summer of 1999 less than total female fitness based on seedling number and size 4% of the within-individual movements were in the increased more than proportionally with maternal ‘wrong’ direction, i.e. allowed self-pollen transfer to a plant size in Digitalis purpurea. Although the positive fertile stigma (J.M. Grindeland, unpublished data). effects of increasing seed mass on fitness traits of the The model of Iwasa et al. (1995) thus seems inappro- seedling are well documented (Gross 1984; Stanton priate for this species, and visitation rate to male phase 1984; Wulff 1986a,b; Schmid & Dolt 1994), this pos- flowers should be a fairly direct measure of pollen sible advantage of a larger plant size has rarely been export. Even though total number of flowers visited considered in studies discussing the evolution of the increased with plant size in the 1999 study, the propor- biennial life-history strategy. tion of male phase flowers visited decreased. A bumble- bee starting on one of the lowermost female flowers would often not reach any male flower at all before   ending the foraging bout. These results suggest that the Monocarpy in facultative biennials has usually been relationship between size and pollen removal in D. pur- explained in terms of increasing reproductive output purea is decelerating rather than accelerating. with size. In this study no disproportionate increase in reproductive output with plant size could be detected   for D. purpurea. The regression coefficient for the log- log relation did not deviate significantly from one, and There are few data on possible size-effects on offspring seed number increased strictly proportionally with quality in facultative biennials. Klinkhamer & de Jong straw mass. This result is in accordance with most other (1987) found no positive relationship between maternal studies, where the relationship has either been decreas- size and seed mass or germination success in Cynoglossum ing (Klinkhamer et al. 1992) or varying (between years officinale, and the relation has been overlooked in most in Cynoglossum officinale: Klinkhamer & de Jong 1987; other studies. In Digitalis purpurea, seed mass, as well de Jong & Klinkhamer 1989; Klinkhamer et al. 1990; as probability and speed of germination, was positively between populations occupying different habitats in correlated with maternal plant size. Estimates of total Centaurium erythraea: Schat et al. 1989). In this context female fitness of maternal plants covering the period up it is somewhat surprising that the possible existence of to offspring age of 14 weeks all increased more than seed quality differences has received so little attention. proportionally with size. In view of the optimal alloca- © 2002 British Ecological Society, Iwasa et al. (1995) suggested that the reproductive tion models that have been developed (Pugliese 1988; Journal of Ecology, advantage of increasing size could be mediated more Iwasa & Cohen 1989), a monocarpic perennial life his- 90, 958–966 through male (pollen export) than through female tory may therefore be optimal under these conditions. 964 If a prolonged period of vegetative growth enables the compensated by producing more seeds per flower. This N. Sletvold plant to produce better-provided seeds, translating into pattern demonstrates size dependent sex-allocation higher total fitness of the plant, this may select for (Charnov 1987). The calculated phenotypic femaleness monocarpy and delayed reproduction. of individuals (Lloyd & Bawa 1984) was positively cor- It was pointed out by Young (1990) that size differ- related with size, meaning that small plants invested ences in reproductive success in Mount Kenya Lobelia more in male function (pollen), whereas larger plants species was likely to be the result of a correlation invested more in female function (seeds). This is similar between plant size and habitat quality, and conse- to what is found in many animal-pollinated mono- quently had no relevance for life-history evolution. carpic plants (e.g. Kelly 1984; Klinkhamer & de Jong If the fitness advantage of large D. purpurea plants 1987; de Jong & Klinkhamer 1989), and seems to indi- only reflects good growing conditions, the same would cate that male fitness levels off more rapidly with size be true for this species. There was, however, no obvi- than female fitness. ous environmental heterogeneity present in the study To conclude, this study has shown that maternal size population, and large size differences were common variation can influence offspring quality in a way that among neighbouring reproductive plants. In addition, may select for monocarpy in facultative biennial spe- an even larger amount of size variation in reproductive cies. The inclusion of possible effects of plant size on plants was found in a common garden experiment seed quality and offspring performance in studies of where all plants were grown in a uniform environment facultative biennials may bring new insight into the (N. Sletvold, unpublished data). selective pressures acting on life-history variation in The non-germinating seeds in this experiment were these species. regarded as dead. Van Baalen (1982) found that soil contact induces an obligatory light requirement for Acknowledgements germination in seeds of D. purpurea, but that after experimental light treatment nearly all seeds will ger- I would like to thank J.M. Grindeland, I. Nordal and J. minate. D. purpurea is consequently not believed to Ågren for critically reading the manuscript. Valuable have any innate dormancy, and all viable seeds should comments from L. Haddon, P.G.L. Klinkhamer and germinate immediately under favourable conditions. two anonymous reviewers also improved the manu- Competition intensity had a large impact on off- script. This research was financially supported by the spring size both after 8 and 14 weeks of growth, but as Norwegian Research Council (grant no. 123845/410) there was no significant interaction term with maternal as a part of a PhD programme, and by the Nansen straw mass at either of the two dates, competition did Foundation (grant no. 73/99). Fieldwork was sup- not exacerbate the effects of maternal size. This result is ported by the University of Oslo (Foundation of Pro- similar to that obtained by Weiner et al. (1997), who fessor Rathke). examined the effects of seed size on offspring growth in Centaurea maculosa at different levels of competition. References However, in several other studies including some sort Aarssen, L.W. & Burton, S.M. (1990) Maternal effects at four of stress, offspring growth variation originating from levels in Senecio vulgaris (Asteraceae) grown on a soil nutri- differences in seed size have been maintained and even ent gradient. American Journal of Botany, 77, 1231–1240. increased in later life stages (Parrish & Bazzaz 1985; Alexander, H.M. & Wulff, R. (1985) Experimental ecological Wulff 1986b; Stratton 1989). The absence of an inter- genetics in Plantago. X. 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