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Floral longevity and reproductive assurance: seasonal patterns and an experimental test with Kalmia latifolia (Ericaceae)¹

Department of Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, Michigan 48109-1048 USA

Received for publication November 22, 2002. Accepted for publication April 18, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Floral longevity is assumed to reflect a balance between the benefit of increased pollination success and the cost of flower maintenance. Flowers of Kalmia latifolia (Ericaceae), mountain laurel, have a long duration and can remain viable up to 21 d if unpollinated. I experimentally tested whether this long duration increases pollination success by clipping stigmas to reduce functional floral longevity to 3–4 d. Clipping stigmas decreased fruit set from 65% to only 10%. Flowers with natural life spans were not pollination-limited, demonstrating that long floral duration ensured female reproductive success. The long floral duration of K. latifolia was unique in this site (the Great Swamp, Rhode Island, USA). Coflowering shrubs in summer had a mean floral life span of 3.4 d. Spring-flowering species had significantly longer mean floral durations (7.2 d). These duration differences may reflect seasonal variation in pollinator availability. However, K. latifolia flowers in summer, when its bumble bee pollinators are abundant but it is a poor competitor for bees because its flowers produce little nectar. The long floral duration allows K. latifolia to outlast coflowering competitors and attract sufficient pollinators. I hypothesize that the long floral duration of K. latifolia functions as a mechanism for competitive avoidance and reproductive assurance.

Key Words: competition for pollinators • competitive avoidance • Ericaceae • floral longevity • flowering phenology • fruit set • Great Swamp, Rhode Island • induced floral senescence • Kalmia latifolia • pollination limitation • reproductive assurance

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
In pollination studies, much attention has focused upon floral traits that assure successful pollination by increasing the attraction or rewards for animal pollinators (e.g., Waser, 1983 ; Bell, 1985 ; Real and Rathcke, 1991 ; Caruso, 2000 ) or by increasing the effectiveness of pollen transfer (e.g., Campbell et al., 1996 ). More recently, it has been recognized that floral longevity (the length of time a flower remains open and functional) is a trait that could ensure successful pollination in habitats where pollinators are sparse or uncertain (Primack, 1985 ; Ashman and Schoen, 1994 , 1996 ; Khadari et al., 1995 ). For example, plants growing in alpine habitats, which typically have few, unpredictable pollinators, have longer floral durations than plants at lower elevations with more abundant, predictable pollinators (Arroyo et al., 1981 ; Stratton, 1989 ; Bingham and Orthner, 1998 ; Blionis and Vokou, 2001 ; Blionis et al., 2001 ). Ashman and Schoen (1994 , 1996 ) have proposed a mathematical model in which the duration of floral life is determined by a balance between female fitness accrual rates (pollen receipt), male fitness accrual rates (pollen dissemination), and the cost of flower maintenance (see also Charnov, 1996 ). By measuring pollen deposition and removal and estimating maintenance costs, Ashman and Schoen (1994) found a significant positive correlation between floral longevities observed in the field and predicted floral longevities for 11 plant species. However, they note that there was not a close quantitative fit and that environmental factors, including pollination, may influence floral longevity in the field. Also, they did not measure effects on fruit or seed set. No experimental tests have demonstrated that longer durations of floral life increase fruit or seed set of flowers in the field.

Here I present an experimental test of the pollination benefit of long floral duration for mountain laurel, Kalmia latifolia L. (Ericaceae). Flowers can remain viable for 2–3 wk if unpollinated, and they rapidly senesce after pollination (Rathcke, 1988a ; Nagy et al., 1999 ). I reduced the functional longevity of flowers by clipping stigmas and then compared fruit sets (fruits initiated per number of flowers) of experimental and control flowers. I also did procedure controls, and I tested whether control (naturally pollinated) flowers were pollination limited by augmenting flowers with cross-pollen. I compare the floral longevity of K. latifolia with other shrub species growing in the same habitat and for different flowering seasons (spring and summer). I discuss possible reasons for the maintenance of long floral duration in this species.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
This study was done in the Great Swamp, Rhode Island, USA (41°29' N, 72°32' W) (for a description of the site, see Rathcke [1988a , b ]). Kalmia latifolia, mountain laurel, is a tall evergreen shrub that grows in the understory of forests and on mountains in eastern North America. Shrubs have tubular, pink-white flowers displayed in inflorescences (corymbs) of 10–300 flowers, and individual shrubs may have thousands of flowers open at the same time. Bumble bees were the only flower visitors and pollinators of K. latifolia (Rathcke, 1988a , b ). In this low elevation habitat, flowers do not self-pollinate (Rathcke, 1988a ), although flowers can self-pollinate at the end of flower life in a montane habitat in Virginia (Rathcke and Real, 1993 ; Levri, 1998 ; Nagy et al., 1999 ). Fruits are dry capsules with hundreds of tiny seeds.

Reduction of floral duration
For the experiment, I tagged four budded inflorescences on each of 14 shrubs. For each shrub, two inflorescences were assigned to controls and two were assigned to experimental treatments. For each assigned inflorescence, new flowers (in anthesis) were tagged with color-coded yarn every 2 d throughout the flowering season (12–29 June). To reduce the functional floral longevity for female function, I clipped the stigmas of all flowers after they had been receptive (glistening and sticky) for 3–4 d. I chose 3–4 d because most other shrubs that coflowered with K. latifolia in June had floral longevities of 3–4 d (Rathcke, 1988b ). Stigmas were clipped with scissors just below the sticky surface, and the scissors were sterilized with ethanol after each use. Experimental flowers and control flowers were exposed to natural pollination, and subsequent fruit sets were measured and compared.

I also did a procedural control treatment to ascertain whether stigma clipping harmed flowers or normal fruit development. I augmented other flowers with cross-pollen and clipped their stigmas the following day. To determine whether stigma clipping would preclude subsequent pollination and fertilization as assumed, I clipped stigmas of virgin flowers and immediately added pollen to the remaining styles. Fruit sets were compared between these treatments and controls.

Fruit set
Fruit set was calculated as the percentage of flowers that initiated fruits. Percentage fruit set was calculated for each inflorescence, and a mean was calculated for each individual shrub for each treatment. Fruit initiation after a month of development was assumed to indicate that flowers had been successfully pollinated and fertilized. Previous studies showed that little fruit abortion occurs after initiation, so these values also closely reflect final fruit set (Rathcke, 1988a ). Seeds were not measured.

Pollination limitation
To test for pollination limitation of fruit set of flowers, I compared the fruit set of naturally pollinated flowers to the fruit set of flowers augmented with cross-pollen. In the pollen-augmentation treatment, all open flowers on one inflorescence on each of the experimental shrubs were augmented every 2 d with cross-pollen mixed from four other distant shrubs. The pollination limitation of entire plants was not determined because each shrub typically has thousands of flowers. It is known that augmented pollination of a subset of flowers on a plant may influence the fruit set of other flowers or may reduce fruit set in subsequent years because of resource reallocation (Zimmerman and Pyke, 1988 ). However, the experimental flowers in these shrubs were usually <1% of the total flowers on a shrub and seem unlikely to have strongly affected the fruit set of other inflorescences.

A relative index of pollination limitation (PL) was calculated based on fruit sets (FS) of control flowers with natural longevity (control) and of experimental flowers with shortened longevity (experimental) as follows:

(1)

If the fruit sets of control and experimental flowers are equal, then PL = 0%. If fruit set were zero for experimental flowers and 100% for control flowers, then PL would equal 100% (Rathcke, 2000 ).

Comparisons of floral longevities
In a previous study, longevities of bagged flowers were measured for other shrub species growing in this site (Rathcke, 1988b ). Flowers were bagged with bridal veiling to exclude pollinators, and flowers were individually tagged, color-coded by day of opening, and monitored daily until flowers senesced. For five of these species, the floral longevities of naturally pollinated flowers were also measured and compared with that of unpollinated flowers (Rathcke, 1988b ).

For this study, the mean floral longevities of spring and summer flowering shrubs were compared to each other and to K. latifolia. To test whether the floral longevity of K. latifolia reflected a trait shared with other species in the same family (Ericaceae), the same comparisons were made using only ericaceous shrubs.

Statistical analyses
Differences in fruit sets among different treatments were tested with nonparametric Friedman's paired-sample sign tests because percentage data are not distributed normally and because individual shrubs can differ in fruit set for other reasons and can be considered blocks. In this test, the ranking in one block (plant) is independent of the ranking in another block (Sokal and Rohlf, 1981 ). Tests are based on plants (blocks), not flowers, although the numbers of flowers are shown. Differences in floral longevities of different species were tested with Student's t tests because the data met the assumption of homogeneity of variances based on Bartlett's tests. Statistical analyses were done using SYSTAT, version 5.01 (Systat Inc., Evanston, Illinois, USA).

	RESULTS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

 
Floral longevity experiment
Flowers with reduced functional floral longevity had only 10% fruit set, whereas control flowers with natural longevities had 65% fruit set (Table 1). Reduced floral longevity resulted in 85% pollination limitation of fruit set. Stigma clipping did not harm flowers or fruit development. The mean fruit set of flowers that were hand-pollinated and then clipped on the following day was not significantly different from the mean fruit set of hand-pollinated, unclipped flowers (Table 2). Stigma clipping precluded subsequent pollination and fertilization as assumed; stigmas of virgin flowers that were clipped and the styles pollinated produced no fruit (Table 2).

	DISCUSSION

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Although pollinator abundance is lower in spring, the longer floral life spans of spring-flowering shrubs could also reflect cooler temperatures, which may reduce metabolism and slow development. Arroyo et al. (1981) reported that mean longevity of flowers was 4.1 d at a low elevation site (2320 m asl) and 9.0 d at a high elevation site (3550 m asl) in the Andes, and they attributed the difference to slower floral development at cooler temperatures. However, cooler temperatures could be a proximate cue for the ultimate factor of low pollinator availability. In either circumstance, K. latifolia again is an exception because it blooms during the warmer days of June but has long floral duration.

The long floral duration of K. latifolia is not a trait shared by other shrubs in the same family (Ericaceae) that were growing in this site. Kalmia latifolia has greater floral longevity than other ericaceous species that coflower in summer, including one congeneric species, K. angustifolia (Table 6). Although the mean floral longevity (8 d) of K. latifolia was not significantly different from spring-flowering Ericaceae, the maximum floral longevity for K. latifolia, up to 21 d, was much longer than the floral maximum of any other shrub in the community.

The long floral duration of 21 d in K. latifolia is unusual for plant species in general. Floral longevities of 2 wk or longer have been reported for relatively few species. These species include some orchids (Primack, 1985 ), two fig species (Khadari et al., 1995 ), and some wind-pollinated species (Primack, 1985 ), and their long flower durations have been attributed to low probabilities of pollination (Primack, 1985 ). Most plant species have floral longevities of 2–4 d (Primack, 1985 ; Stratton, 1989 ). Tropical species often have floral longevities of only 1 d, possibly because of high rates of predation or bacterial and fungal decay (Primack, 1985 ). Risk of disease has been proposed to shorten floral longevities (Shykoff et al., 1996 ). In this study, neither flower predation nor infection of flowers was evident for any of the species, although fungal pathogens on leaves have been shown to increase fruit abortion and affect mating in K. latifolia in Virginia (Levri and Real, 1998 ). The apparent lack of floral predation or disease may allow long floral duration of K. latifolia in this site.

What is a possible explanation for the long floral duration of K. latifolia? I propose that long floral duration is advantageous because K. latifolia is a poor competitor for pollinators and that it is a mechanism for competitive avoidance. Flowers of K. latifolia produce little nectar and are infrequently visited by bumble bees when other shrub species are flowering (Rathcke, 1988a ). Bumble bees are the only pollinators of K. latifolia in this habitat, and they preferentially visit species with higher nectar rewards. The long floral duration of K. latifolia increases the probability that flowers will be pollinated after competing species cease flowering (Rathcke, 1988a ). Because unpollinated flowers remain viable, they extend the flowering phenology of K. latifolia into a competition-free period. In addition, long floral duration may provide pollination assurance when pollinators are scarce and allow K. latifolia to tolerate conditions of low pollinator activity. Such tolerance of low pollinator activity may be an alternative strategy to having greater attraction or rewards and greater competitive ability for pollinators (Rathcke, 1988a ).

Species that tolerate low resources have been termed "good response competitors" in contrast to "good effect competitors" that can deplete resources effectively (Goldberg, 1990 ; Miller and Travis, 1996 ). The ability of K. latifolia to tolerate low rates of pollinator visitation and its inability to compete effectively for pollinators make it a good response competitor. Why is K. latifolia a good response competitor, rather than a good effect competitor for pollinators? Higher nectar production per flower would make it more competitive with coflowering species (Rathcke, 1988a ). However, greater nectar rewards would probably cause bees to forage longer within individual shrubs and would promote geitonogamy (within-plant pollination) and selfing (Rathcke and Real, 1993 ). Because K. latifolia shows strong inbreeding depression with selfing, a strategy of higher nectar rewards could be detrimental to both female and male reproductive success (Rathcke, 1992 ; Rathcke and Real, 1993 ). Higher nectar production would also be costly in resources. Other traits could also increase attractiveness (Waser, 1983 ; Caruso, 2000 ), but higher rewards would be necessary to increase the ability of K. latifolia to compete effectively for pollinators.

Another benefit of long floral duration can be to increase the size of the floral display and attract more pollinators (Ishii and Sakai, 2001 ). However, large floral displays can also increase geitonogamy (within-plant pollination), which would be detrimental to K. latifolia because of strong inbreeding depression (Rathcke and Real, 1993 ). The cost from geitonogamy can be reduced by pollination-induced senescence, which removes flowers from the display when they are pollinated. Flowers of K. latifolia show pollination-induced senescence; corollas wilt and fall the day after they are successfully pollinated (Rathcke, 1988a ). Pollination-induced senescence can also direct pollinators to viable flowers and increase the effectiveness of pollinator visits (van Doorn, 1997 ). In addition, flexible floral longevity can decrease transpirational water loss and energy costs of maintaining flowers (Primack, 1985 ; Stratton, 1989 ), which have been shown to be substantial in some species (Ashman and Schoen, 1997 ; Galen et al., 1999 ).

Whether the long floral duration of K. latifolia evolved as a response to competition for pollinators remains speculative. Instead, this trait may have evolved in areas such as montane habitats, where pollinators are sparse or unpredictable as has been reported for other species (Arroyo et al., 1981 ; Stratton, 1989 ; Bingham and Orthner, 1998 ; Blionis and Vokou, 2001 ; Blionis et al., 2001 ). The trait may have been maintained when it proved to be advantageous in more favorable sites where competition for pollinators was strong. Other species have evolved self-pollination, rather than long floral duration, for reproductive assurance in situations in which pollinators are sparse (Baker, 1955 ; Lloyd, 1980 ; Fausto et al., 2001 ). Self-pollination has also been invoked as a mechanism for competitive avoidance in Arenaria uniflora (Fishman and Wyatt, 1999 ). This study suggests that long floral life span can be an alternative to self-pollination under conditions of uncertain pollination and competition. In fact, K. latifolia exhibits both long floral duration and the ability to self-pollinate in a montane site in Virginia where pollinators are more unpredictable and where competition with other shrubs is also strong (Rathcke and Real, 1993 ; Nagy et al., 1999 ; B. J. Rathcke, unpublished data). Despite having both of these mechanisms for reproductive assurance, fruit set was pollination-limited in Virginia (Rathcke and Real, 1993 ). In contrast, fruit set was not pollination-limited in this habitat in Rhode Island, indicating that pollinators were more available, especially after competing shrubs ceased flowering. This greater availability of pollinators may explain why K. latifolia does not self-pollinate and has only prolonged floral duration as a mechanism for reproductive assurance in this Rhode Island habitat.

Floral senescence in K. latifolia is induced by pollen deposition, rather than pollen removal, which suggests that female function is less rapidly completed than male function (Proctor and Harder, 1995 ; Bell and Cresswell, 1998 ; Evanhoe and Galloway, 2002 ). This response supports the assumption made here that female function and maintenance of fruit set is the important factor selecting for long floral duration in K. latifolia. Whatever the benefits and the adaptive origin of long floral duration of K. latifolia, these results demonstrate that long floral duration can function as a mechanism to avoid competition for pollinators and provide reproductive assurance. Whether long floral duration serves this function in other species remains to be determined.

	FOOTNOTES

 
¹ The author thanks Lee Kass, Carol Landry, Hitashi Sato, Rachel Simpson, and two anonymous reviewers, who provided many helpful suggestions on the manuscript; Irene Stuckey, who provided a home for me near the Great Swamp so I could do this research; and William Crepet, who provided me with an office and facilities in the L. H. Bailey Hortorium during my sabbatical at Cornell University.

² brathcke{at}umich.edu

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