Pollen limitation and cleistogamy in subalpine Viola praemorsa Jessica Forrest and James D. Thomson Abstract: Early-flowering species may be especially susceptible to occasional pollen limitation and, therefore, may benefit from a mixed-mating strategy that provides reproductive assurance. We studied cleistogamous (CL) and chasmogamous (CH) fruit set of spring-flowering Viola praemorsa Dougl. ex Lindl. along an elevational gradient in the Rocky Mountains, testing whether pollen limitation or allocation to CL reproduction covaried with timing of flowering onset, within and across sites. Contrary to predictions, we found no pollen limitation of reproduction at any site, and variation among sites in the pattern of allocation to cleistogamy was not related to growing season length. Differences in reproductive strategy between early- and late-flowering plants within sites were attributable to differences in plant size, with relative allocation to cleistogamy increasing with size. This pattern has been found in some other cleistogamous species, and may indicate a cost of large CH floral displays, perhaps associated with geitonogamy or herbivory. We found no experimental evidence for resource reallocation in response to CH reproductive output, although a weak negative relationship between CH and CL fruit set across a larger sample of unmanipulated plants suggests such a trade-off. The significance of cleistogamy may be clarified by studying how pollinator visitation, self-pollination, and herbivore damage vary temporally and with floral display size. Key words: allometry, cleistogamy, plasticity, pollen limitation, Rocky Mountain Biological Laboratory, Violaceae. Re ´sume ´: Les espe `ces a ` floraison ha ˆtives peuvent e ˆtre particulie `rement sujettes a ` la limitation pollinique, et conse ´quem- ment pourraient be ´ne ´ficier d’une strate ´gie mixte de fe ´condation assurant la reproduction. Les auteurs ont e ´tudie ´ la mise a ` fruit cle ´istogame (CL) et chasmogame (CH) du Viola praemorsa Dougl. ex Lindl. a ` floraison printanie `re, le long d’un gra- dient altitudinal dans les Montagnes Rocheuses, afin de ve ´rifier si la limitation pollinique ou l’allocation a ` la reproduction CL covarient avec le moment de la mise a ` fleur, sur et entre les sites. Contrairement a ` ce qu’on attendait, les auteurs ne constatent aucune limitation pollinique de la reproduction, quel que soit le site, et la variation entre les sites du patron d’allocation a ` la cle ´istogamie ne montre aucune relation avec la dure ´e de la saison de croissance. Ils attribuent les diffe ´ren- ces dans la strate ´gie de reproduction entre les plantes a ` floraison pre ´coce et tardive a ` une diffe ´rence dans la grosseur des plantes, l’allocation a ` la cle ´istogamie augmentant avec la dimension. On retrouve ce patron chez d’autres espe `ces cle ´isto- games, qui pourrait indiquer un cou ˆt e ´leve ´ de l’exposition florale CH volumineuse, associe ´e a ` la ge ´ı ¨tonogamie ou l’herbi- vorie. Les auteurs n’ont obtenu aucune preuve expe ´rimentale de la re ´allocation des ressources en re ´action au cou ˆt reproductif CG, alors qu’une faible corre ´lation entre la mise a ` fruit CH et CL dans un e ´chantillon plus vaste de plantes non-manipule ´es sugge `re un tel e ´change mutuel. On pourrait clarifier la signification de la cle ´istogamie en e ´tudiant com- ment les visites des pollinisateurs, l’autofe ´condation et les dommages par les herbivores varient temporellement et avec la dimension des expositions florales. Mots-cle ´s : allome ´trie, cle ´istogamie, plasticite ´, limitation pollinique, Rocky Mountain Biological Laboratory, Violaceae. [Traduit par la Re ´daction] Introduction Spring-flowering plants that depend on insects for pollination may be especially vulnerable to occasional pollen-limitation of reproduction. Cold, unpredictable weather may prevent emergence of pollinators or limit their activity (Schemske et al. 1978; Kudo 2006). Simple flowers that allow autogamous self-pollination or xenogamous polli- nation by a range of visitors have been suggested as com- mon strategies for early-spring plants to avoid reproductive failure in the event of pollinator shortages (Motten 1986). Because timing of flowering of early-season plants can be particularly sensitive to variation in the cues that signal the onset of the growing season (Fitter and Fitter 2002; Dunne et al. 2003), while insects may be responding to different cues for emergence, these species may be especially at risk of flowering at a time when their pollinators are not yet ac- tive. An unusually warm spring, and correspondingly early flowering, have been implicated in reduced pollinator visita- tion and seed set in Corydalis ambigua Cham et Schlecht. (Kudo et al. 2004). Violets (Viola spp., Violaceae) have zygomorphic flowers that tend to open in early spring throughout their range Received 27 September 2007. Published on the NRC Research Press Web site at botany.nrc.ca on 22 April 2008. J. Forrest 1 and J.D. Thomson. Department of Ecology and Evolutionary Biology, University of Toronto, 25 Harbord St., Toronto, ON M5S 3G5, Canada; Rocky Mountain Biological Laboratory, P.O. Box 519, Crested Butte, CO 81224, USA. 1 Corresponding author (e-mail: forrestj@zoo.utoronto.ca). 511 Botany 86: 511–519 (2008) doi:10.1139/B08-020 # 2008 NRC Canada