Constraints on Reproductive Potential at the Level of the Shoot Module in Three Ericaceous Shrubs

Reproductive potential at the level of the shoot was examined in three species of Ericaceous shrubs representing a range of growth forms (Rhododendron maximum L., an evergreen species with long-lived leaves; Kalmia latifolia L., an evergreen species with short-lived leaves and Rhododendron nudiflorum (L.) Torr., a deciduous species). A subsample of the shoot populations on 10 individuals of each species was used to determine the relationship between shoot properties (size and age) and (1) the probability of flowering and (2) the components of reproduction (flower birds per inflorescence, flowers per flower bud and capsules per flower). Physiological characteristics (nitrogen and nonstructural carbohydrate content) of shoots associated with size and age were also determined. In all three species, shoot size was shown to be a better predictor of reproductive potential than shoot age. The positive relationship of reproductive potential with shoot size appears to be related to internal resource availability. Where shoot age was related to flowering, this relationship could be partially explained by an age-size correlation. Information collected at the level of the shoot was also used to assess differences in flowering potential between individuals within species and between species. Variation in flowering potential between individuals appears to be determined at the stage of flower initiation and is largely due to differences in the number and size distribution of shoots between individuals. Differences between species appear to be related to leaf longevity and leaf structure. Key-words: Rhododendron maximum, Rhododendron nudiflorum, Kalmia latifolia, shoot module, reproductive potential, nitrogen, non-structural carbohydrates * Reprint requests to J.B. McGraw. Introduction An understanding of reproductive ecology in wild plants is important in many ecological and evolutionary issues (Willson, 1983). Many studies in plant reproductive ecology have been aimed at understanding how flowering is regulated in plants, e.g. what factors determine the initiation of flowering and the number of flowers, fruits and viable seeds produced by a plant. Overall plant size has been shown to be an important determinant in many species of annuals (Dolan & Sharitz, 1984; Silvertown, 1985), biennials (Werner, 1975; Werner & Caswell, 1977; Gross, 1981; Hirose & Kachi, 1982), herbaceous perennials (Sohn & Policansky, 1977; Solbrig, 1981; Aker, 1982; Young, 1985) and woody perennials (Matthews, 1963; Robinson & Wareing, 1969; Schmidtling, 1969; Kozlowski, 1971). Several studies have suggested that a positive relationship between flowering and plant size may be explained by the high energy cost of reproduction (Davis, 1957; Werner, 1975; Hirose & Kachi, 1982). Although a number of other factors may act as cues for flowering (i.e. environmental factors such as vernalization or photoperiod and internal factors such as hormones), these are generally not effective until the plant has reached a minimum threshold size (Robinson & Wareing, 1969; Gross, 1981). Most of the studies that focused on the control of flowering in plants dealt with populations of genets. However, the control of reproduction can also be examined within a 'metapopulation': the collection of branches, buds, shoots or other units comprising each plant (White, 1979). The metapopulation provides a convenient unit for studying controls over reproduction in large, long-lived perennials (McGraw & Wulff, 1983; McGraw & Antonovics, 1983). An important advantage of this approach is that it allows subsampling of the individual. This means that the control of reproduction can be studied in very large plants where plant size is difficult to measure and seeds are too numerous to collect and count or in plants with clonal growth where individual genets cannot be readily defined. Other advantages are that it can be This content downloaded from 157.55.39.120 on Mon, 05 Sep 2016 06:16:55 UTC All use subject to http://about.jstor.org/terms 98 utilized in the natural environment and applied in S. D. Cooper & a non-destructive manner, allowing one to follow J. B. McGraw individuals over time (Harper, 1980). In demographic studies of plant populations both age and size have been used to classify individuals. Although age is the traditional method of classification, size appears to be a better predictor of survival and reproduction in many species of plants at the level of the individual. In this study we were interested in determining whether size or age of a plant part, in particular the shoot module (sensu White, 1984), is a better predictor of reproductive potential. Most studies that have used the shoot demography approach have dealt with descriptive aspects of growth and have not examined why size, age or other shoot properties are correlated with flower and seed production. Therefore, in this study we also examined internal shoot properties associated with the initiation of flowering and which may determine the components of reproductive success thereafter. In particular, we examined both concentration and total shoot nitrogen and carbohydrate content as related to age and size in search of an explanation for age and size effects on reproduction. The study was carried out with three coexisting, closely-related species having contrasting growth forms: a deciduous shrub, Rhododendron nudiflorum, (L.) Torr.; an evergreen shrub with short-lived leaves, Kalmia latifolia L.; and an evergreen shrub with long-lived leaves Rhododendron maximum L. In addition to the fundamental question of age and size effects, three questions based on the above objectives were considered: (1) What similarities exist between the three species studied in terms of shoot properties affecting reproductive potential? (2) Are there specific similarities which exist between the two closely related species (R. maximum and R. nudiflorum)? and (3) Is there variation between genets in the control of reproduction within each