Effects of age and size on life histories and population growth of Rhododendron maximum shoots

Ageand size-specific shoot life histories were studied with population censuses in June 1984 and June 1985 in an evergreen understory shrub, Rhododendron maximum. Most shoots (65%) survived without branching or flowering, and lesser numbers branched (2%), flowered (20%), or died (23%) during the year between censuses. The probabilities of surviving, branching, flowering or dying were both ageand size-dependent. Small, young shoots increased in leaf area. Flowering occurred most prominently in 3to 6-year old shoots that had exceeded a leaf area of 200 cm2, and the rate of flowering increased proportionately with size above this threshold. Branching normally occurred in the year following flowering. The age and size distributions of the population shifted significantly between years, indicating a nonequilibrium population. The survival schedule was Deevey Type I, indicating a high degree of "parental care" of young shoots. Ageand age + size-based demographic models predicted a rapid decline of the shoot population over a decade, while a size-based model predicted a much slower decline in shoot numbers. A sensitivity analysis of the models showed that overall shoot population growth was positively influenced by branching shoots and shoots that added leaf area, and negatively influenced by shoots that lost leaf area, died, or flowered. The role of shoot life histories in determining individual plant fitness and ecological dominance is discussed. PLANTS COMPRISE POPULATIONS at two levels; the "traditional" population of individuals, each of which developed from a single seed, and the population of parts such as shoots, leaves or buds that is found within each individual (White, 1979; Harper, 1980). The demography of plant parts has become an important tool for determining growth rates of plants, especially under field conditions where a nondestructive method of growth analysis is desirable (Garbutt and Bazzaz, 1983; McGraw and Antonovics, 1983; Chester and McGraw, 1983; Fetcher and Shaver, 1983; Cooper and McGraw, 1988). In addition, demographic models applied to populations of plant parts can potentially predict future growth of the plant. Demographers traditionally classify individuals by age because significant changes in survival and reproduction occur with development through time. However, developmental rate is highly plastic in plant populations. Size is therefore correlated with survival and fecundity, while age is a relatively poor predictor of plant performance. This has led to a prefI Received for publication 17 September 1986; revision accepted 24 May 1988. The author would like to thank Hal Caswell and David Peterson for critical reviews of this manuscript, and Hal Caswell for performing the log-linear analysis. erence for size-based models (Lefkovitch, 1965) of populations of whole plants (Werner and Caswell, 1977; Caswell and Werner, 1978; reviewed in Caswell, 1986). The appropriate basis for classification of plant parts for demographic analyses has not been established. A priori, age-based models may seem more appropriate since plant parts tend to be much less variable in size than whole genets (White, 1979, 1984). However, variability in shoot size that does exist within plant parts may reflect important differences in internal resource availability or resource capturing ability, and thus be more relevant than age as a classification variable. Both ageand size-based demographic models have been employed for plant parts (Bazzaz and Harper, 1977; Fetcher and Shaver, 1983; McGraw and Antonovics, 1983; Chester and McGraw, 1983; Huenneke and Marks, 1987), although direct comparisons of the two approaches have not been made on the same population. A third demographic model has also been developed that allows classification by both age and size (Law, 1983), although it has not been applied to populations of plant parts. In the present study, I simultaneously applied age-, size-, and age + size-based models to the same shoot population. In addition to its use as a tool in growth analysis, understanding the demography of plant parts can explain variation in individual