Density‐dependent growth and survival in a natural population of the facultative biennial Digitalis purpurea

Abstract

Summary 1 Density-dependent effects on vital rates may vary in both magnitude and direction at different stages of the life cycle. In monocarpic perennials, however, it is often assumed that recruitment is the stage most affected by density. 2 The spatial pattern of newly emerged individuals of the facultative biennial Digitalis purpurea was recorded and followed in five 0.5-m 2 plots censused twice during each of three seasons. 3 To examine effects of local density on growth and survival, a plant’s neighbourhood was defined as the number of individuals inside a circle of fixed radius around it. Species identity of the nearest neighbour was established to see whether the effects of intra- and interspecific interactions differed. 4 Relative growth rate (RGR) was negatively related to local density during both of the first two summers, but not during winter. In 1998 the negative effects were stronger when the closest neighbour was conspecific. 5 Contrasting patterns of size-dependent growth were found during summer and winter, with summer RGR an increasing, and winter RGR a decreasing, function of size. 6 Summer survival was generally high, and was positively related to spring size. There was a significant negative effect of increasing local density on survival probability in 1998. No effect of neighbour identity was detected. 7 Winter survival was low, and mainly dependent on autumn size, with no significant effects of local density in any season. During the first winter, individuals with a conspecific as the nearest neighbour had lower survival probabilities. 8 There was no evidence of a tradeoff between summer RGR and survival probability. 9 Density-dependent effects may be significant beyond the recruitment stage in monocarpic perennials such as D. purpurea . Density dependence was strongest at early life stages although, because effects on growth persisted into the second year, it is likely that local density may influence timing of reproduction in this population. The use of detailed studies investigating the timing and magnitude of density dependence across the entire life cycle may provide new insight into life history evolution.