Does size matter? Ontogenetic responses of an Andean shrub to conspecific density-dependence

Abstract

Dispersal, population spatial structure and intraspecific density-dependent processes are critical determinants of plant population dynamics. However, we lack empirical data on how the interplay between these factors changes across the plant species life cycle, particularly for long-lived plants. This is in great part due to the difficulty in implementing experimental approaches to discriminate the relative effects of these factors. Here, we studied the spatial pattern of adults and recruits of the Andean shrub Croton wagneri Müll. Arg. (Euphorbiaceae) and assessed the role of dispersal, population spatial structure and intraspecific interactions on individual plant performance. For this purpose, we developed a spatially explicit approach in which plant spatial position is combined with vegetative and reproductive attributes. The spatial pattern of C. wagneri showed small-scale aggregation. This is consistent with the species' short-distance seed dispersal. The spatial pattern of recruits was dependent on the spatial pattern of adults, signifying a high relevance of adult-recruit interactions in the spatial pattern creation. However, once established, recruits seemed to be subjected to self-thinning (i.e., negative density dependence). Finally, a positive density-dependent effect of the conspecific neighbourhood was found on adult reproductive performance, while plant growth was affected by intraspecific competition. Overall, dispersal, population spatial structure and intra-specific interactions combine to influence vegetative and reproductive performance of the species. We also found that plant interaction at the intraspecific level can be a composite of facilitative and competitive effects that combine in complex ways to influence population spatial structure and individual plant performance. Furthermore, such effects did not always leave a spatial signature, which highlights the importance of complementing plant position with quantitative fitness attributes in spatial pattern analysis in plant ecology.