Demography and size structure of the giant milkweed shrub Calotropis procera (Aiton) W.T. Aiton Tarek M. Galal 1 • Emad A. Farahat 1 • Maha M. El-Midany 1 • Loutfy M. Hassan 1 Received: 9 June 2015 / Accepted: 23 October 2015 Ó Accademia Nazionale dei Lincei 2015 Abstract Studies on plant population demography and size structure provide important base for monitoring and managing plant species. The present study investigated the monthly variation in plant demography, population dynamics and size structure of Calotropis procera in urban areas, South Cairo, Egypt. Sixty-three permanent quadrats were selected to represent the monthly variation in the characteristics of C. procera population all over one year. The highest plant density was attained in June and the lowest in February and March. The highest biomass was recorded in November, but the lowest in March. The maximum individual’s height was recorded from August to January, while the minimum was in late winter. The monthly size distribution indicated that C. procera had three different size distributions along the whole year: more or less inverse J-shape, positively skewed and bimodal size distribution. The maximum plant survival was correlated with the availability of soil moisture. The demographic flux indicated that the new branches of plant were mainly formed during April and June, followed by significant mortality in July and October. This study may contribute in planning for managing and conserving this medicinal plant. Keywords Demographic flux Á Natality Á Mortality Á Survival Á Invasiveness Á Calotrope 1 Introduction Despite the dynamics of semi-desert communities, many shrubs and trees may be considered pioneer species. Their high germination ability, elevated growth rates during early stages and tolerance to high radiation levels allow them to colonize open spaces, thus providing micro-sites for the germination and establishment of many other species under their canopies (Nobel and Franco 1989; Pugnaire et al. 1996). This process contributes to create the heterogeneous landscape that characterizes semi-desert ecosystems. As a result of the above reason, shrubs and trees of semi-desert communities could be considered keystone species on which many community and ecosystem processes depend (Pugnaire et al. 1996). Demographic studies in plant populations provide useful information on population dynamics and can be used to examine the biotic and abiotic factors affecting them. The information obtained through demographic studies could be used in restoration of degraded lands. Many researchers emphasize the need for studying and understanding the population dynamics of keystone species to find the best way of managing and preserving them within or out of their natural habitat (Bekele 2000). The contribution of demo- graphic studies in assessing the status of a population (Schemske et al. 1994) and the role of plant population studies in examining demographic variation in relation to temporal and spatial environmental variation, succession and nature management has provided important insights (O’Connor 1994). Although even-aged mono-specific stands of plants are the simplest plant populations, they exhibit some interest- ing behaviors (Purves and Lawd 2002). As the plants grow, the population develops a size hierarchy, with a few large and many small individuals (Hara 1988), and this pattern & Maha M. El-Midany maha_elmidany@yahoo.com 1 Department of Botany and Microbiology, Faculty of Science, Helwan University, Cairo, Egypt 123 Rend. Fis. Acc. Lincei DOI 10.1007/s12210-015-0487-1