Population and reproductive structure of an endangered juniper from coastal pine forests

Abstract

Plant species adapted to coastal systems are suffering severe declines. This is the case of the critically endangered Juniperus navicularis, which colonizes fossil dunes in the southwestern Iberian Peninsula and grows in the understory of pine plantations. This endemic juniper presents low reproduction rates and a decreasing number of populations has been reported in its western distribution edge. However, age structure and the demographic situation of its easternmost populations, that have been recently discovered, remain unknown. We used dendrochronological procedures to estimate the age structure of the easternmost isolated population of J. navicularis, considering three subpopulations under contrasting management regimes and selecting plots under pine cover and in open areas. We also assessed sex ratio, age at maturity and reproductive effort. We observed the predominance of 20–40 years-old stems in all study plots and a remarkable lack of young stems in open areas. Stem density was higher under pine canopies. There, junipers were, on average, younger than pine trees, meaning that junipers had grown in already forested areas. Only one sex per plot was found in all study plots but one, in which no reproductive stems were recorded, suggesting the existence of clonal growth. Reproductive effort increased with age in females, but was independent in males. We hypothesize that clonal growth might occur as a survival strategy to endure the harsher conditions of distribution edges. These results will help to develop more effective management strategies to preserve this endangered species, contributing to our understanding of dynamics of coastal ecosystems and setting the basis for future genetic studies.