Disruption of Juniperus thurifera woodland structure in its northwestern geographical range: potential drivers and limiting factors

Abstract

Enhancement of Juniperus thurifera recruitment and colonisation by oak and pine species has been related at the local level to changes in livestock pressure. We used forest inventory data from Castilla y Leon Autonomous Region (Central Spain), an area comprising 34% of the world range of J. thurifera, to assess whether this process is occurring at a larger scale. We compared tree composition and density in a set of 659 permanent plots over a 10-year period. Logistic models and redundancy analysis were used to assess the effect on this process of parameters such as livestock pressure, propagule availability and climatic conditions. Between 1992 and 2002, juniper woodlands became denser (1.31% juniper stem year−1) and tree diversity increased due to rapid colonisation by oaks and pines (2.21% occupied plots year−1). In addition, the presence of juniper increased in other types of forests at a moderate rate (0.6% y−1). Thus, we observed both a disruption of the borders between current forest types and a generalised increase in α-diversity of tree species. The seed source was the main factor explaining colonisation rate, suggesting that the pace of colonisation is critically constrained by the spatial configuration of the landscape and the local propagule availability of the colonising species. If the current colonisation trends continue, monospecific juniper woodlands will become very scarce by the end of the twenty-first century.