Forest canopy and mistletoe infestation alter the facilitative effects of Juniperus oxycedrus s.l. on woody seedlings on Mount Lebanon (Lebanon)

Abstract

Questions:In this study we investigated the direct effect of the shrub Juniperus oxycedrus s.l. on Mediterranean woody species regeneration, and how this interaction can be modified by the juniper habit, the forest canopy and/or infestation of the juniper by the plant parasite Arceuthobium oxycedri. We also aimed at identifying species response groups in relation to their ecological requirements and successional strategies.Location: North of Mount Lebanon, in Horsch Ehden natural reserve, a humid montane Mediterranean forest. Methods:We compared the regeneration of all occurring woody species below and outside 111 Juniperus oxycedrus canopies using the random pair sampling method. We calculated eight relative interaction intensity indexes (RII Juniper ) corresponding to the eight treatment combinations of the three factors: forest canopy (forest/gap), mistletoe infestation (with/without) and juniper habit (spreading/erect). We analyzed the RII values at three different levels: community (all species grouped), species group and species.Results: Overall, the regeneration of the woody community was facilitated by Juniperus oxycedrus. However, the forest cover significantly reduced the facilitative effect of Juniperus oxycedrus when the latter was not infested by Arceuthobium oxycedri. We identified three species response groups (A: highly facilitated; B: slightly facilitated to neutral; and C: neutral) determined by their response to both Juniperus oxycedrus canopy and forest cover. The most facilitated species were shade-tolerant/ drought-intolerant late-successional species. Conclusions:The results suggest the importance of shade tolerance in the complex mechanism of forest regeneration that is also affected by a parasite plant (Arceuthobium oxycedri), which boosts regeneration dynamics under forest cover by reducing shrub canopy cover.