Spatiotemporal dynamics of recruitment in Aleppo pine (Pinus halepensis Miller)

Abstract

Spatial and temporal aspects of recruitment play a central role in plant population and community dynamics and have important basic and applied implications. Here we summarize and discuss the results from studies of spatiotemporal dynamics of recruitment stages (seeds-seedlings-saplings) in Aleppo pine (Pinus halepensis) throughout the species' native range. Seed release is induced either by fire (pyriscence) or by drying atmospheric conditions (xeriscence), each generating a distinct temporal pattern. In both cases most seeds travel relatively short distances (<30 m). Pyriscence is not expected to promote long-distance seed dispersal, but xeriscence is associated with relatively strong winds that can transport seeds over 1 km and more, as predicted by a new mechanistic dispersal model. In the absence of fire, seed survival increases with distance from adults because of higher competition with adults and sibs and higher seed predation near the seed sources. New data provide further evidence for such distance-dependent seed predation and also show that predation rates vary among habitats and are lowest in times of high seed abundance. The resulting recruitment patterns in the absence of fire are characterized by rapid spread and complex spatiotemporal dynamics that are fairly unpredictable and give rise to variable age structure. In contrast, the spatial pattern of the first post-fire generation is highly predictable because it explicitly replicates the spatial pattern of the pre-fire population. Unlike fire-free regeneration in which multiple factors operating at various stages are likely to be important, post-fire regeneration is governed by a fairly specific set of factors (the chemical properties of ash) operating during a specific stage (saplings) and at a specific location (the canopy projection of large burned pines). Post-fire forests are therefore even-aged and have predictable spatiotemporal dynamics. Most studies of Aleppo pine recruitment have focused on the seedling and sapling stages and on post-fire regeneration; we call for greater attention to the seed stage and to more extensive sampling of all recruitment stages in both space and time.