Factores determinantes del reclutamiento de encinas (Quercus ilex) en mosaicos agroforestales. Dispersión de bellotas por urracas (Pica pica) y establecimiento de plántulas

Abstract

Forest restoration is a major ecological challenge in many regions of the world. In Mediterranean environments, aridity, lack of propagules and their dispersers, seed predation, herbivory, and some plant-plant interactions such as competition, are major limitations for the restoration of forests in old fields. The main objective of this PhD Thesis was to assess the effect of different ecological factors on the recruitment of holm oak (Quercus ilex) in old fields. The Thesis focuses on the dispersal of acorns by magpie (Pica pica) and the emergence, the survival, and the establishment of holm oak seedlings. The Thesis contains seven chapters. Chapter 1 is a general introduction in which the main factors that determine oak recruitment are presented. There are four research chapters. Chapters 2 and 3 quantify and assess acorn dispersal by magpie. Chapters 4 and 5 study the post-dispersal processes driving emergence, early survival and establishment of holm oak juveniles. Chapter 6 integrates and discusses the results of the research chapters in the framework of oak colonization and proposes applications for old field restoration and future research lines. Finally, Chapter 7 summarizes the main conclusions of this Thesis. In Chapter 2 we quantified the acorn dispersal capacity by magpie, according to their breeding status (breeding individuals vs. juveniles) and sex. The magpies removed more than half of the annual production of acorns from the trees. Breeding magpies were the main acorn scatter-hoarders, especially breeding males, which dispersed around twice as many acorns as breeding females. Magpies cache 86% of the acorns they remove, and one magpie can store more than 1,300 acorns for 6 weeks at an average distance of 32.5 m (1.4-210 m) from the holm oak. Up to 2.4% of the acorns dispersed by magpies were not recovered or predated, resulting in a high density of emerged oak seedlings (56–439 seedlings/ha). These results indicate that the magpie is a massive and effective acorn disperser and may play a key role in recruitment of oaks in open agroforestry systems. In Chapter 3 we studied the role of caching territories and spatial preferences at different scales by magpies to cache acorns. The selection of sites to cache acorns followed a hierarchical decision-making process in which the caching territories constrained both the dispersal distance and the availability at different spatial scales of sites to cache the acorns. At the coarsest spatial scale (habitat), magpies selected the tilled areas over the oak plantation and this woodland over the old fields. At the sub-habitat scale, magpies selected the areas with high plantproductivity within the plantation. At the microsite and type of material scale (the finest scale), they selected the structures built by animals and the least abundant material in each site to cache the acorns. Habitat-scale preferences matched the density of emerged seedlings in different habitat types. Therefore, magpie caching territoriality indirectly drives the emergence of oak seedlings, and thereby potential oak recruitment and forest regeneration. In Chapter 4 we analyse how post-dispersive acorn and seedling predation and the orientation and distance to planted woodland islets affect the emergence and early survival of seedlings. The 96.9% of the unprotected acorns were predated. The least harsh microclimate in the least sun exposed orientation (north) promoted the emergence of twice as many seedlings (42.9%) than in the south orientation (13.2%). Survival of the spontaneously established seedlings was 28.6% after the first summer. Drought (45%) and seedling herbivory (35%) were the main causes of mortality. In Chapter 5 we evaluate the effect of woodland islets, which were planted 25 years ago to assist natural holm oak regeneration in old fields, on oak seedling emergence and short-term survival for four consecutive years and the long-term establishment of juveniles. Wet springs and summers increased both seedling emergence and survival. Seedlings emerged in wet springs had higher survival after summer drought. Emergence and survival were highest in the less sunexposed orientations. Microsites close to the islets in less sun-exposed orientations and far from the islets in the most sun-exposed orientation enhanced oak seedling emergence and early survival but did not affected annual growth rate. Herb biomass was lower near the islets due to the competition of the planted oaks. Twenty-five years after the plantation of the holm oak forest islets, 92 new individuals have been established spontaneously, resulting in a recruitment rate of 5.7 individuals ha-1 year-1. Planted islets were effective for the passive restoration of Mediterranean woodland on old fields. In conclusion, this Thesis shows that magpies are massive and effective acorn scatter-hoarders, which are keys for oak recruitment in old fields. Magpie territories constrain dispersal distance and the availability of places to cache acorns. Habitat types preference for acorn caching matched the density of emerged seedlings in the different habitats. The distance and orientation to the seed source determines the fate of acorns as they drive the emergence, early survival, and establishment of new individuals. Holm oak woodland islets provide the necessary acorns for the restoration of old fields where the remnants of this type of vegetation are scarce. Conservation and promotion of magpie populations, woodland islets plantation, ploughing old fields after abandonment and controlling acorn predators and herbivores could speed oak forest restoration in old fields.