Limitation de l’installation du chêne vert (Quercus ilex subsp. ballota (Desf.) Samp.) dans un écosystème de savane forestière méditerranéenne

Abstract

Abstract• Tree recruitment in Mediterranean savannas is generally hampered, in contrast with the original oak forests where these savannas are derived from. We asked whether this difference in recruitment success can be explained by differential post-dispersal survival. For one year we monitored experimentally cached holm oak acorns in a savanna -forest ecosystem in Central Spain, and recorded cache pilferage, type of pilferer, boar rooting, seedling emergence, seedling survival and the cause of mortality.• Cache pilferage was significantly lower in savanna (8%) than in forest (21%). However, the higher cache survival was more than offset by lower seedling emergence and, particularly, by nine times higher seedling mortality in savanna, mainly due to desiccation. Wild boar rooting did not differ between experimental caches and controls without acorns, indicating that individual cached acorns do not trigger rooting activity.• Our results indicate that the difference in post-dispersal survival between savanna and forest is due to lower emergence and, primarily, higher seedling mortality in savanna, not to higher cache pilferage. Absence of safe sites such as shrubs, abundantly present in the forest, may well explain the lack of recruitment in the savanna. Management measures appear necessary for long-term persistence of Mediterranean savannas in general. Mots-clés :installation des semis / prédation des graines / recrutement des arbres / dehesa Résumé -Limitation de l'installation du chêne vert (Quercus ilex subsp. ballota (Desf.) Samp.) dans un écosystème de savane forestière méditerranéenne.• Le recrutement des arbres dans les savanes méditerranéennes est généralement entravé, ce qui contraste avec les forêts de chênes originelles dont ces savanes sont issues. Nous nous sommes demandé si cette différence dans le succès du recrutement pouvait être expliqué par une différence de survie post-dissémination. Pendant une année, nous avons suivi expérimentalement des glands de chêne vert cachés dans un écosystème de savane forestière en Espagne centrale, et nous avons enregistré les chapardages des caches, les types de chapardage, la fouille des sangliers, l'émergence des semis, la survie des semis et les causes de la mortalité • Le chapardage dans les caches a été significativement plus faible dans la savane (8 %) que dans la forêt (21 %). Toutefois, la survie plus élevée dans les caches été plus que compensée par une baisse de l'émergence des semis et, en particulier, par une mortalité des semis neuf fois plus élevée dans la savane, principalement due à la dessiccation. La fouille des racines par les sangliers n'est pas diffé-rente entre les caches expérimentales et les témoins sans glands, indiquant que les mises en cache des glands ne déclenchent pas activité de fouille des racines.• Nos résultats indiquent que la différence de survie post-dissémination entre la savane et la forêt est due à la baisse d'émergence et en premier lieu, à une mortalité plus élevée des semis dans la savane, et non pas à des chapardages de...

Predation and aridity slow down the spread of 21-year-old planted woodland islets in restored Mediterranean farmland

“…Similarly, we did not find higher seedling emergence at closer distances to the islets where we expected higher emergence due to micro-climate amelioration by islet canopy shade. This contrasts with results in Smit et al (2009) showing higher emergence in shaded microsites than in open microsites and suggests a lack of ''safe'' microsites for recruitment in our experimental field (Pulido and Díaz 2005). It is possible that microclimatic amelioration by islet canopy was not enough to maintain high soil moisture during the dry spring, but we did not measure soil water content to assess this explanation.…”

“…For instance, Smit et al (2009) observed emergence values [90 % while Pulido and Díaz (2005) observed 73 % emergence in their study. Low emergence values in our study are probably a consequence of the dry spring, particularly in the few weeks after seeding.…”

Tree-shrub associations in grazed woodlands: first rodents, then cattle?

“…We selected 80 viable acorns following the same procedure as before and buried one acorn per microhabitat on all transects on October 23rd, 2007. Single acorns were buried horizontally at a depth of 2 cm to mimic a mouse-made cache (Den Ouden et al 2005;Smit et al 2009). To monitor the rates of seed removal in the different microhabitats, we checked the presence of the acorns at 3, 10, 85, 108, 135, and 198 days after burial.…”

“…Those studies that measured high post-dispersal predation of acorns (Li and Zhang 2003;Iida 2006), therefore questioned the effectiveness of wood mice as dispersers. In contrast, many other studies underline the importance of seed dispersal by wood mice for tree regeneration despite high predation rates, as even a tiny fraction of acorns that escapes from post-dispersal seed predation may be sufficient for a substantial tree recruitment over time (Gómez 2004;Den Ouden et al 2005;Vander Wall et al 2005;Gómez et al 2008;Smit et al 2009). …”

“…Thus far, studies on grazing and spatial associations in grazed woodlands have focused on the sapling stage (C1 year) of trees, while seed and seedling stage (\1 year) are rarely studied (but, see Smit et al 2008Smit et al , 2009). However, the seed and seedling stages are considered as most vulnerable, hence most relevant for plant dynamics (Gómez 2004;Arrieta and Suárez 2005).…”