Elimination of cone and seed pests by wildfire opened a five-year regeneration window in a non-regenerating incense-juniper (Juniperus thurifera L.) stand

Abstract The sanitary status of seed cones of incense juniper, Juniperus thurifera, has been surveyed from 2001 to 2011 in a relict population of the French Pyrenees (Rié Mountain, Haute-Garonne, France) which has been severely affected by a wildfire during the summer 2003. Damage to seed and cones was compared on the same trees before and following the 2003 fire. Before the fire, natural regeneration was severely hindered by the joint damage of a seed mite, Trisetacus quadrisetus, and a seed chalcid, Megastigmus thuriferana which limited to 25-55 the number of sounds seeds falling on the ground per tree. The fire destroyed a large part of the stand but simultaneously killed most of insect seed predators. No flowering was observed the following year (2004), thus contributing to eliminate the remaining predators. In spite of large annual fluctuations, more than 200 sound seeds were dispersed on the average per tree in 2006 and 2009, i. e. 4-10 times more than before the fire. Seed chalcids and mites only began to recolonize the stand during summer 2008, probably originating from nearby stands of J. thurifera (chalcids) or J. communis (mites). Although the pest populations regularly increased every year, cone damage remains concentrated on less than ten trees. Unexpected events such as wildfires may thus play a regulating role upon seed predation by arthropods and opened a regeneration window of five years at least, as it can be observed from the sudden appearance of numerous seedlings during this period. Résumé L’évolution du potentiel de régénération naturelle de la population pyrénéenne relique de Juniperus thurifera de la montagne de Rié (Haute-Garonne, France) a été surveillée de 2001 à 2011, avant et après l’incendie qui a détruit une large partie de ce peuplement en 2003. La production de galbules, leur état sanitaire de la floraison à la maturité, et le nombre de graines viables résultantes ont été mesurés chaque année sur les dix mêmes branches de dix arbres différents. Avant l’incendie, l’impact conjugué du chalcidien des graines, Megastigmus thuriferana, et des acariens des galbules, Trisetacus quadrisetus, réduisait de manière drastique le potentiel de régénération naturelle du Genévrier thurifère, avec seulement 25 à 55 graines pleines tombant au sol en moyenne par arbre. L’incendie de 2003, survenu pendant une période où les ravageurs ne pouvaient s’échapper des galbules, a conduit à l’élimination complète de leurs populations sur le site. Aprè