Influence of pruning and the climatic conditions on acorn production in holm oak (Quercus ilex L.) dehesas in SW Spain

Abstract

-Acorn production by Quercus ilex L. ssp. ballota (Desf.) Samp. in SW Spain was assessed, and variations between years and the influence of pruning on it were examined. To this end, an experimental study was conducted at two different sites (Calañas and San Bartolomé, in the province of Huelva) where trees were subjected to traditional (light, moderate or heavy) pruning and also to a new (crown-regeneration) pruning method. Acorn yield was quantified over a period of 5 years in the Calañas plot and 4 in the San Bartolomé plot, and found to average at 95.61 ± 0.76 g DM/m 2 , which is equivalent to 6.5 ± 0.05 kg DM/tree; however, yield figures varied markedly between years depending on the particular climatic conditions. The average acorn production was correlated with the water potential in mid summer (end of July); the annual, spring and autumn rainfall; and the actual evapotranspiration for the period from September (previous year) to August. No significant differences in acorn production between traditional pruning intensities were detected; in fact, there were only hints that heavy pruning might result in decreased acorn yields. The new pruning method used, crown-regeneration, seems promising with a view to increasing acorn yield; however, it should be tested on larger sample sizes before any final conclusions can be drawn in this respect. Based on the results, the present health status of holm oaks in southwestern Spain (a result of sustained decline) and the low value of firewood -which used to be a very important source of income from pruning a few decades ago -, the authors recommend reducing the frequency and intensity of pruning in the dehesas of the study area.holm oak / acorn production / pruning / masting Résumé -Influence de la taille et des conditions climatiques sur la production de glands par le chêne vert (Quercus ilex L.) en dehesas dans le sud ouest de l'Espagne. La production de glands par Quercus ilex L. ssp. ballota (Desf.) Samp. dans le sud ouest de l'Espagne a été mesurée, et l'influence de la taille sur les variations interannuelles a été examinée. Dans ce but, une étude expérimentales a été conduite dans deux sites (Calañas et San Bartolomé dans la province de Huelva) où des arbres ont été soumis à une taille traditionnelle (légère, modérée ou forte) et aussi à une nouvelle méthode de taille (régénération de couronne). La production de glands a été quantifiée sur une période de 5 ans sur le site de Calañas et de 4 ans sur celui de San Bartolomé et était en moyenne de 95,61 ± 0,76 g de matière sèche/m 2 , ce qui équivaut à 6,5 ± 0,05 kg de matière sèche/arbre. Cependant la production a varié de façon marquée entre années en relation avec les particularités climatiques. La moyenne de production de glands était corrélée avec le potentiel hydrique au milieu de l'été (fin juillet) ; avec les précipitations annuelles, du printemps et de l'automne, et avec l'évapotranspiration pour les périodes allant de septembre (année précédente) à août. Aucune différence significative de productio...

Do ecological and silvicultural factors influence acorn mass in Holm Oak (southwestern Spain)?

“…Pruning has traditionally been carried out to obtain fuelwood, but also because it was thought to improve acorn production and acorn mass, probably because opening the canopy implies that the tree receives more solar radiation, what would be supposed to improve flowering and therefore acorn production. However, this hypothesis has been questioned (Carbonero et al 2002;Cañellas et al 2006;Alejano et al 2008) and significant differences in acorn production from pruned and non pruned trees have not been found (Alejano et al 2008). But the effect of pruning on acorn mass, as a component of acorn production, has not been well documented in these papers.…”

“…The pruning operation was performed by workers hired by the forest owners. In the absence of previous criteria, the pruning intensity was established as follows: light pruning involved removing sucker and dead branches only; heavy pruning coincided with the usual practice in the area; and moderate pruning was in between the previous two (Alejano et al 2008). Once work was completed, pruning intensity was estimated as a function of the weight of pruned branches and tree diameter.…”

“…The exact ages of the trees are unknown because locally absent rings and false bands are common in this species (Hidalgo-Fernández and Heras 2003). We estimated age in both locations as being 40-90 yearsold, based on size and historical knowledge of land owners, managers, and forest rangers (Alejano et al 2008).…”

“…Pruning treatments were discriminated in terms of intensity by using the branch dry weight (DW) to tree diameter (D) ratio; thus, DW/D was less than 0.8 for light pruning, greater than 1.7 for heavy pruning and in between these two levels for moderate pruning (Alejano et al 2004). Every Holm Oak harvested for acorns was classified into its original group, so no tree was changed to a different pruning treatment (Alejano et al 2008). …”

“…All previous studies related with acorn production (Martin et al 1998;Á lvarez et al 2002;Carbonero et al 2002;Gea-Izquierdo et al 2006;Poblaciones et al 2006;Cañellas et al 2007;Alejano et al 2008) have not considered these two components separately. Nevertheless, to get a better understanding of the acorn production process it is very important to analyze both components separately, as silvicultural and ecological factors will affect them differently.…”