Raúl Tapias

Life histories of Mediterranean pines

The life history of Spanish pines and their relation to fire as the main disturbance factor in their ecosystems was analysed. The primary ecological attributes studied were the canopy seed bank (onset of cone production, percentage and persistence of serotinous cones), seed and cone morphology, sprouting and bark thickness. Four ecological groups were separated using multivariate cluster analysis and their life-history characteristics are discussed. Serotiny and early flowering in Pinus halepensis and P. pinaster reflect their evader strategy in relation to fire as this character is advantageous to survive frequent crown fires and to attain successful post-fire recruitment. Late flowering and absence of serotinous cones in P. nigra, P. sylvestris and P. uncinata indicate that their natural forest did not evolve under frequent crown fires. P. canariensis and P. pinea appeared in two single groups because of their sprouting capability and their seed size respectively. Intraspecific variation in P. pinaster was also analysed using the same criteria and high variability was found in its life history traits. A group of P. pinaster populations showed high levels of serotiny and thin bark as a possible adaptation to frequent stand-replacing crown fires. In contrast, a group of non- or weakly-serotinous populations seems to have evolved under a low-intensity fire regime where the best fitness corresponds to thick-barked individuals capable of surviving ground fires. Intermediate strategies were also evident in this species and were discussed in relation to the effect of different fire regimes caused by the understorey vegetation.

Fire adaptations in the Canary Islands pine (Pinus canariensis)

A wide set of phenotypic characteristics related to life history were studied in mature stands of Pinus canariensis throughout its natural range of distribution in the Canary archipelago. Natural forests ranging from those located in xeric areas through to the sub-tropical cloud forests and high mountain stands were classified into eight ecological regions according to their main climatic features. The recent history of forest fires (covering the last 30 years) was taken into account using a categorical factor with three levels. The phenotypic variables studied included those related to seed dispersal (cone size, number of seed scales, seed and wing size and percentage of serotinous trees) and stem growth both on the breast height section (bark thickness, radial growth at various ages and sapwood and heartwood sizes) and on the entire stem (height growth related to age). The average percentage of serotinous trees present in the ecological regions studied varied from 3 to 35%. Average bark thickness in adult trees ranged from 22 to 49 mm and was found to be unrelated to age or diameter. Growth both in height and diameter was found to decline after an average of 25 years, although clear trends in relation to this could not be established across the ecological regions. A high correlation was found to exist between annual rainfall, fire frequency, serotiny and bark thickness at a regional level. Sapwood area per hectare proved to be a valuable indirect site-quality index for the objectives of this paper. Favourable sites (characterised by a high sapwood area per hectare) displayed the highest levels of both bark thickness and serotiny. These particular areas are those, which have suffered more frequent and intense fires over the last decades. The evolutionary implications of this trend and of other general traits of the species, such as vegetative resprouting, are discussed here in relation to the role of understorey vegetation in fire regimes, competition and volcanic history of the islands.

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

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/m2, 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.RésuméLa 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/m2, 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 1’évapotranspiration pour les périodes allant de septembre (année précédente) à août. Aucune différence significative de production de glands n’a été détectée, entre les intensités de taille traditionnelle. La nouvelle méthode utilisée est prometteuse et permet une augmentation de la production de glands; cependant elle devra être testée sur des lots d’arbres plus importants avant d’arriver à des conclusions définitives sur ce sujet. Sur la base de ces résultats, en prenant en compte de l’état de santé actuel du chêne vert dans le sud ouest de l’Espagne (résultat d’un déclin prolongé) et la faible valeur actuelle du bois de feu qui était habituellement une source très importante de revenus provenant de la taille il y a quelques dizaines d’années, les auteurs recommandent la réduction de la fréquence et de l’intensité de la taille dans les dehesas.

Nursery fertilisation affects the frost-tolerance and plant quality of Eucalyptus globulus Labill. cuttings

Eucalyptus globulus is widely used in productive exotic plantations but the expansion of these plantations is limited by low temperatures, as its cold hardening capacity is limited (0.5 to 3.0 °C). It is not well understood how nursery fertilisation affects the field performance of plants. This led us to study the effect of three mineral nutrients (N, P and K) on both plant quality and frost tolerance. The experiment comprised eight growth treatments in which a high dose (H-) or a low dose (L-) of each nutrient was applied. Nitrogen was the nutrient that determined shoot growth, new root growth after transplanting (root egress), frost tolerance and field performance. Performance was better with treatment H-N than with treatment L-N, leaf nitrogen contents being 1.53 and 0.89% respectively. The effects of phosphorus and potassium were not significant between treatments for any parameter. The exception was P which, when interacting with N, favoured root egress for the H-N treatment. It was concluded that nursery fertilisation offers a management tool for eucalyptus growers concerned with plant stock quality.RésuméEucalyptus globulus est largement utilisé dans des plantations exotiques productives, mais l’expansion de ces plantations est limitée par les basses températures, étant donné que l’endurcissement potentiel au froid de cette espèce est limité (0,5 à 3,0 °C). On ne comprenait pas bien comment la fertilisation en pépinière pouvait affecter la performance en plantation des plants. Ceci nous a amené à étudier l’effet de trois nutriments minéraux (N, P et K) sur la qualité des plants et la résistance au froid. L’expérimentation a comporté huit traitements pour l’étude de la croissance pour lesquels une forte dose (H-) ou une faible dose (L-) de chaque nutriment a été apportée. L’azote a été le nutriment qui a déterminé la croissance de la pousse, la croissance de nouvelles racines après transplantation (émission de racines), la résistance au froid et la performance en plantation. Les performances étaient meilleures avec le traitement H-N que avec le traitement L-N, la teneur en azote des feuilles atteignant respectivement 1,53 et 0,89 %. Les effets du phosphore et du potassium n’ont été significatifs pour aucun des paramètres. L’exception a concerné le phosphore qui lorsqu’il était en interaction avec l’azote a favorisé l’émission de racines dans le traitement H-N. On conclut de cette étude que la fertilisation en pépinière offre un outil de gestion pour les producteurs d’eucalyptus confrontés au problème de la qualité des plants.