The main objective of this study was to identify differential anatomical features between Ulmus pumila L. and Ulmus minor Mill. clones resistant to Dutch elm disease and U. minor clones susceptible to Dutch elm disease, with a focus on the intervascular pits and medullary rays. Resistant elms showed lower mean values than susceptible elms for pit membrane diameter, pit aperture area, pit membrane abundance per vessel-wall area, ray width, and ray tangential area. A principal component analysis of the parameters measured revealed slight differentiation between species but clearly grouped U. minor clones according to their susceptibility group. In comparison with susceptible elms, the pit structure observed in resistant elms may limit passive fungal spread within the sapflow, lower the probability of fungal cells passively reaching pit membranes, and reduce the vulnerability of the xylem to cavitation. Similarly, the ray structure observed in the resistant elms is likely to reduce the amount of easily accessible nutrients available for fungal growth as well as the rate of radial colonization in comparison with susceptible elms. Examination of the principal component loadings suggested that susceptible U. minor clones were mainly characterized by enhanced values of pit membrane abundance per vessel-wall area relative to resistant U. minor trees.Résumé : L'objectif principal de cette étude était d'identifier les différences dans les caractéristiques anatomiques entre des clones d'Ulmus pumila L. et d'U. minor Mill. résistants à la maladie hollandaise de l'orme et des clones d'U. minor sensibles à la maladie hollandaise de l'orme en mettant l'accent sur les ponctuations intervasculaires et les rayons médul-laires. Le diamètre des membranes de ponctuations, la superficie de l'ouverture des ponctuations et l'abondance des membranes de ponctuations par unité de surface des parois des vaisseaux, de largeur des rayons et de superficie tangentielle des rayons étaient en moyenne plus faibles chez les ormes résistants. Une analyse en composantes principales des paramètres qui ont été mesurés a révélé une légère différentiation entre les espèces mais a clairement regroupé les clones d'U. minor selon leur groupe de sensibilité. Comparativement aux ormes sensibles, la structure des ponctuations observée chez les ormes résistants pourrait limiter la propagation passive du champignon avec le mouvement de la sève, réduire la probabilité que les cellules fongiques atteignent passivement les membranes de ponctuations et diminuer la vulnérabilité du xylème à la cavitation. De la même façon, la structure des rayons observée chez les ormes résistants réduit probablement la quantité de nutriments facilement accessibles et disponibles pour la croissance du champignon ainsi que le taux de colonisation radiale comparativement aux ormes sensibles. L'examen de la saturation des composantes principales indique que les clones sensibles d'U. minor étaient principalement caractérisés par des valeurs plus élevées de l'abondance des membranes de po...

A study was conducted on the variation in growth, biomass, juvenile wood anatomy, and needle morphology of Pinushalepensis Mill. from three Spanish regions of provenance characterized by environmental differences, without the influence of the site factor. Seeds collected from two progenies in each region were planted in a single plot, and the trees were felled at the age of 7 years. The results showed significant differences between provenances, as well as the genetic or environmental influence on the traits analyzed. Trees adapted to moderate summer drought conditions (Inland Catalonia region) are primarily characterized by higher average values for height, diameter, biomass, cell wall thickness, inter-tracheid wall strength, pit membrane diameter, torus diameter, bordered pit aperture diameter, and ray tracheid abundance in comparison with trees adapted to severe summer drought conditions (Southern region and Balearic Islands region). The greater structural requirements of trees from the Inland Catalonia region, subjected to higher weight and wind loads, resulted in thick cell walls. Moreover, the large pits and more abundant ray tracheids in trees from this provenance would allow more efficient water transport and greater water storage capacity, respectively. The differences found between provenances suggest the adaptive nature of the anatomy of this species, which demonstrates the importance of the region of provenance when choosing reproduction material for reforestation.

The conifer forests of the Mediterranean Basin have been subjected to overuse by humans since ancient times. Some species have survived in inaccessible refuges but the ranges of other species have been greatly reduced by the effects of clearance for agriculture, livestock raising, illegal felling and, in some cases, fire. The firs are no exception and some now exist only as relict species. Abies pinsapo is an example, with the species surviving in only three enclaves in southern Spain and two in northern Morocco. Until the mid 20th century A. pinsapo forests were subject to major anthropogenic pressures, and in Spain they were under constant threat of overuse until they were acquired by the State. Conservation efforts have now, however, been undertaken in both Spain and Morocco, and the fact that all the A. pinsapo forests are covered by some form of protection preserves them from further inappropriate use or exploitation. These forests are now recovering after years of intensive grazing and use of their timber for construction, firewood and charcoal making. However, these relict forests face the new threats of climate change, arson and the appearance of pests. The limited area occupied by these forests makes them highly vulnerable to disturbance.