Dendroecological analysis of relict pine forests in the centre of the Iberian Peninsula

Pinus nigra subsp. salzmannii is found in the east and centre of the Iberian Peninsula, in the south of France and in North Africa. This subspecies occupies the westernmost position of the species’ general range. The persistence on the Iberian Peninsula of very long-lived specimens of Pinus nigra subsp. salzmannii, along with their sensitivity to climate, has drawn the attention of many researchers, but to date the importance of dendroecological studies relating to conservation of biodiversity or the genetic resources of this taxon had not been stressed. In the present paper we use dendroecological methods to analyse the relict pine forest in Navalacruz, an interesting and endangered genetic forestry resource on the northern slopes of the Gredos mountains (in Spain’s Central System Range) at the subspecies’ south-western global limit. This forest provides a prime example for demonstrating the potential application of dendroecology for studying the origin, dynamics, local variability, relationships with climate and anthropogenic disturbances of relict forest populations. We dated 93 growth sequences from 47 trees ranging from 1809 to 2006 and we have determined that interspecific competition is the most relevant factor as regards differences in the diameter growth of these trees. Moreover, we detected great variability and numerous common growth disturbances unrelated to climatic oscillations. These quasi-periodic disturbances alternate between suppression and release suggesting continuous management cycles of different intensities. Despite its high level of disturbance, the pine forest presents a certain degree of climatic sensitivity. Comparing with others Pinus nigra subsp. salzmannii populations, we denoted a temporal grading of the growth response to precipitation that is indicative of differences in the start and length of the vegetative period. Furthermore, we compiled different dendroecological and palaeobiogeographical data to demonstrate that this dense, homogeneous and relatively younger P. nigra population is of an indigenous nature. This study aims to provide data for improved management and conservation of this exceptional and highly endangered bastion of biodiversity.