Marcescent Forests of the Iberian Peninsula: Floristic and Climatic Characterization

Forests dominated by marcescent oak species represent the transition between deciduous forests adapted to rainy summers and cold winters, on the one hand, and evergreen sclerophyllous Mediterranean forests. In the Iberian Peninsula marcescence is shown by some oak species, including Quercus pubescens, Q. pyrenaica, Q. faginea and Q. canariensis; it suggests an old evergreen habit forced to become deciduous by the cold winters. In this paper we analyse the floristic diversity of marcescent forests in the Iberian Peninsula and their proportion of evergreen broad-leaved and sub-Mediterranean species, and relate them to climatic conditions. This analysis uses 494 phytosociological relevés from the Information System of Iberian and Macaronesian Vegetation (SIVIM) and the BIOVEG data-bases. The data-set was submitted to an agglomerative clustering, which produced four clusters. An NMDS gradient analysis was also applied, in order to assess the relationship between the clusters and bioclimatic variables. The hierarchical and syntaxonomical classifications show a high correspondence, as reflected in the dominance of different Quercus species in each cluster. Quercus broteroi and Q. canariensis forests show a higher proportion of evergreen broad-leaved species, while Q. pubescens and Q. faginea forests are characterized by sub-Mediterranean species. As for climatic relationships, Q. broteroi and Q. canariensis forests present the highest values of thermicity (It) and the lowest values of the ombrothermic (Io₂) index. These values indicate their Mediterranean, thermophilous character, which in turn is related to their high proportions of evergreen broad-leaved species. As a conclusion, it can be stated that marcescence is not related to evergreeness, the latter being linked to mediterraneity. Therefore, the idea of considering marcescence as a residual feature of ancient evergreen laurophyll forests is not supported by our results. Those marcescent forests, particularly the basophilous ones, are related to the sub-Mediterranean floristic element.