Coexistence in the Mediterranean-Temperate transitional border: Multi-century dynamics of a mixed old-growth forest under global change

- Historical shifts in forest dominance between temperate and sub-Mediterranean species.
- Recent Fagus sylvatica growth decline unprecedented in the last two hundred years.
- Higher climate sensitivity of temperate F. sylvatica than oak species.
- Oak species better suited to current environmental conditions.

Old-growth forests, particularly those located at the interface between different bioregions, are unevaluable sources of long-term vegetation dynamics and historical stand response to natural and anthropogenic disturbances. Although old-growth forests are scarce, the information gathered studying them may assist forest ecosystem restoration and management under forthcoming climate and land-use changes.We analysed how complementary dynamics of a mixed old-growth forest composed by temperate (Fagus sylvatica, Quercus petraea) and submediterranean (Quercus pyrenaica) tree species were driven in response to global changes in the last two centuries. The old-growth forest, named El Hayedo de Montejo, is located at the interface between the Mediterranean and temperate bioregions in the centre of the Iberian Peninsula. The populations of temperate species growing in El Hayedo de Montejo (F. sylvatica and Q. petraea) are at the dry and warm edges of their natural distribution area in Europe, whereas the submediterranean species Q. pyrenaica is at the core of the distribution range. In order to analyse the long-term dynamics, we developed basal area increment and disturbance chronologies for each of the tree species under study. Furthermore, we assessed the climate influence on tree growth during the most recent decades.Our results reveal historical shifts in forest dominance (as reflected by growth) induced by changes in climate and forest management between temperate and sub-Mediterranean species. This was particularly noticeable for F. sylvatica and Q. pyrenaica the least and most drought-tolerant species, respectively. A reduction in growth of F. sylvatica unprecedented in the context of the last two hundred years was observed during the last decades concurrent with forest densification and marked changes in climate. Conversely, both oak species seem to be better suited to current environmental conditions as expressed by increasing growth rates.