Canopy Cover Loss of Mediterranean Oak Woodlands: Long-term Effects of Management and Climate

Although forest areas have been growing in Europe, some forest types have been declining regionally. Since the 1980 s, there have been reports of defoliation and mortality in evergreen oak woodlands of the Iberian Peninsula. However, long-term and large-scale trends of Iberian oak decline are still poorly understood. We quantified trends in canopy cover for cork oak (Quercus suber) and holm oak (Q. rotundifolia) woodlands in mainland Portugal for a 50 year period (1965 to 2015). We considered loss of canopy cover a proxy of forest degradation and asked how it changed over time, in association with human activity and climate. Between 1965 and 2015, both cork oak and holm oak canopy cover showed declining trends (57.9% and 71.1% of sampling points, respectively). Canopy cover dynamics, however, differed across time. Between 1965 and 1990, canopy cover gains (over 35% of sampling points) prevailed over losses (under 20%), with larger differences for cork oak than holm oak. Between 1990 and 2015, canopy cover losses (over 70% of sampling points) were much higher than gains (under 9%). Oak canopy cover decrease was associated with flatter areas (usually having more intensive land use) for both oak species, higher cattle numbers (holm oak only) and higher mean temperature (cork oak only). Contrastingly, loss of holm oak canopy cover seems to have occurred in less hot and dry regions that enable more intensive land uses. Active restoration is urgently needed to reverse the current decline in canopy cover in Mediterranean evergreen oak woodlands.