Spatio-temporal patterns of defoliation in agro-forestry Quercus ilex L. systems

Agroforestry systems such as holm oak (Quercus ilex subsp. ballota) dehesas face increased vulnerability due to the abandonment of sustainable practices, extreme droughts, and emerging pathogens. Despite the adaptability of Q. ilex to adverse conditions, signs of deterioration such as defoliation, reduced growth, and increased mortality have been observed in recent years. However, few national-wide studies have quantified the spatial and temporal components of defoliation in Q. ilex dehesas. This study aims to evaluate the spatio-temporal dynamics of defoliation in Q. ilex dehesa systems distributed across western Spain. Furthermore, we aim to study the potential effects of climate and site conditions on defoliation in Q. ilex stands. Defoliation was analysed over a 31-year period (1987–2018) in 254 monitoring plots, examining correlations with site variables (elevation, latitude, slope, orientation, soil texture, nitrogen, and carbon) and climate factors (temperature, precipitation). Average defoliation ranged from 17.9% to 21.5%, with northern Q. ilex dehesas in cooler, wetter regions exhibiting less defoliation than southern populations in harsher environments. Between 1987 and 1998, defoliation increased markedly across the region, especially in the northern dehesas of the study area (up to an 88% rise), compared to a 40–50% increase at more favourable sites. Defoliation was negatively correlated with soil nitrogen content, latitude, and elevation, and positively correlated with clay content and average annual temperature. Therefore, the forecasted harsher climatic conditions may contribute to increased defoliation of entire populations of Q. ilex growing in dehesas. This could threaten the persistence of these agroforestry systems and the ecosystem services provided by them, especially in the southern populations.