Shrub Understorey Clearing and Drought Affects Water Status and Growth of Juvenile Quercus Suber Trees

Shrub encroachment is a global phenomenon occurring in a wide range of ecosystems, including in tree plantations, which is known to affect ecosystem functioning and productivity. Forest management in tree plantations (e.g. shrub clearing, tree thinning and pruning) often aims to increase wood and non-woody products yield while optimizing other ecosystems services such as carbon sequestration. However, little is known on the effect of shrub clearing on tree growth and tree water status in juvenile tree stands. Under climate change scenarios that predict increased frequency of extreme events such as droughts, management during the early stages of stand plantations may be crucial for its sustainability. This study aimed to explore the combined effect of shrub understorey removal and drought in two cork oak (Quercus suber L.) juvenile plantations, with similar stand age but distinct soil types and forest management, on tree water status, assessed through leaf water potential measurement, and on tree growth. The study was conducted during two consecutive years, including a drought year. Results revealed that the shrub understorey removal positively affected the physiological response of trees in both plantations but only affected tree growth (i.e. total height and leaf area) in the plantation with a higher tree density and shrub biomass. Shrub biomass and species composition most probably had a determinant effect on tree growth due to soil water competition. Our results emphasize the need for site-specific forest management during the early stages of cork oak plantation for maintaining its future sustainability and provision of ecosystem services.