The silent shareholder in deterioration of oak growth: common planting practices affect the long-term response of oaks to periodic drought

Oaks play an important role in forest ecosystems and are of great economic value. Therefore, the increasing frequency of oak decline incidents in recent decades is of outstanding interest to foresters and researchers in many fields. Despite the general agreement that drought is of major importance in oak decline, very little attention has been devoted to the condition of root systems during drought. In this paper we tested whether cutting the taproot of oak seedlings, a common forest nursery practice, or coppicing, a common management strategy, play a pivotal role in drought-related weakening of oak plantations. Using dendrochronological methods we analyzed whether Quercus robur stands originating from acorns sown directly in the ground, planted seedlings and coppice managed stands differ in response to drought. All studied stands have shown growth depression as a result of prolonged drought conditions. However, stands originating from direct seeding were less severely affected than those from planted seedlings and stands regrown from sprouts after coppicing. In addition, stands established by direct acorns sowing were characterized by lower 13C/12C isotope ratios (δ13C) in wood, indicating better access to water during its deficit. Therefore, it is likely that intact taproot systems of trees developed from direct seeding are able to reach deeper levels of ground water, and as a result oaks are able to cope more effectively with the difficulties created by prolonged drought conditions. Our study has indicated that common forestry practices may exacerbate drought stress in oak stands, which is considered a major factor causing or predisposing oak decline.