Evaluating Quercus ilex afforestation success on former agricultural land under increasing aridity

Abstract

In the 1990s, afforestation of former agricultural land with Quercus ilex was largely applied in Spain under the Common Agricultural Policy in order to recover woodlands especially where climatic conditions were limiting for holm oak natural regeneration. In this study, we investigated if holm oak afforestation success is limited by environmental factors, especially aridity, as it is the case for natural regeneration. Around 25 years after plantation was performed, we evaluated afforestation success of 4620 holm oak trees from 154 holm oak plantations located in the eastern Spanish provinces of Teruel, Castellón and Valencia along a spatial aridity gradient (MAP/PET ratio ranges from 0.32 to 0.68). Afforestation success was determined in terms of tree survival, height and reproduction ability. Afforestation success was analyzed as a function of a set of variables related to climatic (aridity, drought magnitude), soil (fertility, texture, infiltration capacity, rock cover), topographic (slope) and planting (tree density) conditions. Overall, afforestation success was high and no aridity threshold prevented tree survival, growth or reproduction. More than 70% of the trees planted survived 25 years after planting. Almost one third of them exceeded 2 meters and over half were able to reproduce. Afforestation success was positively influenced by soil fertility and infiltration capacity, but negatively affected by aridity, drought magnitude, slope and planting density. Random effects, such as tree nursery and plantation owner, also strongly influenced overall afforestation success. Beyond identifying limiting factors for Q. ilex afforestation success, our results allow a better understanding of ecological filters that constrain Q. ilex natural regeneration in harsh climatic conditions.