Limited seed dispersal, allelopathy and unfavorable microclimatic conditions prevent the recovery of oak forests within Eucalyptus plantations

Abstract

Plantations of exotic trees were proposed as an alternative to restore native forests in degraded lands, because they can attract seed dispersers and modulate the microclimate beneath their canopies. However, the large-scale introduction of exotic tree plantations can negatively affect native species by high competitiveness for soil resources and light, while they can also release allelopathic compounds that inhibit the germination and growth of other plants. In this study, we tested the suitability of Eucalyptus camaldulensis plantations to assist the restoration of degraded lands in central Mexico, which were originally covered with oak forests. For this, we sampled soil seed banks within forest remnants and plantations, which showed that the animal-mediated dispersal of oak propagules from forests inwards plantations is reduced. Additionally, we conducted germination trials that evidenced that the leaves of Eucalyptus trees contain allelopathic compounds that inhibit the germination of oaks seeds, also showing that these compounds accumulate in the soil of plantations. We finally conducted field experiments in which soil form forests and plantations were translocated between these two habitats, and acorns were sowed on both soil types. These field experiments revealed that, besides the accumulation of allelopathic compounds in soils of plantations, oak recruitment in these habitats is also impaired by unfavorable microclimatic conditions. Therefore, we conclude that Eucalyptus plantations are not promoting the recovery of native oak forests in central Mexico and, instead, alternative ecological restoration methods should be applied for reaching this aim.