Forest recovery through applied nucleation: effects of tree islet size and disperser mobility on tree recruitment in a temperate landscape

The plantation of tree islets within deforested areas (applied nucleation) has been proposed as a restoration strategy able to catalyze tree colonization in the matrix with reasonable costs and labor compared to traditional restoration methods such as extensive plantations. However, recent work has questioned its effectiveness in comparison to natural forest regeneration, and observational studies have eventually reported a limited expansion of tree islets after years of planting. To infer the processes affecting the effectiveness of applied nucleation, we used a stochastic model able to predict avian movement and fruit consumption, tree seed dispersal and seedling establishment in a fragmented temperate forest landscape of northern Spain. First, we simulated scenarios of natural regeneration and compared them to applied nucleation in terms of tree colonization in the remaining deforested matrix. Subsequently, in applied nucleation scenarios we assessed the effects of islet size and matrix area recovered on tree recruitment into the matrix. Also, we evaluated if such effects were mediated by the scale of movement of frugivores. Our results showed that applied nucleation triggers matrix colonization by trees at higher rates than natural regeneration because it avoids frugivore entrapment within forest edges. Also, that large tree islets can hamper matrix colonization despite being visited more frequently. If large tree islets offer abundant resources, animals do not need to leave their boundaries, concentrating seed deposition within islets. As expected, such effects depended on the scale of movement of frugivores. Our work highlights that in nucleated landscapes, islets harboring fruit-producing trees should attract animal dispersers, and also foster animal inter-islet movement. Both conditions should be met to ensure seed deposition and seedling establishment in the deforested matrix, promoting forest recovery independent of islet expansion through vegetative growth. To achieve these restoration goals, we propose establishing islets whose size and inter-islet distances matches the scale of movement of main seed dispersers present in the area.