Through thorns to trees : regeneration pathways in Southern Swedish pastures landscapes

Sammanfattning

Wood-pastures in Europe are shaped by the interaction between grazing, shrub dynamics, and tree regeneration. In these landscapes, thorny shrub patches have been proposed to function as protective refugia that facilitate tree regeneration establishment and browsing escape. This study investigated regeneration pathways in grazed pastures in Southern Sweden and evaluated whether shrub characteristics influence the establishment and survival of tree regeneration under herbivore pressure. Field data were collected in eight cattle-grazed pasture systems differing in woodland structure and connectivity to surrounding forest. Shrub patch size, shrub species composition, distance to forest edge, and regeneration characteristics were measured along transects. Generalized Additive Mixed Models (GAMMs) and Linear Mixed Effect Models (LMMs) were used to assess relationships between shrub structure, regeneration establishment, and browsing escape thresholds. Tree regeneration establishment is positively associated with shrub patch size. In pastures bordering forests, regeneration probability declined with increasing distance from the forest edge, indicating spatial limitations in dispersal and establishment. Larger shrub patches partially compensated for this effect, allowing regeneration to establish at greater distances from forest edges. Shrub patch size requirements increased substantially between demographic stages: larger shrub patches were required for saplings to reach browsing escape heights than for initial establishment. Shrub species composition also influenced regeneration patterns. Patches containing Rubus and Juniperus were associated with significantly higher regeneration probabilities, suggesting that structural and defensive shrub traits play an important role in browsing protection. These findings indicate that thorny shrub patches are strongly associated with regeneration establishment and allow regeneration to escape grazing/browsing in wood-pasture systems. I conclude that shrubs provide an important pathway for tree regeneration under grazing/browsing pressure, with shrub establishment and growth strongly shaping woodland dynamics in semiopen landscapes.