Successional patterns of Icelandic subarctic mountain birch woodlands after decades of grazing exclusion

Abstract

Introduction: Degradation of Iceland's native woodlands has mainly been driven by extensive land use including year-round grazing. Large-scale woodland protection and restoration have been initiated in recent decades, but understanding the effects of grazing exclusion and revegetation requires long-term monitoring, a step often overlooked in restoration planning. Objectives: We studied the effects of exclusion of sheep grazing combined with initial revegetation treatments on the succession of two areas of 37.2 km2 formerly grazed lands in Southern Iceland. The natural vegetation is dominated by mountain birch (Betula pubescens ssp. tortuosa) woodlands, the only native woodland-forming tree species in Iceland. Methods: The areas were re-surveyed almost 30 years after livestock grazing ceased. Woodland structural parameters, such as density of trees and recruitment were recorded, as well as the diversity and cover of the understory vegetation. Results: Both areas showed significant increases in tree density, which on average increased from 500 to more than 9000 trees/ha after almost three decades of grazing exclusion. Recruitment also increased significantly, indicating more active natural regeneration. One of the two areas had a significantly increased overstory height. Species richness and diversity of the understory plant community increased within both areas, and taller vegetation had replaced mosses and bare soil. Conclusions: Our study serves as long-term empirical evidence of woodland successional processes after grazing exclusion, and demonstrates that this can be an efficient low-intensity restoration strategy for subarctic woodlands. The study furthermore emphasizes the importance of monitoring to determine the outcome of restoration efforts to guide decision-making in ecological restoration.