Why the present is not the key to past or future: moving beyond restricted relict habitat conditions to improve outcomes in mountain woodland restoration

Abstract

Introduction: Mountain woodland restoration can enhance upland biodiversity and contribute to nature-based solutions mitigating climate change impacts. However, high-altitude planting of trees and shrubs requires considerable commitment of time and practical effort and should be evidence-based to prioritise scarce resources for efficient restoration at scale. Many mountain woodlands have experienced substantial anthropogenic degradation into remnants that may not reflect past complexity and function. Objectives: We examine where planting can deliver effective outcomes in mountain woodland restoration outside the constraints of using present-day distribution to indicate future potential. Following centuries of woodland clearance and overgrazing, Scotland has pioneered montane scrub restoration for over 30 years. This action offers the opportunity to study whether restoration replicating remaining habitat risks bias, or if outcomes can be improved though wider selection of planting locations. Methods: Our research uses a long-term case-study of Salix lapponum restoration planting in the Scottish Highlands to investigate the abiotic and biotic factors affecting treeline shrub health and productivity. Results: Overall, only a small amount of variation in shrub growth and catkin presence was attributed to local environmental variables related to relict habitat conditions restricted to cliff ledges. There were positive associations with increasing soil fertility, vegetation height, soil depth, and tall herb cover and richness; but negative associations with increasing sedge and rush cover, soil wetness, and (contrary to expectations) soil pH. Conclusions: Using present-day relict habitat as the basis for mountain woodland restoration planning constrains planting to only a subset of potential locations.