Pervasive moose browsing in boreal forests alters successional trajectories by severely suppressing keystone species

Anders Lorentzen Kolstad,James D. M. Speed,Erling J. Solberg,Gunnar Austrheim,Laurent De Vriendt

doi:10.1002/ecs2.2458

Abstract

AbstractLarge herbivores can shape young forest stands and determine the successional trajectory of forested ecosystems by selectively browsing palatable species at the sapling stage. Moose (Alces alces) is the dominant vertebrate herbivore in Fennoscandian boreal forests, and high population densities have raised concerns about potential negative effects on ecosystem functioning and properties including biological diversity and timber production. We used 31 herbivore exclosures in Norway to investigate how forests developed after clear‐cutting with or without moose present. We tested how tree demography, abundances of understory plant functional groups, community composition, and plant diversity (including bryophytes) across multiple scales varied with moose exclusion. After seven years, the exclosures were dominated by deciduous trees, including many large rowan (Sorbus aucuparia) individuals, a functionally important keystone species. In contrast, the open plots subject to moose impacts (browsing, trampling, defecation) were dominated by economically important coniferous trees and there was next to no rowan recruitment to taller height classes. The biomass of large herbs and ferns was much greater inside exclosures. This study emphasizes the large immediate effect of moose on early successional boreal forest stands. Landscape‐level alterations caused by reduced deciduous dominance, and a reduction in large flowering herbs is likely to lead to cascading effects on ecosystem functioning. The management of boreal production forests needs to account for the combined effects of silvicultural practices and ungulate herbivory to ensure ecosystem functioning, but this management goal may be jeopardized in our study regions due to drastically reduced abundance of keystone species.

Highlights

Cervids are key drivers of ecosystem dynamics across boreal and temperate forest biomes, where their continuous feeding, trampling, and defecation are important determinants of vegetation dynamics and ecosystem functioning (Hobbs 1996, Fuller and Gill 2001, Wardle et al 2001, Edenius et al 2002, Kuijper et al 2010, Co^te et al 2014, Schulze et al 2014)
As early successional forests are common (e.g., 22% of productive forest area in Norway is in the first and second development classes, typically meaning trees are less than 12 m tall [Statistics Norway 2017]), we interpret the strong immediate effect of moose in our study to represent a landscape-level ecosystem moderation with implications for ecosystem functioning and biodiversity
As cervid browsing in early successional forests has been shown to have long-lasting effects on tree species composition, we predict that the reduced sapling recruitment of the keystone species rowan will have important and negative ramifications in the future

Summary

Introduction

Cervids (deer) are key drivers of ecosystem dynamics across boreal and temperate forest biomes, where their continuous feeding, trampling, and defecation are important determinants of vegetation dynamics and ecosystem functioning (Hobbs 1996, Fuller and Gill 2001, Wardle et al 2001, Edenius et al 2002, Kuijper et al 2010, Co^te et al 2014, Schulze et al 2014). In boreal forests, selective browsing on palatable tree species by cervids shapes successional trajectories and forest canopy composition, favoring the less palatable tree species (Mcinnes et al 1992, Hidding et al 2013). Boreal forests usually become dominated by fast-growing deciduous trees which over the course of a few decades become outcompeted by slow-growing conifers. Conflicts and synergies between herbivory and the forestry sector are common and depend on whether the browsed tree species are economically important for timber or pulp production, in which case herbivores diminish forest revenue (Glo€de et al 2004, Wam et al 2016a). Browsing may favor forestry interests by reducing competition from deciduous species on the commercial tree species (Heikkila et al 2003)

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