Moose effects on soil temperatures, tree canopies, and understory vegetation: a path analysis

Anders Lorentzen Kolstad,Erling J Solberg,Bente J Graae,James D M Speed,G Richard Strimbeck,Gunnar Austrheim

doi:10.1002/ecs2.2966

Anders Lorentzen Kolstad, Erling J Solberg + Show 4 more

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https://doi.org/10.1002/ecs2.2966

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Abstract

AbstractLarge vertebrate herbivores are ubiquitous and increasingly numerous in boreal forests where they are known to influence ecosystems in many ways. However, separating the direct effects of herbivores from their indirect effects on plant communities via forest structural changes and microclimate remains unexplored, limiting the predictability of herbivore impacts. We used an exploratory path analysis approach to investigate potential mechanistic pathways between herbivore removal, forest canopy cover, soil temperature, and understory vegetation dynamics. Moose (Alces alces) were excluded from 15 recently clear‐cut boreal forest sites in Norway using 20 × 20 m fences with paired open un‐fenced plots. Soil temperatures and vegetation data were recorded after eight years (total n = 98). Moose exclusion reduced summer soil temperatures but not winter soil temperatures, leading to no net effect over 344 d. Path analysis showed a strong positive effect of moose exclusion on canopy cover and subsequently reduced summer soil temperatures. There was some support for an increase in the dominant grass species Avenella flexuosa with increasing summer soil temperatures, but neither temperature, canopy cover, nor A. flexuosa had clear links to plant species densities. Moose exclusion directly increased herb biomass and shifted understory species composition toward less shrub‐dominated communities, resulting in increased species densities of vascular plants and bryophytes. Our results indicate that in early‐successional boreal forests, direct effects of large herbivores on plant communities are clearly visible, and indirect effects are detectable but much weaker.

Highlights

Large herbivores can be seen as biological disturbance agents that modify and shape the structure of ecosystems from the level of single trees (Danell et al 2003) to entire forests (Kuijper et al 2010, Eichhorn et al 2017)
A recent systematic review of the effect of large herbivores on multiple aspects of vegetation and diversity highlighted the large number of contingent factors that makes it difficult to reach a synthesis for a generalizable mechanism (Bernes 2018)
In 2017, summer soil temperatures were on average 0.62°C (Æ0.28 standard error [SE]) higher in open plots compared to exclosures (Appendix S1: Fig. S6), and this cooling effect inside exclosures was stronger in late evening when soil temperatures were the highest (Appendix S1: Fig. S7)

Summary

Introduction

Large herbivores can be seen as biological disturbance agents that modify and shape the structure of ecosystems from the level of single trees (Danell et al 2003) to entire forests (Kuijper et al 2010, Eichhorn et al 2017). By altering vegetation characteristics such as canopy structure and composition, or compacting or mixing the soil, these animals can indirectly affect ecosystem properties such as the microclimate, litter quality and quantity, decomposition, and nutrient mineralization (McInnes et al 1992, Kielland et al 1997, Kielland and Bryant 1998, Ritchie et al 1998, Kolstad et al 2018a). This again influences ground and soil biota (Wardle et al 2001, Suominen et al 2008, Andriuzzi and Wall 2017), tree seedling growth, and mycorrhizal infection. Long-term effects (>40 yr) of deer activities on diversity are less studied than immediate effects, but are probably common, and result from alternate successional pathways and legacy effects from early plant regeneration stages (Hidding et al 2013, Nuttle et al 2014)

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