Bryophyte species composition at the stand scale (1 ha) – Differences between secondary stands half a century after clear-cutting and older semi-natural boreal forests

Mats Dynesius,Jörgen Olsson,Joakim Hjältén,Therese Löfroth,Jean-Michel Roberge

doi:10.1016/j.foreco.2020.118883

Mats Dynesius, Jörgen Olsson + Show 3 more

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https://doi.org/10.1016/j.foreco.2020.118883

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Abstract

A growing proportion of the boreal biome consists of managed even-aged secondary forest stands regenerated after clear-cutting. Many disturbance-intolerant species may not be able to recolonize or reach their original abundance in these stands before the next clear-cutting, potentially causing large-scale biodiversity losses. Boreal bryophytes (mosses and liverworts) include many species intolerant to clear-cutting, and at small spatial scales species richness and occupancy has been shown to remain changed in secondary stands half a century after logging. To assess if such persistent changes occur also at the stand scale, we listed and estimated cover of all bryophyte species in 1-ha plots, comparing 14 secondary stands originating from clear-cutting 40–60 years earlier with 14 older semi-natural stands. The large plots also made it possible to assess differences in occupancy and abundance for more bryophyte species than in previous studies. Species composition differed significantly for both mosses and liverworts, but unlike earlier studies, we could not detect any significant difference between stand types in species numbers. Thirteen species were significantly associated with semi-natural stands and the total cover of liverworts was less than half in secondary stands. Secondary stands had significantly fewer species typically occurring under shady conditions and/or mostly growing on “tree substrates” (dead wood and/or bases and stems of living trees). Ordination analysis further emphasized the importance of shade and suitable deadwood substrates; the among-plot variation in bryophyte species composition was related to amount of coarse deadwood as well as to gradients from shady spruce dominated to open pine dominated stands and from polar- to equator-facing slopes. Besides lack of suitable habitat conditions in secondary stands, dispersal limitation may have caused a colonization time lag for some species. The clear importance of stand scale habitat conditions for bryophyte species composition calls for management adaptions to facilitate life boating and/or recolonization by ensuring availability of shade, coarse decomposing logs, and specific deciduous tree species (Populus, Salix, Sorbus) in secondary stands.

Highlights

After World War II, a forestry system based on even-aged stands created by clear-cutting has become dominant in the vast boreal biome (Ostlund et al, 1997, Josefsson and Ostlund, 2011, Bergeron and Fen ton, 2012, Kuuluvainen et al, 2012)
We discuss how the differences in bryophyte species compo sition between secondary and semi-natural stands could be explained by differences in habitat conditions and by a recolonization lag
These correlations are most likely the results of the sensitivity to microclimate of many bryophyte species as well as of variation in amount of decomposing wood, potentially excluding some dead-wood species from the stands having least of this substrate (e.g., Hofmeister et al, 2015, Taborskaet al., 2020). Could these factors cause differences in bryophyte communities between semi-natural and secondary stands, i.e. do they differ between the two forest stand types? volume of coarse dead wood was more than three times higher in semi-natural than in secondary stands (Table 1), reflecting the long period after clear-cutting with small trees and low or no input of coarse dead-wood (Fridman and Walheim, 2000, Stenbacka et al, 2010)

Summary

Introduction

After World War II, a forestry system based on even-aged stands created by clear-cutting has become dominant in the vast boreal biome (Ostlund et al, 1997, Josefsson and Ostlund, 2011, Bergeron and Fen ton, 2012, Kuuluvainen et al, 2012). The less affected of these semi-natural forests support relatively many wood-inhabiting and late-successional organisms (Gustafsson et al, 2004, Stenbacka et al, 2010) Because such forests are continuously clear-felled, Swedish boreal forests will in the future consist almost entirely of secondary even-aged stands or protected areas

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