Abstract
Green-tree retention is a forest management method in which some living trees are left on a logged area. The aim is to offer ‘lifeboats’ to support species immediately after logging and to provide microhabitats during and after forest re-establishment. Several studies have shown immediate decline in bryophyte diversity after retention logging and thus questioned the effectiveness of this method, but longer term studies are lacking. Here we studied the epiphytic bryophytes on European aspen (Populus tremula L.) retention trees along a 30-year chronosequence. We compared the bryophyte flora of 102 ‘retention aspens’ on 14 differently aged retention sites with 102 ‘conservation aspens’ on 14 differently aged conservation sites. We used a Bayesian community-level modelling approach to estimate the changes in bryophyte species richness, abundance (area covered) and community structure during 30 years after logging. Using the fitted model, we estimated that two years after logging both species richness and abundance of bryophytes declined, but during the following 20–30 years both recovered to the level of conservation aspens. However, logging-induced changes in bryophyte community structure did not fully recover over the same time period. Liverwort species showed some or low potential to benefit from lifeboating and high potential to re-colonise as time since logging increases. Most moss species responded similarly, but two cushion-forming mosses benefited from the logging disturbance while several weft- or mat-forming mosses declined and did not re-colonise in 20–30 years. We conclude that retention trees do not function as equally effective lifeboats for all bryophyte species but are successful in providing suitable habitats for many species in the long-term. To be most effective, retention cuts should be located adjacent to conservation sites, which may function as sources of re-colonisation and support the populations of species that require old-growth forests.
Highlights
Despite their great importance for biodiversity conservation, ecosystem services and climatic control, only 12.5% of the world’s forests are under legal protection while the rest are being exploited or converted for multiple purposes [1]
We address three specific questions: 1) To what extent do bryophytes occupy retention aspens after logging, i.e. are retention aspens promoting lifeboating of bryophytes? 2) To what extent are bryophytes able to re-colonize retention aspens after a stand has re-established, i.e. are retention aspens functioning as structural enrichment for bryophytes? 3) Can retention aspens substitute conservation aspens in terms of maintaining biodiversity and ensuring the long-term persistence of populations? To address these questions, we build a hierarchical Bayesian model that utilises both specieslevel and community-level information in the data, and we use the parameterised model to ask how bryophyte species richness, abundance and community structure may change on an aspen after either retention or conservation
The occurrence and abundance of bryophytes were greater on conservation aspens than on retention aspens (Fig. 2, site type: the 95% highest posterior distribution of m3 is positive in both models)
Summary
Despite their great importance for biodiversity conservation, ecosystem services and climatic control, only 12.5% of the world’s forests are under legal protection while the rest are being exploited or converted for multiple purposes [1]. Retention forestry is an approach where some structures and organisms of the forest are intentionally retained during logging actions, mimicking the biological legacies left by natural disturbances [6,7,9,10,11]. It is applied widely in boreal and temperate forests for biological, ecological and social reasons [10]. Lifeboating, landscape connectivity and structural enrichment for disturbance-phase species are temporally relevant immediately after logging and continuously during the lifespan of the retained structures their importance decreases as the surrounding forest re-establishes. The importance of structural enrichment for forest species increases during the lifespan of the structures
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