Recovery of mature forest microclimate and bryophyte communities in logged forests

Globally, many forest ecosystems that undergo harvesting are adapted to natural disturbances such as fire. However, harvested forests can have depleted biodiversity due to different ecological impacts between harvesting and natural disturbance. To minimise the ecological changes due to forest harvesting, management techniques have been developed that better mimic the effects of natural disturbance. These techniques focus on retaining structural components of the pre-disturbance forest within harvested areas. Retained structures create more mature environmental conditions within the harvested area; these mature conditions encourage recolonisation and development of mature communities. Aggregated retention is an example of a management technique that retains mature forest elements. It involves the retention of patches (aggregates) of trees within harvested units. Retained aggregates have multiple benefits; they provide areas for mature forest species to persist through the disturbance period (“lifeboating”), and they can create more mature forest like conditions within the harvested area through “forest influence” – edge effects that occur within disturbed forest due to proximity to a nearby standing mature forest. Forest influence works through mechanisms such as shading and the reduction of dispersal distance. Aggregated retention and other similar techniques have been implemented across many forest systems throughout the world. However, little is known about how effective they are for various groups of organisms. In Tasmanian forests, aggregated retention has been identified as an effective harvesting method for sustainable harvesting, yet information on how forest influence impacts environmental conditions and community structure is limited. Gaining knowledge on how increasing environmental maturity and the ways in which forest influence can aid in the return of pre-disturbance biological communities is crucial in the adaptive management of retention harvesting systems both in Tasmania and globally. In this thesis, I investigate the impacts of forest influence on important abiotic components, specifically microclimate. I then assess the benefits of increased environmental maturity on recolonisation of bryophyte flora and the role that forest influence can have in bryophyte recolonisation. These topics are addressed across four experimental chapters. The first experimental chapter investigates how environmental conditions within harvested forest impacts on the level of maturity of bryophyte communities (Chapter 2). This is achieved by creating a measure of bryophyte community maturity within harvested forest. Various environmental conditions were then tested to see if they impact the recolonisation of mature bryophyte flora. Following on from this, I investigate whether distance from a mature forest edge (forest influence) can adjust the microclimate of disturbed forests (Chapter 3). Patterns in forest influence on bryophyte communities are then investigated (Chapter 4) to determine whether plots closer to a mature forest edge experience more rapid recolonisation by bryophytes. These questions are tested by sampling microclimate and bryophyte community composition across mature forest/harvested edges and analysing how condition change with distance from an edge. Additionally, as restoration of harvested areas is highly dependent on successional processes, the benefits of increased maturity and forest influence need to be assessed through time. Consequently, Chapters 2 - 4 were based on a chronosequence (space for time) study of sites that were previously clearfelled, to enable temporal impacts to be assessed. Finally, this thesis examines whether isolated aggregates generate the same level of microclimatic forest influence as the forest bordering the harvested area (Chapter 5). This comparison will help determine whether aggregated retention silviculture is an effective technique in generating environmental maturity within harvested forest. Results of the thesis showed that mature environmental conditions within harvested areas did increase the maturity of bryophyte community compositions. Microclimate conditions were shown to be of particular importance in determining the community maturity of bryophytes within harvested forest. Results from Chapter 3 then showed that forest influence is an ecological process that can create mature forest microclimate conditions. Areas next to a mature forest edge experienced microclimatic conditions that were more similar to mature forest conditions within harvested areas. As well as its impact on microclimate, Chapter 4 showed that forest influence also impacted bryophyte communities. Bryophyte communities recover quicker in areas next to an edge compared to areas further away. In the final experimental chapter (Chapter 5), results showed that aggregated retention is an effective method to create microclimate forest influence within harvested areas. Overall, I have shown that forest influence is effective at creating mature forest conditions within regenerating harvested forest, and that bryophyte recolonisation is aided by increased environmental maturity. This information can be used to refine management techniques, such as aggregated retention, which are designed to encourage forest influence for facilitating the successful restoration of harvested areas. The temporal response to forest influence and the development of mature environmental conditions shows that future studies on the benefits of retention forestry should consider time since disturbance when reporting results.