Biodiversity and forest management
Plantation forest management has changed considerably over the last century. Plantations were once established and managed principally for timber biomass production. Increasing awareness over the last 20 years of the benefits of exploiting the potential of plantations to provide ecosystem and social services (such as water quality, biodiversity conservation, carbon sequestration and recreation) has led to the integration of environmental objectives in forest management plans and policy.
The degree to which forest management focusses on biodiversity considerations depends on the purpose of the forest plantation. However, the majority of forest management plans developed in the 21st century include measures aimed at biodiversity conservation, as attention continues to focus on halting biodiversity loss.
In Ireland a number of initiatives support forest management for biodiversity, including the National Biodiversity Plan, Forest Biodiversity Guidelines, the Native Woodland Scheme, and the Forestry Environment Protection Scheme. However, the conservation status of some important protected habitats and species continue to decline, according to the most recent report by NPWS to the European Commission on the status of protected habitats and species in Ireland.
Research has demonstrated that afforestation may have either a positive or negative impact on biodiversity. The magnitude and direction of this effect is influenced by the land use that preceded forest planting and by forest management practices that include the tree species planted, the forest configuration and techniques used to thin and harvest crop trees.
The BIOFOREST research project and the PLANFORBIO research programme were developed to address gaps in the knowledge of forest biodiversity in Ireland. The ultimate goal is to inform future forest management policy and practice, with a view to developing an environmentally sustainable forestry sector in Ireland.
Forest management in Ireland:
This body of research has shown that plantation forests in Ireland are home to a wealth of plants and animals, including species of conservation concern. Shrub and understorey vegetation, including non-crop broadleaves that may colonise open space in forest plantations, makes an important contribution to the biodiversity of these forests. As well as plantation forests research has also been undertaken in semi-natural woodlands to underpin the protection of existing native woodlands and derive an understanding of factors that are important for biodiversity in these woodlands which will help inform plantation management.
In Ireland, one of the major constraints on the expansion of plantation forests at present is the restriction on planting in certain habitats, in order to avoid negative impacts on Ireland’s objectives under the EU Birds and Habitats Directives. The BIOPLAN project demonstrated that targeting areas of particularly poor biodiversity value for afforestation (such as improved pasture) is desirable to maximise the contribution of plantation forests to biodiversity conservation. Combined analysis of data from the BIOFOREST and FORESTBIO projects has shown that afforestation of sites on former woodland or adjacent to existing woodland increases the biodiversity of woodland specialist, with plantations established using native tree species having the most similar communities to native woodlands.
The FORESTBIO project demonstrated that species richness is typically high and community composition distinct at the beginning and/or end of the commercial forest cycle and richness lowest during the closed-canopy middle stages, particularly in conifer plantations. This is due to the influence of the canopy on light levels within the plantation. Higher light levels lead to better structural diversity, which is important for invertebrates and birds. The practice of clear-felling and then replanting of large areas of forest provides few benefits to biodiversity, though some species are seen increase in abundance during the second rotation. Although some retention of species between rotations is observed, particularly for birds, most species found in commercially mature forest do not persist after clear-felling, and forest communities begin to develop again upon replanting. While it is difficult to study the effects of planting mixed tree species on biodiversity of commercially mature plantations, due to the lack of mixed forests of suitable ages in Ireland, the FORESTBIO project provided some evidence that the inclusion of a broadleaved species in conifer plantations has little impact on biodiversity, though does provide increased habitat for (and thus incidence of), species adapted to living in broadleaved trees. It is likely that mixed tree species planting can only have a significant positive impact on forest biodiversity where either choice of species mixes, planting configuration or subsequent management of the forest ensure that the growth of secondary tree species is not suppressed by faster growing commercial conifers.
This research revealed that patterns of biodiversity observed in plantation forests are related to the complexity of vegetation structure beneath the forest canopy, particularly shrub and understorey layers. More complex habitats support a greater diversity of species, and the provision of structural complexity will often be a central aim of management for biodiversity. Detailed comparison of native woodland with plantation forests during the FORESTBIO project revealed the environmental divers most likely to enhance plantation biodiversity; these can then be accommodated within management plans. To facilitate the implementation of biodiversity management, the FORESTBIO project also developed a suite of environmental biodiversity indicators. These include coarse woody debris which is positively related to diversity of a range of taxa in Irish plantations, despite the very low levels of deadwood in Irish forests, particularly in plantations, by international standards.
Open spaces can contribute to higher biodiversity within plantation forests as they allow for the development of structural complex layers. The Irish Forest Biodiversity Guidelines (2000) state that 15% of forested area should be incorporated into Areas for Biodiversity Enhancement (ABE). Forest roads enhance habitat diversity within plantation, though beyond the recommended minimum of 15m no significant increase in biodiversity is observed.
Structural complexity can, in some contexts, be affected by grazing and the BIOPLAN project identified a relationship between plant species composition and abundance and herbivore grazing levels.
Forest management policies and practices into the future will have to be developed in the context of climate change. The BIOPLAN project investigated the potential risk of climate change to Irish forest biodiversity and concluded that conservation management plans need greater focus on the potential impacts of climate change on species’ ranges, and inter-specific interactions, in order to ensure that conservation measures are effective in the long-term.