Dynamics of deadwood and saproxylic beetles in forest reserves

Deadwood is a vital prerequisite for a quarter of all organisms living in a forest. The future intensification of forest management will pose a threat to many wood inhabiting (saproxylic) insect species. In forest reserves we quantify the formation and decay process of deadwood and investigate the importance of micro-habitats for saproxylic beetles.

Deadwood is a key element for the survival of saproxylic species (species associated with the decay of wood on living and dead trees) which represent more than 20% of the forest species. It is also known to play an important role in carbon sequestration, nutrient cycling, water retention and for the natural regeneration in mountain forests. Furthermore, deadwood can also contribute to protection against falling rocks on steep slopes. Deadwood is therefore an essential component of the forest ecosystem and is considered to be an important indicator for forest biodiversity, especially for invertebrate diversity, and for sustainable near-to-nature forest management in Europe. The amount of deadwood is an excellent indicator for the conservation value of a forest.

Similarly to Central Europe, deadwood has strongly decreased in Switzerland during the last centuries due to intense forest exploitation and fuelwood harvesting. Moreover, modern forestry is usually based on rotations shorter than the natural longevity of tree species. Consequently, the number of large old trees which are likely to be an important source of deadwood in the forest is therefore relatively low. Nevertheless, the amount of deadwood has increased in Europe in the last decades. In Switzerland, since approximately World War II, deadwood amount has been increasing and reaches today an average amount of more than 30 m³/ha. This increase is due to windthrows, to an increased acceptance of the ecological value of deadwood and to the low wood prices, which makes the exploitation of many forest stands uneconomical. Although this average amount of dead wood permits the conservation of many saproxylic species, deadwood quantities are distributed very heterogeneously and this substrate is lacking in many forest stands. Furthermore, at the regional scale, deadwood amounts are still small in the Jura Mountains and on the Swiss Plateau. Consequently, many saproxylic species are still suffering from habitat loss and are threatened.

Several years ago, Switzerland has launched an initiative to increase deadwood for example through setting aside natural forest reserves and deadwood islands and also to promote deadwood in managed forests. Forest stands with no logging activity for a long period may have features similar to primeval forests, such as veteran trees, suitable habitat structures and continuously available deadwood. In such stands, many saproxylic beetle species can survive, whereas they have largely disappeared from managed forests. However, deadwood alone cannot guarantee the survival of all saproxylic species. Further environmental factors such as connectivity, habitat tradition, exposition, stand openness and tree micro-habitats are also influencing the saproxylic species.

In order to improve deadwood management and saproxylic species conservation in Switzerland, we launched a research project focusing on both the deadwood dynamics and on saproxylic beetles.

Deadwood dynamics

For the long-term conservation of saproxylic species, deadwood should be available at the right place and time with the right quality and quantity in forests. It is therefore crucial to know more about tree mortality and decomposition rates when planning deadwood conservation strategies. However, there is a lack of knowledge on forests developing under near-natural conditions such as natural forest reserves. It is therefore compulsory to improve the knowledge in this field in order to establish guidelines for deadwood management. The following main issues are addressed in this project part: (i) Establish the decay rate for different tree species abundant in Switzerland and (ii) establish a computer model simulating the deadwood turnover under Swiss conditions.

Saproxylic beetles

From the perspective of biodiversity conservation, there should be enough deadwood in the forests to maintain the saproxylic species and their communities. But how much deadwood is enough for the Swiss saproxylic species assemblages? In order to convince forest owners and policy makers, more hard data on saproxylic beetles living under specific conditions are needed. The following main questions are addressed in this project part: (i) How does deadwood influence the saproxylic beetle fauna? Are there thresholds? (ii) How does the saproxylic beetle fauna develop after abandoning management? (iii) Which ecological factors influence the saproxylic beetles in view of an efficient conservation?

The common goal of both parts of the project is to establish guidelines for forest owners and foresters to improve the conservation of saproxylic species in forest.

Since 2007, WSL monitors, in cooperation with ETH Zurich and the Federal Office for the Environment, 49 natural forest reserves covering different forest types in Switzerland. Many of these forests reserves have already been inventoried in the past and the available data represent a unique opportunity to study the mortality and the decay of deadwood over a long period. In these forest reserves, which have been under protection for 30 to 50 years, many trees have been individually identified in permanent plots. This allows to determine the date of the death of each single tree. The obtained chronosequences represent the base to establish a deadwood dynamic model.

To study the saproxylic beetles, we focus on beech forests, as it is the natural dominant tree species in Central Europe. To study the effect of forest management on saproxylic beetles we established a management intensity gradient from managed forest to young and old forest reserves.

Saproxylic beetles are sampled using flight interception traps. For further insects groups such as epigeic species, we also installed a pitfall trap on each sampling plot. Several environmental factors (deadwood, tree microhabitats, temperature, insolation, soil-vegetation, etc.) are inventoried around each trap.