Deadwood residence time in alluvial hardwood temperate forests – A key aspect of biodiversity conservation

Abstract There are hundreds of scientific papers dealing with the issues of biodiversity and extinction debt in temperate forests, both of which are partly related to the absence of deadwood. Some studies have derived habitat thresholds for deadwood (according to species, size, spatial distribution) as baselines for management recommendations. But a majority of studies have one important knowledge gap – how long does it take for deadwood to decompose? To derive the residence time of deadwood, we used a 40-year time series of 4 censuses in European lowland hardwood temperate forests. The deadwood residence of 2252 logs of 5 main tree species was calculated using Bayesian Survival Trajectory Analysis. Pedunculate oak ( Quercus robur ) had the longest residence time, with the largest logs decomposing for an average of 62 years. Next was narrow-leaved ash ( Fraxinus angustifolia ) with a residence time of about 42 years. Field maple ( Acer campestre ) and hornbeam ( Carpinus betulus ) had a similar residence time – with the largest logs averaging 37–39 years. Elm ( Ulmus sp.) had the fastest decomposition of large-sized logs – estimated at 24 years. Generally, small-sized logs of all investigated species decomposed quickly. Conversely, large-sized logs of hornbeam and ash decomposed as quickly as their intermediate-sized logs. Large-sized logs of elm decayed even faster than intermediate-sized logs. An analysis of the position of field maple logs during decomposition showed that those decomposing in the air or in water decomposed slower than logs “on the ground”, with a half-life of 40 years (compared to 28 years on the ground) and a residence time of 54 years (44 years on the ground). Our results suggest that the management of deadwood should focus on (i) leaving the largest logs, which took the longest time to reach advanced decay stages, and (ii) a diversified approach regarding the individual residence time of different tree species.