In this post Ludovic Henneron discusses his recent paper ‘Forest management adaptation to climate change: a Cornelian dilemma between drought resistance and soil macro-detritivore functional diversity‘

Climate change is a major threat for world’s forests. Hence, an increasing number of climate-induced forest die-offs are expected to occur in the future as a result of more frequent and intense droughts. This could greatly alter ecosystem services supply and have a positive feedback on climate change as it will alter the carbon uptake and sequestration capacity of forest ecosystems.

Forest management adaptation to climate change is therefore necessary to increase the resilience of forest ecosystems to climate-related stress. A growing number of studies show that a reduction of stand density through an intensification of the thinning regime can increase their resistance to drought by reducing competition for water. This is thus a promising management option that could help mitigate the increasing water stress risk and subsequent tree mortality resulting from climate change.

However, the impact of this management change on soil biodiversity has been poorly assessed, despite the growing recognition of its importance for terrestrial ecosystem functioning. Among soil organisms, soil macro-detritivores such as earthworms play a key role for ecosystem processes such as litter decomposition, carbon and nutrient cycling, soil structure maintenance and for the preservation of soil fertility and forest productivity.

In our study ‘Forest management adaptation to climate change: a Cornelian dilemma between drought resistance and soil macro-detritivore functional diversity’, we took advantage of a large-scale, multi-site experimental network of long-term forestry sites experimentally manipulating tree canopy biomass through thinning operations in oak Quercus petraea forests to assess if forest management adaptation to climate change by stand density reduction alters soil macro-detritivore assemblages. A total of 33 stands located within a wide geographic area, i.e. the northern half of France, were studied. It encompasses a variety of soil types and climatic conditions, as well as a large gradient of stand density, i.e. stand basal area varying from 2.5 to 43.7 m².ha^-1, and stand age, i.e. from 18 to 174-year-old.

Though the global soil macro-detritivore abundance and diversity showed little response to stand density management, this hides substantial changes of community composition. Hence, we observed contrasted responses of some taxonomic and functional groups. Millipede abundance and diversity were strongly impeded by stand density reduction because of a less buffered microclimate and a subsequent higher frequency of extreme microclimatic events such as drought or frost.

More importantly, we found in mull stands that earthworm species that act as ‘soil engineers’ were strongly affected and this had a cascading effect on soil functioning. Endogeic earthworms were highly favoured by stand density reduction as a result of a well-developed herbaceous vegetation and was slightly associated with an increase of soil organic matter mineralization. Anecic earthworm abundance decreased and was strongly associated with a decline of forest floor dynamics.

Overall, our study provides strong evidence that reductions of stand density will have substantial impacts on soil macro-detritivore assemblages and cascading effects on soil functioning, particularly in mull stands. As low density stands have a less buffered microclimate, the frequency and severity of extreme microclimatic events on the forest floor is likely to further increase in the future with a warmer and more extreme climate. Therefore the impacts of stand density reduction will likely be even more pronounced than observed with the conditions today. In this context, preserving the highest biodiversity inherited from past management will be particularly important to increase the probability to host species being adapted to these new climatic conditions and thus increase the probability of maintaining a high functional diversity level and an efficient ecosystem functioning, i.e. the so-called insurance hypothesis. Hence, managing stand density of oak forests at an intermediate level, i.e. 25 m² ha^-1, appears to be best to optimize the trade-off between improving forest resistance to climate change and ensuring the conservation of functional diversity to preserve forest ecosystem functioning and stability.