Temporal and spatial dynamic of stool uprooting in abandoned chestnut coppice forests

Chestnut (Castanea sativa Mill.) coppice is a man-made forest type that has been managed for centuries in short rotations to rapidly produce woody biomass. These forests, which nowadays cover significant areas within Europe, experience a general neglect and are subsequently being abandoned. Most of them are now over-aged, very dense, and highly monotone. Little is known about their development. The increasing frequency of uprooting events of stools (i.e. a whole stump including the shoots that originated after coppicing), is raising concern among forest managers who fear a progressive expansion of the phenomenon. Our objective was (i) to describe the temporal and spatial patterns of the ongoing uprooting processes, (ii) to identify causes and (iii) to estimate future developments. We have analysed the stool uprooting dynamics in a 100 ha abandoned chestnut coppice and have built an empirical, predictive model to estimate the uprooting probability based on topographic, stand and stool characteristics. Finally, detailed uprooting dynamics were reconstructed at the single gap level for three case studies to characterise the process of gap expansion. Tree-rings were used to date the relevant events. We found that uprooting is primarily caused by precarious tree statics rather than external forcing agents. The empirical model clearly predicts that tall stools located in hollows and gullies are the most likely to uproot. In fact, in this particular situation a non-extreme environmental event suffices to disturb the equilibrium between the higher tree-induced gravitational loads and the weaker root anchorage, resulting in a collapse. Since the stool uprooting is mainly an endogenous process, we expect a progressive increase of this phenomenon with the ageing of abandoned coppices. From the forest manager's perspective, this situation favours a progressive rejuvenation and diversification of the forest structure. On steep slopes, however, where the forests also play an important role in protecting infrastructure, uprooting events might entail some additional risks. Our results have important management implications for foresters.