Heavy crop tree thinning can enhance net overyielding in Scots pine-sessile oak mixed stands by reducing pine tree mortality

Scots pine ( Pinus sylvestris L.) and sessile oak ( Quercus petraea (Matt.) Liebl.) are species associated with drought resistance and biodiversity, respectively. Therefore, mixed stands of the two species promise stable, diverse and productive populations. However, the information about how to manage this mixture is still scarce. Here, we studied the growth of mixed and monospecific stands of Scots pine and sessile oak along an ecological gradient across Europe (France, Germany Poland and Spain), focusing on the impact of heavy tree crop thinning compared to unthinned conditions. Firstly, we analysed the effect of tree crop thinning on the gross and net stand growth in mixed compared with monospecific stands. Predominantly, the effects were negative on gross growth and neutral or positive on net growth. Secondly, we revealed that due to the similar ecological traits of both species, there was no significant overyielding in terms of gross growth and even underyielding in net growth. However, by heavy tree crop thinning underyielding was avoided. Thirdly, we found that although the productivity of pure and mixed stands was relatively similar, without thinning the mortality in mixed stands increases. Heavy tree crop thinning counteracted the decline of pine, promoted oak growth, and avoided growth losses at stand level. The results mean that future research into mixed stands and especially studies and comparisons between pure and mixed stands should pay more attention to the previous silvicultural treatment, possible anticipation of mortality, and the resulting difference between gross and net productivity of the stocks. • Heavy crop tree thinning decreases gross productivy in both mixed and pure stands. • Net productivy is higher in thinned pine-oak mixed stands than in unthinned ones. • Sessile oak benefits from Scots pine admixture in both thinned and unthinned stands. • Scots pine shows lower growth in mixed than in monospecific stands. • Scots pine mortality increases in unthinned stands when mixed with sessile oak.