Complex biotic interactions mediated by shrubs: revisiting the stress-gradient hypothesis and consequences for tree seedling survival

1. Using nurse shrubs to improve tree seedling establishment in stressed environments is a common practice in forestry. Recent refinements of the stress-gradient hypothesis suggest that positive nurse effects occur under intermediate stress and decline in the harshest conditions. Additionally, indirect facilitation is expected in low-stressed/productive systems according to the initial stress-gradient hypothesis. However, there have been fewer investigations into the use of nurse shrubs to decrease herbivore impacts in stressed systems compared with the role of nurses on stress alleviation. This raises the need to clarify conditions in which the use of nurse shrubs would be most appropriate. 2. We conducted a field experiment in the sand dune forest of southwest France characterised by severe summer water shortages and the presence of different types of wild herbivores. In 2015 and 2016, Pinus pinaster seedlings were planted under shrubs (Arbutus unedo) or in shrub-removed locations, in...