Maud Deniau, Vincent Jung, Cécile Le Lann, Harald Kellner, Benoît Béchade, Thibault Morra and Andreas Prinzing
In 1970, ecologists Janzen and Connell suggested that seedlings located under an adult tree of the same species had a lower survival rate than seedlings at distance, as adult trees attract enemies like detrimental fungi that attack seedlings under this adult. However, some species nevertheless exhibit a high seedling survival under an adult of the same species. This could be due to soil fungi helping, rather than attacking, seedlings under an adult. Indeed, an adult also attracts beneficial soil fungi, like mutualistic mycorrhiza or decomposers, which could improve the nutritional status of seedlings under this adult. This help may increase when the adult is large and surrounded by related tree species.
We studied oaks in a temperate forest. We planted oak seedlings at different distances from adults and followed seedling mortality, budburst and leaf consumption. To determine the effect of soil fungi on seedlings, we applied a fungicide on half of the seedlings. We quantified seedling infection by mycorrhizas and the decomposition rate of oak leaves and of cellulose, a compound found in leaves of all tree species.
We showed that under an adult, seedlings suffered from fungi: specifically, fungi increased leaf consumption by herbivores. Surprisingly, the fungi involved are likely the supposedly beneficial decomposers of oak leaves, which were numerous under an adult and increased the nutritional quality of seedlings, thereby attracting leaf consumers. However, if the adult was surrounded by related adult trees, seedlings profited from fungi: fungi accelerated seedling budburst and thereby photosynthesis. Beneficial soil fungi involved could be one particular genus of mycorrhiza which was abundant under an adult surrounded by related tree species and accelerated seedling budburst. Effects of fungi on seedlings did not depend on the size of the nearby adult.
Overall, under an adult oak, seedlings suffered from fungal decomposers increasing the nutritional quality of seedlings, leading to greater leaf consumption. However, under an adult oak surrounded by related tree species, seedlings were helped by mycorrhizal fungi facilitating resource acquisition. Interactions between adults and offspring seem to be more complex than imagined in 1970, at least for trees.