Offspring may succeed well next to their relatives, but it needs particular traits

There is ongoing debate about whether offspring perform best next to phylogenetically distantly related adult neighbours (due to the scarcity of enemies and competitors) or next to closely related adults (due to the abundance of mutualists). Here we hypothesize that relatedness of adult neighbours affects which traits confer performance rather than performance itself.

We studied seed removal, seed germination and sapling growth in sessile oaks (Quercus petraea and hybrids) and how they depend on size, shape and other traits, under both closely and distantly related canopies, manipulating offspring density, presence of insects and fungi, and spatial proximity to oaks.

We found that phylogenetic distance of adult neighbours affects only little the performance of offspring but strongly affects which traits confer performance on offspring, in particular the size and shape of seeds and saplings. Differences in trait-performance relationships mostly disappear once insects or conspecific competitors are excluded (albeit exclusion of fungi reinforced these differences). Effects of phylogenetic distance of neighbours were not replaceable by the percentage of gymnosperms among neighbours or by the environmental conditions considered.

We suggest that, by responding to a biotic micro-mosaic of selection pressures, sessile oak flexibly succeeds in diverse neighbourhoods. Sessile oak might maintain the potential for both convergence with and divergence from phylogenetically distantly related species, thereby reinforcing or eroding phylogenetic signal of niches.