Plant reproductive traits in old and recently-restored temperate forest understories

Recently-restored forests on former agricultural land have impoverished herbaceous understories. This is partly due to the poor colonizing capacity of understory plants, which makes them slow in responding to land use change. Functional traits can be used to compare old and recent understories and provide insight about which plant syndromes are missing after restoration intervention. In this work, we focus on the seed and germination traits of herbaceous understories, investigating how they influence community assembly in restored woodlands. We studied temperate deciduous oak forests in two locations at the centre and southern limit of the European Atlantic region (Loder Valley Nature Reserve, England; and Tragamon Oak Grove, Spain). In each location, we sampled a pair of old and recently-restored forest sites and established 10 permanent plots, where we surveyed understory species abundances every two months for one year. We recorded plant reproductive traits for the species found and measured germination traits in laboratory experiments with seeds collected in situ. Finally, we calculated community-weighted means (CWM) for each trait and plot. The reproductive traits of old and recent forest understory species had significant differences in both locations. Old forest communities had shorter plants that flowered earlier and were more dependent on vegetative reproduction. They produced less seeds, which were heavier and had higher terminal velocity. The germination of these seeds occurred at colder temperatures and was less dependent on light. However, some dormancy-related traits seem more related to bioclimatic differences between the two locations (England and Spain). Old and recently-restored communities vary consistently for several reproductive traits, with old community traits reflecting a poor dispersal capacity and a specialized germination niche in shaded habitats under a deciduous canopy. When designing species mixes for restoration efforts, plants with particular old understory traits should be given priority.