Spatial patterns of seedling emergence and survival as a critical phase in holly (Ilex aquifolium L.) woodland recruitment in Central Spain

Abstract

Seedling emergence and survival was monitored over 3 years in a holly woodland and the surrounding habitats, located in Central Spain. Ten random quadrats were established in each of the following microhabitats: inside the closed holly woodland, in small canopy gaps of this woodland, at the woodland edge, in the adjacent open grassland, under isolated fleshy-fruited shrubs in the grassland and in an adjacent oak woodland.

Seedling emergence was maximum in closed holly woodland, from 7 to 51 seedlings m−2, and in the gaps, from 3 to 40 seedlings m−2. At the edge and under the shrubs emergence was lower than in the holly woodland (between 1 and 4 seedlings m−2), while no germination was detected in the grassland microhabitat. Under the oak woodland canopy, seedling emergence was approx. 4 seedlings m−2. Survival after the first year following germination was low inside the holly woodland, from 0 to 2% under the closed holly canopy and from 0 to 8% in the canopy gap. Survival was higher at the edge (between 25 and 54%) and under the oak woodland (27%), while under the shrubs the mortality showed a high interannual variability (from 9 to 80%). Mortality inside the holly woodland was due to the combination of a very low light availability and a high soil disturbance, the establishment in the gaps being slightly better than under the closed canopy. The oak woodland presents better conditions for survival, due to higher light levels, and lower disturbance episodes. At the edge and under the shrubs greater light levels, a higher incidence of invertebrate herbivory and lower edaphic humidity were present.

These holly populations show patchy regeneration dynamics, with an apparent propagule-seedling conflict, due to the uncoupling of emergence and survival, resulting in a low net annual incorporation. Regeneration within the holly woodland is very limited and vegetative self-maintenance is the most viable alternative means. However, holly appears to exploit the forest ecotones and shrub invasion through sexual propagation of a young population away from parental influence.

This work deepens the knowledge of the recruitment probabilities of these singular patchy woods, helping to develop good land management policies. This management should be based on the combination of the increase in self-maintenance probabilities of the previously established woodlands, and the promotion of those natural processes of population propagation documented here.